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UBC Theses and Dissertations

Spectral analysis of human evoked potentials Davis, Alan Edwin 1975

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SPECTRAL ANALYSIS OF HUMAN EVOKED POTENTIALS by ALAN EDWIN DAVIS B . A . S c . , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1969 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREEE OF DOCTOR OF PHILOSOPHY i n the Department o f PHYSIOLOGY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d / ^ g ^ h d a r d THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1975 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may b e g r a n t e d b y t h e H e a d o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t b e a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 ABSTRACT The purposes o f t h i s i n v e s t i g a t i o n were: (1) t o a p p l y s p e c t r a l o r f r e q u e n c y a n a l y s i s t o v i s u a l and a u d i t o r y evoked p o t e n t i a l s (EPs) r e c o r d e d from t h e s c a l p ; (2) t o m e a n i n g f u l l y r e l a t e the r e s u l t i n g f r e q u e n c y - d o m a i n d e s c r i p t i o n s t o p h y s i o l o g i c a l f a c t o r s such as speech d o m i n a n c e , s t i m u l u s m o d a l i t y and c o r t i c a l a r e a ; (3) t o d e t e r m i n e i f s i m p l e f l a s h and c l i c k s t i m u l i c o u l d g e n e r a t e l e f t - t o - r i g h t h e m i s p h e r i c d i f f e r e n c e s , and i f s o ; (4) to s t u d y v a r i o u s a s p e c t s o f t h e s e d i f f e r e n c e s , such as t h e i r r e l a t i o n s h to handedness, speech dominance and s t i m u l u s m o d a l i t y . I t was h y p o t h e s i z e d t h a t s p e c t r a l a n a l y s i s might d e t e c t EP d i f f e r e n c e s t h a t were not o b s e r v a b l e by measures o f peak a m p l i t u d e s and l a t e n c i e s , and t h a t a s y m m e t r i c r e s p o n s e s c o u l d be g e n e r a t e d by s i m p l e s t i m u l i . Reviewed e v i d e n c e s u g g -e s t e d t h a t v e r b a l s t i m u l i , such as speech o r l e t t e r s , were p r o c e s s e d w i t h i n t h e l e f t , s p e e c h - d o m i n a n t h e m i s p h e r e , w h i l e n o n - v e r b a l s t i m u l i , such as g e o m e t r i c p a t t e r n s o r m e l o d i e s , were p r o c e s s e d w i t h i n t h e r i g h t , speech non-dominant h e m i s p h e r e . The r e s u l t s showed t h a t EP a m p l i t u d e s were l a r g e s t o v e r t h e s p e c i f i c p r o j e c t i o n c o r t e x o f t h e s t i m u l a t e d m o d a l i t y , but h e m i s p h e r i c d i f f e r e n c e s o f a m p l i t u d e s were not r e l a t e d t o the known speech dominance o f e p i l e p t i c p a t i e n t s . In c o n t r a s t , c o h e r e n c e o r s i m i l a r i t y o f form between p a i r s o f EPs was r e l a t e d t o speech d o m i n a n c e , and was g r e a t e r o v e r t h e sp eech-dominant hemisphere f o r c l i c k s t i m u l i , and o v e r the speech non-dominant hemisphere f o r f l a s h s t i m u l i . These r e s u l t s s u g g e s t e d t h a t t h e a m p l i t u d e o f r e s p o n s e s r e p r e s e n t e d a b i l a t e r a l c o r t i c a l r e s p o n s e t o the s e n s o r y s t i m u l u s , w h i l e t h e l a t e r a l i z e d , c o h e r e n t s p r e a d r e p r e s e n t e d p e r c e p t u a l p r o c e s s i n g o r e x t r a c t i o n o f meaning from t h a t s t i m u l u s . i i TABLE OF CONTENTS I . INTRODUCTION 1 A. General i n t r o d u c t i o n o f purpose 1 B. The averaged evoked p o t e n t i a l (EP) 2 C. EP a n a l y s i s 5 D. S p e c t r a l a n a l y s i s 7 E. The bases of h e m i s p h e r i c asymmetry 12 1 . I n t r o d u c t i o n 12 2 . L e s i o n s t u d i e s 12 3 . S t u d i e s o f s p l i t - b r a i n p a t i e n t s 16 4 . D i c h o t i c l i s t e n i n g 18 5 . Asymmetries w i t h i n the v i s u a l system 22 6 . Handedness 24 7. E l e c t r o p h y s i o l o g i c a l asymmetries 26 8 . C o n c l u s i o n s on h e m i s p h e r i c asymmetries 31 F. Statement o f purpose 38 I I . METHODS 41 A. I n t r o d u c t i o n 41 B. S p e c t r a l a n a l y s i s 41 C. S u b j e c t s 44 D. E l e c t r o d e placements 45 E. Equipment and e x p e r i m e n t a l c o n d i t i o n s 47 F. Experiment I - Power s p e c t r a l a n a l y s i s 48 G. Experiment II - Coherence a n a l y s i s 49 1 . I n t r o d u c t i o n 49 i i i 2 . E s t i m a t i o n o f c o h e r e n c e . 50 3 . A n a l y s i s o f v a r i a n c e 51 H. Experiment I I I - D i s c r i m i n a n t a n a l y s i s 53 1. I n t r o d u c t i o n 53 2 . S p e c t r a l a n a l y s i s 55 3 . Asymmetry c a l c u l a t i o n s 56 4 . D i s c r i m i n a n t a n a l y s i s 56 I I I . RESULTS 58 A. Experiment I - Power s p e c t r a l a n a l y s i s 58 1 . The EP power s p e c t r a 58 2 . S t a t i s t i c a l a n a l y s i s 61 3 . The temporal form o f t h e f r e q u e n c y groups 61 B. Experiment II - Coherence a n a l y s i s 64 1 . Comparison o f h e m i s p h e r i c asymmetries 64 2 . M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e 67 C. Experiment I I I - D i s c r i m i n a n t a n a l y s i s 72 1. Coherence asymmetries 72 2. D i s c r i m i n a n t e q u a t i o n s o f c o h e r e n c e asymmetries 72 3 . Speech dominance o f normal s u b j e c t s 76 4 . D i s c r i m i n a n t a n a l y s i s o f power s p e c t r a 78 5 . Coherence asymmetries and handedness 79 IV. DISCUSSION 8 3 A. Experiment I > Power s p e c t r a l a n a l y s i s 83 B. Experiment I I - Coherence a n a l y s i s 8 8 C. Experiment I I I - D i s c r i m i n a n t a n a l y s i s 97 i v V. CONCLUSIONS 105 A! Summary o f s p e c t r a l a n a l y s i s 105 B. P h y s i o l o g i c a l s i g n i f i c a n c e o f t h e s p e c t r a l components 107 C. V e r b a l and n o n - v e r b a l s t i m u l i 108 D. I m p l i c a t i o n s f o r h e m i s p h e r i c f u n c t i o n a l asymmetries 109 E. F i n a l c o n c l u s i o n s 113 V I . BIBLIOGRAPHY 114 V I I . CURRICULUM VITA 136 V L I S T OF T A B L E S I . G r o u p d i s t r i b u t i o n o f m a x i m u m V E P a n d A E P c o h e r e n c e s 6 5 I I . V E P a n d A E P c o h e r e n c e s f o r s i n g l e s u b j e c t s 6 6 I I I . M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e o f c o h e r e n c e a s y m m e t r i e s 6 8 I V . M e a n c o h e r e n c e a s y m m e t r i e s 7 0 V . D i s c r i m i n a n t a n a l y s i s o f c o h e r e n c e a s y m m e t r i e s 7 3 V I . D i s t r i b u t i o n o f s p e e c h d o m i n a n c e i n e a c h s u b j e c t g r o u p 7 7 V I I . D i s c r i m i n a n t a n a l y s i s o f p o w e r s p e c t r a l a s y m m e t r i e s 7 9 V I I I . A n a l y s i s o f v a r i a n c e o f r i g h t a n d l e f t - h a n d e d s u b j e c t ' s c o h e r e n c e a s y m m e t r i e s 8 1 v i LIST OF FIGURES 1 . T y p i c a l averaged evoked p o t e n t i a l s 3 2. E l e c t r o d e placements f o r r e c o r d i n g s 46 3 . P r i m a r y p r o j e c t i o n a r e a EPs and s p e c t r a 59 4 . N o n - p r i m a r y p r o j e c t i o n a r e a EPs and s p e c t r a 60 5 . M e d i a n , o c c i p i t a l - m i n u s - t e m p o r a l s p e c t r a 62 6. D i g i t a l l y f i l t e r e d 0 - 5 h z . and 6-12 h z . components o f EPs 63 ACKNOWLEDGEMENTS I would l i k e t o thank t h e f o l l o w i n g p e o p l e f o r t h e i r h e l p d u r i n g my g r a d u a t e c a r e e r . In t h e Department o f P h y s i o l o g y , D r s . H a r o l d Copp, Hugh McLennan and Tony P e a r s o n , and Mr. K u r t Henze gave much a s s i s t a n c e and f r i e n d l y encouragement i n c o u r s e w o r k , t e a c h i n g l a b s , v a r i o u s a p p l i c a t i o n s and e n c o u n t e r s w i t h t h e b u r e a u c r a c y . In t h e D i v i s i o n o f N e u r o l o g i c a l S c i e n c e s , Anne Hamm and Ed Jung h e l p e d i n n u m e r a b l e t i m e s i n k e e p i n g equipment w o r k i n g p r o p e r l y , r u n n i n g e x p e r i m e n t s , and b e i n g g e n e r a l l y i n t e r e s t e d i n my work. I e s p e c i a l l y thank Dr. Juhn A. Wada, my s u p e r v i s o r , f o r m o s t l y " d o i n g n o t h i n g " i n a zen s e n s e , w h i c h I hope h e ' l l a p p r e c i a t e . Much o f t h e o r i g i n a l i t y o f my work has a r i s e n because i t was not i n i t i a l l y r e s t r i c t e d t o any p a r t i c u l a r a r e a o r i d e a , - e i t h e r mine o r Dr. W a d a ' s . F i n a l l y , I warmly a p p r e c i a t e my w i f e , H e a t h e r , who has been a q u i e t background o f warm f e e l i n g a g a i n s t my o f t e n f r e n e t i c a c t i v i t i e s . A l a n J u l y E. D a v i s 1 5 , 1975. 1 INTRODUCTION Evoked p o t e n t i a l s ( E P s ) , w h i c h a r e s m a l l EEG t r a n s i e n t s g e n -e r a t e d i n r e s p o n s e t o p e r i p h e r a l s e n s o r y s t i m u l a t i o n , have been u s e f u l i n t h e i n v e s t i g a t i o n o f t h e c o r t i c a l mechanisms o f s e n s o r y i n f o r m a t i o n p r o c e s s i n g . Among t h e s e have been t h e d e m o n s t r a t i o n t h a t EP asymmetries may be r e l a t e d t o h e m i s p h e r i c asymmetries such as speech dominance, and p o s s i b l y handedness. Most i n v e s t i g a t i o n s , however, have c o n c e n t r a t e d upon the temporal form o f the E P s , and o n l y a few upon t h e i r f r e q u e n c y c h a r a c t e r i s t i c s . The purpose o f t h e p r e s e n t i n v e s t i g a t i o n was to a p p l y f r e q u e n c y a n a l y s i s o f EPs t o the s t u d y o f v a r i o u s c o r t i c a l a s y m m e t r i e s . To d e f i n e t h i s purpose more c l e a r l y , however, a r e v i e w i s n e c e s s a r y o f evoked p o t e n t i a l a n a l y s i s , f r e q u e n c y o r s p e c t r a l a n a l y s i s , and a s p e c t s o f h e m i s p h e r i c asymmetry. 2 B. The averaged evoked p o t e n t i a l Evoked p o t e n t i a l s (EPs) a r e s m a l l a m p l i t u d e t r a n s i e n t s i n t h e EEG r e c o r d e d from t h e s c a l p . They a r e t y p i c a l l y o f 1 t o 20 m i c r o v o l t s i n a m p l i t u d e a g a i n s t a background EEG w h i c h ranges from 2 t o 100 t i m e s l a r g e r . EPs can be g e n e r a t e d i n r e s p o n s e t o many t y p e s o f s t i m u l i , but i n t h i s t h e s i s o n l y v i s u a l evoked p o t e n t i a l s (VEPs) and a u d i t o r y evoked p o t e n t i a l s (AEPs) w i l l be i n v e s t i g a t e d . F i g . 1 shows some t y p i c a l EPs g e n e r a t e d w i t h f l a s h and c l i c k s t i m u l i . A f a i r degree o f v a r i a t i o n i s p r e s e n t both w i t h i n the EPs o f one s u b j e c t r e c o r d e d from d i f f e r e n t a r e a s and between s u b j e c t s . Many schemes have been s u g g e s t e d t o l a b e l t h e peaks i n a c o n s i s t e n t morphology ( f o r example: C i g a n e k , 1961; G a s t a u t and R e g i s , 1965; G o f f , M a t s u m i y a , A l l i s o n and G o f f , 1969). None o f t h e s e schemes has g a i n e d f u l l a c c e p t a n c e , t h o u g h , p r i m a r i l y because o f the v a r i a b i l i t y o f peak l a t e n c i e s , a m p l i t u d e s and appearance between r e c o r d i n g a r e a s and between s u b j e c t s . S i n c e t h e EP a m p l i t u d e s a r e o f v e r y much s m a l l e r a m p l i t u d e than t h e background EEG (2 t o 100 t i m e s ) , a n a l y s i s o f a s i n g l e EP i s a l m o s t i m p o s s i b l e . To a c c e n t u a t e t h i s s m a l l waveform a g a i n s t t h e l a r g e r ongoing a c t i v i t y , the a v e r a g i n g t e c h n i q u e has been d e v e l o p e d . U s i n g a s m a l l l a b o r a t o r y c o m p u t e r , many EPs are averaged w h i c h i n e f f e c t r e t a i n s EEG changes which c o n s i s t e n t l y o c c u r w i t h the p r e s e n t a t i o n o f t h e s t i m u l u s , and r e j e c t s t h o s e t h a t do n o t . . A.K. A.D. VEP O , ^ Y ^ ~ ^ 1.0 Seq.. 5/JV. AEP o, ' \ r ^ ^ -02>^Vw^^ VE P o/\^^^^ 02 T3 T 4 AEPo o2' F i g u r e 1 Some t y p i c a l , averaged (N=40) evoked p o t e n t i a l s f o r f l a s h s t i m u l i (VEPs) and c l i c k s t i m u l i ( A E P s ) . L e f t and r i g h t o c c i p i t a l (01 and 02) and l e f t and r i g h t temporal (T3 and T4) e l e c t r o d e p l a c e m e n t s . Two s u b j e c t s (AK and AD). Each EP i s 1 second d u r a t i o n . 4 The a v e r a g i n g t e c h n i q u e i s w i d e l y used because o f i t s s i m p l i c i t y and because o f t h e many i n s i g h t s i t has g i v e n i n t o c o r t i c a l r e s p o n s e s t o s e n s o r y s t i m u l i . I t d o e s , however, make c e r t a i n a s s u m p t i o n s about the EP " s i g n a l " and t h e EEG background " n o i s e " . The s i g n a l i s assumed to be t i m e - l o c k e d t o t h e s t i m u l u s p r e s e n t a t i o n , w h i l e the n o i s e i s assumed t o be independent o f t h e s t i m u l u s . I t i s f u r t h e r assumed t h a t n e i t h e r t h e s i g n a l nor the n o i s e change w i t h r e p e a t e d p r e s e n t a t i o n s o f the s t i m u l u s . In p a r t i c u l a r , t h i s means t h a t t h e s t i m u l u s p r e s e n t a t i o n has no e f f e c t on the background EEG t h a t would be r e c o r d e d i f no s t i m u l u s were p r e s e n t . The EEG c l e a r l y does not e x a c t l y s a t i s f y the a s s u m p t i o n s o f a v e r a g i n g . For i n s t a n c e , EP a m p l i t u d e s and l a t e n c i e s change as t h e s u b j e c t h a b i t u a t e s to t h e s t i m u l u s ( W a l t e r , 1 9 6 4 ) , so the EP form i s not c o n s t a n t o v e r t i m e . Complex i n t e r a c t i o n s a l s o o c c u r between t h e EP and EEG. Background EEG i s a f f e c t e d by the s t i m u l u s p r e s e n t a t i o n (Gibson and B r o u g h t o n , 1969;- Woods and B r o u g h t o n , 1 9 6 9 ) , and EPs r e c o r d e d d u r i n g low a l p h a c o n d i t i o n s a r e o f g r e a t e r a m p l i t u d e and a r e l e s s v a r i a b l e than t h o s e r e c o r d e d d u r i n g h i g h a l p h a c o n d i t i o n s ( S p i l k e r , K a m i y a , C a l l a w a y and Y e a g e r , 1 9 6 9 ) . Many more examples c o u l d be p r e s e n t e d t o show t h a t t h e EEG does n o t s a t i s f y t h e t h e o r e t i c a l a s s u m p t i o n s n e c e s s a r y t o d e r i v e averaged EPs. The key q u e s t i o n , however, i s whether t h e s e v a r i a t i o n s from t h e i d e a l a r e s m a l l enough t o i g n o r e . S i n c e i t s i n t r o d u c t i o n by Dawson (1951; 1 9 5 3 ) , s i g n a l a v e r a g i n g o f p h y s i o l o g i c a l d a t a has p r e s e n t e d many u s e f u l i n s i g h t s . 5 G e n e r a l l y , i t seems t h a t i f t h e v a r i a b i l i t y o f i n d i v i d u a l EPs i s c o n s i d e r e d i n the d e s i g n o f e x p e r i m e n t s , and i f s i t u a t i o n s a r e a v o i d e d where t h i s c o u l d become a major f a c t o r , the averaged EP t e c h n i q u e can be used t o d e r i v e r e s u l t s which agree w i t h and e x t e n d o t h e r b e h a v i o r a l and e l e c t r o p h y s i o l o g i c a l d a t a . T h i s o b s e r v a t i o n has been made many t i m e s , and the r e s u l t s o f t h i s t h e s i s w i l l demonstrate i t a g a i n . C. EP a n a l y s i s The EP c o n s i s t s o f an o s c i l l a t i n g waveform. Two b a s i c approaches e x i s t f o r t h e a n a l y s i s o f t h i s waveform and each makes a s s u m p t i o n s about the n a t u r e o f EP g e n e r a t i o n . One i s c a l l e d t i m e - d o m a i n a n a l y s i s and the o t h e r f r e q u e n c y - d o m a i n a n a l y s i s . The t i m e - d o m a i n approach assumes t h a t the s e r i e s o f peaks i n t h e EP r e p r e s e n t s some a s p e c t o f a sequence o f temporal e v e n t s . That i s , t h e peak a t one t i m e i n t e r v a l o f t h e EP i s g e n e r a t e d by a d i f f e r e n t p r o c e s s than t h e peak a t a n o t h e r t i m e . The EP i s thus c o n s i d e r e d t o be a sequence o f e v e n t s which a r e t r i g g e r e d one a f t e r the o t h e r . The f r e q u e n c y - d o m a i n approach assumes t h a t t h e f r e q u e n c y components o f the EP r e p r e s e n t some a s p e c t o f t h e c o r t i c a l r e s p o n s e t o the s t i m u l u s . In o t h e r w o r d s , the EP components a t d i f f e r e n t f r e q u e n c i e s a r e r e l a t e d t o d i f f e r e n t p r o c e s s e s . From t h i s v i e w p o i n t , t h e v a r i o u s peaks o f t h e EP would be c o n s i d e r e d t o be p r i m a r i l y g e n e r a t e d by one o s c i l l a t o r y p r o c e s s i f they were a l l i n t h e same f r e q u e n c y r a n g e . 6 The t i m e and f r e q u e n c y - d o m a i n a n a l y s e s thus make f u n d a m e n t a l l y d i f f e r e n t a s s u m p t i o n s about the way i n which EPs a r e g e n e r a t e d . Time-domain a n a l y s i s assumes t h a t d i f f e r e n t peaks a t d i f f e r e n t times a r e produced by d i f f e r e n t p r o c e s s e s , w h i l e f r e q u e n c y - d o m a i n a n a l y s i s assumes t h a t d i f f e r e n t f r e q u e n c i e s a r e produced by d i f f e r e n t p r o c e s s e s . The a s s u m p t i o n s o f EP g e n e r a t i o n i m p l i c i t i n t h e s e two approaches have been n e i t h e r proved nor d i s p r o v e d , and the i n v e s t i g a t i o n o f the mechanisms remains an a c t i v e a r e a o f r e s e a r c h . Both peak and f r e q u e n c y components have been shown to be r e l a t e d t o p h y s i o l o g i c a l e v e n t s , and e i t h e r can be u s e f u l i n o b t a i n i n g i n f o r m a t i o n on c o r t i c a l r e s p o n s e s t o s e n s o r y s t i m u l i (Regan, 1972). 7 D. S p e c t r a l a n a l y s i s o f EPs One way o f e x a m i n i n g the f r e q u e n c y components o f evoked p o t e n t i a l s i s t h r o u g h t h e use o f s p e c t r a l a n a l y s i s . T h i s a p p l i c a t i o n o f t h e t e c h n i q u e i s a l m o s t c o m p l e t e l y u n e x p l o r e d , so most o f t h e i n t r o d u c t i o n below has few r e f e r e n c e s t o EP s t u d i e s . In e s s e n c e , s p e c t r a l a n a l y s i s decomposes a s i g n a l o r waveform i n t o a sum o f s i n u s o i d a l components o f d i f f e r e n t f r e q u e n c i e s . The a m p l i -tude and phase s h i f t o f each component w i t h r e s p e c t t o t h e temporal o r i g i n o f the s i g n a l a r e such t h a t when summed t o g e t h e r , t h e y w i l l produce t h e o r i g i n a l waveform. Measures o f a m p l i t u d e and phase a r e thus a v a i l a b l e from t h i s d e c o m p o s i t i o n f o r each f r e q u e n c y component. In a d d i t i o n , t h e s i m i l a r i t y o f form and phase r e l a t i o n s h i p s between each p a i r o f waveforms can be d e r i v e d . The m a t h e m a t i c a l t e c h n i q u e s o f s p e c t r a l a n a l y s i s a r e w e l l d e v e l o p e d (Blackman and T u k e y , 1956; J e n k i n s and W a t t s , 1968; Hannan, 1 9 7 0 ) , and are based on c a l c u l a t i n g the f o u r i e r t r a n s f o r m o r spectrum o f t h e s i g n a l . T h i s s p e c t r u m i s a s e t o f numbers which d e s c r i b e s t h e a m p l i t u d e and phase o f each f r e q u e n c y component o f t h e waveform. In t h i s t h e s i s , t h e s p e c t r a a r e c a l c u l a t e d u s i n g c o n v e n t i o n a l p r o c e d u r e s i n c o r p o r a t e d i n a s t a n d a r d s p e c t r a l a n a l y s i s program from t h e UCLA B i o m e d i c a l Computing F a c i l i t y ( D i x o n , 1973). A b r i e f o u t l i n e o f the program i s i n c l u d e d i n t h e Methods s e c t i o n . 8 Power s p e c t r a l a n a l y s i s i s a method o f e x a m i n i n g t h e power o r a m p l i t u d e o f each component i n the spectrum i n d e p e n d e n t l y o f i t s phase r e l a t i o n s h i p s . T h i s phase removal a l l o w s s i m p l e comparisons between s i g n a l s o f t h e d i s t r i b u t i o n o f a m p l i t u d e s a t d i f f e r e n t f r e q -u e n c i e s . A n a l y s i s o f the power s p e c t r a o f EEG has been u s e f u l i n a t t e m p t s to q u a n t i f y t h e v i s u a l r e a d i n g o f r e c o r d s f o r a b n o r m a l i t i e s ( f o r example: W a l t e r , 1963; H o r d , J o h n s o n , L u b i n , N a i t o l i , Nute and A u s t i n , 1 9 6 9 ; Wennberg and Z e t t e r b u r g , 1971; G i a n n i t r a p a n i and K a y t o n , 1974). However, t h e r e a r e many problems c o n c e r n i n g t h e s e l e c t i o n o f s a m p l e s , sample l e n g t h , and t h e t i m e - v a r y i n g c h a r a c t e r i s t i c s o f t h e EEG. In a d d i t i o n , t h e many d i v e r g e n t t h e o r i e s o f EEG g e n e r a t i o n have l i m i t e d t h e u s e f u l n e s s o f power s p e c t r a l a n a l y s i s i n the i n t e r p r e t a t i o n o f t h e normal c o r t i c a l p r o c e s s i n g o f s e n s o r y i n f o r m a t i o n . In c o n t r a s t t o the many a p p l i c a t i o n s o f power s p e c t r a l a n a l y s i s t o EEG, v e r y few a t t e m p t s have been made t o s i m i l a r l y a n a l y z e EP waveforms ( S h i p l e y , Jones and F r y , 1968; Emrich and M i c h a e l , 1970; Regan, 1972). B u t , t h e s e waveforms a r e i n some ways i d e a l f o r t h i s method s i n c e t h e y have a p r e c i s e l y known s t a r t i n g p o i n t and a r e f a i r l y s t a b l e and r e p e a t a b l e . T h e i r s h o r t d u r a t i o n , however, c r e a t e s s e v e r a l d i s a d v a n t a g e s . The s p e c t r a l e s t i m a t e s a r e q u i t e v a r i a b l e and the o r i g i n a l d a t a may not be c l o s e l y g a u s s i a n o r normal as assumed by the t e c h n i q u e . These two problems can be overcome by p o o l i n g t h e r e s u l t s o f many s u b j e c t s so t h a t t h e p o p u l a t i o n s approach n o r m a l i t y ( c e n t r a l l i m i t theorem o f s t a t i s t i c s ; Snedecor and C o c h r a n , 1967) and by s t a t i s t i c a l l y a n a l y z i n g the r e s u l t s o f a l l s u b j e c t s as a g r o u p . 9 When two o r more waveforms a r e a v a i l a b l e , c o m p a r i s o n s can be made between them by u s i n g t h e c r o s s - s p e c t r u m . T h i s i s d e r i v e d from t h e spectrum o f each waveform and e x p r e s s e s t h e t o t a l i t y o f r e l a t i o n s h i p s between a l l t h e waveforms. I t can be shown ( J e n k i n s and W a t t s , 1968) t h a t the power spectrum o f each waveform and t h e s i m i l a r i t y o f form between each p a i r o f waveforms, d e r i v e d from t h e c r o s s - s p e c t r u m , c o n s t i t u t e t h i s t o t a l d e s c r i p t i o n . S i m i l a r i t y o f form i s measured by t h e c o h e r e n c e , which i s somewhat s i m i l a r t o c o r r e l a t i o n , e x c e p t t h a t c o h e r e n c e measures c o r r -e l a t i o n between two waveforms a f t e r c o n s t a n t phase s h i f t s between them have been removed. For e x a m p l e , i f an o c c i p i t a l EP has e x a c t l y t h e same waveform but o c c u r s e a r l i e r than a temporal EP, t h e y w i l l be s h i f t e d i n phase w i t h r e s p e c t t o each o t h e r . T h e i r c o r r e l a t i o n w i l l t h u s be l e s s than u n i t y s i n c e t h e y do not behave t h e same a t each t i m e . The c o h e r e n c e between them, however, w i l l be n e a r l y u n i t y s i n c e they w i l l be h i g h l y c o r r e l a t e d a f t e r t h e phase s h i f t between them has been removed. In t h i s s e n s e , c o h e r e n c e measures s i m i l a r i t y o f form between two waveforms, i n d e p e n d e n t l y o f whether t h e y o c c u r t o g e t h e r o r a t d i f f e r e n t t i m e s . Once s p e c t r a l q u a n t i t i e s such as t h e power s p e c t r a and c o h e r e n c e s have been c a l c u l a t e d , a t e c h n i q u e i s n e c e s s a r y f o r i d e n t i f y i n g the d i f f e r e n c e s t h a t o c c u r w i t h d i f f e r e n t s t i m u l u s c o n d i t i o n s o r s u b j e c t s . D i s c r i m i n a n t a n a l y s i s (Snedecor and C o c h r a n , 1967; A f i f i and A z e n , 1970) i s such a t e c h n i q u e , and i s based on s e l e c t i n g p r o p e r t i e s o f t h e d a t a which 10 b e s t d i s c r i m i n a t e between t h e groups o f i n t e r e s t . G e n e r a l l y , t h e c r i t e r i o n o f " b e s t " i s a p r o b a b i l i s t i c one i n w h i c h the s i g n i f i c a n c e o f each p r o p e r t y ' s r e l a t i o n s h i p t o the group i s e v a l u a t e d . As an example o f i t s u s e , d i s c r i m i n a n t a n a l y s i s c o u l d be used t o i d e n t i f y which f r e q u e n c y o r peak components o f t h e EP were most a f f e c t e d by e x p e r i m e n t a l c o n d i t i o n s , such as s t i m u l u s m o d a l i t y o r a t t e n t i o n , o r by s u b j e c t c h a r a c t e r i s t i c s , such as handedness o r speech dominance. P r e v i o u s a p p l i c a t i o n s o f d i s c r i m i n a n t a n a l y s i s o f evoked p o t e n t i a l s have been made i n the i d e n t i f i c a t i o n o f changes i n t h e i r t i m e - d o m a i n c h a r a c t e r i s t i c s d u r i n g s h i f t s o f s e l e c t i v e a t t e n t i o n ( D o n c h i n , 1967) ; i n s c h i z o p h r e n i c v e r s u s normal s t a t e s ( D o n c h i n , C a l l a w a y and J o n e s , 1 9 7 0 ) ; and d u r i n g v a r i o u s t y p e s o f c o g n i t i v e t a s k s ( D o n c h i n , Kubovy, K u t a s , Johnson and H e r n i n g , 1973). U s i n g a computer s i m u l a t i o n o f EP g e n e r a t i o n , Donchin and H e r n i n g (1975) have shown t h a t t h e t e c h n i q u e i s a p o w e r f u l and r e l i a b l e means o f d e t e c t i n g s i g n i f i c a n t d i f f e r e n c e s between EPs. However, no use has y e t been made o f i t i n d e t e c t i n g d i f f e r e n c e s i n the f r e q u e n c y c h a r a c t e r i s t i c s o f EPs. D i f f e r e n c e s between d i f f e r e n t r e s p o n s e s can a l s o be d e t e c t e d w i t h a n a l y s i s o f v a r i a n c e (Snedecor and C o c h r a n , 1967; W i n e r , 1971). T h i s t e c h n i q u e assumes t h a t the o b s e r v e d r e s p o n s e s can be broken down i n t o a number o f components, c a l l e d e f f e c t s , and i n t e r a c t i o n s between t h e s e components. For e x a m p l e , t h e a m p l i t u d e o f a c e r t a i n peak o f the EP c o u l d be o b s e r v e d w i t h . d i f f e r e n t s t i m u l u s i n t e n s i t i e s and c o l o r s . I n t e n s i t y and c o l o r would be the two e f f e c t s , and t h e i r i n t e r a c t i o n would measure 11 how the p a t t e r n o f r e s p o n s e t o d i f f e r e n t i n t e n s i t i e s depended on t h e c o l o r . A l t h o u g h t h e a n a l y s i s o f v a r i a n c e i s a w e l l - k n o w n t e c h n i q u e i n the b e h a v i o r a l s c i e n c e s , i t s use i n t h e e v a l u a t i o n o f EEG o r o t h e r s p e c t r a seems t o have been l i m i t e d . T h i s may be due t o the f a c t t h a t many i n v e s t i g a t o r s w i t h s u f f i c i e n t m a t h e m a t i c a l background t o use s p e c t r a l a n a l y s i s , such as e l e c t r i c a l e n g i n e e r s o r g e o p h y s i c i s t s , o f t e n have a poor u n d e r s t a n d i n g o f g e n e r a l s t a t i s t i c s . R e c e n t l y , however, t h i s t r e n d seems t o be r e v e r s i n g . For e x a m p l e , G i a n n i t r a p a n i and Kayton (1974) have used an a n a l y s i s o f v a r i a n c e o f the EEG power s p e c t r a o f s c h i z o -p h r e n i c p a t i e n t s and normal s u b j e c t s t o show d i f f e r e n c e s i n t h e i r dominant f r e q u e n c i e s . A note on c o h e r e n c e : C o h e r e n c e , as used i n t h i s t h e s i s , i s a c t u a l l y the magnitude o f a more g e n e r a l q u a n t i t y c a l l e d t h e complex c o h e r e n c e , w h i c h c o n s i s t s o f both a magnitude and a phase. The magnitude measures the degree o f s i m i l a r i t y of form between two waveforms, w h i l e t h e phase term measures the phase s h i f t between them a t w h i c h the magnitude i s measured. I n t u i t i v e l y , a complete s e t of any number o f waveforms can then be d e s c r i b e d by t h e i r a m p l i t u d e s o r power s p e c t r a , t h e i r s i m i l a r i t i e s o f form or magnitude o f c o h e r e n c e s , and the' ;phase o f c o h e r e n c e s between them. 12 E. The bases o f h e m i s p h e r i c asymmetries 1 . I n t r o d u c t i o n F u n c t i o n a l d i f f e r e n c e s between t h e r i g h t and l e f t c e r e b r a l hemispheres have been known s i n c e t h e e a r l y 1800s. From t h a t time u n t i l t h e 1 9 6 0 s , t h i s knowledge was based a l m o s t c o m p l e t e l y on n e u r o -l o g i c a l s t u d i e s o f p a t i e n t s w i t h l e s i o n s l o c a l i z e d i n v a r i o u s c o r t i c a l a r e a s . In the l a s t d e c a d e , however, a v a r i e t y o f b e h a v i o r a l and e l e c t r o p h y s i o l o g i c a l t e c h n i q u e s have become a v a i l a b l e f o r t h e e l u c i d -a t i o n o f h e m i s p h e r i c d i f f e r e n c e s w i t h a wide v a r i e t y o f s t i m u l u s c o n d i t i o n s and t a s k s u s i n g normal s u b j e c t s . The purpose o f t h i s s e c t i o n i s t o r e v i e w t h e s e f i n d i n g s , both o l d and new, and t o p l a c e them i n a s u i t a b l e p e r s p e c t i v e f o r the i n v e s t i g a t i o n s o f h e m i s p h e r i c asymmetry u s i n g s p e c t r a l a n a l y s i s . 2 . L e s i o n s t u d i e s B r a i n asymmetries were f i r s t o b s e r v e d i n n e u r o l o g i c a l p a t i e n t s w i t h l e s i o n s i n v a r i o u s c o r t i c a l a r e a s . A l t h o u g h the i n f e r e n c e o f f u n c t i o n on t h e b a s i s o f t h e i n t e r r u p t i o n o f some a s p e c t o f t h a t f u n c t i o n must always be made c a u t i o u s l y , a l a r g e body o f r e l i a b l e knowledge has a c c u m u l a t e d . The f i r s t c o n t r i b u t i o n s t o t h i s knowledge o f t h e r e l a t i o n s h i p o f b r a i n l e s i o n s t o b e h a v i o r a l d e f i c i t s o f speech and language were noted by Broca (1861) and Wernicke (1874). T h e i r c r i t i c a l o b s e r v a t i o n was t h a t many v a r i e t i e s o f speech d i s o r d e r s were a l m o s t i n v a r i a b l y 13 l i n k e d w i t h l e s i o n s i n the l e f t h e m i s p h e r e . Over t h e y e a r s , many a s p e c t s o f t h e s e d i s o r d e r s became a p p a r e n t , such as t h e n e c e s s i t y o f communicating l i n k s t o and from t h e v i s u a l c e n t r e s f o r r e a d i n g and w r i t i n g , and the a u d i t o r y a r e a s f o r u n d e r s t a n d i n g spoken w o r d s . Today, the i n t e r r e l a t i o n s h i p s o f t h e s e a r e a s a r e w e l l u n d e r s t o o d from a n e u r o -l o g i c a l v i e w p o i n t . That i s , most d e f i c i t s can be u n d e r s t o o d i n terms o f t h e pathways and c e n t r e s which a r e i n t e r r u p t e d o r d e s t r o y e d by the l e s i o n . The modern c o n t r i b u t i o n s t o d i f f e r e n t i a l l e f t and r i g h t h e m i s p h e r i c i n v o l v e m e n t i n the b e h a v i o r o f n e u r o l o g i c a l p a t i e n t s have been m o s t l y based on i n t e r r u p t i v e p r o c e s s e s , e i t h e r p a t h o l o g i c a l as i n t u m o u r s , s t r o k e s , e t c . , o r i n t e n t i o n a l as i n s u r g e r y . M i s s i l e o r . p e n e t r a t i n g wounds ( L u r i a , 1969) have been u s e f u l b u t the u n c e r t a i n t i e s o f s i t e s and e n t e r i n g paths have l i m i t e d t h e l o c a l i z a t i o n o f l e s i o n s . S t i m u l a t i o n o f the c o r t e x d u r i n g s u r g e r y ( P e n f i e l d and R o b e r t s , 1959) has a l s o been l i m i t e d because o f the s h o r t t ime t h a t i s a v a i l a b l e w i t h e a c l u p a t i e h t , and because o n l y c e r t a i n s t r u c t u r e s a r e a c c e s s i b l e . Temporal lobectomy ( M i l n e r , 1961) f o r the t r e a t m e n t o f e p i l e p s y has produced a l a r g e amount o f i n f o r m a t i o n . However, the e x t e n t o f l o n g - t i m e a b n o r m a l i t i e s and t h e p o s s i b i l i t y o f compensating mechanisms make t h e s e o b s e r v a t i o n s u n c e r t a i n i n t h e i r e x t e n s i o n t o normal s u b j e c t s ( G e s c h w i n d , 1970). The Wada t e s t (Wada, 1949; Wada and Rasmussen, 1 9 6 0 ) , i n w h i c h sodium amytal i s i n j e c t e d i n t o one c a r o t i d a r t e r y , has been a major s o u r c e o f i n f o r m a t i o n c o n c e r n i n g the c e r e b r a l l a t e r a l i z a t i o n o f language f u n c t i o n . 14 A l l o f t h e above l i n e s o f e v i d e n c e , i n w h i c h c o r t i c a l p r o c e s s e s a r e i n t e r r u p t e d , have s u g g e s t e d r e l a t i v e l y c l e a r c u t d i f f e r e n c e s between t h e f u n c t i o n s o f t h e l e f t and r i g h t h e m i s p h e r e s . L e s i o n s o f the l e f t , p a r i e t o - t e m p o r a l a r e a s g e n e r a l l y i n t e r f e r e w i t h some a s p e c t o f language p r o d u c t i o n o r comprehension ( H e c a e n , 1 9 6 2 ) , and o f t e n a t t h e h i g h e s t l e v e l s o f t h e language p r o c e s s . For e x a m p l e , t h e p a t i e n t may produce s e e m i n g l y n o r m a l , g r a m m a t i c a l l y c o r r e c t s e n t e n c e s , but which a r e a l m o s t empty o f meaning ( G e s c h w i n d , 1970). In c o n t r a s t , t h e r i g h t , p a r i e t o - t e m p o r a l a r e a s a r e more i n v o l v e d i n p r o c e s s e s t h a t r e q u i r e n o n - v e r b a l a n a l y s i s . L e s i o n s o f t h e s e a r e a s r e s u l t i n d e c r e a s e d s c o r e s on s p a t i a l r e a s o n i n g o r d i s c r i m i n a t i o n t e s t s ( W e i n s t e i n , 1 9 6 2 ) ; d e c r e a s e d a b i l i t y t o l o c a l i z e sounds ( N e f f , 1 9 6 2 ) ; and an i n a b i l i t y t o u n d e r s t a n d t h e meaning o f p i c t u r e s ( C r i t c h l e y , 1 9 6 2 ) . P a t i e n t s who have had r i g h t temporal l o b e c t o m i e s a r e i m p a i r e d on a u d i t o r y t a s k s u s i n g n o n - v e r b a l m a t e r i a l s , but cannot be d i f f e r e n t i a t e d f r o m normal s u b j e c t s i n t h e i r performance on v e r b a l t e s t s ( S h a n k w e i l e r , 1 9 6 6 ) . These same p a t i e n t s p e r f o r m l e s s w e l l than normal s u b j e c t s on t e s t s o f v i s u a l d i s c r i m i n a t i o n ( M e i r and F r e n c h , 1965; De Renzi and S p i n d l e r , 1 9 6 6 ) . These d e f i c i t s a r e e s p e c i a l l y pronounced w i t h p a t t e r n s l e s s l i a b l e t o v e r b a l i d e n t i f i c a t i o n ( M i l n e r , 1967). Asymmetries between the l e f t and r i g h t hemispheres a l s o extend t o t h e mechanisms o f memory and l e a r n i n g . F o c a l l e s i o n s o f the l e f t temporal l o b e u s u a l l y i n t e r f e r e w i t h the e f f i c i e n c y o f v e r b a l memory w h i l e s p a r i n g n o n - v e r b a l s k i l l s , w h i l e the c o n v e r s e i s t r u e w i t h r i g h t temporal l e s i o n s ( F e d i o and M i r s k y , 1969; M i l n e r , 1970) . 15 For v i s u a l l y p r e s e n t e d m a t e r i a l , M i l n e r (1968) has shown p e r c e p t u a l and r e c o g n i t i o n d i s o r d e r s among r i g h t - t e m p o r a l p a t i e n t s on t a s k s which use n o n - v e r b a l s t i m u l i which a r e n o t e a s i l y decoded i n t o v e r b a l s y m b o l s . C o n v e r s e l y , l e f t - t e m p o r a l p a t i e n t s are c o n s i s t e n t l y i m p a i r e d i n the m e m o r i z a t i o n o f v i s u a l l y p r e s e n t e d v e r b a l m a t e r i a l ( M i l n e r , 1969; S a m u e l s , 1971). G e r n e r , Ommaya and F e d i o (1972) f u r t h e r e l u c i d a t e d t h e mechanisms i n v o l v e d i n t h e s e asymmetries o f memory. They showed t h a t b o t h l e f t and r i g h t - t e m p o r a l p a t i e n t s had an i n t a c t immediate memory span f o r a l o n g s t r i n g o f v e r b a l o r n o n - v e r b a l m a t e r i a l . However, when t h e t e s t sequence exceeded t h i s memory s p a n , t h e l e f t - t e m p o r a l p a t i e n t s d i d p o o r l y on v e r b a l m a t e r i a l , and t h e r i g h t , on n o n - v e r b a l m a t e r i a l . The complex i n t e r a c t i o n s t h a t o c c u r between s e e i n g , c l a s s i f y i n g and a s s o c i a t i n g shapes have been i n v e s t i g a t e d by B i s i a c h and F a g l i o n i (1974). They found t h a t p a t i e n t s w i t h l e f t b r a i n damage were s l o w e r than t h o s e w i t h r i g h t b r a i n damage i n r e c o g n i z i n g a s s o c i a t i o n s between random s h a p e s . T h i s seems a t v a r i a n c e w i t h e a r l i e r s t u d i e s which have s u g g e s t e d t h a t s p a t i a l p r o c e s s i n g o f p a t t e r n s i s a r i g h t h e m i s p h e r i c f u n c t i o n . However, t h e y s u g g e s t e d t h a t t h e i r r e s u l t s s u p p o r t t h e h y p o t h e s i s t h a t t h e t r a n s f o r m a t i o n o f m e a n i n g l e s s shapes i n t o m e a n i n g f u l shapes may be a l e f t h e m i s p h e r i c f u n c t i o n which i s independent o f v e r b a l l a b e l l i n g . T h i s same h y p o t h e s i s had been s u g g e s t e d e a r l i e r by B o i l e r and De Renzi ( 1 9 6 7 ) . In s u p p o r t o f t h i s , they showed t h a t l e f t - b r a i n damaged p a t i e n t s had t h e p o o r e s t performance f o r p a i r e d s t i m u l i w i t h low measures o f a s s o c i a t i o n . That i s , l e f t h e m i s p h e r i c l e s i o n s i n t e r f e r e d w i t h t h e a b i l i t y t o make a s s o c i a t i o n s between v i s u a l 16 o r s p a t i a l p a t t e r n s . 3 . S t u d i e s o f s p l i t - b r a i n p a t i e n t s A n o t h e r s o u r c e o f e v i d e n c e f o r t h e l a t e r a l i z a t i o n o f p e r c e p t u a l f u n c t i o n s i s b e h a v i o r a l d a t a c o l l e c t e d from e p i l e p t i c p a t i e n t s who have had t h e i r corpus c a l l o s u m b i s e c t e d a l o n g t h e m i d l i n e . O r i g i n a l l y , t h i s r a d i c a l t e c h n i q u e was performed as a f i n a l a t t e m p t t o c o n t r o l t h e s p r e a d o f i n t r a c t a b l e s e i z u r e s from one hemisphere t o t h e o t h e r ( S p e r r y , 1964; S p e r r y , G a z z a n i g a and Bogen, 1 9 6 9 ) . I t soon became a p p a r e n t , however, t h a t t h e s e s p l i t - b r a i n p a t i e n t s , as t h e y became known, d i s p l a y e d some r e m a r k a b l e p e r c e p t u a l and motor asymmetries w h i c h c o u l d o n l y be e x p l a i n e d on t h e b a s i s o f the l a t e r a l i z a t i o n o f v e r b a l and n o n - v e r b a l p r o c e s s i n g c a p a b i l i t i e s ( G a z z a n i g a , 1970; S p e r r y , 1973). In s p l i t - b r a i n p a t i e n t s , s t i m u l i can be s e l e c t i v e l y t r a n s m i t t e d t o t h e r i g h t o r l e f t hemisphere by p r e s e n t a t i o n i n t h e l e f t o r r i g h t v i s u a l f i e l d f o r v i s u a l s t i m u l i , o r t o t h e l e f t , o r r i g h t e a r f o r a u d i t o r y s t i m u l i . Because o f t h e s e c t i o n e d c o r p u s c a l l o s u m , no i n t e r h e m i s p h e r i c communication e x i s t s and so one hemisphere cannot r e c e i v e i n f o r m a t i o n from t h e o t h e r . U s i n g t h i s t e c h n i q u e , a p a t i e n t can name an o b j e c t shown t o him i n t h e r i g h t v i s u a l f i e l d b u t not when i t i s shown i n t h e l e f t . P r e s u m a b l y , t h i s o c c u r s because an o b j e c t i n t h e r i g h t v i s u a l f i e l d i s p e r c e i v e d w i t h i n the l e f t , l a n g u a g e r d o m i n a n t h e m i s p h e r e , so t h e p a t i e n t can name i t . P r e s e n t a t i o n i n t h e l e f t v i s u a l f i e l d , however, o n l y i n f o r m s t h e r i g h t h e m i s p h e r e . S i n c e t h i s hemisphere 17 has l i t t l e o r no l i n g u i s t i c c a p a b i l i t y , t h e p a t i e n t cannot communicate i n words what he has s e e n . He can p o i n t t o a p i c t u r e o f what he has s e e n , thus i n d i c a t i n g t h a t t h e r i g h t hemisphere does have p e r c e p t u a l c a p a c i t i e s , but t h e s e a r e n o t v e r b a l l y o r i e n t e d ( S p e r r y , 1964, 1969; G a z z a n i g a , 1970). The s u p e r i o r i t y o f the r i g h t hemisphere f o r s p a t i a l r e a s o n i n g can be shown by a v a r i e t y o f t e s t s . For i n s t a n c e , t h e p a t i e n t ' s r i g h t h a n d , under t h e c o n t r o l o f t h e l e f t h e m i s p h e r e , can w r i t e a l m o s t n o r m a l l y , but i s p r a c t i c a l l y u n a b l e t o copy g e o m e t r i c f i g u r e s . C o n v e r s e l y , w i t h h i s l e f t h a n d , c o n t r o l l e d by t h e r i g h t h e m i s p h e r e , t h e p a t i e n t can copy g e o m e t r i c f i g u r e s b u t c a n n o t w r i t e words (Bogenand G a z z a n i g a , 1965, 1970; S p e r r y , 1 9 6 8 ) . As w i t h l e s i o n s t u d i e s , t h e s p l i t - b r a i n p a t i e n t s have a l s o p r o v i d e d e v i d e n c e f o r t h e l a t e r a l i z a t i o n o f d i f f e r e n t a s p e c t s o f v e r b a l and n o n - v e r b a l memory. M i l n e r and T a y l o r (1972) have shown t h a t the r i g h t hemisphere i s s u p e r i o r i n t h e t a c t i l e p a t t e r n r e c o g n i t i o n o f p r e v i o u s l y p r e s e n t e d p a t t e r n s . Teng and S p e r r y (1974) demonstrated t h a t t h e r i g h t hemisphere i s f a s t e r and more a c c u r a t e than t h e l e f t hemisphere i n apprehending t h e number o f d o t s i n a b r i e f l y p r e s e n t e d p a t t e r n . They s u g g e s t e d t h a t t h i s may due t o t h e r i g h t h e m i s p h e r e ' s g r e a t e r a b i l i t y t o remember p a t t e r n s , so c o u n t i n g c o u l d c o n t i n u e a f t e r the p a t t e r n had d i s s a p e a r e d . B e t t e r dot c o u n t i n g a b i l i t y has a l s o been o b s e r v e d i n t h e r i g h t hemisphere o f normal s u b j e c t s ( K i m u r a , 1968). 18 Evidence from s p l i t - b r a i n p a t i e n t s has a l s o shown t h a t the l e f t and r i g h t hemispheres n o r m a l l y s h i f t i n t h e i r predominance. T h i s s h i f t depends on t h e t y p e o f c o g n i t i v e t a s k b e i n g p e r f o r m e d . For i n s t a n c e , v e r b a l t a s k s such as r e a d i n g , s p e a k i n g o r w r i t i n g seem t o be p r i m a r i l y l e f t h e m i s p h e r i c , w h i l e n o n - v e r b a l t a s k s such as l i s t e n i n g t o m e l o d i e s o r d o i n g g e o m e t r i c p u z z l e s a r e p r i m a r i l y r i g h t h e m i s p h e r i c f u n c t i o n s . In s p l i t - b r a i n humans, both v e r b a l and n o n - v e r b a l p r o c e s s i n g can t a k e p l a c e s i m u l t a n e o u s l y t o a g r e a t e r degree than i n normal s u b j e c t s . For e x a m p l e , d i f f e r e n t s t i m u l i can be s i m u l t a n e o u s l y p r e s e n t e d t o each h e m i s p h e r e , and d i f f e r e n t s i m u l t a n e o u s r e s p o n s e s o b t a i n e d from t h e l e f t and r i g h t hands (Teng and S p e r r y , 1973; L e v y , T r e v a r t h e n and S p e r r y , 1972; G a z z a n i g a and H i l l y a r d , 1973; Teng and S p e r r y , 1974). These r e s u l t s s u g g e s t t h a t i n t h e normal p e r s o n , the f u n c t i o n s o f the l e f t and r i g h t hemispheres c a n n o t o p e r a t e i n d e p e n d e n t l y and s i m u l t a n e o u s l y . 4 . D i c h o t i c l i s t e n i n g The s t u d y o f n e u r o l o g i c a l and s p l i t - b r a i n p a t i e n t s has p r o v i d e d a g r e a t d e a l o f i n f o r m a t i o n about l e f t and r i g h t h e m i s p h e r i c d i f f e r e n c e s . However, a l l o f t h i s has been based on p a t i e n t s whose a b n o r m a l i t i e s i n v o l v e t h e v e r y f u n c t i o n s on which t h e c o n c l u s i o n s a r e b a s e d . S i n c e about 1960, s e v e r a l new t e c h n i q u e s have become a v a i l a b l e t o c i r c u m v e n t t h i s problem by s t u d y i n g h e m i s p h e r i c asymmetries i n normal s u b j e c t s . 19 D i c h o t i c l i s t e n i n g i s one o f t h e s e new t e c h n i q u e s . I t has been e s p e c i a l l y u s e f u l i n more c l e a r l y d e f i n i n g what type o f sounds are " v e r b a l " and what type a r e " n o n - v e r b a l " . Kimura (1961, 1963) f i r s t used the s i m u l t a n e o u s p r e s e n t a t i o n o f two a u d i t o r y s t i m u l i , a d i f f e r e n t one t o each e a r , t o show t h a t g e n e r a l l y one o r the o t h e r e a r was c o n s i s t e n t l y more a c c u r a t e i n h e a r i n g . Ear s u p e r i o r i t y can s h i f t t o e i t h e r the l e f t o r t h e r i g h t , depending on t h e s t i m u l u s c h a r a c t e r i s t i c s and the t a s k i n w h i c h t h e s u b j e c t i s engaged. T e s t s o f t h i s s u p e r i o r i t y have m o s t l y been based on a c c u r a c y o f r e c a l l , a b i l i t y t o make f i n e d i s t i n c t i o n s between s o u n d s , and r e a c t i o n t i m e s . The r e c a l l s u p e r i o r i t y o f t h e r i g h t e a r has been shown f o r d i c h o t i c a l l y p r e s e n t e d d i g i t s ( K i m u r a , 1961; B r y d e n , 1 9 6 3 ) ; nonsense s y l l a b l e s ( K i m u r a , 1 9 6 7 ) ; word sequences ( B a r t z , S a t z , F e n n e l l and L a l l y , 1967) ; and backwards and f o r w a r d s speech sounds (Kimura and F o l b , 1 9 6 8 ) . P r e s u m a b l y , t h e r i g h t e a r s u p e r i o r i t y f o r a l l o f t h e s e speech sounds r e f l e c t s the p r e p o t e n c y o f t h e c o n t r a l a t e r a l a u d i t o r y pathway and the l e f t h e m i s p h e r e ' s s p e c i a l i z a t i o n f o r the p e r c e p t i o n o f speech ( K i m u r a , 1961; S t u d d e r t - K e n n e d y and S h a n k w e i l e r , 1970). Ingram (1975) r e p o r t e d t h a t t h e s e r i g h t e a r s u p e r i o r i t i e s o c c u r r e d i n c h i l d r e n as young as 3 y e a r s o f a g e , which may r e f l e c t t h e v e r y e a r l y development o f speech s p e c i a l i z a t i o n w i t h i n t h e l e f t h e m i s p h e r e . 20 In c o n t r a s t t o t h e s e s u p e r i o r i t i e s o f t h e r i g h t e a r f o r speech s o u n d s , t h e l e f t e a r i s g e n e r a l l y s u p e r i o r f o r melody r e c o g n i t i o n ( K i m u r a , 1967; B a r t h o l o m e u s , 1974). T h i s was i n t e r p r e t e d as meaning t h a t the l e f t hemisphere predominated f o r v e r b a l p r o c e s s i n g and the r i g h t hemisphere f o r n o n - v e r b a l p r o c e s s i n g . These were o p e r a t i o n a l d e f i n i t i o n s , however, s i n c e t h e c o n c e p t s o f " v e r b a l " and " n o n - v e r b a l " were s t i l l d e f i n e d i n terms o f how t h e b r a i n r e a c t e d t o them, r a t h e r than some p r e c i s e c h a r a c t e r i s t i c o f the s t i m u l u s ( K i m u r a , 1967). L e f t e a r , and c o n s e q u e n t l y i n f e r r e d r i g h t h e m i s p h e r i c s u p e r i o r i t i e s were a l s o found f o r o t h e r n o n - v e r b a l s t i m u l i . The r e c o g n i t i o n o f v o i c e s (Doehring and B a r t h o l o m e u s , 1971); f r e q u e n c i e s (Yund and E f r o n , 1974; E f r o n and Yund, 1974); and t o n a l c o n t o u r s o f n o n - s p e e c h sounds ( B l u m s t e i n and C o o p e r , 1974) were among t h e s e . The above r e s u l t s s u g g e s t e d t h a t d i f f e r e n t t y p e s o f sounds were p r o c e s s e d d i f f e r e n t l y . L a n g u a g e - r e l a t e d sounds such as s p e e c h , nonsense s y l l a b l e s , and d i g i t s were p r o c e s s e d more e f f i c i e n t l y by the l e f t h e m i s p h e r e , w h i l e n o n - v e r b a l sounds such as v o i c e s , m e l o d i e s and tones were p r o c e s s e d w i t h i n the r i g h t h e m i s p h e r e . S e v e r a l w o r k e r s have a t t e m p t e d t o d e t e r m i n e t h e p r i m a r y c h a r a c t e r i s t i c s o f s t i m u l i which d i r e c t t h e i r p r o c e s s i n g t o e i t h e r t h e r i g h t o r l e f t h e m i s p h e r e . Kimura and F o l b (1968) showed t h a t s t i m u l u s f a m i l i a r i t y , m e a n i n g f u l n e s s and c o n c e p t u a l c o n t e n t had no e f f e c t on e a r s u p e r i o r i t i e s . S h a n k w e i l e r and S t u d d e r t - K e n n e d y (1967) demonstrated a s t r o n g r i g h t - e a r 21 s u p e r i o r i t y f o r c o n s o n a n t - v o w e l s y l l a b l e s , but o n l y a weak r i g h t - e a r s u p e r i o r i t y f o r v o w e l s . From t h i s and o t h e r w o r k , S t u d d e r t - K e n n e d y and S h a n k w e i l e r (1970) proposed t h a t the e s s e n t i a l f e a t u r e t h a t i n i t i a t e d r i g h t - e a r s u p e r i o r i t y was t h e p r e s e n c e o f p h o n e t i c i n f o r m a t i o n encoded i n t h e s o u n d s . They f u r t h e r s u g g e s t e d t h a t both t h e l e f t and r i g h t hemispheres were equipped f o r p r o c e s s i n g t h e a u d i t o r y parameters o f s p e e c h , such as p i t c h , l o u d n e s s , t i m b r e and d u r a t i o n , hence t h e a l m o s t equal a b i l i t y o f each t o respond t o v o w e l s . The l e f t h e m i s p h e r e , however, was supposed t o be more a b l e t o e x t r a c t l i n g u i s t i c parameters o r meanings from t h e sounds as r e p r e s e n t e d by t h e v o w e l - c o n s o n a n t c o m b i n a t i o n s ( S t u d d e r t -Kennedy, S h a n k w e i l e r and P i s o n i , 1972). The t h e o r i e s o f S t u d d e r t - K e n n e d y and S h a n k w e i l e r i g n o r e a c r i t i c a l p o i n t , however. The t e c h n i q u e o f d i c h o t i c l i s t e n i n g p r e s e n t s s t i m u l i which must be decoded and a c t e d upon by t h e s u b j e c t ; f o r i n s t a n c e i n the r e c o g n i t i o n , r e c a l l o r r e p e t i t i o n o f d i g i t s , phonemes, e t c . Bartholomeus (1974) has s u g g e s t e d t h a t not o n l y the s t i m u l u s p r o p e r t i e s , b u t r e c e p t o r p r o p e r t i e s ( i e . - the s u b j e c t ' . ) a r e i m p o r t a n t i n d e t e r m i n i n g e a r dominance. She showed t h a t when t h e same sequence o f l e t t e r s was sung by two d i f f e r e n t s i n g e r s , and t o two d i f f e r e n t m e l o d i e s , the s u b j e c t s showed a l e f t e a r s u p e r i o r i t y when they were a t t e m p t i n g to i d e n t i f y t h e m e l o d y , and a r i g h t e a r s u p e r i o r i t y when t h e y were t r y i n g t o i d e n t i f y t h e l e t t e r s . Thus f o r the same s t i m u l u s , e a r s u p e r i o r i t y can s h i f t t o e i t h e r the l e f t o r r i g h t , depending on the type o f c o g n i t i v e t a s k i n which t h e s u b j e c t i s engaged. 22 5 . Asymmetries w i t h i n the v i s u a l system D i c h o t i c p r e s e n t a t i o n o f a u d i t o r y i n f o r m a t i o n can be used t o e l i c i t h e m i s p h e r i c d i f f e r e n c e s i n the r e s p o n s e t o v e r b a l and n o n - v e r b a l s t i m u l i . A n a l a g o u s l y , v i s u a l s t i m u l i can be p r e s e n t e d to e i t h e r the r i g h t o r l e f t v i s u a l f i e l d s , t h u s c h a n n e l i n g v i s u a l s e n s o r y i n p u t t o e i t h e r the l e f t o r r i g h t h e m i s p h e r e s . O v e r a l l , t h i s t e c h n i q u e has shown t h a t p e r c e p t i o n o f m a t e r i a l which can be encoded i n words i s s u p e r i o r i n t h e r i g h t v i s u a l f i e l d , and hence l e f t h e m i s p h e r e . C o n v e r s e l y , p e r c e p t i o n o f s e n s o r y m a t e r i a l w i t h complex s p a t i a l a t t r i b u t e s i s s u p e r i o r i n the l e f t v i s u a l f i e l d ( M i l n e r , 1971; K i m u r a , 1973; B e r l u c c h i , 1974). These asymmetries have been a t t r i b u t e d t o : (1) t h e a n a t o m i c a l arrangement o f r e t i n o -c o r t i c a l c o n n e c t i o n s , p r o v i d i n g f o r t h e r e p r e s e n t a t i o n o f each h a l f o f the v i s u a l f i e l d i n the c o n t r a l a t e r a l h e m i s p h e r e ; and (2) t h e s p e c i a l i z a t i o n o f t h e l e f t hemisphere f o r v e r b a l a n a l y s i s , and o f t h e r i g h t hemisphere f o r s p a t i a l a n a l y s i s . R e a c t i o n t i m e s , which presumably r e f l e c t t h e s p e e d . o f p e r c e p t u a l p r o c e s s i n g , a r e s i m i l a r l y l a t e r a l i z e d . P r e s e n t a t i o n o f s t i m u l i i n the r i g h t v i s u a l f i e l d , and hence l e f t h e m i s p h e r e , a l l o w s f a s t e r naming o f a l p h a b e t i c l e t t e r s ( M o s c o v i t c h and C a t l i n , 1970; McKeever and G i l l , 1972). G e f f e n , Bradshaw and W a l l a c e (1971) found t h a t the speed and a c c u r a c y o f r e s p o n s e to v e r b a l m a t e r i a l was g r e a t e r i n t h e r i g h t v i s u a l f i e l d , and t o s p a t i a l m a t e r i a l i n t h e l e f t v i s u a l f i e l d . D a v i s and S c h m i t ( 1 9 7 1 ) , however, found no d i f f e r e n c e s t o v e r b a l m a t e r i a l . 23 The r e c o g n i t i o n o f w o r d s , r a t h e r than l e t t e r s o r g e o m e t r i c s h a p e s , i s more a c c u r a t e i n t h e r i g h t v i s u a l f i e l d (McKeever and H u l i n g , 1971) , and independent o f t h e i r o r i e n t a t i o n (MacKavey, C u r c i o and R o s e o , 1975). Mere d e t e c t i o n o f a v e r b a l s t i m u l u s , however, i s n o t s u f f i c i e n t t o produce a s y m m e t r i e s . G e f f e n , Bradshaw and W a l l a c e (1971) showed t h a t when a s i n g l e , m e a n i n g l e s s v e r b a l r e s p o n s e was made t o a d i g i t p r e s e n t e d i n e i t h e r v i s u a l f i e l d , t h e r e were no d i f f e r e n c e s i n r e a c t i o n t i m e s . B u t , when the d i g i t s . h a d t o be named, numbers i n t h e r i g h t v i s u a l f i e l d were responded t o more q u i c k l y . The p o s s i b l e mechanisms o f the r i g h t and l e f t v i s u a l f i e l d asymmetries have o n l y been r e c e n t l y i n v e s t i g a t e d . R i z z o l a t t i , U m i l t a and B e r l u c c h i (1971) proposed t h a t v e r b a l and n o n - v e r b a l i n f o r m a t i o n m i g h t be t r a n s f e r r e d a t d i f f e r e n t r a t e s w i t h i n each h e m i s p h e r e . However, t h i s seems u n l i k e l y s i n c e i t would r e q u i r e t h e o p t i c t r a c t , f o r i n s t a n c e , t o "know" t h a t i t i s c a r r y i n g a p a t t e r n o f v e r b a l i n f o r m a t i o n , such as a w o r d , o r o f s p a t i a l i n f o r m a t i o n , such as a g e o m e t r i c s h a p e . K i n s b o u r n e (1970, 1972) h y p o t h e s i z e d t h a t t h e f a s t e r r e a c t i o n t i m e s f o r some s t i m u l i m i g h t be due t o l a t e r a l asymmetries o f a t t e n t i o n , w h i c h were d i r e c t e d t o the l e f t hemisphere f o r v e r b a l s t i m u l i and t o t h e r i g h t f o r n o n - v e r b a l s t i m u l i . However, B e r l u c c h i , B r i z z o l a r i , R i z z o l a t t i and U m i l t a (1974) showed t h a t t h e asymmetries p e r s i s t e d w i t h randomized p r e s e n t a t i o n o f v e r b a l and n o n - v e r b a l m a t e r i a l , t h u s p r e c l u d i n g t h e s u b j e c t ' s a t t e n t i o n t o one t y p e o f i n f o r m a t i o n . The f i n a l h y p o t h e s i s , which i s s t i l l unproven by t h e v i s u a l p r e s e n t a t i o n t e c h n i q u e s , i s t h a t t h e asymmetries a r e a r e s u l t o f t h e s p e c i a l i z a t i o n o f the l e f t hemisphere f o r t h e p r o c e s s i n g o f v e r b a l m a t e r i a l , and o f t h e r i g h t hemisphere f o r the p r o c e s s i n g o f n o n - v e r b a l m a t e r i a l . 24 6. Handedness Handedness i s a more o b v i o u s e x p r e s s i o n o f asymmetric f u n c t i o n than some o f t h e more s u b t l e d i f f e r e n c e s d e s c r i b e d above. However, i t s r e l a t i o n s h i p , i f a n y , t o t h e s e o t h e r asymmetries i s s t i l l v e r y u n c l e a r . I n i t i a l l y , t h e p o s s i b l e r e l a t i o n s h i p o f r i g h t - h a n d e d n e s s t o t h e r e p r e s e n t a t i o n o f speech f u n c t i o n w i t h i n the l e f t hemisphere was s u g g e s t e d by t h e n e u r o l o g i c a l f i n d i n g t h a t most p a t i e n t s w i t h l e f t speech dominance, as i t came t o be c a l l e d , were a l s o r i g h t - h a n d e d . In a d d i t i o n , t h e speech o f l e f t - h a n d e d p a t i e n t s was a l s o r e p o r t e d to be l e s s a f f e c t e d by l e f t temporal l e s i o n s , i n d i c a t i n g a p o s s i b l y b i l a t e r a l o r even r i g h t h e m i s p h e r i c r e p r e s e n t a t i o n o f s p e e c h . These r e s u l t s were noted f o r p a t h o l o g i c a l l e s i o n s ( B e n t o n , 1962; Hecaen and A j u r i a g u e r r a , 1 9 6 4 ) ; temporal l o b e c t o m i e s ( M i l n e r , 1 9 6 2 ) ; p h a r m a c o l o g i c a l p a r a l y s i s ( B r a n c h , M i l n e r and Rasmussen, 1 9 6 4 ) ; and f o r e l e c t r o s t i m u l a t i o n o f t h e exposed c o r t e x ( P e n f i e l d and R o b e r t s , 1 9 5 9 ) . In t h e y e a r s s i n c e t h e s e o b s e r v a t i o n s were made, many e x c e p t i o n s have been r e p o r t e d . I t i s i s now g e n e r a l l y a c c e p t e d t h a t c e r e b r a l l a t e r a l i t y f o r language and handedness a r e n o t d i r e c t l y l i n k e d , and one does n o t d e t e r m i n e t h e o t h e r ( H e i l m a n , C o y l e , Gonyea and G e s c h w i n d , 1973; H e i l m a n , Gonyea and G e s c h w i n d , 1974). In normal s u b j e c t s , however, s i g n i f i c a n t d i f f e r e n c e s seem to e x i s t between t h e c o g n i t i v e a b i l i t i e s o f r i g h t and l e f t - h a n d e d s u b j e c t s . E a r l y s t u d i e s showed t h a t l e f t - h a n d e d c h i l d r e n s c o r e d l o w e r on IQ t e s t s than 25 t h e i r r i g h t - h a n d e d c l a s s m a t e s ( C l a r k , 1 9 5 7 ) , but more r e c e n t work s u g g e s t s t h a t t h e s e decrements a r e c o n f i n e d t o mixed o r a m b i d e x t r o u s i n d i v i d u a l s ( Z a n g w i l l , 1962; Berman, 1971). Other p e r c e p t u a l c a p a c i t i e s may a l s o be d e p r e s s e d i n l e f t - h a n d e d p e r s o n s . Levy (1969) showed t h a t , w h i l e r i g h t - h a n d e r s s c o r e d about e q u a l l y w e l l on v e r b a l and s p a t i a l o r n o n - v e r b a l r e a s o n i n g t e s t s , l e f t - h a n d e r s w i t h e q u a l l y h i g h v e r b a l s c o r e s d i d s u b s t a n t i a l l y worse on n o n - v e r b a l t e s t s . A n o t h e r s t u d y by M i l l e r ( 1 9 7 1 ) , however, found t h i s p a t t e r n t o be o n l y p r e s e n t i n mixed handed i n d i v i d u a l s . Nebes (1971) a l s o demonstrated t h a t l e f t - h a n d e r s performed more p o o r l y i n t h e p e r c e p t i o n o f p a r t - w h o l e r e l a t i o n s h i p s t h a n d i d r i g h t - h a n d e r s . T h i s i s n o r m a l l y c o n s i d e r e d t o be a r i g h t h e m i s p h e r i c , n o n - v e r b a l f u n c t i o n . Nebes and B r i g g s (1974) extended t h e d e f i c i t s o f l e f t - h a n d e r s t o t h e r e t e n t i o n and r e c a l l o f v i s u a l m a t e r i a l . They showed t h a t r i g h t -handers d i d b e t t e r than e i t h e r mixed o r l e f t - h a n d e r s a t r e p r o d u c t i o n from memory o f s e t s o f g e o m e t r i c s h a p e s . T h i s s u p p o r t e d a h y p o t h e s i s by Levy ( 1 9 6 9 ) , who s u g g e s t e d t h a t the p o s s i b l e l a c k o f h e m i s p h e r i c s p e c i a l i z a t i o n i n t h e s e n o n - r i g h t - h a n d e d i n d i v i d u a l s was d e t r i m e n t a l , and due t o a b a s i c i n c o m p a t a b i l i t y between v e r b a l and v i s u o - s p a t i a l modes o f p r o c e s s i n g . T h u s , i f n o n - r i g h t - h a n d e r s had t h e i r s p a t i a l f u n c t i o n s p r i m a r i l y w i t h i n the r i g h t hemisphere as w e l l as speech and motor c o n t r o l o f t h e i r l e f t s i d e , t h e f u n c t i o n s would t e n d t o i n t e r f e r e . T h i s p o s s i b i l i t y i s s u p p o r t e d by the o b s e r v a t i o n t h a t s p l i t - b r a i n p a t i e n t s can c a r r y on a v e r b a l and n o n - v e r b a l t a s k s i m u l t a n e o u s l y to a g r e a t e r degree than can normal s u b j e c t s . 7. E l e c t r o p h y s i o l o g i c a l 26 asymmetries The l e s i o n and p s y c h o p h y s i c a l e v i d e n c e p r e s e n t e d f o r p e r c e p t u a l asymmetries both have d e f e c t s . L e s i o n d a t a i s l i m i t e d because i n f e r e n c e s about normal p r o c e s s e s a r e made from abnormal c o n d i t i o n s , w h i l e measurements o f s u b j e c t r e s p o n s e s such as r e a c t i o n t i m e , r e c a l l a c c u r a c y and so f o r t h i n v o l v e complex c o r t i c a l p r o c e s s e s whose p r i n c i p l e s a r e o n l y v e r y v a g u e l y u n d e r s t o o d . P r i m a r i l y w i t h i n t h e l a s t d e c a d e , e l e c t r o p h y s i o l o g i c a l approaches have been d e v e l o p e d which e l i m i n a t e t h e s e p r o b l e m s , a l t h o u g h p o s i n g a few new ones as w e l l . W i t h t h e s e t e c h n i q u e s , t h e p e r c e p t u a l asymmetries o f normal s u b j e c t s can be i n v e s t i g a t e d , e i t h e r w i t h o r w i t h o u t the s u b j e c t ' s o v e r t p a r t i c i p a t i o n . In t h e s e e x p e r i m e n t s , i t i s assumed t h a t the e l e c t r i c a l e v e n t s o c c u r r i n g c o n c u r r e n t l y w i t h p h y s i o l o g i c a l r e s p o n s e s a r e i n some way d i r e c t l y r e l a t e d t o t h e u n d e r l y i n g c o r t i c a l p r o c e s s e s . T r a n s i t o r y EEG changes o r evoked p o t e n t i a l s g e n e r a t e d i n r e s p o n s e t o monaural c l i c k s t i m u l i were r e p o r t e d as e a r l y as 1946 by T u n t u r i , and a g a i n i n 1954 by Rosenzweig. Both a u t h o r s r e p o r t e d t h a t t h e s e r e s p o n s e s were o f l a r g e s t a m p l i t u d e o v e r t h e a u d i t o r y c o r t e x c o n t r a l a t e r a l t o the s t i m u l a t e d e a r . Vaughn and R i t t e r (1970) r e p e a t e d t h e s e f i n d i n g s , and a l s o r e p o r t e d t h a t t h e l a r g e s t asymmetries between c o r t i c a l a r e a s o c c u r r e d o v e r the l e f t hemisphere o f r i g h t - h a n d e d s u b j e c t s . However, Ruhm (1971) r e p o r t e d t h a t the temporal r e s p o n s e s were g r e a t e r i n the r i g h t hemisphere than i n the l e f t f o r s t i m u l a t i o n o f t h e r i g h t e a r , and t h a t no c o n s i s t e n t asymmetries were found f o r s i t m u l a t i o n o f t h e l e f t e a r . P e t e r s and 27 Mendel (1974) a l s o were unable t o d i s c r i m i n a t e between t h e peak a m p l i t u d e s o f e a r l y components ( l e s s than 50 m s e c . ) g e n e r a t e d by e i t h e r monaural o r b i n a u r a l s t i m u l i . F i n a l l y , t o c o n f u s e t h e p i c t u r e even more, P e r o n n e t , M i c h e l , E c h a l l i e r and G i r o d (1974) c l a i m e d t h a t r i g h t h e m i s p h e r i c r e s p o n s e s were g r e a t e r than t h o s e o f the l e f t f o r t o n e s t i m u l i . The l a t e n c i e s o f evoked p o t e n t i a l s i n r e l a t i o n s h i p t o h e m i s p h e r i c asymmetries have a l s o been t h e s u b j e c t o f c o n t r o v e r s y . M a j k o w s k i , Bochenek, Bochenek, K n a p i k - F i j a l k o w s k i and Kopec (1971) r e p o r t e d t h a t s h o r t e r l a t e n c i e s o c c u r r e d c o n t r a l a t e r a l t o t h e s t i m u l a t e d e a r i n 22 o f 29 s u b j e c t s . P e t e r s and Mendel (1974) however, were u n a b l e t o d i s c r i m i n a t e between the peak l a t e n c i e s o f e a r l y components g e n e r a t e d i n r e s p o n s e t o monaural o r b i n a u r a l s t i m u l i . To d e t e r m i n e t h e n a t u r e o f s t i m u l i w h i c h would e l i c i t a s y m m e t r i c a l r e s p o n s e s , s e v e r a l t y p e s o f s t i m u l i have been used w h i c h were more s t r u c t u r e d than s i m p l e c l i c k s . M o r r e l l and Salamy (1971) used n a t u r a l speech s t i m u l i t o produce evoked p o t e n t i a l s which were l a r g e s t o v e r t h e l e f t h e m i s p h e r e , p a r t i c u l a r l y the t e m p o r o - p a r i e t a l a r e a s . M o l f e s e (1972) r e c o r d e d evoked p o t e n t i a l s which were o f g r e a t e r a m p l i t u d e i n t h e l e f t hemisphere f o r speech s t i m u l i , and i n t h e r i g h t hemisphere f o r m u s i c a l s t i m u l i . I m p o r t a n t l y , t h e s e asymmetries were d e t e c t e d i n b a b i e s o f l e s s than one y e a r ' s a g e , w h i c h s u g g e s t s t h a t l a t e r a l i z a t i o n may o c c u r a t an e a r l y a g e . The d i s t i n c t i o n between the r e s p o n s e s t o v e r b a l and n o n - v e r b a l s t i m u l i was f u r t h e r i n v e s t i g a t e d by Wood and G o f f ( 1 9 7 1 ) , u s i n g b i n a u r a l s t i m u l i d u r i n g two a u d i t o r y d i s c r i m i n a t i o n t a s k s . One o f t h e s e t a s k s 28 i n v o l v e d t h e a n a l y s i s o f a c o u s t i c parameters f o r making a c o n s o n a n t o r l i n g u i s t i c d i s t i n c t i o n , w h i l e t h e o t h e r r e q u i r e d t h e a n a l y s i s o f f r e q u e n c y parameters w i t h no l i n g u i s t i c i n f o r m a t i o n . They found t h a t t h e magnitude o f evoked p o t e n t i a l s g e n e r a t e d i n r e s p o n s e t o t h e s e 2 s t i m u l u s t y p e s d i f f e r e d w i t h i n t h e l e f t hemisphere b u t n o t w i t h i n the r i g h t . In c o n t r a s t to v e r b a l and n o n - v e r b a l s t i m u l i , M a t s u m i y a , T a g l i a s c o , Lombroso and Goodglass (1972) showed t h a t h e m i s p h e r i c asymmetries o f a m p l i t u d e s were a l s o dependent on t h e m e a n i n g f u l n e s s o f t h e s t i m u l u s . The s u b j e c t s were r e q u i r e d t o a t t e n d to e i t h e r s i n g l e i s o l a t e d words embedded i n n o n - s p e e c h s o u n d s , o r t o f u l l s e n t e n c e s o f i n s t r u c t i o n s . The l a r g e s t , l e f t - t o - r i g h t , h e m i s p h e r i c asymmetries o f a m p l i t u d e were o b t a i n e d when t h e s u b j e c t had t o use each word m a x i m a l l y i n the f u l l s e n t e n c e s . S m a l l e r asymmetries were o b t a i n e d when t h e s u b j e c t s had to count t h e number o f w o r d s . I t was c o n c l u d e d t h a t s t i m u l i which were a c o n s i s t e n t p a r t o f a l a r g e r w h o l e , such as words o f a s e n t e n c e , produced l a r g e r a m p l i t u d e asymmetries than those same words embedded i n a m e a n i n g l e s s b a c k g r o u n d . S i m i l a r r e l a t i o n s t o s t i m u l u s meaning were d e m o n s t r a t e d by S h a r r a r d ( 1 9 7 3 ) , who found t h a t evoked p o t e n t i a l . a m p l i t u d e s were g r e a t e r f o r a 64 word message p l a y e d f o r w a r d s , then f o r t h e same message w i t h each word r e v e r s e d . Words w i t h s y m m e t r i c a l r i s e and f a l l t i m e s a l s o produced t h i s r e s u l t , but no c o n s i s t e n t l a t e r a l i t y e f f e c t s were o b s e r v e d w i t h e i t h e r t y p e o f s t i m u l u s s e q u e n c e . 29 Evoked p o t e n t i a l s g e n e r a t e d i n r e s p o n s e t o v i s u a l s t i m u l i have a l s o demonstrated a s y m m e t r i e s . In r i g h t - h a n d e d s u b j e c t s , Eason and White (1967) showed t h a t l a t e n c i e s were s h o r t e r i n t h e r i g h t hemisphere f o r f l a s h s t i m u l i , but C u l v e r , T a n l e y and Eason (1970) were u n a b l e t o r e p l i c a t e t h e s e f i n d i n g s . Buchsbaum and F e d i o ( 1 9 6 9 , 1970) a t t e m p t e d t o e l i c i t asymmetries w i t h v i s u a l s t i m u l i c o n t a i n i n g v e r b a l o r n o n - v e r b a l i n f o r m a t i o n . They r e c o r d e d t h e evoked p o t e n t i a l s g e n e r a t e d w i t h p r e s -e n t a t i o n o f random d o t s , words composed o f d o t s , and g e o m e t r i c p a t t e r n s composed o f d o t s . The a m p l i t u d e s o f r e s p o n s e s were found to d i f f e r more, and were more v a r i a b l e w i t h i n t h e l e f t h e m i s p h e r e . T h i s s u g g e s t e d t h a t v e r b a l s t i m u l i were p r o c e s s e d w i t h i n a more l o c a l i z e d a r e a o f the l e f t h e m i s p h e r e . However, Regan (1972) has p o i n t e d out t h a t s c a l p p o t e n t i a l s a s s o c i a t e d w i t h p o s s i b l e movements o r p r e p a r a t i o n f o r v o c a l i z a t i o n o f t h e p r e s e n t e d words c o u l d have caused t h e s e a s y m m e t r i e s . The above o b s e r v a t i o n s o f h e m i s p h e r i c asymmetries o f EPs have been q u e s t i o n e d by F r i e d m a n , S i m s o n , R i t t e r and Rapin (1975). U s i n g s t a t i s t i c a l t e s t s which they c o n s i d e r e d to be more a p p r o p r i a t e than t h o s e used e a r l i e r , t h e y were o n l y a b l e t o d e t e c t a m p l i t u d e asymmetries g e n e r a t e d w i t h speech sounds i n 2 o u t 8 s u b j e c t s . A r e v i e w o f e a r l i e r r e s u l t s " p o i n t e d up f l a w s o f d e s i g n , s t a t i s t i c a l t e c h n i q u e and i n c o n s i s t e n c i e s i n r e p o r t e d f i n d i n g s . They s u g g e s t e d t h a t w h i l e evoked p o t e n t i a l s may some-t i m e s r e f l e c t d i f f e r e n c e s i n h e m i s p h e r i c f u n c t i o n i n g , t h i s e f f e c t i s m a r g i n a l a t b e s t . T h e i r o p i n i o n s were based on t h e a n a l y s i s o f a m p l i t u d e and l a t e n c y asymmetries o f i n d i v i d u a l peaks o f t h e waveforms. 30 EEG s t u d i e s have a l s o s u g g e s t e d l a t e r a l i t y o f f u n c t i o n . G a l i n and O r n s t e i n (1971) showed t h a t t h e p r o p o r t i o n o f t o t a l EEG a l p h a power d e c r e a s e d i n t h e hemisphere engaged i n t h e c o g n i t i v e t a s k . For e x a m p l e , when t h e s u b j e c t was p e r f o r m i n g a v e r b a l t a s k , t h e p r o p o r t i o n o f l e f t h e m i s p h e r i c power d e c r e a s e d ; when he was p e r f o r m i n g a s p a t i a l t a s k such as f i t t i n g g e o m e t r i c p u z z l e s , t h e p r o p o r t i o n o f r i g h t h e m i s p h e r i c a l p h a power d e c r e a s e d . C o n t i n u i n g t h e s e i n v e s t i g a t i o n s o f a l p h a asymmetry, McKee, Humphrey and McAdam (1973) showed t h a t t h e p r o p o r t i o n a t e power w i t h i n t h e l e f t hemisphere dropped d u r i n g a l i n g u i s t i c t a s k , and w i t h i n the r i g h t hemisphere d u r i n g a m u s i c a l t a s k . A l p h a asymmetries a l s o e x i s t f o r t a s k s i n v o l v i n g d i f f e r e n t t y p e s o f mental i m a g e r y . Robbins and McAdam (1974) measured t h e h e m i s p h e r i c p r o p o r t i o n o f a l p h a power when t h e s u b j e c t s were engaged i n c o v e r t imagery modes: shapes and c o l o r s , w o r d s , o r both shapes and w o r d s . A l p h a power dropped i n t h e l e f t hemisphere f o r the v e r b a l imagery mode, and i n t h e r i g h t hemisphere f o r t h e v i s u a l imagery mode. G a l i n and E l l i s (1975) a l s o o b s e r v e d asymmetries o f evoked p o t e n t i a l a m p l i t u d e s which were r e l a t e d t o c o g n i t i v e t a s k . As w i t h t h e EEG, t h e p r o p o r t i o n o f evoked p o t e n t i a l power and a m p l i t u d e s d e c r e a s e d i n t h e hemisphere engaged i n the c o g n i t i v e t a s k . These f i n d i n g s were a l s o extended t o memory and r e c a l l . For t h e v i s u a l t a s k , t h e s u b j e c t s were r e q u i r e d to memorize a g e o m e t r i c p a t t e r n , and then r e c o n s t r u c t i t . For 31 the v e r b a l t a s k , they were r e q u i r e d t o memorize a word p a s s a g e , and then r e w r i t e i t from memory. S i m i l a r asymmetries were o b s e r v e d as d e s c r i b e d above. In an i n t e r e s t i n g e x t e n s i o n o f t h e s e f i n d i n g s , Dumas and Morgan (1975) found t h a t e n g i n e e r s and a r t i s t s d i d n o t show s i g n i f i c a n t d i f f e r e n c e s i n the s h i f t i n g o f a l p h a power as a f u n c t i o n o f c o g n i t i v e mode. That i s , both groups d e c r e a s e d t h e i r a l p h a power i n t h e hemisphere engaged i n t h e t a s k and t h e s e d e c r e a s e s were t h e same f o r both g r o u p s . The a u t h o r s had o r i g i n a l l y h y p o t h e s i z e d t h a t a r t i s t s m i g h t t e n d t o be more n o n - v e r b a l l y o r s p a t i a l l y o r i e n t e d , and e n g i n e e r s more v e r b a l l y o r a n a l y t i c a l l y o r i e n t e d . The EEG f i n d i n g s , however, d i d not s u p p o r t t h i s h y p o t h e s i s . 8 . C o n c l u s i o n s on h e m i s p h e r i c asymmetry E v i d e n c e f o r h e m i s p h e r i c asymmetries have come from many d i v e r s e s o u r c e s : b e h a v i o r a l d e f i c i t s i n n e u r o l o g i c a l p a t i e n t s , o b s e r v a t i o n s o f s p l i t - b r a i n p a t i e n t s , and d i c h o t i c l i s t e n i n g , v i s u a l p r e s e n t a t i o n and e l e c t r o p h y s i o l o g i c a l i n v e s t i g a t i o n s i n normal s u b j e c t s . The c o r r e l a t i o n between l e s i o n s i t e and b e h a v i o r a l d e f i c i t s has s u p p l i e d t h e o l d e s t and l a r g e s t body o f knowledge about h e m i s p h e r i c asymmetries o f speech and v i s u a l f u n c t i o n s . Most o f i t i n d i c a t e s a l a t e r a l i z a t i o n o f v e r b a l p r o c e s s i n g t o t h e l e f t hemisphere and o f n o n - v e r b a l 32 s p a t i a l p r o c e s s i n g t o t h e r i g h t . More r e c e n t o b s e r v a t i o n s , however, s u g g e s t t h a t most c o r t i c a l p r o c e s s i n g o f s e n s o r y s t i m u l i engages both hemispheres i n a c o o r d i n a t e d manner which depends upon t h e s t i m u l u s c o m p l e x i t y , meaning and l e v e l o f a b s t r a c t i o n r e q u i r e d t o g e n e r a t e an a p p r o p r i a t e r e s p o n s e . The e v i d e n c e from humans who have had t h e i r corpus: c a l l o s u m b i s e c t e d c o n f i r m s r a t h e r d r a m a t i c a l l y t h e c o n c l u s i o n s based on c o r t i c a l ' l e s i o n s . T h a t i s , t h e l e f t hemisphere engages i n c o g n i t i v e t a s k s r e q u i r i n g v e r b a l , l i n g u i s t i c and c o u n t i n g a b i l i t i e s , w h i l e the r i g h t hemisphere i s o r i e n t e d towards more n o n - v e r b a l , s p a t i a l p r o c e s s i n g . In the s p l i t - b r a i n p a t i e n t s , t h e s e f u n c t i o n s can o p e r a t e s i m u l t a n e o u s l y and i n d e p e n d e n t l y t o a much g r e a t e r degree than i n normal s u b j e c t s . T h i s s u g g e s t s t h a t t h e normal i n t e r a c t i o n o f t h e hemispheres i n v o l v e s a s h i f t i n g o f f u n c t i o n from one hemisphere t o t h e o t h e r , o r a t most a s i m u l t a n e o u s p r o c e s s i n g o f d i f f e r e n t a s p e c t s o f t h e same s e n s o r y m a t e r i a l . The p r i m a r y problem w i t h both l e s i o n and s p l i t - b r a i n s t u d i e s i s t h a t i n f e r e n c e about normal f u n c t i o n a r e made on t h e b a s i s o f a b n o r m a l i t i e s o f t h a t same f u n c t i o n . T h i s r e a s o n i n g may be v a l i d f o r s i m p l e systems i n which an i n p u t s i g n a l d i r e c t l y a f f e c t s an o u t p u t . The s i t u a t i o n i s much more c o m p l e x , however, f o r feedback systems such as would be e x p e c t e d t o be an i n t e g r a l p a r t o f t h e c o n t r o l mechanisms o f the b r a i n . In t h i s c a s e , the i n p u t s a f f e c t t h e o u t p u t s , which then i n t u r n a f f e c t t h e s y s t e m ' s r e s p o n s e t o t h e n e x t i n p u t s . The e f f e c t s o f the i n t e r r u p t i o n o f pathways w i t h i n systems w i t h s e v e r a l feedback l o o p s a r e a l m o s t i m p o s s i b l e t o p r e d i c t . In many c a s e s , t h e system becomes c o m p l e t e l y 33 u n s t a b l e and responds i n an e r r a t i c and abnormal f a s h i o n t o random n o i s e w i t h i n the system ( L a t h i , 1970). D i c h o t i c l i s t e n i n g has p r o v i d e d a b e h a v i o r a l o r p s y c h o p h y s i o l -o g i c a l way o f s t u d y i n g asymmetries i n normal s u b j e c t s . With t h i s t e c h n i q u e , h i g h l y s o p h i s t i c a t e d f u n c t i o n s such as t h e a n a l y s i s and r e c o g n i t i o n o f speech and m e l o d y , have been s t u d i e d . A g a i n , the e v i d e n c e s u g g e s t s a l a t e r a l i z a t i o n o f v e r b a l p r o c e s s i n g t o t h e ; l e f t hemisphere and o f n o n - v e r b a l , s p a t i a l p r o c e s s i n g t o t h e r i g h t . D i c h o t i c l i s t e n i n g has f u r t h e r h e l p e d i n d e f i n i n g what c h a r a c t e r i s t i c s c o n s t i t u t e t h e fundamental d i s t i n c t i o n between a v e r b a l and n o n - v e r b a l s i g n a l . A l t h o u g h s e v e r a l a t t e m p t s have been made t o do t h i s , i t i s s t i l l not a p p a r e n t what t h e e s s e n t i a l d i f f e r e n c e i s t h a t causes t h e words o f a song t o be p r o c e s s e d more e f f i c i e n t l y w i t h i n the l e f t h e m i s p h e r e , and t h e melody o f t h a t same song w i t h i n t h e r i g h t . The s e l e c t i v e p r e s e n t a t i o n o f v i s u a l m a t e r i a l t o e i t h e r t h e r i g h t o r l e f t v i s u a l f i e l d has a l s o p r o v i d e d p s y c h o p h y s i o l o g i c a l e v i d e n c e o f l a t e r a l i t y i n normal s u b j e c t s . In a manner s i m i l a r t o d i c h o t i c l i s t e n i n g , t h e s e t e c h n i q u e s have shown t h a t t h e p e r c e p t i o n and r e c a l l o f v i s u a l l y p r e s e n t e d m a t e r i a l than can be encoded i n words i s s u p e r i o r i n t h e r i g h t v i s u a l f i e l d , and c o n s e q u e n t l y by i n f e r e n c e , the l e f t h e m i s p h e r e . C o n v e r s e l y , p e r c e p t i o n and r e c a l l o f n o n - v e r b a l m a t e r i a l w i t h complex s p a t i a l a t t r i b u t e s i s more e f f i c i e n t w i t h i n t h e r i g h t h e m i s p h e r e . 34 C o n c l u s i o n s from the p r e s e n t a t i o n o f sounds t o e i t h e r h e m i s p h e r e , as i n d i c h o t i c l i s t e n i n g , and from the p r e s e n t a t i o n o f v i s u a l s t i m u l i t o e i t h e r hemisphere a r e n o t n e c e s s a r i l y c o n g r u e n t . Both g e o m e t r i c shapes and m e l o d i e s appear t o be p r e f e r e n t i a l l y p r o c e s s e d w i t h i n t h e r i g h t h e m i s p h e r e , but t h e i r communality i s not e v i d e n t . S i m i l a r l y , words t h a t a r e h e a r d , and words t h a t a r e s e e n , both seem t o be p r o c e s s e d more w i t h i n t h e l e f t h e m i s p h e r e , but the common c h a r a c t e r i s t i c between them i s n o t c l e a r . One c o u l d say t h a t both a r e " v e r b a l " o r " n o n - v e r b a l " b u t t h i s does n o t d e f i n e t h e i r d i f f e r e n c e s ; i t m e r e l y a t t a c h e s d i f f e r e n t l a b e l s t o how the b r a i n responds t o them. One p o s s i b l e d i f f e r e n c e i s t h a t v e r b a l m a t e r i a l i s a n y t h i n g t h a t can be l a b e l l e d o r a s s o c i a t e d w i t h a known speech sound. N o n - v e r b a l o r s p a t i a l m a t e r i a l s , such as m e l o d i e s o r p a t t e r n s would n o t n e c e s s a r i l y have t h i s c h a r a c t e r i s t i c . Kimura ( 1 9 6 8 ) , who advanced t h i s t h e o r y , s u p p o r t e d i t by showing t h a t l e f t v i s u a l f i e l d s u p e r i o r i t y f o r t h e r e c o g n i t i o n o f g e o m e t r i c p a t t e r n s d e c r e a s e d as t h e p a t t e r n became more easy t o v e r b a l l y l a b e l . Handedness i s a more o b v i o u s e x p r e s s i o n o f l a t e r a l i t y o f f u n c t i o n than t h e o t h e r s d i s c u s s e d above. In p a t i e n t s , i t has become c l e a r t h a t handedness and speech l a t e r a l i t y a r e not c a u s a l l y r e l a t e d , and one cannot be a c c u r a t e l y p r e d i c t e d from the o t h e r . L e f t - h a n d e d s u b j e c t s , however, seem t o demonstrate l e s s h e m i s p h e r i c s p e c i a l i z a t i o n , o r more b i l a t e r a l r e p r e s e n t a t i o n o f speech than do r i g h t - h a n d e d s u b j e c t s . A f a i r amount o f e v i d e n c e s u g g e s t s t h a t t h i s may r e s u l t i n i n t e r f e r e n c e between v e r b a l and n o n - v e r b a l p r o c e s s i n g c a p a b i l i t i e s . T h i s i s s u p p o r t e d 35 by t h e o b s e r v a t i o n t h a t i n s p l i t - b r a i n p a t i e n t s , t h e s e p r o c e s s e s can proceed i n d e p e n d e n t l y t o a g r e a t e r degree than i n normal s u b j e c t s . As w i t h t h e s t u d i e s o f the e f f e c t s o f v a r i o u s l e s i o n s , the p s y c h o p h y s i o l o g i c a l methods o f s t u d y i n g l a t e r a l i t y such as d i c h o t i c l i s t e n i n g and v i s u a l f i e l d p r e s e n t a t i o n have t h e i r p a r t i c u l a r d i s a d -v a n t a g e s . The p r i m a r y problem i s t h e r e q u i r e m e n t t h a t the s u b j e c t respond t o t h e s t i m u l u s i n o r d e r f o r t h e i n v e s t i g a t o r t o measure some a t t r i b u t e . T h i s reponse b r i n g s i n many c o m p l i c a t i n g f a c t o r s such as i n t e r h e m i s p h e r i c communications f o r t h e t r a n s f e r o f s e n s o r y i n f o r m a t i o n t o the motor s y s t e m s , and d e c i s i o n - m a k i n g o r d i s c r i m i n a t i o n f u n c t i o n s . E l e c t r o p h y s i o l o g i c a l t e c h n i q u e s , such as EEG and evoked p o t e n t i a l a n a l y s i s , do not have t h i s p a r t i c u l a r problem o f s u b j e c t p a r t i c i p a t i o n , a t l e a s t i n some e x p e r i m e n t a l p a r a d i g m s . They a r e t h u s w e l l - s u i t e d f o r the s t u d y o f t h e p r o c e s s i n g o f s e n s o r y i n f o r m a t i o n i n t h e normal s u b j e c t w i t h o u t the f u r t h e r c o m p l i c a t i o n o f s u b j e c t p a r t i c i p a t i o n t o measure t h a t r e s p o n s e . The a m p l i t u d e and l a t e n c i e s o f evoked p o t e n t i a l s g e n e r a t e d i n r e s p o n s e t o a u d i t o r y s t i m u l i a r e o f i n t e r e s t i n more c l e a r l y d e f i n i n g t h e d i f f e r e n c e between v e r b a l and n o n - v e r b a l s t i m u l i . A l t h o u g h t h e s e have o n l y been e l u s i v e l y r e l a t e d t o c l i c k s t i m u l i , more complex s t i m u l i w i t h v e r b a l and m e a n i n g f u l a s s o c i a t i o n s have i n d i c a t e d a p r e - e m i n e n c e o f t h e l e f t hemisphere f o r t h e i r p r o c e s s i n g . The e v i d e n c e s u g g e s t s t h a t as the s t i m u l u s becomes more embedded i n a m e a n i n g f u l whole o r s e q u e n c e , l e f t h e m i s p h e r i c i n v o l v e m e n t becomes g r e a t e r . 36 T h i s same f i n d i n g c a r r i e s o v e r t o t h e v i s u a l s y s t e m , where a v i s u a l p a t t e r n c o n s i s t i n g o f a g e o m e t r i c p a t t e r n o f d o t s c a u s e s s m a l l e r , l e f t h e m i s p h e r i c asymmetries than a p a t t e r n a r r a n g e d i n a w o r d . The r e p o r t s o f asymmetry o f EEG a l p h a when t h e s u b j e c t i s engaged i n d i f f e r e n t c o g n i t i v e t a s k s s u p p o r t t h e c o n t e n t i o n t h a t t h e l e f t hemisphere p r o c e s s e s v e r b a l i n f o r m a t i o n , w h i l e t h e r i g h t hemisphere p r o c e s s e s n o n - v e r b a l i n f o r m a t i o n . A g a i n , however, t h e d e f i n i t i o n s o f v e r b a l and n o n - v e r b a l a r e o p e r a t i o n a l . In a d d i t i o n , t h e s e EEG s t u d i e s assume t h a t t h e a l p h a s t a t e r e p r e s e n t s a r e s t i n g c o r t i c a l c o n d i t i o n , s i n c e i t i s o b s e r v e d t h a t t h e a l p h a power d e c r e a s e s p r o p o r t i o n a t e l y i n t h e hemisphere engaged i n t h e c o g n i t i v e t a s k . . I t i s f u r t h e r assumed t h a t t h i s i s the l e f t hemisphere f o r v e r b a l t a s k s and t h e r i g h t hemisphere f o r n o n - v e r b a l t a s k s . N o t h i n g i n t h e i r \ d a t a shows t h a t t h e o p p o s i t e c o n t e n t i o n c o u l d n o t be t r u e , t h a t i s , t h a t s i n c e power i n c r e a s e s p r o p o r t i o n a t e l y w i t h i n t h e r i g h t hemisphere f o r v e r b a l t a s k s , t h i s must be t h e hemisphere more i n v o l v e d i n t h e p r o c e s s i n g , s i n c e i t s power i s g r e a t e r . In a l l o f t h e s e s t u d i e s , t h e c r i t i c a l q u e s t i o n i s : what i s t h e d i f f e r e n c e between a " v e r b a l " and a " n o n - v e r b a l " s t i m u l u s ? One p o s s i b i l i t y may be i t s s e q u e n t i a l c h a r a c t e r i s t i c s . T h i s i s s u g g e s t e d by the f o l l o w i n g o b s e r v a t i o n s : F i r s t , d i c h o t i c l i s t e n i n g showed t h a t c o n s o n a n t - v o w e l c o m b i n a t i o n s caused l a r g e r asymmetries than vowels a l o n e . S e c o n d , sounds embedded i n a s e n t e n c e caused l a r g e r EP asymmetries than t h e same sounds embedded i n a m e a n i n g l e s s b a c k g r o u n d . T h i r d , d o t s a r r a n g e d i n t o a word caused l a r g e r asymmetries o f r e c o g n i t i o n than d o t s a r r a n g e d i n a g e o m e t r i c p a t t e r n . In a l l o f t h e s e , t h e p r i m a r y c h a r a c t e r i s t i c o f t h e 37 v e r b a l s t i m u l i i s t h a t t h e y a r e a r r a n g e d i n t o a sequence o f sounds o r f o r m s , w h i l e t h e n o n - v e r b a l s t i m u l i a r e i s o l a t e d o r a s i n g l e e n t i t y w i t h no r e l a t i o n to e a r l i e r s t i m u l i . Based on t h e s e o b s e r v a t i o n s , v e r b a l s t i m u l i would be c l a s s i f i e d as s e q u e n t i a l , w h i l e n o n - v e r b a l s t i m u l i would be c l a s s i f i e d as s i m u l t a n e o u s . With t h e s e d e f i n i t i o n s , t h e two d i f f e r e n t p r o c e s s i n g modes would be termed s e q u e n t i a l o r s e r i a l f o r t h e l e f t h e m i s p h e r i c f u n c t i o n , and p a r a l l e l , s i m u l t a n e o u s , h o l i s t i c o r g e s t a l t - l i k e f o r the r i g h t hemisphere ( L e v i - A g r e s t i and S p e r r y , 1968; Diamond and Beaumont, 1972; Cohen, 1973). In c o n c l u s i o n , t h e e x p e r i m e n t a l d a t a on human h e m i s p h e r i c asymmetry s u p p o r t s v e r y s t r o n g l y t h e s e p a r a t i o n o f v e r b a l l y a n a l y t i c f u n c t i o n s t o the l e f t hemisphere and n o n - v e r b a l o r s p a t i a l l y a n a l y t i c f u n c t i o n s t o the r i g h t . The d i s t i n c t i o n between t h e v e r b a l and n o n - v e r b a l a t t r i b u t e s i s u s e f u l , a l t h o u g h c l e a r l y t o o s i m p l i s t i c . A l a r g e body o f e v i d e n c e s u g g e s t s t h a t most c o r t i c a l p r o c e s s i n g o f s e n s o r y d a t a engages both hemispheres w i t h v a r y i n g degrees o f i n t e r a c t i o n , depending on t h e s t i m u l u s c o m p l e x i t y , meaning and l e v e l o f a b s t r a c t i o n r e q u i r e d f o r t h e t a s k i n which t h e i n d i v i d u a l i s engaged. 38 F. Statement o f purpose Two ways e x i s t f o r the s t u d y o f evoked p o t e n t i a l s , and each makes a s s u m p t i o n s about t h e way i n w h i c h the EP i s g e n e r a t e d . Time-domain a n a l y s i s s t u d i e s temporal c h a r a c t e r i s t i c s such as peak a m p l i t u d e s o r l a t e n c i e s , and assumes t h a t v a r i o u s components a r e s e q u e n t i a l l y g e n e r a t e d as d i s c r e t e e v e n t s . Frequency-domain a n a l y s i s , on the o t h e r hand, assumes t h a t d i f f e r e n t f r e q u e n c i e s a r e g e n e r a t e d by d i f f e r e n t p r o c e s s e s , and s t u d i e s the a m p l i t u d e , phase and c o h e r e n c e r e l a t i o n s h i p s o f t h e s e p r o c e s s e s . Most EP s t u d i e s have c o n c e n t r a t e d upon t h e i r temporal c h a r a c t e r i s t i c s , and o n l y v e r y few upon t h e i r f r e q u e n c y c h a r a c t e r i s t i c s . However, f r e q u e n c y a n a l y s i s i s o f t e n a more a p p r o p r i a t e means f o r s t u d y i n g o s c i l l a t o r y p r o c e s s e s , and the r h y t h m i c waveform o f t h e EP s u g g e s t s t h a t such an a p p l i c a t i o n might be u s e f u l . Time-domain a n a l y s i s o f peak a m p l i t u d e and l a t e n c i e s o f EPs and o t h e r e l e c t r o p h y s i o l o g i c a l e v e n t s have been used t o s t u d y h e m i s p h e r i c a s y m m e t r i e s . Most o f t h e s e have s u g g e s t e d t h a t t h e l e f t , s p e e c h - d o m i n a n t hemisphere p r o c e s s e s v e r b a l i n f o r m a t i o n , and t h a t t h e r i g h t , n o n -speech-domina nt hemisphere p r o c e s s e s n o n - v e r b a l o r s p a t i a l i n f o r m a t i o n . A l a r g e body o f e v i d e n c e from t h e e f f e c t s o f c o r t i c a l l e s i o n s , o b s e r v a t i o n o f s p l i t - b r a i n p a t i e n t s , d i c h o t i c l i s t e n i n g and v i s u a l f i e l d p r e s e n t a t i o n e x p e r i m e n t s s u p p o r t t h i s v i e w p o i n t . The e s s e n t i a l problem w i t h a l l o f t h e s e s t u d i e s has been t o d e f i n e t h e d i f f e r e n c e between " v e r b a l " and " n o n - v e r b a l " s t i m u l i o r p r o c e s s i n g modes. A l m o s t a l l r e p o r t s , no m a t t e r what t h e t e c h n i q u e , have had t o r e l y 39 upon complex s t i m u l i , such as words o r m e l o d i e s , to e l i c i t c o n s i s t e n t h e m i s p h e r i c a s y m m e t r i e s . T h i s c o m p l i c a t i o n has f u r t h e r c o n f u s e d t h e attempt t o d e f i n e t h e c e n t r a l d i f f e r e n c e . D e s p i t e many d e m o n s t r a t i o n s o f a v a r i e t y o f a s p e c t s o f t h e p r o b l e m , i t i s s t i l l n o t c l e a r what c h a r a c t e r i s t i c o f some s t i m u l i t r i g g e r s l e f t h e m i s p h e r i c p r o c e s s i n g , and what c h a r a c t e r i s t i c o f o t h e r s t i m u l i t r i g g e r s r i g h t h e m i s p h e r i c p r o c e s s i n g . A p o s s i b l e s o l u t i o n t o t h i s problem may l i e i n d e m o n s t r a t i n g t h a t h e m i s p h e r i c asymmetries e x i s t f o r s i m p l e f l a s h and c l i c k s t i m u l i . T h i s would remove t h e a m b i g u i t y o f the " v e r b a l v e r s u s n o n - v e r b a l " d i s t i n c t i o n , and show t h a t asymmetries o f h e m i s p h e r i c f u n c t i o n were r e l a t e d t o p r o c e s s e s much more fundamental than t h o s e o f l a n g u a g e . However, h e m i s p h e r i c asymmetries have not been c l e a r l y demonstrated f o r s i m p l e s t i m u l i , e s p e c i a l l y w i t h EPs (Friedman e t a l , 1975). T h i s may be due t o t h e p o s s i b i l i t y t h a t d i f f e r e n t asymmetries do not e x i s t f o r f l a s h and c l i c k s t i m u l i , o r because t h e a n a l y t i c t e c h n i q u e s have been e i t h e r too weak o r based upon t h e wrong a s s u m p t i o n s . I t i s h y p o t h e s i z e d t h a t t h i s f a i l u r e i s due t o a p p l y i n g the t i m e -domain a s s u m p t i o n s t o t h e a n a l y s i s o f t h e EP waveform. A l m o s t a l l w o r k e r s have c o n c e n t r a t e d upon t h e b e h a v i o r o f one o r a few peaks o f t h e EP, and i f the t i m e - d o m a i n a s s u m p t i o n o f s e q u e n t i a l e v e n t s i s c o r r e c t , no o t h e r approach i s v a l i d . However, Friedman e t a l (1975) have c a r e f u l l y r e v i e w e d a l l EP s t u d i e s i n v o l v i n g many d i f f e r e n t t y p e s o f s t i m u l i , and have 40 c o n c l u d e d t h a t none have c l e a r l y demonstrated d i f f e r e n c e s i n a m p l i t u d e s o r l a t e n c i e s which c o u l d be r e l a t e d t o o n l y the r i g h t o r l e f t h e m i s p h e r e . I f t h e EP i s t r u l y g e n e r a t e d by an o s c i l l a t o r y p r o c e s s , t h i s f a i l u r e becomes u n d e r s t a n d a b l e , f o r t h e n , peaks o r t r o u g h s a r e n o t s p e c i a l e v e n t s w i t h more s i g n i f i c a n c e than o t h e r p o i n t s . T h e i r v a r i a b i l i t y i s as g r e a t as t h a t o f any o t h e r i n s t a n t on the waveform. To p r o p e r l y a n a l y z e an o s c i l l a t i n g p r o c e s s , the e n t i r e waveform must be c o n s i d e r e d . Each segment o f t h a t waveform may be h i g h l y v a r i a b l e , but the o v e r a l l shape and i t s r e l a t i o n t o o t h e r waveforms may be e x t r e m e l y c o n s i s t e n t and u n v a r y i n g . I f o n l y a few f r e q u e n c y components a r e n e c e s s a r y t o d e s c r i b e the waveform, t h e n t h e d e s c r i p t i o n w i l l be a much more condensed and a c c u r a t e r e p r e s e n t a t i o n than t h a t based on any p a r t i c u l a r p a r t . The i n t e n t , t h e n , i s t o a p p l y s p e c t r a l a n a l y s i s , which i s a form o f f r e q u e n c y a n a l y s i s , t o t h e EP waveforms g e n e r a t e d by s i m p l e f l a s h and c l i c k s t i m u l i . I f t h e EPs a r e g e n e r a t e d by an o s c i l l a t o r y p r o c e s s , s p e c t r a l a n a l y s i s w i l l p r o b a b l y demonstrate EP c h a r a c t e r i s t i c s which a r e n o t o b s e r v a b l e w i t h t h e a n a l y s i s o f p e a k s . An a t t e m p t w i l l be made t o m e a n i n g f u l l y r e l a t e t h e f r e q u e n c y components o f t h e EPs t o p h y s i o l o g i c a l f a c t o r s , i n p a r t i c u l a r t o speech dominance and handedness. The i m p l i c a t i o n s o f t h e r e s u l t s a r e g r e a t f o r t h e d i f f e r e n t ways i n which each hemisphere p r o c e s s e s s e n s o r y i n f o r m a t i o n . 41 METHODS A. I n t r o d u c t i o n Three e x p e r i m e n t s a r e d e s c r i b e d . The f i r s t i s on t h e a m p l i t u d e or power s p e c t r a l c h a r a c t e r i s t i c s o f EP f r e q u e n c y components and t h e i r r e l a t i o n s h i p t o s t i m u l u s m o d a l i t y and the c o r t i c a l a r e a from which t h e y a r e r e c o r d e d . The second e x p e r i m e n t shows the r e l a t i o n s h i p o f coherence t o speech dominance, and the f i n a l e x p e r i m e n t d e m o n s t r a t e s t h e r e l a t i o n s h i p s o f both coherence and power s p e c t r a l asymmetries t o speech dominance and handedness. In each e x p e r i m e n t , the p r i m a r y o b j e c t i v e was t o m e a n i n g f u l l y r e l a t e t h e s p e c t r a l o r f r e q u e n c y c h a r a c t e r i s t i c s o f t h e EPs t o p h y s i o l -o g i c a l f a c t o r s such as c o r t i c a l a r e a , s t i m u l a t e d s e n s o r y m o d a l i t y , speech dominance and handedness. B. S p e c t r a l a n a l y s i s S p e c t r a l a n a l y s i s o f EPs was the b a s i c method o f d a t a r e d u c t i o n i n a l l t h r e e e x p e r i m e n t s . By means o f t h i s t e c h n i q u e , t h e o s c i l l a t o r y EP waveforms were t r a n s f o r m e d t o a s e t o f s i n u s o i d a l components. Each o f t h e s e had an a m p l i t u d e and phase s h i f t such t h a t summing a l l o f them t o g e t h e r would r e p r o d u c e the o r i g i n a l waveform. The f o u r i e r t r a n s f o r m was t h e b a s i s o f t h e s p e c t r a l a n a l y t i c t r a n s f o r m a t i o n o f the temporal waveform t o i t s f r e q u e n c y components, o r 42 f r e q u e n c y - d o m a i n d e s c r i p t i o n . From t h e magnitude o f t h e components o f t h e t r a n s f o r m , the a m p l i t u d e o r power spectrum o f each waveform was c a l c u l a t e d . The c o h e r e n c e , o r s i m i l a r i t y o f form between each two waveforms was c a l c u l a t e d from b o t h t h e a m p l i t u d e and phase o f t h e components o f t h e i r c o r r e s p o n d i n g f o u r i e r t r a n s f o r m s . The s p e c t r a l a n a l y s i s was done w i t h t h e UCLA B i o m e d i c a l computer program BMD 03T ( D i x o n , 1973). The d e t a i l s o f the methods i n t h e program were c o m p l e t e l y c o n v e n t i o n a l ( J e n k i n s and W a t t s , 1968) and so w i l l be o n l y b r i e f l y d e s c r i b e d . P r i o r t o t h e f o u r i e r t r a n s f o r m a t i o n , s e v e r a l s t e p s were t a k e n i n p r e p a r a t i o n o f t h e d a t a . These were i n c l u d e d as p a r t o f t h e program and were d e s i g n e d t o handle d a t a which c o n t a i n e d much more n o i s e and l a r g e r d i s c o n t i n u i t i e s o r a r t i f a c t s than found i n EPs. The d a t a p r e p a r a t i o n had t h e c h a r a c t e r i s t i c t h a t waveforms t h a t d i d n o t r e q u i r e the p r o c e s s i n g were u n a f f e c t e d by t h e p r o c e d u r e s . The f i r s t s t e p i n d a t a p r e p a r a t i o n was to remove f r e q u e n c i e s above and below t h o s e o f i n t e r e s t by d i g i t a l f i l t e r i n g . The low f r e q u e n c y c u t o f f o f the EEG was s e t a t 0 . 3 h z . , so f r e q u e n c i e s below t h i s , f o r i n s t a n c e l i n e a r t r e n d s , were c o n s i d e r e d a r t i f a c t u a l and removed. L i k e w i s e , o b s e r v a t i o n o f t h e EP waveforms showed t h a t t h e major o s c i l l a t i o n s were i n t h e 1-20 hz . r a n g e , w h i l e h i g h e r f r e q u e n c i e s w h i c h sometimes o c c u r r e d were u s u a l l y due t o 60 c y c l e , l i n e i n t e r f e r e n c e o r myogenic p o t e n t i a l s i n nervous s u b j e c t s . T h e r e f o r e , f r e q u e n c i e s above about 30 h z . were a l s o d i g i t a l l y f i l t e r e d o u t . F i n a l l y , the f i r s t and l a s t 50 msec, o f each EP 43 was t a p e r e d so t h a t t h e waveforms begun and ended s m o o t h l y . T h i s was n e c e s s a r y f o r t h e c a s e o f an a b r u p t jump i n t h e b e g i n n i n g o r e n d i n g o f the d a t a , which would be i n t e r p r e t e d as a v e r y l a r g e a m p l i t u d e , h i g h f r e q u e n c y component. However, s i n c e t h e major c y c l i c components o f t h e EP d i d n o t b e g i n u n t i l a f t e r 50 msec, and were m o s t l y c o m p l e t e by 400 m s e c , t h i s s t e p had no e f f e c t . A f t e r the f i l t e r i n g , removal o f t r e n d , and t a p e r i n g , t h e EPs were f o u r i e r t r a n s f o r m e d t o g i v e the a m p l i t u d e and phase o f each f r e q u e n c y component. The power s p e c t r a were then c a l c u l a t e d d i r e c t l y from t h e a m p l i t u d e s . To c a l c u l a t e the c o h e r e n c e s between each p a i r o f E P s , the f o l l o w i n g s t e p s were t a k e n . The c r o s s - s p e c t r u m o r p r o d u c t o f t h e f o u r i e r t r a n s f o r m s a t each f r e q u e n c y were f i r s t c a l c u l a t e d f o r each p a i r o f EPs. The c r o s s - s p e c t r u m r e p r e s e n t e d t h e amount o f a c t i v i t y i n one waveform t h a t c o u l d be p r e d i c t e d as a s i m p l e p r o p o r t i o n o f t h e a c t i v i t y i n the o t h e r waveform. Coherence between t h e two waveforms was c a l c u l a t e d as t h e s q u a r e - r o o t o f t h e r a t i o o f t h e magnitude o f t h e i r c r o s s - s p e c t r u m to t h e p r o d u c t o f t h e i r power s p e c t r a i n each f r e q u e n c y r a n g e . Coherence thus r e p r e s e n t e d a r a t i o o f s i m i l a r a c t i v i t y between 2 w a v e f o r m s , d i v i d e d by t h e magnitude o f t h e i r i n d i v i d u a l a c t i v i t i e s . U s i n g t h e above d e f i n i t i o n , two p r o c e s s e s w i l l be h i g h l y c o h e r e n t i f t h e s i m i l a r a c t i v i t y between them i s n e a r l y as g r e a t as t h e i r i n d i v i d u a l a c t i v i t y . As mentioned e a r l i e r , a n o t h e r way o f s t a t i n g t h i s i s t h a t coherence measures s i m i l a r i t y o f form between two waveforms. High coherence thus means t h a t t h e f o r m o f one waveform can be a c c u r a t e l y p r e d i c t e d from the form o f t h e o t h e r . 44 C. S u b j e c t s The averaged EPs o f f o u r groups o f s u b j e c t s were i n v e s t i g a t e d : r i g h t - h a n d e d normal s u b j e c t s , l e f t - h a n d e d normal s u b j e c t s , r i g h t s p e e c h -dominant e p i l e p t i c s , and l e f t speech-dominant e p i l e p t i c s . The e p i l e p t i c p a t i e n t s , 6 l e f t s p e e c h - d o m i n a n t (LSD) and 5 r i g h t speech-dominant (RSD), 8 male and 3 f e m a l e , ages 1 3 - 4 7 , were a l l b e i n g e v a l u a t e d f o r p o s s i b l e s u r g i c a l t r e a t m e n t o f e p i l e p t i c s e i z u r e s . Speech dominance was a s c e r t a i n e d by t h e c a r o t i d amytal t e s t , u s i n g f e m o r a l c a t h e t e r i z a t i o n , and r e p e a t e d on both s i d e s a t l e a s t 2 days a p a r t . Handedness i n the normal s u b j e c t s was d e t e r m i n e d a c c o r d i n g t o c r i t e r i a proposed by A n n e t t ( 1 9 6 7 ) . A l l were " p u r e " i n t h a t t h e y used o n l y t h e i r l e f t s i d e , o r o n l y t h e i r r i g h t s i d e , f o r a l l of t h e f o l l o w i n g t a s k s : w r i t i n g , b a l l t h r o w i n g , match s t r i k i n g , s c i s s o r h o l d i n g , n e e d l e t h r e a d i n g , d e a l i n g c a r d s , hammering a n a i l , h o l d i n g a t o o t h b r u s h , u n s c r e w i n g a j a r l i d , h o l d i n g a t e n n i s r a c k e t , l o o k i n g t h r o u g h a a m i c r o s c o p e , and k i c k i n g a b a l l . The normal s u b j e c t s were u n i v e r s i t y s t u d e n t s . The f i n a l groups i n c l u d e d 12 " p u r e " r i g h t - h a n d e d p e r s o n s , 7 male and 5 f e m a l e , and 12 " p u r e " l e f t - h a n d e d p e r s o n s , 6 male and 6 f e m a l e . 45 D. E l e c t r o d e placements f o r EP r e c o r d i n g EEG r e c o r d i n g s were made from e l e c t r o d e placements d e t e r m i n e d from t h e I n t e r n a t i o n a l 10-20 system ( J a s p e r , 1958). T h i s system a t t e m p t s t o p l a c e p a r t i c u l a r e l e c t r o d e s o v e r p a r t i c u l a r b r a i n -i n d e p e n d e n t l y o f s k u l l s i z e . P o s i t i o n s a r e d e t e r m i n e d as p e r c e n t a g e s o f t h e d i s t a n c e from t h e n a s i o n ( b r i d g e o f t h e nose) t o t h e i n i o n ( o c c i p u t ) , and on t h e s k u l l c i r c u m f e r e n c e . In t h i s t h e s i s , evoked p o t e n t i a l s t o v i s u a l and a u d i t o r y s t i m u l i were o f most i n t e r e s t . E l e c t r o d e placements were t h e r e f o r e chosen t o be o v e r t h e s p e c i f i c p r o j e c t i o n c o r t e x o f t h e v i s u a l and a u d i t o r y m o d a l i t i e s , t h a t i s , the o c c i p i t a l and s u p e r i o r temporal a r e a s . R e f e r r i n g t o F i g . 2 , r e c o r d i n g s were made from t h e l e f t and r i g h t o c c i p i t a l ( l a b e l l e d 01 and 0 2 ) , and from the l e f t and r i g h t temporal a r e a s , midway between T3 and C3 on t h e l e f t s i d e , and midway between T4 and C4 on the r i g h t . The temporal placements were l o c a t e d a p p r o x -i m a t e l y o v e r the s u p e r i o r temporal g y r u s , j u s t p o s t e r i o r and s u p e r i o r t o t h e i n t e r s e c t i o n o f the R o l a n d i c and S y l v i a n f i s s u r e s . To make them e a s i l y comparable t o t h e o c c i p i t a l e l e c t r o d e s 01 and 0 2 , t h e temporal placements were c a l l e d T3 and T4. The p o t e n t i a l s a t t h e f o u r e l e c t r o d e s were r e f e r r e d t o a l i n k e d r i g h t and l e f t e a r l o b e r e f e r e n c e . These were chosen as p o i n t s o f r e l a t i v e i n d i f f e r e n c e t o the evoked a c t i v i t y . 46 F i g u r e 2 The I n t e r n a t i o n a l 10-20 system o f EEG e l e c t r o d e placements and the r e c o r d i n g p o i n t s 1 to 4 used f o r r e c o r d i n g EPs. B a s e l i n e measurements a r e made a l o n g t h e n a s i o n t o i n i o n d i s t a n c e ( v e r t i c a l l i n e ) , the l e f t - t o -r i g h t , p r e a u r i c u l a r d i s t a n c e ( h o r i z o n t a l l i n e ) , and t h e s k u l l c i r c u m -f e r e n c e ( o u t e r c i r c l e ) . The i n n e r c i r c l e o r temporal l i n e o f e l e c t r o d e s i s 10% o f t h e l e f t - r i g h t , p r e a u r i c u l a r d i s t a n c e . A l l o t h e r e l e c t r o d e s a r e p l a c e d a t 20% i n t e r v a l s a l o n g t h e i r r e s p e c t i v e b a s e l i n e s . 47 E. Equipment and e x p e r i m e n t a l c o n d i t i o n s The s u b j e c t s s a t u p r i g h t i n an 8' by 8 ' by 8' s o u n d p r o o f e d room, i l l u m i n a t e d from above by a d i f f u s e 40 w a t t c e i l i n g lamp. The f l a s h s t i m u l u s was g e n e r a t e d by a Grass PSI p h o t o s t i m u l a t o r , 10 m i c r o s e c o n d d u r a t i o n , i n t e n s i t y 8 , and p r e s e n t e d t h r o u g h a 6 " c i r c u l a r a p e r t u r e p l a c e d 24" from t h e s u b j e c t ' s n a s i o n . The c l i c k s t i m u l u s was g e n e r a t e d w i t h a Grass S4C c l i c k s t i m u l a t o r , 0.1 msec, d u r a t i o n , 4 v o l t a m p l i t u d e , and p r e s e n t e d t h r o u g h s t e r e o headphones ( S a n s u i - S S l , 8 ohms) w i t h a c o m f o r t a b l e l i s t e n i n g l e v e l o f a p p r o x i m a t e l y 60 dB. F l a s h and c l i c k t r i g g e r p u l s e s were randomly mixed w i t h an a v e r a g e i n t e r - s t i m u l u s -i n t e r v a l o f 3 s e c o n d s , minimum o f 2 s e c o n d s , on a p r e v i o u s l y p r e p a r e d c o n t r o l t a p e w h i c h was used f o r a l l s u b j e c t s . Four groups o f 40 randomly mixed f l a s h and c l i c k s t i m u l i were p r e s e n t e d w i t h a 1 minute r e s t between g r o u p s . The s u b j e c t s were i n s t r u c t e d t o keep t h e i r eyes open and f i x a t e d on a s m a l l ' x ' i n t h e m i d d l e o f t h e f l a s h a p e r t u r e , t o remain q u i e t , t o a v o i d e y e , tongue o r jaw movements, and t o remain p a s s i v e l y aware o f the s t i m u l i . Monopolar o r r e f e r e n t i a l r e c o r d i n g s were made from l e f t and r i g h t o c c i p i t a l a r e a s (01 and 0 2 ) , and l e f t and r i g h t temporal a r e a s (T3 and T 4 ) , u s i n g l i n k e d r i g h t and l e f t e a r l o b e s as a r e f e r e n c e w i t h a f o r e h e a d g r o u n d . Grass E5GH e l e c t r o d e s were a p p l i e d t o t h e s c a l p w i t h p a s t e . 48 The s c a l p p o t e n t i a l s were a m p l i f i e d w i t h Grass P511 a m p l i f i e r s ( f r e q u e n c y r e s p o n s e 0 . 3 - 1 0 0 h z . ) and r e c o r d e d on a T e c h n i c a l Measurement C o r p o r a t i o n Model 1400 FM tape r e c o r d e r . These were l a t e r p l a y e d back and averaged i n t o groups o f 40 o r 160 s t i m u l i , and a n a l y s i s t i m e s o f 500 msec, o r 1 s e c , depending on t h e e x p e r i m e n t . A T e c h n i c a l Measurement C o r p o r a t i o n CAT 400B t r a n s i e n t a v e r a g e r w i t h a s a m p l i n g r a t e o f 1 0 0 / s e c . was u s e d . The d i g i t i z e d , averaged EPs were then r e c o r d e d on a P r e c i s i o n I n s t r u m e n t PI-1167 d i g i t a l tape r e c o r d e r f o r subsequent a n a l y s i s on an IBM 3 7 0 . c o m p u t e r . F. Experiment I - Power s p e c t r a l a n a l y s i s In the f i r s t e x p e r i m e n t , the a m p l i t u d e c h a r a c t e r i s t i c s o f t h e EP f r e q u e n c y components were i n v e s t i g a t e d u s i n g t h e power s p e c t r a o f the r i g h t - h a n d e d s u b j e c t s . The f a s t f o u r i e r t r a n s f o r m o f the EPs was used t o c a l c u l a t e t h e a m p l i t u d e o r magnitude o f t h e s p e c t r a a t 1 h z . i n t e r v a l s a t 0 , 1 , 2 , 3 hz . e t c . The d u r a t i o n o f each EP was 1 s e c o n d . The r e l a t i v e l y f i n e r e s o l u t i o n o f 1 h z . f o r t h e s p e c t r a was n e c e s s a r y t o show the d e t a i l o f t h e d i s t r i b u t i o n o f a m p l i t u d e s i n the o c c i p i t a l and temporal a r e a s , and how t h e s e changed w i t h f r e q u e n c y and s t i m u l u s m o d a l i t y . The W i l c o x s o n , n o n - p a r a m e t r i c t e s t f o r p a i r e d comparisons ( S i e g e ! , 1956) was used to e v a l u a t e the s i g n i f i c a n c e o f the o b s e r v e d d i f f e r e n c e s between the o c c i p i t a l and temporal a m p l i t u d e s o f t h e VEPs .and AEPs. 49 G. Experiment II - Coherence a n a l y s i s 1. I n t r o d u c t i o n In t h e second e x p e r i m e n t , t h e c o h e r e n c e o r s i m i l a r i t y o f form between the EPs from v a r i o u s a r e a s were i n v e s t i g a t e d . The VEPs and AEPs o f t h e r i g h t - h a n d e d s u b j e c t s , and t h e r i g h t and l e f t speech-dominant e p i l e p t i c p a t i e n t s were used t o show t h e r e l a t i o n s h i p s o f c o h e r e n c e asymmetries t o t h e speech dominance o f t h e s u b j e c t . In c o n t r a s t t o the s i m p l e f o u r i e r t r a n s f o r m a t i o n o f t h e EPs which was used i n t h e f i r s t e x p e r i m e n t , t h e EP a n a l y s i s o f Experiment I I was done w i t h t h e more s o p h i s t i c a t e d s p e c t r a l a n a l y s i s p r o g r a m , BMD 03T - Time s e r i e s spectrum e s t i m a t i o n ( D i x o n , 1973). T h i s program a l l o w e d f o r n o i s e , l i n e a r t r e n d and o t h e r a r t i f a c t s w i t h i n t h e d a t a , which c o u l d p o s s i b l y produce d i s t o r t e d r e s u l t s i f unaccounted f o r . The r e s u l t s o f Experiment I s u g g e s t e d t h a t a 1 h z . r e s o l u t i o n o f t h e s p e c t r a was f i n e r than n e c e s s a r y , so a 3 h z . r e s o l u t i o n was chosen t o g i v e e s t i m a t e s a t 0 , 3 , 6 , 9 , 1 2 h z . e t c . In a d d i t i o n , o b s e r v a t i o n o f the EPs showed t h a t o n l y t h e f i r s t 500 msec, c o n t a i n e d c y c l i c components (see f o r e x a m p l e , F i g s . 3 o r 4 ) , so t h e a n a l y s i s was c o n f i n e d t o t h i s r a n g e . T h i s i n e f f e c t assumed t h a t any v a r i a t i o n s a f t e r 500 msec, were not s u f f i c i e n t l y t i m e - l o c k e d t o t h e s t i m u l u s t o be i n c l u d e d i n e i t h e r t h e average o r t h e s p e c t r a l a n a l y s i s . Both o f t h e s e assume t h a t t h e s i g n a l p r o p e r t i e s , such as f r e q u e n c y and p h a s e , do not change w i t h t i m e . 50 2. E s t i m a t i o n o f coherence The r e s u l t s o f Experiment I s u g g e s t e d t h a t the 6-12 h z . component o f t h e EP was most r e l a t e d to the s t i m u l u s m o d a l i t y , so f u r t h e r a n a l y s i s was c o n f i n e d t o t h a t f r e q u e n c y band. E s t i m a t i o n o f c o h e r e n c e was done i n two ways. F i r s t , a s i n g l e measure o f o c c i p i t a l t o temporal coherence was made f o r each h e m i s p h e r e , by a v e r a g i n g the 6 , 9 and 12 h z . coherence e s t i m a t e s o v e r the 4 r e p l i c a t i o n s o f each s u b j e c t . T h i s a v e r a g e , composed o f t h e 12 i n d i v i d u a l c o h e r e n c e s , was then used to compare t h e c o h e r e n c e o f t h e r i g h t hemisphere to t h a t o f t h e l e f t on a s i m p l e , g r e a t e r - o r - l e s s than b a s i s f o r each s u b j e c t . A v e r a g i n g o f c o h e r e n c e s o v e r f r e q u e n c i e s and o v e r r e p l i c a t i o n s assumes t h a t t h e y do n o t change w i t h each component o f t h a t a v e r a g e . To more r i g o r o u s l y i n v e s t i g a t e t h i s a s s u m p t i o n , a n a l y s i s o f v a r i a n c e was u t i l i z e d w i t h each f r e q u e n c y and r e p l i c a t i o n as a f a c t o r i n t h e a n a l y s i s . Each coherence e s t i m a t e thus r e p r e s e n t e d a s i n g l e o b s e r v a t i o n , which when averaged t o g e t h e r f o r each s u b j e c t o v e r t h e 3 f r e q u e n c i e s and 4 r e p l i c a t i o n s , would g i v e t h e same v a l u e o f c o h e r e n c e as w i t h t h e s i m p l e r p r o c e d u r e d e s c r i b e d a b o v e . A v e r a g i n g and a n a l y s i s of v a r i a n c e both assume t h a t t h e d a t a i s n o r m a l l y d i s t r i b u t e d , which i s not t h e c a s e f o r any o f t h e s p e c t r a l q u a n t i t i e s ( J e n k i n s and W a t t s , 1 9 6 8 ) . In the f i r s t e x p e r i m e n t , t h i s problem was e l i m i n a t e d by u s i n g n o n - p a r a m e t r i c s t a t i s t i c s . However, f o r 51 the t y p e o f a n a l y s i s done on the c o h e r e n c e s , no n o n - p a r a m e t r i c p r o c e d u r e s e x i s t , so a n o t h e r approach was u s e d . T h i s i n v o l v e d the t r a n s f o r m a t i o n o f the d a t a t o a n o r m a l , b e l l - s h a p e d d i s t r i b u t i o n by a s c a l i n g t e c h n i q u e . The c o h e r e n c e s produced a d i s t r i b u t i o n which was skewed o r pushed t o the l e f t , w i t h many low v a l u e s o f coherence and o n l y a few h i g h o n e s . A t r a n s f o r m a t i o n was used t o make t h e p r o p o r t i o n o f low c o h e r e n c e s a p p r o x i m a t e l y e q u a l t o t h e p r o p o r t i o n o f h i g h c o h e r e n c e s . The coherences were t r a n s f o r m e d w i t h the F i s h e r Z - t r a n s f o r m ( J e n k i n s and W a t t s , 1968) which i s t h e a r c t a n h o f t h e c o h e r e n c e . T h i s t r a n s f o r m a t i o n i s a l s o used t o c o n v e r t o r d i n a r y c o r r e l a t i o n c o e f f i c i e n t s t o a more n o r m a l l y d i s t r i b u t e d form (Snedecor and C o c h r a n , 1967). T h e r e f o r e , each average o f c o h e r e n c e s was composed o f t h e a r c t a n h o f the c o h e r e n c e e s t i m a t e s a t 6 , 9 and 12 h z . and 4 r e p l i c a t i o n s . These same a r c t a n h - t r a n s f o r m e d c o h e r e n c e s were used i n the a n a l y s i s o f v a r i a n c e . 3 . A n a l y s i s o f v a r i a n c e The s i m p l e comparison o f r i g h t and l e f t h e m i s p h e r i c coherences was done f o r a l l t h r e e s u b j e c t g r o u p s . A s i m p l e d e c i s i o n o f coherence g r e a t e r on t h e r i g h t o r l e f t s i d e was made f o r each s u b j e c t and each m o d a l i t y . However, o t h e r asymmetries were p o s s i b l e between t h e f o u r r e c o r d i n g a r e a s . M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e was used t o i n v e s t i g a t e t h e s e as w e l l as t o c l a r i f y the dependence o f the c o h e r e n c e s on f r e q u e n c y of t h e s p e c t r a l e s t i m a t e . 52 For the 4 r e c o r d i n g a r e a s , l e f t and r i g h t o c c i p i t a l (01 and 02) and l e f t and r i g h t temporal (T3 and T 4 ) , t h e r e were s i x p o s s i b l e p a i r s o f c o h e r e n c e s : ( 0 1 : 0 2 ) , ( 0 1 : T 3 ) , ( 0 1 : T 4 ) , ( 0 2 : T 3 ) , - ( 0 2 : T 4 ) and ( T 3 : T 4 ) . Three p o s s i b l e i n d e p e n d e n t asymmetries o f c o h e r e n c e c o u l d thus be c a l c u l a t e d . These w e r e : between hemispheres (01:T3 compared t o 0 2 : T 4 ) , between p r o j e c t i o n a r e a s o f each m o d a l i t y ( 0 1 : 0 2 compared t o T 3 : T 4 ) , and between hemispheres and p r o j e c t i o n a r e a s (01:T4 compared to 0 2 : T 3 ) . These coherence p a i r s were g e n e r a t e d i n r e s p o n s e t o t h e same s t i m u l u s , and were r e c o r d e d from the same s u b j e c t , and so were i n g e n e r a l c o r r e l a t e d . Each o f t h e s e c o r r e l a t e d asymmetries c o u l d have been a n a l y z e d i n a s e p a r a t e a n a l y s i s o f v a r i a n c e , but a b e t t e r method u t i l i z i n g a m u l t i v a r i a t e a n a l y s i s o f v a r i a n c e , a l l o w e d a s i m u l t a n e o u s comparison o f t h e t h r e e a s y m m e t r i e s . A n a l y s i s o f v a r i a n c e a l l o w s an i n v e s t i g a t i o n o f the e f f e c t s and i n t e r r e l a t i o n s h i p s o f v a r i o u s independent f a c t o r s on a dependent v a r i a b l e . M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e e x t e n d s t h i s c o n c e p t to m u l t i p l e dependent v a r i a b l e s , i n t h i s c a s e t h e coherence a s y m m e t r i e s , which a r e c o r r e l a t e d . The e x a c t use and i n t e r p r e t a t i o n o f t h e t e c h n i q u e w i l l become c l e a r e r i n the R e s u l t s and D i s c u s s i o n . In most r e s p e c t s , ; m u l t i v a r i a t e a n a l y s i s o f v a r i a n c e has d i r e c t a n a l o g s w i t h the u n i v a r i a t e c o n c e p t s o f t h e F - t e s t , main e f f e c t s and i n t e r a c t i o n s . The U - s t a t i s t i c i s t h e m u l t i v a r i a t e a n a l o g o f t h e u n i v a r i a t e F - s t a t i s t i c , and i s used t o t e s t the s i g n i f i c a n c e o f the d i f f e r e n c e between two s e t s o f means. In t h i s e x p e r i m e n t , i t was used t o t e s t t h e h y p o t h e s i s t h a t t h e t h r e e coherence asymmetries were s i g n i f i c a n t l y d i f f e r e n t f o r f l a s h and c l i c k s t i m u l i . Main e f f e c t s 53 d e s c r i b e t h e e f f e c t s o f each o f t h e i n d e p e n d e n t f a c t o r s on t h e dependent v a r i a b l e s . I n t e r a c t i o n s d e s c r i b e how two o r more f a c t o r s i n t e r a c t t o a f f e c t the dependent v a r i a b l e s i n a way t h a t c a n n o t p r e d i c t e d from any f a c t o r by i t s e l f . The t e c h n i q u e s o f m u l t i v a r i a t e a n a l y s i s o f v a r i a n c e a r e d e s c r i b e d by Anderson (1958) and a r e i n c o r p o r a t e d i n t h e UCLA B i o m e d i c a l computer program BMD X69 - M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e and c o v a r i a n c e ( D i x o n , 1973). The program was used t o a n a l y z e t h e a r c t a n h - t r a n s f o r m e d coherence a s y m m e t r i e s , w i t h each 3 h z . r e s o l u t i o n e s t i m a t e a t 6 , 9 and 12 h z . used as a s i n g l e o b s e r v a t i o n . The f a c t o r s and t h e i r l e v e l s were s u b j e c t s S ( 1 2 ) , s t i m u l u s m o d a l i t y M ( 2 ) , f r e q u e n c y band o f t h e c o h e r e n c e e s t i m a t e F ( 3 ) , and t h e r e p l i c a t i o n s f o r each s u b j e c t R ( 4 ) . The c o r r e l a t e d , dependent v a r i a b l e s were the t h r e e c o h e r e n c e asymmetries c a l c u l a t e d a s p a i r e d d i f f e r e n c e s ( 0 1 : 0 2 - T 3 : T 4 ) , ( 0 1 : T 3 - 0 2 : T 4 ) and ( 0 1 : T 4 - 0 2 : T 3 ) . D i f f e r e n c e s between each a r c t a n h - t r a n s f o r m e d c o h e r e n c e were c a l c u l a t e d f o r each s u b j e c t , m o d a l i t y , f r e q u e n c y and r e p l i c a t i o n . The e x p e r i m e n t a l d e s i g n was a 3-way f a c t o r i a l w i t h 4 r e p l i c a t i o n s and 3 dependent v a r i a b l e s , and was i n t e n d e d t o i n v e s t i g a t e t h e e f f e c t s o f s u b j e c t , s t i m u l u s m o d a l i t y , and f r e q u e n c y component on t h e t h r e e coherence a s y m m e t r i e s . H. Experiment I I I - D i s c r i m i n a n t a n a l y s i s I . I n t r o d u c t i o n In t h e t h i r d e x p e r i m e n t , d i s c r i m i n a n t a n a l y s i s o f t h e EP s p e c t r a was used t o f u r t h e r i n v e s t i g a t e the r e l a t i o n s h i p s o f both c o h e r e n c e and 54 a u t o s p e c t r a l asymmetries t o speech dominance. In a d d i t i o n , a computer t e s t f o r the EP d e t e r m i n a t i o n o f speech dominance was c o n s t r u c t e d . T h i s was then a p p l i e d t o the normal s u b j e c t s t o o b t a i n d i s t r i b u t i o n s o f speech dominance i n the n o r m a l , r i g h t o r l e f t - h a n d e d p o p u l a t i o n . A p r i m a r y aim o f t h i s e x p e r i m e n t was t o c l a r i f y some o f t h e m e t h o d o l o g i c a l u n c e r t a i n t i e s o f t h e p r e v i o u s e x p e r i m e n t . The f i r s t o f t h e s e c e n t r e d around t h e e s t i m a t i o n o f the 6-12 h z . c o h e r e n c e as an average o f t h e 6 , 9 and 12 hz . c o h e r e n c e s o v e r 4 r e p l i c a t i o n s . A more c o n v e n t i o n a l , though n o t n e c e s s a r i l y more a c c u r a t e p r o c e d u r e , would have been t o c a l c u l a t e a s i n g l e c o h e r e n c e o v e r the e n t i r e 6-12 h z . range f o r one EP. T h i s was done by u s i n g s p e c t r a o f 6 h z . r e s o l u t i o n c a l c u l a t e d from a s i n g l e , averaged EP o f 160 s t i m u l i . A c r o s s - v a l i d a t i o n was then p o s s i b l e f o r the c o n c l u s i o n s based on t h e novel and c o n v e n t i o n a l p r o c e d u r e s . The second problem i n v o l v e d the e v a l u a t i o n o f w h i c h s p e c t r a l asymmetries were most r e l a t e d t o speech dominance. T h i s was o n l y approached q u a l i t a t i v e l y i n t h e second e x p e r i m e n t , b u t i n t h i s e x p e r i m e n t , d i s c r i m i n a n t a n a l y s i s was used to e v a l u a t e t h e s i g n i f i c a n c e o f each asymmetry's r e l a t i o n s h i p t o speech dominance. 55 2. S p e c t r a l a n a l y s i s The UCLA s p e c t r a l a n a l y s i s program was a g a i n u s e d , t h i s t i m e t o c a l c u l a t e s p e c t r a w i t h 6 hz . r e s o l u t i o n from t h e 500 msec. EPs. The s p e c t r a l e s t i m a t e c e n t r e d a t 6 h z . , and e x t e n d i n g o v e r t h e f r e q u e n c y range o f 3 - 9 hz.^ was used f o r a l l a n a l y s e s . The 3 - 9 h z . bandwidth produced by t h e program d i d n o t n u m e r i c a l l y o v e r l a p the 6-12 hz. range examined p r e v i o u s l y , but i t was t h e c l o s e s t one a v a i l a b l e . A l s o , from a p r a c t i c a l v i e w p o i n t , t h e two were a l m o s t e q u a l . The 3 - 9 h z . band and t h e 6-12 h z . band c o n t a i n e d t h e l a r g e s t a m p l i t u d e component o f any i n t h e 500 msec, s p e c t r a , and encompassed from 60-80% o f the t o t a l power i n a l l s u b j e c t s . In a d d i t i o n , t h e power s p e c t r a o f a l l s u b j e c t s showed t h a t most o f t h e low f r e q u e n c y , 0 - 5 hz . power was below 3 h z . , and most o f t h e h i g h f r e q u e n c y , 6-12 h z . power was below 10 h z . ( f o r e x a m p l e , see F i g . 3 o f t h e R e s u l t s ) . The 3-9 h z . c o n v e n t i o n a l spectrum was thus a good o v e r l a p w i t h t h a t o f t h e 6-12 h z . b a n d , but w i t h the advantage t h a t a v e r a g i n g o v e r f r e q u e n c i e s f o r e i t h e r coherences o r power s p e c t r a was n o t n e c e s s a r y . T h i s was u s e f u l i n c o n f i r m i n g the c o n c l u s i o n s based on t h e n o v e l e s t i m a t i o n o f c o h e r e n c e , b u t i t was n e c e s s a r y f o r t h e d i s c r i m i n a n t a n a l y s i s , s i n c e some o f i t s t h e o r e t i c a l r e q u i r e m e n t s l i m i t e d i t s a p p l i c a t i o n t o a s i n g l e coherence o r power s p e c t r a l e s t i m a t e . As w i t h t h e a n a l y s i s o f v a r i a n c e , d i s c r i m i n a n t a n a l y s i s r e q u i r e d a normal d i s t r i b u t i o n o f d a t a . Coherences were a r c t a n h - t r a n s f o r m e d as b e f o r e , and power s p e c t r a were 1 o g - j Q - t r a n s f o r m e d s i n c e t h i s s c a l e i s n o r m a l i z i n g f o r them ( J e n k i n s and W a t t s , 1968; Hannan, H970,).. 56 3 . Asymmetry c a l c u l a t i o n s Coherence asymmetries were c a l c u l a t e d i n t h e same way as i n t h e l a s t e x p e r i m e n t . That i s , from t h e 4 r e c o r d i n g a r e a s 0 1 , 0 2 , T3 and T 4 , s i x coherence p a i r s were c a l c u l a t e d . From t h e s e , t h r e e independent asymmetries o f c o h e r e n c e were d e r i v e d : ( 0 1 : 0 2 - T 3 : T 4 ) , ( 0 1 : T 3 - 0 2 : T 4 ) , and ( 0 1 : T 4 - 0 2 : T 3 ) . Power s p e c t r a l asymmetries were c a l c u l a t e d between homologous a r e a s o f each h e m i s p h e r e . These were l e f t - m i n u s - r i g h t o c c i p i t a l (01-02) and l e f t - m i n u s - r i g h t temporal ( T 3 - T 4 ) . Asymmetries were c a l c u l a t e d f o r b o t h AEPs and VEPs, so t h e f i n a l d a t a i n c l u d e d 6 asymmetries o f c o h e r e n c e and 4 o f a u t o s p e c t r a f o r each s u b j e c t . 4 . D i s c r i m i n a n t a n a l y s i s The c o h e r e n c e and power s p e c t r a l asymmetries were each s e p a r a t e l y s u b j e c t e d t o d i s c r i m i n a n t a n a l y s i s ( A f i f i and A z e n , 1 9 7 0 ) , u s i n g the UCLA B i o m e d i c a l computer program BMD 07M - S t e p w i s e d i s c r i m i n a n t a n a l y s i s ( D i x o n , 1973). Two problems were s e t up: one w i t h 6 c o h e r e n c e a s y m m e t r i e s , and one w i t h t h e 4 power s p e c t r a l asymmetries o f each s u b j e c t as i n p u t . 57 In each o f t h e above p r o b l e m s , t h e program e v a l u a t e d t h e s i g n i f -i c a n c e o f each asymmetry's r e l a t i o n s h i p t o speech dominance. T h i s was based on a comparison o f t h e known speech dominance o f each p a t i e n t and h i s c o r r e s p o n d i n g s p e c t r a l a s y m m e t r i e s . E q u a t i o n s were d e r i v e d by t h e program which q u a n t i f i e d t h e s e r e l a t i o n s h i p s f o r asymmetries s i g n i f i c a n t l y r e l a t e d t o dominance a t a p r o b a b i l i t y o f p l e s s than 0 . 0 5 These e q u a t i o n s were then a p p l i e d t o the s p e c t r a l asymmetries o f t h e normal s u b j e c t s t o p r e d i c t t h e i r speech dominance. 58 RESULTS A. Experiment I - Power s p e c t r a l a n a l y s i s 1. The evoked p o t e n t i a l power s p e c t r a F i g . 3 shows the VEPs, AEPs and power s p e c t r a o f two s u b j e c t s . The r e s p o n s e s i n the p r i m a r y p r o j e c t i o n a r e a o f the s t i m u l a t e d m o d a l i t y ( i e . - o c c i p i t a l f o r f l a s h o r temporal f o r c l i c k ) c o n s i s t e d o f two major f r e q u e n c y components, a 0 - 5 h z . component, l a b e l l e d as Group I , and a 6-12 h z . component, l a b e l l e d as Group I I . These two groups were p r e s e n t i n t h e power s p e c t r a o f a l l 4 a r e a s i n a l l 12 s u b j e c t s , and c o n s t i t u t e d a t l e a s t 95% o f t h e t o t a l power i n each r e s p o n s e . F i g . 4 shows t h e c o r r e s p o n d i n g power s p e c t r a o f EPs i n a r e a s which were not s p e c i f i c t o t h e s t i m u l u s m o d a l i t y , f o r e x a m p l e , the o c c i p i t a l r e s p o n s e to c l i c k s t i m u l i , o r t h e temporal r e s p o n s e t o f l a s h s t i m u l i . Both the Group I and II f r e q u e n c i e s were s t i l l p r e s e n t , but the Group I I , 6-12 h z . component v a r i e d more than i n t h e p r i m a r y p r o j e c t i o n a r e a s and u s u a l l y r e p r e s e n t e d a s m a l l e r p r o p o r t i o n o f t h e t o t a l EP power. In some s u b j e c t s , t h e Group II r e s p o n s e d i s s a p p e a r e d a l m o s t e n t i r e l y . 59 BC F L A S H CLICK AUD->. 1.0 SEC. 0 5 10 15 20 25 FREQUENCY (HZ) A D CLICK POWER S P E C T R A Ol 02 AUD 2 . 1.0 SEC. > 0 5 10 15 20 25 FREQUENCY (HZ) F i g u r e 3 P r i m a r y p r o j e c t i o n a r e a E P s a n d p o w e r s p e c t r a o f 2 s u b j e c t s . N o t e t h e t w o m a j o r f r e q u e n c y g r o u p s ( I a n d I I ) i n t h e s p e c t r a . 0 1 , 0 2 = l e f t , r i g h t o c c i p i t a l ; A U D I , AUD2 = l e f t , r i g h t t e m p o r a l . F i g u r e 4 N o n - p r i m a r y p r o j e c t i o n a r e a EPs and s p e c t r a o f same s u b j e c t s as F i g . 3. Note t h e more v a r i a b l e Group I I as compared t o t h e p r i m a r y a r e a , r e s p o n s e s o f F i g . 3. 61 2 . S t a t i s t i c a l a n a l y s i s Comparisons o f t h e magnitudes o f r e s p o n s e s p e c t r a i n the p r i m a r y and n o n - p r i m a r y p r o j e c t i o n a r e a s were made w i t h the W i l c o x s o n , n o n -p a r a m e t r i c , p a i r e d samples t e s t ( S i e g e l , 1 9 5 6 ) . The t e s t was done a t each f r e q u e n c y t o d e t e r m i n e the f r e q u e n c y d i s t r i b u t i o n o f a m p l i t u d e changes between p a i r s o f r e c o r d i n g a r e a s . Each t e s t i n v o l v e d 48 p a i r s , composed o f 4 VEPs and 4 AEPs from each r e c o r d i n g a r e a o f each o f t h e 12 s u b j e c t s . A t w o - t a i l e d , p l e s s than 0 . 0 5 l e v e l o f s i g n i f i c a n c e was u s e d . F i g . 5 shows the m e d i a n , p a i r e d d i f f e r e n c e s and a s s o c i a t e d p r o b a b i l i t i e s between t h e o c c i p i t a l and temporal a r e a s p e c t r a o f each h e m i s p h e r e . Two major f r e q u e n c y groups o f d i f f e r e n c e s appeared i n the same ranges as the i n d i v i d u a l s u b j e c t s p e c t r a . The low f r e q u e n c y , 0 - 5 h z . , Group I d i f f e r e n c e s ( o c c i p i t a l minus temporal r e s p o n s e ) were always s i g n i f i c a n t l y n e g a t i v e f o r both f l a s h and c l i c k s t i m u l i . In o t h e r w o r d s , t h e temporal r e s p o n s e s were always g r e a t e r than t h e o c c i p i t a l . In c o n t r a s t , t h e Group I I , 6-12 h z . r e s p o n s e s were s i g n i f -i c a n t l y g r e a t e r i n the o c c i p i t a l a r e a s f o r f l a s h and i n the temporal a r e a s f o r c l i c k . The r i g h t hemisphere VEP a l s o showed t h i s p a t t e r n , but the d i f f e r e n c e s were n o t s i g n i f i c a n t . 3 . The temporal form o f t h e f r e q u e n c y groups The 0 - 5 h z . and 6-12 h z . f r e q u e n c y components were e x t r a c t e d by d i g i t a l l y f i l t e r i n g the EPs shown i n F i g . 3 . The f i l t e r e d components 62 LEFT HEMISPHERE RIGHT HEMISPHERE 2.0 F L A S H (0|-AUD|>-P R O B — > -1.0 -20 20 1.0 -1.0 - 2 0 /••I ; \ l \ I \ i \ i r v ^ r i _ \ 1 / 10 15 FREQ CHZ) . 0 0 . 0 1 >-. 0 2 J CO . 0 3 < CD . 0 4 § =.05 ° -F L A S H (02-AUD2>— PROB-—.00 CLICK ( O r A L C \ ) — PROB . 0 0 . 0 1 >• . 0 2 h-_l . 0 3 ABI . 0 4 CO o CC * . 0 5 CL i JK n so is ^FREQ CHZJ F i g u r e 5 M e d i a n , o c c i p i t a l (01 o r 02) minus temporal (AUDI o r AUD2) s p e c t r a and a s s o c i a t e d p r o b a b i l i t i e s o f a l l s u b j e c t s . Group I , 0 - 5 h z . r e s p o n s e s a r e g r e a t e r i n the temporal a r e a s f o r both f l a s h and c l i c k , w h i l e Group I I , 6-12 h z . r e s p o n s e s a r e g r e a t e r i n the p r i m a r y p r o j e c t i o n a r e a o f t h e s t i m u l u s m o d a l i t y . 63 B C TOTAL EVOKED RESPONSE FLASH CLICK GROUP I 0-5 HZ. 0| GROUPH 6-12 HZ. AUD' 1.0 SEC. > AD TOTAL EVOKED RESPONSE FLASH 0| CLICK AUD, AUD2-GROUP I 0-5 HZ. GROUP n 6-12 HZ. A U D i ^ / V ^ ^ AUD2_/\/\^ 1.0 SEC. > 3*. F i g u r e 6 D i g i t a l l y f i l t e r e d Group I ( 0 - 5 h z . ) and Group I I , (6-12 h z . ) components o f t h e same EPs as F i g . 3 . 64 a r e shown i n F i g . 6. Group I , 0 - 5 h z . f r e q u e n c i e s c o n s i s t e d o f a s l o w , b i p h a s i c wave, v e r y s i m i l a r f o r b o t h f l a s h and c l i c k i n t h e f i r s t 300 m s e c , and c o r r e s p o n d i n g t o p o s i t i v e peaks a t about 100 and 300 msec. Group I I , 6-12 h z . f r e q u e n c i e s f i r s t appeared a t about 50 m s e c , r e c u r r e d a t about 50 m s e c i n t e r v a l s , and were d i s s i m i l a r f o r t h e f l a s h and c l i c k s t i m u l i . B. Experiment I I - Coherence a n a l y s i s 1 . Comparison o f h e m i s p h e r i c c o h e r e n c e s T a b l e I shows t h e hemisphere i n which t h e maximum VEP and AEP coherence o c c u r r e d f o r the r i g h t and l e f t - s p e e c h dominant e p i l e p t i c s , and t h e n o r m a l , r i g h t - h a n d e d s u b j e c t s , who were presumably l e f t -speech dominant. Coherences were c a l c u l a t e d as t h e average o f the 6 , 9 and 12 h z . e s t i m a t e s o v e r 4 r e p l i c a t i o n s f o r each s u b j e c t . For t h e g r e a t m a j o r i t y o f s u b j e c t s , the c l i c k r e s p o n s e s o r AEPs were more c o h e r e n t on t h e speech dominant s i d e . C o n v e r s e l y , the f l a s h r e s p o n s e s o r VEPs were more c o h e r e n t on t h e speech non-dominant s i d e i n most s u b j e c t s . T a b l e I I shows t h e coherences o f s u b j e c t s who b e s t i l l u s t r a t e d t h e t r e n d s o f T a b l e I . TABLE I Number o f s u b j e c t s w i t h maximum o c c i p i t a l - t e m p o r a l coherence i n the dominant hemisphere f o r a u d i t o r y evoked p o t e n t i a l s (AEPs) and i n the non-dominant hemisphere f o r v i s u a l evoked p o t e n t i a l s ( V E P s ) . L,R = l e f t , r i g h t . S u b j e c t T o t a l number S u b j e c t s w i t h maximum S u b j e c t s w i t h maximum group o f s u b j e c t s AEP coherence on VEP coherence on dominant s i d e non-dominant s i d e N o r m a l , R. handed 12 10 9 (assume L. dominant) L. dominant amytal 6 5 5 R. dominant amytal 5 4 3 66 TABLE I I O c c i p i t a l - t e m p o r a l , 6-12 h z . c o h e r e n c e s i n the l e f t and r i g h t h e m i s p h e r e s . For 4 s u b j e c t s b e s t i l l u s t r a t i n g t h e t r e n d s o f each s u b j e c t g r o u p . A coherence o f 0 . 5 0 means t h a t 0 . 5 x 0 . 5 = 0 . 2 5 o f t h e v a r i a n c e o f one r e s p o n s e can be e x p l a i n e d as a l i n e a r v e r s i o n o f t h e o t h e r . S u b j e c t dominance VEP h e m i s p h e r i c coherence AEP h e m i s p h e r i c c o h e r e n c e L e f t R i g h t L e f t R i g h t L e f t 0 . 8 9 0 . 9 2 0 . 8 8 0 . 8 4 R i g h t 0.72 0 . 5 9 0 . 7 6 0.81 R i g h t - b i l a t e r a l 0 . 7 0 0 . 7 0 0 . 6 5 0 . 6 6 N o r m a l , r i g h t - h a n d e d 0 . 7 5 0 . 8 0 0 . 7 6 0 . 6 8 67 2 . M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e T a b l e I I I shows t h e r e s u l t s o f t h e m u l t i v a r i a t e a n a l y s i s o f v a r i a n c e o f a l l t h r e e p o s s i b l e coherence asymmetries as the dependent v a r i a b l e s . T h i s a n a l y s i s was d e s i g n e d t o i n v e s t i g a t e t h e e f f e c t s and i n t e r a c t i o n s o f t h e s u b j e c t , s t i m u l u s m o d a l i t y and f r e q u e n c y o f c o h e r e n c e e s t i m a t e on t h e coherence a s y m m e t r i e s . The i n t e r p r e t a t i o n o f t h e m u l t i v a r i a t e a n a l y s i s o f v a r i a n c e t a b l e i s s i m i l a r t o t h e t h a t o f u n i v a r i a t e a n a l y s i s . The s o u r c e s r e p r e s e n t the e f f e c t s o r i n t e r a c t i o n s o f i n t e r e s t , which are t h e s u b j e c t s ( S ) , m o d a l i t y ( M ) , f r e q u e n c y (F) and t h e i r i n t e r a c t i o n s . The g e n e r a l i z e d v a r i a n c e , U - s t a t i s t i c and degrees o f freedom a r e t h e m u l t i v a r i a t e a n a l o g s o f t h e v a r i a n c e , F - s t a t i s t i c , and degrees o f f r e e d o m , and measure t h e v a r i a b i l i t y and r e l i a b i l i t y o f t h e e f f e c t s . F i n a l l y , the p r o b a b i l i t y t h a t t h e o b s e r v e d U - s t a t i s t i c c o u l d be due t o chance i s used t o e v a l u a t e s i g n i f i c a n c e l e v e l s . For i n s t a n c e , i f an e f f e c t had a U - p r o b a b i l i t y o f 0 . 0 1 5 , t h i s would mean t h a t t h e r e were 1 . 5 chances out o f 100 t h a t t h e o b s e r v e d e f f e c t was due t o c h a n c e . However, t h e a n a l y s i s i t s e l f o n l y d e t e r m i n e s whether the means o f t h e e f f e c t s a r e s i g n i f i c a n t l y d i f f e r e n t . To i n t e r p r e t j u s t what the s i g n i f i c a n t e f f e c t i s and what form i t t a k e s , r e q u i r e s an e x a m i n a t i o n o f t h e means. T a b l e I shows t h a t w i t h a p r o b a b i l i t y o f p l e s s than 0 . 0 1 , t h e s i g n i f i c a n t main e f f e c t s on c o h e r e n c e asymmetries were s u b j e c t (S) and s t i m u l u s m o d a l i t y (M). The s i g n i f i c a n t i n t e r a c t i o n s were s u b j e c t x m o d a l i t y (SM) and s u b j e c t x m o d a l i t y x f r e q u e n c y (SMF). TABLE I I I 68 M u l t i v a r i a t e a n a l y s i s o f v a r i a n c e o f a r c t a n h - t r a n s f o r m e d coherence a s y m m e t r i e s . Dependent v a r i a b l e s were t h e asymmetries o f c o h e r e n c e . 0 1 , 0 2 = l e f t , r i g h t o c c i p i t a l ; T 3 , T 4 = l e f t , r i g h t t e m p o r a l . F a c t o r s and l e v e l s were s u b j e c t S ( 1 2 ) , M o d a l i t y M ( 2 ) , Frequency F(3) o f coherence e s t i m a t e , and r e p l i c a t i o n s R ( 4 ) . * = s i g n i f i c a n t a t p l e s s than 0 . 0 1 . Dependent v a r i a b l e s = ( 0 1 : 0 2 - T 3 : T 4 ) , ( 0 1 : T 3 - 0 2 : T 4 ) , and ( 0 1 : T 4 - 0 2 : T 3 ) . Source Log U - s t a t i s t i c Degrees o f P r o b a b i l i t y o f ( g e n e r a l i z e d freedom g r e a t e r than U v a r i a n c e S 13.67176 0.453242 3 11 216 0 . 0 0 0 0 * M 12.93489 0.946997 3 1 216 0.0087 * F 12.90911 0.971728 3 2 216 0 . 4 0 5 2 SM 13.23715 0.699971 3 11 216 0 . 0 0 0 0 * SF 13.24435 0.694952 3 22 216 0.0907 MF 12.93388 0.947956 3 2 216 0.9737 SMF 13.36088 0.618509 3 2 216 0 . 0 0 0 9 * R(SMF) 13.88043 6 9 From t h i s a n a l y s i s , the f o l l o w i n g i n f e r e n c e s were made. The s i g n i f i c a n t s u b j e c t (S) main e f f e c t showed t h a t d i f f e r e n t s u b j e c t s had d i f f e r e n t magnitudes o f c o h e r e n c e a s y m m e t r i e s . The s i g n i f i c a n t m o d a l i t y (M) e f f e c t showed t h a t t h e coherence asymmetries d i f f e r e d s i g n i f i c a n t l y f o r VEPs and AEPs. The s i g n i f i c a n t m o d a l i t y x s u b j e c t (SM) i n t e r a c t i o n showed t h a t not a l l s u b j e c t s responded i n t h e same way t o t h e same s t i m u l u s m o d a l i t y . That i s , some s u b j e c t s had a p a t t e r n o f coherence asymmetries which were d i f f e r e n t than the m a j o r i t y . T h i s a grees w i t h t h e r e s u l t s shown i n T a b l e I , which show t h a t n o t a l l t h e n o r m a l , r i g h t - h a n d e d s u b j e c t s f o l l o w e d a l e f t - s p e e c h dominant p a t t e r n . The s i g n i f i c a n t s u b j e c t x m o d a l i t y x f r e q u e n c y (SMF) i n t e r a c t i o n i n d i c a t e d t h a t the VEP and AEP coherence asymmetries depended i n some complex f a s h i o n on t h e f r e q u e n c y . To i n t e r p r e t more c l e a r l y t h e s i g n i f i c a n t r e l a t i o n s h i p s o f t h e f a c t o r s , o b s e r v a t i o n o f t h e mean c o h e r e n c e s was n e c e s s a r y . T a b l e IV shows the s i g n i f i c a n t m o d a l i t y (M) e f f e c t f o r the f l a s h and c l i c k , mean c o h e r e n c e s and asymmetries i n both t h e i r o r i g i n a l and a r c t a n h -t r a n s f o r m e d , n o r m a l l y d i s t r i b u t e d f o r m s . The ( 0 1 : 0 2 - T 3 : T 4 ) asymmetries changed i n degree w i t h t h e s t i m u l u s m o d a l i t y . The ( 0 1 : T 4 - 0 2 : T 3 ) asymmetries remained about t h e same f o r both m o d a l i t i e s . Only the r i g h t - t o - l e f t hemisphere asymmetries ( 0 1 : T 3 - 0 2 : T 4 ) changed i n d i r e c t i o n o r s i g n w i t h the s t i m u l u s m o d a l i t y . That i s , f o r VEPs, t h e o c c i p i t a l -t o - t e m p o r a l coherence was g r e a t e r i n t h e r i g h t h e m i s p h e r e , w h i l e f o r A E P s , t h i s coherence was g r e a t e r i n the l e f t h e m i s p h e r e . T a b l e IV a l s o shows t h a t i n g e n e r a l , t h e AEP c o h e r e n c e s were s m a l l e r than the VEP 70 TABLE IV Coherence and c o h e r e n c e asymmetry means f o r 12 s u b j e c t s . 0 1 , 0 2 = l e f t , r i g h t o c c i p i t a l ; T3,T4 = l e f t , r i g h t t e m p o r a l . Coherence asymmetries ( a r c t a n h - t r a n s f o r m e d ) ( 0 1 : 0 2 - T 3 : T 4 ) ( 0 1 : T 3 - 0 2 : T 4 ) ( 0 1 : T 4 - 0 2 : T 3 ) VEP AEP - 0 . 0 8 0 7 - 0 . 0 9 1 2 +0.0809 - 0 . 2 6 8 8 +0.0775 +0.0752 Coherences ( 0 1 : 0 2 ) (T3:T4) (01:T3) ( 0 2 : T 4 ) (01:T4) ( 0 2 : T 3 ) VEP AEP 0 . 9 2 0 . 9 3 0 . 8 2 0 . 8 4 0 . 8 5 0 . 8 2 0 . 8 7 0.91 0 . 7 4 0 . 7 0 0 . 7 3 0 . 7 0 71 c o h e r e n c e s . A l s o , t h e h i g h e s t coherences o c c u r r e d between homologous a r e a s o f t h e l e f t and r i g h t hemisphere ( 0 1 : 0 2 and T 3 : T 4 ) , but t h e s e d i d not seem t o be c l e a r l y r e l a t e d t o s t i m u l u s m o d a l i t y . The n a t u r e o f t h e s i g n i f i c a n t i n t e r a c t i o n s was i n d i c a t e d by o b s e r v a t i o n o f a l l the means ( n o t shown). The s u b j e c t x m o d a l i t y (SM) i n t e r a c t i o n was due t o some s u b j e c t s h a v i n g c o h e r e n c e asymmetries which were o p p o s i t e o f t h o s e o f t h e m a j o r i t y . The s u b j e c t x m o d a l i t y x f r e q u e n c y (SMF) i n t e r a c t i o n was due t o VEP c o h e r e n c e s t e n d i n g to be maximal a t 9 o r 12 h z . , w h i l e AEP c o h e r e n c e s were g e n e r a l l y maximal a t 6 o r 9 h z . T h i s s u g g e s t e d t h a t VEP a c t i v i t y was g e n e r a l l y o f s l i g h t l y h i g h e r f r e q u e n c y than AEP a c t i v i t y . These f r e q u e n c y dependencies were o b s e r v e d f o r both the c o h e r e n c e s t h e m s e l v e s and t h e i r a s y m m e t r i e s . 72 C. Experiment I I I - D i s c r i m i n a n t a n a l y s i s 1. Coherence a n a l y s i s T a b l e V shows t h e d i s c r i m i n a n t a n a l y s i s o f c o h e r e n c e a s y m m e t r i e s . There were o n l y two asymmetries w h i c h were r e l a t e d t o speech d o m i n a n c e . a t a s i g n i f i c a n c e l e v e l o f p l e s s than 0 . 0 5 . These were the l e f t - m i n u s - r i g h t h e m i s p h e r i c coherence ( 0 1 : T 3 - 0 2 : T 4 ) f o r VEPs and AEPs. The LSD and RSD group means o f t h e s e two asymmetries were s i g n i f i c a n t l y d i f f e r e n t ( p l e s s than 0 . 0 0 5 ) , as shown by the U - s t a t i s t i c produced by t h e program. The d i s c r i m i n a n t e q u a t i o n s produced by t h e program w e r e : 1 .79 - 5 . 9 2 (F) + 14.67 (C) (1) 1 .14 + 1 . 9 3 (F) - 11 .59 (C) S LSD SRSD where S ^ and S ^ a r e t h e LSD and RSD s c o r e s r e s p e c t i v e l y , and (F) = f l a s h coherence asymmetry ( 0 1 : T 3 - 0 2 : T 4 ) , and (C) = c l i c k coherence asymmetry ( 0 1 : T 3 - 0 2 : T 4 ) . U s i n g t h e s e e q u a t i o n s , an unknown s u b j e c t ' s coherence asymmetries a r e i n s e r t e d i n t o t h e (F) and (C) t e r m s , and he i s a s s i g n e d t o t h e speech dominance group f o r w h i c h he a t t a i n s t h e most p o s i t i v e s c o r e . 73 TABLE V D i s c r i m i n a n t a n a l y s i s o f a r c t a n h - t r a n s f o r m e d c o h e r e n c e a s y m m e t r i e s . 0 1 , 0 2 = l e f t , r i g h t o c c i p i t a l ; T 3 , T 4 = l e f t , r i g h t t e m p o r a l . F,C = f l a s h , c l i c k . LSD.RSD = l e f t , r i g h t - s p e e c h dominant. D i s c r i m i n a t i o n p r o b a b i l i t y e v a l u a t e s t h e asymmetry's c o n t r i b u t i o n t o speech dominance d e t e r m i n a t i o n . Coherence LSD mean RSD mean D i s c r i m i n a t i o n asymmetry (6 p a t i e n t s ) (5 p a t i e n t s ) p r o b a b i l i t y ( 0 1 : 0 2 - T 3 : T 4 ) F + . 0 2 + . 0 8 . 2 8 6 ( 0 1 : 0 2 - T 3 : T 4 ) C - . 1 8 - . 0 6 . 2 4 3 ( 0 1 : T 3 - 0 2 : T 4 ) p - . 3 4 - . 1 9 .037 ( p l e s s than . 0 5 ) ( 0 1 : T 3 - 0 2 : T 4 ) C +.11 - . 2 4 .001 ( p l e s s than . 0 0 5 ) ( 0 1 : T 4 - 0 2 : T 3 ) p - . 3 6 + . 0 9 . 2 4 0 ( 0 1 : T 4 - 0 2 : T 3 ) C - . 0 5 - . 1 7 .941 74 The p r o b a b i l i t i e s , P L S D and P R S D , t h a t a s u b j e c t has been m i s c i a s s i f i e d a r e : P. LSD 1 + exp (S, LSD (2) P. RSD = 1 - P, LSD 1 + exp (S RSD S u b j e c t s were c l a s s i f i e d as b i l a t e r a l i f t h e i r p r o b a b i l i t i e s o f m i s c l a s s i f i c a t i o n were g r e a t e r than 0 . 0 5 . These e q u a t i o n s were d e r i v e d on the b a s i s o f t h e c o h e r e n c e asymmetries o f the p a t i e n t s and were then used to e v a l u a t e t h e speech dominance o f t h e normal s u b j e c t s . In a d d i t i o n , by i n s e r t i n g the coherence asymmetry v a l u e s o f the p a t i e n t s i n t o t h e e q u a t i o n s , " p o s t e r i o r " p r o b a b i l i t i e s were e v a l u a t e d . These gave a measure o f how w e l l the e q u a t i o n s c l a s s i f i e d the p a t i e n t s whose speech dominance was a l r e a d y known. U s i n g t h e coherence asymmetries o f t h e p a t i e n t s , 5/6 LSD and 5/5 RSD p a t i e n t s were c o r r e c t l y c l a s s i f i e d , w i t h p o s t e r i o r p r o b a b i l i t i e s o f p l e s s than 0 . 0 5 i n 5/6 LSD and 3/5 RSD p a t i e n t s . The two RSDs w i t h i n s i g n i f i c a n t d i f f e r e n c e s were c l i n i c a l l y c l a s s i f i e d w i t h the c a r o t i d amytal t e s t as b i l a t e r a l , b u t w i t h major r e p r e s e n t a t i o n o f speech i n t h e r i g h t h e m i s p h e r e . 75 As an example o f t h e use o f t h e s e e q u a t i o n s , the c o h e r e n c e asymmetries o f the l e f t s p e e c h - d o m i n a n t p a t i e n t RG w e r e : (F) = l e f t h e m i s p h e r i c VEP coherence minus r i g h t h e m i s p h e r i c VEP coherence = 0 . 8 8 - 0 . 9 8 = - 0 . 1 0 (C) = l e f t h e m i s p h e r i c AEP c o h e r e n c e minus r i g h t h e m i s p h e r i c AEP coherence = 0 . 8 9 - 0 . 7 9 = +0.10 The d i s c r i m i n a n t s c o r e s w e r e : S LSD = _ 1 - 7 9 " 5 , 9 2 ( F ) + 1 4 , 6 7 ( C ) = - 1 . 7 9 - 5 . 9 2 ( - . 1 0 ) + 14.67 (+.10) = +.27 SRSD = ~ 1 , 1 4 + 1 , 9 3 ( F ) " 1 1 , 5 9 ( C ) = - 2 . 4 9 S i n c e t h e LSD s c o r e was more p o s i t i v e than t h e RSD s c o r e , t h i s p a t i e n t was c l a s s i f i e d as l e f t s p e e c h - d o m i n a n t . To e v a l u a t e t h e p r o b a b i l i t y t h a t t h i s p a t i e n t was a c t u a l l y RSD, r a t h e r than LSD, t h e e q u a t i o n s (2) were used t o c a l c u l a t e t h e p r o b a b i l i t y o f m i s c l a s s i f i c a t i o n , as f o l l o w s : P L S D = 1 = 3 = 0 ' 0 5 9 1 + exp ( S L S D - S R S D ) 1 + exp ( . 2 7 +2.49 ) PRSD = 1 " P LSD = 0 , 9 4 1 T h e r e f o r e , t h e r e were 5 . 9 chances o u t 100 t h a t t h e person had been 76 m i s c l a s s i f i e d as LSD, when he was a c t u a l l y RSD. The e q u a t i o n s and example show how changes i n o c c i p i t a l - t o -temporal coherence a f f e c t e d t h e p r o b a b i l i t y t h a t the p e r s o n was e i t h e r l e f t o r r i g h t - s p e e c h dominant. I f the l e f t - m i n u s - r i g h t h e m i s p h e r i c VEP asymmetry (F) became more n e g a t i v e , t h e RSD s c o r e would have become s m a l l e r , and t h e LSD s c o r e l a r g e r . S i m i l a r l y , i f the l e f t - m i n u s - r i g h t h e m i s p h e r i c AEP asymmetry (C) became more p o s i t i v e , the p e r s o n ' s RSD s c o r e would have d e c r e a s e d and h i s LSD s c o r e i n c r e a s e d . S i n c e a more p o s i t i v e LSD s c o r e i n c r e a s e d t h e p r o b a b i l i t y t h a t the p e r s o n was LSD, and v i c e - v e r s a f o r t h e RSD s c o r e s , the e q u a t i o n s showed t h e f o l l o w i n g r e l a t i o n s h i p : High AEP c o h e r e n c e s w i t h i n one hemisphere i n c r e a s e d the p r o b a b i l i t y t h a t speech dominance was l o c a l i z e d w i t h i n t h a t h e m i s p h e r e . C o n v e r s e l y , h i g h VEP c o h e r e n c e s w i t h i n one hemisphere i n c r e a s e d t h e p r o b a b i l i t y t h a t speech dominance was l o c a l i z e d w i t h i n the o p p o s i t e h e m i s p h e r e . . 3 . Speech dominance o f normal s u b j e c t s T a b l e VI a p p l i e s t h e above d i s c r i m i n a n t e q u a t i o n s o f speech dominance, as d e t e r m i n e d from the coherence asymmetries and c o r r e s p o n d i n g speech dominance o f t h e p a t i e n t s , t o the c o h e r e n c e asymmetries o f t h e n o r m a l , r i g h t o r l e f t - h a n d e d s u b j e c t s . For t h e 12 r i g h t - h a n d e d s u b j e c t s , 8 were LSD, 3 RSD and 1 b i l a t e r a l . For t h e 12 l e f t - h a n d e d s u b j e c t s , 5 were LSD, 1 RSD and 6 b i l a t e r a l . 77 TABLE VI Speech dominance d e t e r m i n a t i o n u s i n g d i s c r i m i n a t i o n e q u a t i o n s o f the c o h e r e n c e a s y m m e t r i e s . LSD,RSD = l e f t , r i g h t speech d o m i n a n t , BIL = b i l a t e r a l . S u b j e c t s were a s s i g n e d t o e i t h e r LSD o r RSD c l a s s i f i c a t i o n s i f t h e i r d i s c r i m i n a t i o n p r o b a b i l i t i e s were l e s s than . 0 5 , and t o BIL i f g r e a t e r than . 0 5 . S u b j e c t group Number o f s u b j e c t s Coherence asymmetry c l a s s LSD RSD BIL LSD amytal 6 5 1 0 RSD amytal 5 0 3 2 (2 RSD) R i g h t - h a n d e d 12 8 3 1 (1 RSD) L e f t - h a n d e d 12 5 1 6 (2 RSD,& LSD) 78 4 . D i s c r i m i n a n t a n a l y s i s o f power s p e c t r a l asymmetries T a b l e VII shows t h e d i s c r i m i n a n t a n a l y s i s o f t h e power s p e c t r a l asymmetries between t h e l e f t and r i g h t h e m i s p h e r e s . For a l l p a t i e n t s , t h i s f r e q u e n c y component c o n t a i n e d from 60-80% o f t h e t o t a l power o f t h e r e s p o n s e and was t h e l a r g e s t a m p l i t u d e , o r dominant c o m p o n e n t . , There were no s i g n i f i c a n t p r e d i c t o r s o f speech dominance, even a t a reduced s i g n i f i c a n c e l e v e l o f p l e s s than 0 . 2 5 . The U - s t a t i s t i c showed no s i g n i f i c a n t d i f f e r e n c e s between t h e LSD and RSD g r o u p s . D i s c r i m i n a n t a n a l y s i s o f t h e power s p e c t r a t h e m s e l v e s , r a t h e r than t h e d i f f e r e n c e s between t h e l e f t and r i g h t hemispheres a l s o gave i n s i g n i f i c a n t r e s u l t s . 5 . Coherence asymmetries and handedness The coherence asymmetry m e a s u r i n g t h e d i f f e r e n c e between t h e o c c i p i t a l t o temporal c o h e r e n c e s o f t h e l e f t and r i g h t hemispheres ( 0 1 : T 3 - 0 2 : T 4 ) , was t h e o n l y asymmetry w h i c h was s i g n i f i c a n t l y r e l a t e d to speech dominance. C o n s e q u e n t l y , a s t u d y o f t h i s asymmetry by i t s e l f i n normal s u b j e c t s c o u l d be used t o d e r i v e i n f e r e n c e s about t h e r e l a t i o n s h i p o f speech dominance t o handedness. A u n i v a r i a t e a n a l y s i s o f v a r i a n c e o f t h e ( 0 1 : T 3 - 0 2 : T 4 ) c o h e r e n c e asymmetry was done. The e x p e r i m e n t a l d e s i g n was a r e p e a t e d measures f o r m a t , w i t h s u b j e c t s (S) n e s t e d i n handedness (H) and c r o s s e d w i t h s t i m u l u s m o d a l i t y . ( M ) . I n t u i t i v e l y , t h i s meant t h a t t h e 79 TABLE V I I D i s c r i m i n a n t a n a l y s i s o f l o g - ^ - t r a n s f o r m e d power s p e c t r a l a s y m m e t r i e s . 0 1 , 0 2 = l e f t , r i g h t o c c i p i t a l ; T 3 , T 4 = l e f t , r i g h t t e m p o r a l . LSD, RSD = l e f t , r i g h t speech d o m i n a n t . Asymmetry LSD mean RSD mean D i s c r i m i n a t i o n p r o b a b i l i t y VEP 01-02 - . 0 4 8 - . 0 7 4 .763 T3-T4 - . 1 0 6 - . 0 8 1 . 8 4 9 AEP 01-02 - . 0 4 3 - . 0 4 9 . 8 8 0 T3-T4 - . 1 1 5 - . 0 1 7 .466 80 the f l a s h and c l i c k s t i m u l i were both p r e s e n t e d t o the same s u b j e c t ; , so t h e i r i n t e r c o r r e l a t i o n s had to be a c c o u n t e d f o r i n a s p e c i a l t y p e o f a n a l y s i s o f v a r i a n c e c a l l e d r e p e a t e d measures. S u b j e c t s n e s t e d i n handedness meant t h a t t h e 12 s u b j e c t s i n t h e r i g h t - h a n d e d group were d i f f e r e n t than t h e 12 s u b j e c t s i n t h e l e f t - h a n d e d g r o u p , but b o t h groups were n e s t e d o r were p a r t o f t h e f a c t o r c a l l e d handedness. S t i m u l u s m o d a l i t y c r o s s e d w i t h t h e o t h e r f a c t o r s meant t h a t each s t i m u l u s was a p p l i e d a c r o s s a l l s u b j e c t s . A l l o f t h e s e i n t e r r e l a t i o n s h i p s o f f a c t o r s produced a c e r t a i n p a t t e r n o f c o r r e l a t i o n s which were a c c o u n t e d f o r by s p e c i f y i n g t h e e x p e r i m e n t a l d e s i g n when t h e computer program was r u n . T a b l e V I I I shows t h e r e s u l t s o f t h e a n a l y s i s o f v a r i a n c e f o r the 24 normal s u b j e c t s , u s i n g t h e UCLA B i o m e d i c a l computer program BMD 08V - A n a l y s i s o f v a r i a n c e ( D i x o n , 1973). The VEP and AEP means were s i g n i f i c a n t l y d i f f e r e n t as i n d i c a t e d by the s i g n i f i c a n t m o d a l i t y (M) e f f e c t , but t h e l e f t and r i g h t - h a n d e d means were n o t , s i n c e t h e handedness e f f e c t (H) was not s i g n i f i c a n t and no i n t e r a c t i o n o c c u r r e d between handedness and m o d a l i t y (HM). These r e s u l t s i n d i c a t e d t h a t t h e r e were s i g n i f i c a n t d i f f e r e n c e s between t h e VEP and AEP c o h e r e n c e a s y m m e t r i e s , but not between t h o s e o f the l e f t and r i g h t - h a n d e d s u b j e c t s . The l a c k o f i n t e r a c t i o n between m o d a l i t y and handedness i n d i c a t e d t h a t the coherence asymmetries o f both l e f t and r i g h t - h a n d e d s u b j e c t s changed i n the same way f o r f l a s h and c l i c k s t i m u l i . 81 TABLE V I I I A n a l y s i s o f v a r i a n c e o f 12 r i g h t - h a n d e d and 12 l e f t - h a n d e d normal s u b j e c t s . Dependent v a r i a b l e i s l e f t - m i n u s - r i g h t h e m i s p h e r i c , o c c i p i t a l - t o - t e m p o r a l coherence ( 0 1 : T 3 - 0 2 : T 4 ) . a s y m m e t r y . Source F - v a l u e F - p r o b a b i l i t y Mean 2 . 7 3 4 . 1 0 9 Handedness (H) 0 . 4 8 9 . 4 9 8 M o d a l i t y (M) 5 . 9 2 6 . 0 2 2 (p l e s s than 0 . 0 5 ) I n t e r a c t i o n (HM) 1.514 . 2 3 0 Mean, l e f t - m i n u s - r i g h t h e m i s p h e r i c coherences F l a s h C l i c k R i g h t - h a n d e d • - . 0 8 + . 1 3 L e f t - h a n d e d - . 0 5 +.10 82 The mean c o h e r e n c e asymmetries o f T a b l e V I I I show t h e s i g n i f i c a n t changes which o c c u r r e d between t h e VEPs and AEPs o f both s u b j e c t g r o u p s . In both t h e l e f t and r i g h t - h a n d e d s u b j e c t s , t h e l e f t - m i n u s - r i g h t h e m i s p h e r i c , d i f f e r e n c e s were p o s i t i v e f o r A E P s , i n d i c a t i n g g r e a t e r l e f t h e m i s p h e r i c c o h e r e n c e s , and n e g a t i v e f o r VEPs, i n d i c a t i n g g r e a t e r r i g h t h e m i s p h e r i c c o h e r e n c e s . The asymmetries o f t h e l e f t - h a n d e d s u b j e c t s appear t o be s l i g h t l y s m a l l e r i n magnitude than t h e r i g h t - h a n d e d s u b j e c t s , b u t the a n a l y s i s o f v a r i a n c e showed t h a t t h e s e d i f f e r e n c e s were not s i g n i f i c a n t . 8 3 DISCUSSION A. Experiment I - Power s p e c t r a l a n a l y s i s In t h e f i r s t e x p e r i m e n t , the a m p l i t u d e c h a r a c t e r i s t i c s o f E P s , and t h e i r r e l a t i o n s h i p t o s t i m u l u s m o d a l i t y and c o r t i c a l r e c o r d i n g a r e a were i n v e s t i g a t e d i n t h e n o r m a l , r i g h t - h a n d e d s u b j e c t s . The power s p e c t r a o f both t h e v i s u a l and a u d i t o r y evoked p o t e n t i a l s showed two major f r e q u e n c y g r o u p s ; Group I , 0 - 5 h z . , and Group I I , 6-12 h z . These two f r e q u e n c y groups were r e m a r k a b l y c o n s t a n t between t r i a l s and between s u b j e c t s and seemed t o change m a i n l y i n t h e i r a m p l i t u d e s and l a t e n c i e s , r a t h e r than i n t h e i r f o r m . T h i s s u g g e s t s t h a t much o f t h e o b s e r v e d i n t r a - and i n t e r - s u b j e c t v a r i a b i l i t y o f EPs c o u l d be due t o c h a n g i n g a m p l i t u d e and phase r e l a t i o n s h i p s between two f r e q u e n c y components w h i c h a r e t h e m s e l v e s q u i t e c o n s t a n t . The s p a t i a l d i s t r i b u t i o n o f t h e two components was d i f f e r e n t and a f u n c t i o n o f s t i m u l u s m o d a l i t y . Group I I , 6-12 h z . r e s p o n s e s were o f l a r g e s t a m p l i t u d e i n the p r i m a r y p r o j e c t i o n a r e a o f t h e s t i m u l u s m o d a l i t y . The Group I , 0 - 5 h z . r e s p o n s e s were always g r e a t e r i n the temporal a r e a s f o r both VEPs and AEPs. T h i s i n d i c a t e s t h a t the Group I , low f r e q u e n c y r e s p o n s e s r e t a i n a c o n s t a n t s p a t i a l d i s t r i b u t i o n which i s n o t s p e c i f i c a l l y r e l a t e d t o the s t i m u l u s m o d a l i t y . In c o n t r a s t , the h i g h e r f r e q u e n c y , Group I I r e s p o n s e s s h i f t and become g r e a t e s t i n the p r i m a r y p r o j e c t i o n a r e a o f the s t i m u l a t e d m o d a l i t y . 8 4 The Group I I , 6-12 h z . components were g r e a t e r i n t h e s p e c i f i c p r o j e c t i o n a r e a o f t h e s t i m u l a t e d m o d a l i t y f o r a l l a r e a s and m o d a l i t i e s , e x c e p t the r i g h t hemisphere VEP. T h i s anomaly may be r e l a t e d t o speech dominance, s i n c e most r e p o r t e d d i f f e r e n c e s i n e v e n t - r e l a t e d p o t e n t i a l s such as EPs ~or c o n t i n g e n t n e g a t i v e v a r i a t i o n (CNV) have o c c u r r e d on t h e l e f t , presumably s p e e c h - d o m i n a n t s i d e . M o r r e l l and Salamy (1971) found maximum EP a m p l i t u d e s t o n a t u r a l speech s t i m u l i i n the l e f t h e m i s p h e r e . McAdam and W h i t t a k e r (1971) found s l o w , n e g a t i v e p o t e n t i a l s g e n e r a t e d when t h e s u b j e c t s spoke and which were l a r g e r on t h e l e f t s i d e . Buchsbaum and F e d i o ( 1 9 6 9 , 1970) showed t h a t o c c i p i t a l r e s p o n s e s t o t a c h i s t o s c o p i c o r f l a s h p r e s e n a t i o n o f words and s i m i l a r l y p a t t e r n e d v i s u a l s t i m u l i were more d i f f e r e n t on the l e f t , dominant s i d e . These s t u d i e s and o t h e r i n d i c a t e t h a t v i s u a l o r a u d i t o r y s t i m u l i w i t h v e r b a l c o n t e n t g e n e r a t e r e s p o n s e s w h i c h a r e more d i f f e r e n t i n the l e f t hemisphere than i n t h e r i g h t . The power s p e c t r a l a n a l y s i s s u g g e s t s t h a t h e m i s p h e r i c d i f f e r e n c e s may a l s o be demonstrated w i t h s i m p l e f l a s h and c l i c k s t i m u l i . T h i s p o s s i b i l i t y w i l l be d e a l t w i t h more f u l l y i n the n e x t e x p e r i m e n t . The waveform and peak l a t e n c i e s o f each o f t h e EP f r e q u e n c y groups i s shown by t h e d i g i t a l f i l t e r i n g . The Group I , 0 - 5 h z . f r e q u e n c i e s c o n s i s t o f a s l o w , b i p h a s i c wave w i t h peak l a t e n c i e s a t about 100 and 300 msec. Both f l a s h and c l i c k s t i m u l i produce v e r y s i m i l a r Group I waveforms, thus s u p p o r t i n g t h e h y p o t h e s i s t h a t t h e y a r e n o n - s p e c i f i c t o t h e s t i m u l u s m o d a l i t y . The G r o u p . I I , 6-12 h z . r e s p o n s e s appear f i r s t a t about 50 m s e c , and r e c u r a t i n t e r v a l s o f about 50 m s e c The VEPs and AEPs from d i f f e r e n t a r e a s have v i s u a l l y 85 d i f f e r e n t f o r m s , s u g g e s t i n g t h e i r m o d a l i t y and a r e a s p e c i f i c i t y . The p h y s i o l o g i c a l meaning o f the low f r e q u e n c y components i s s u g g e s t e d by t h e i r l a t e n c i e s . Peaks o c c u r a t about 100 and 300 m s e c , which i s too l a t e t o be c a r r y i n g p r i m a r y s e n s o r y i n f o r m a t i o n . The 300 m s e c peak has been i m p l i c a t e d i n a wide v a r i e t y o f mechanisms i n v o l v i n g a s p e c t s o f c o r t i c a l i n f o r m a t i o n p r o c e s s i n g w h i c h can be m a n i p u l a t e d by v a r i o u s t y p e s o f c o g n i t i v e a c t i v i t y ( D o n c h i n , Kubovy, K u t a s , Johnson and H e r n i n g , 1973). P i c t o n and H i l l y a r d (1974) and Schwent and H i l l y a r d (1975) showed t h a t t h e a m p l i t u d e o f both t h e 100 and 300 m s e c peaks o f AEPs c o u l d be r e l i a b l y a l t e r e d by changes i n the s u b j e c t ' s a t t e n t i v e s e t . F r i e d m a n , S i m s o n , R i t t e r and Rapin (1975) r e p o r t e d t h a t the l a t e n c y o f t h e 300 msec. VEP component became g r e a t e r f o r words w i t h i n f o r m a t i o n a l c o n t e n t , than f o r t h o s e which were redundant r e p e t i t i o n s o f e a r l i e r i n f o r m a t i o n . These s t u d i e s s u g g e s t e d t h a t both t h e 100 and 300 msec, components o f t h e EP were r e l a t e d t o p r o c e s s e s which were n o n - s p e c i f i c t o t h e s t i m u l u s m o d a l i t y . The f u n c t i o n a l r o l e o f t h e peaks and Group I , low f r e q u e n c i e s i s a l s o s u g g e s t e d by t h e i r s p a t i a l d i s t r i b u t i o n . These f r e q u e n c i e s were w i d e s p r e a d and o f l a r g e s t a m p l i t u d e i n the temporal a r e a s f o r . b o t h f l a s h and c l i c k s t i m u l i , which a g a i n i n d i c a t e s a p r o c e s s which i s not s p e c i f i c a l l y r e l a t e d t o t h e s t i m u l u s m o d a l i t y . The l a t e components o f t h e EP have a s i m i l a r s p a t i a l d i s t r i b u t i o n . G a s t a u t , R e g i s , L y a g o u b i , Mano and Simon (1967) showed t h a t t h e s e components 86 o f t h e v i s u a l , a u d i t o r y and somatosensory EPs were o f l a r g e s t a m p l i t u d e near t h e v e r t e x , and d e c r e a s e d towards the o c c i p u t . These f i n d i n g s were r e p l i c a t e d f o r A E P s , and t h e i r dependence on t h e s u b j e c t ' s a t t e n t i v e s e t was shown by R i t t e r , Simson and Vaughn ( 1 9 7 2 ) ; P i c t o n and H i l l y a r d ( 1 9 7 4 ) ; H i l l y a r d , C o u r c h e s n e , Krausz and P i c t o n ( 1 9 7 5 ) ; and S q u i r e s , S q u i r e s and H i l l y a r d ( 1 9 7 5 ) . A l l o f the above s t u d i e s t h u s s u g g e s t t h a t the l o n g l a t e n c y components o f both VEPs and AEPs a r e i n v o l v e d i n p r o c e s s e s w h i c h a r e n o t s p e c i f i c a l l y r e l a t e d t o t h e s t i m u l a t e d m o d a l i t y o r r e c o r d i n g a r e a . . The 100 and 300 msec, p e a k s , e s p e c i a l l y , seem t o be a s s o c i a t e d w i t h g e n e r a l i z e d f u n c t i o n s such as a t t e n t i o n and e x t r a c t i o n o f m e a n i n g , as i n d i c a t e d by changes i n t h e i r a m p l i t u d e and l a t e n c y when t h e s e s u b j e c t i v e v a r i a b l e s a r e m a n i p u l a t e d . S i n c e t h e 100 and 300 msec, peaks a r e a l s o the o n l y p o s i t i v e peak components o f t h e Group I , 0 - 5 h z . r e s p o n s e , i t i s r e a s o n a b l e to s u g g e s t t h a t t h e s e two s e p a r a t e peaks may be due t o a s i n g l e o s c i l l a t o r y p r o c e s s . The s p a t i a l d i s t r i b u t i o n o f t h i s p r o c e s s i s w i d e s p r e a d o v e r the c o r t e x , and p o s s i b l y r e l a t e d t o a s p e c t s o f s e n s o r y i n f o r m a t i o n p r o c e s s i n g w h i c h a r e not s p e c i f i c to the m o d a l i t y c o n v e y i n g t h a t i n f o r m a t i o n . The p h y s i o l o g i c a l meaning o f t h e h i g h e r f r e q u e n c y , Group I I , 6-12 h z . component i s a l s o s u g g e s t e d by i t s s p a t i a l d i s t r i b u t i o n and peak l a t e n c i e s . The peaks f i r s t appear a t about 50 m s e c , and then c o n t i n u e a t about 50 msec, i n t e r v a l s . E a r l y components o f the EP, from about 30 t o 100 m s e c , have been c l a s s e d as s p e c i f i c a l l y r e l a t e d 87 t o the s t i m u l u s m o d a l i t y and t o t h e r e c o r d i n g a r e a ( P e r r y and C h i l d e r s , 1967). However, from a f r e q u e n c y v i e w p o i n t , the 6-12 hz . group a l s o i s s t i m u l u s and a r e a s p e c i f i c . A g a i n , as w i t h t h e low f r e q u e n c y components, i n c l u s i o n o f a l l t h e peaks w i t h s p e c i f i c m o d a l i t y o r a r e a c h a r a c t e r i s t i c s i n t o a s i n g l e f r e q u e n c y group would s i m p l i f y t h e i r d e s c r i p t i o n . T h i s u n i f i c a t i o n would a l s o be more m e a n i n g f u l than i n d i v -i d u a l peak d e s c r i p t i o n i f t h e EP was due t o a s i n g l e o s c i l l a t o r y p r o c e s s , r a t h e r than a sequence o f s e p a r a t e e v e n t s . The Group I I , 6-12 h z . component i s l a r g e s t o v e r t h e s p e c i f i c p r o j e c t i o n c o r t e x o f the s t i m u l a t e d m o d a l i t y . I t i s t h e r e f o r e p r o b a b l y r e l a t e d i n some way t o t h e s e n s o r y i n f o r m a t i o n b e i n g t r a n s m i t t e d t o t h e c o r t e x from l o w e r c e n t e r s . I t s l o n g l a t e n c y o f 50 t o 100 msec, makes i t s u n l i k e l y t h a t s e n s o r y i n f o r m a t i o n i s b e i n g c a r r i e d i n the t i m e - v a r y i n g c h a r a c t e r i s t i c s o f i t s waveform, s i n c e a complete c y c l e t a k e s about 100 msec. A more l i k e l y p o s s i b i l i t y i s t h a t the a m p l i t u d e o f t h i s waveform m i g h t be modulated by a f f e r e n t s e n s o r y p a t t e r n s . I n f o r m a t i o n would thus be c a r r i e d i n t h e s p a t i a l l y v a r y i n g o r s p a t i a l f r e q u e n c y c h a r a c t e r i s t i c s o f t h e waveform, r a t h e r than i n i t s temporal f o r m . 88 B. Experiment I I - Coherence a n a l y s i s In t h i s e x p e r i m e n t , the c o h e r e n c e o r form s i m i l a r i t y between p a i r s o f EPs was i n v e s t i g a t e d i n r e l a t i o n to speech dominance and s t i m u l u s m o d a l i t y . C o n s e q u e n t l y , o n l y the 6-12 h z . , m o d a l i t y - s p e c i f i c component was a n a l y z e d . Speech dominance i s t h e b e h a v i o r a l m a n i f e s t a t i o n o f t h e supposed l a t e r a l i z a t i o n o f t h e c e r e b r a l mechanisms o f speech comprehension and p r o d u c t i o n t o one o r t h e o t h e r h e m i s p h e r e s . In most c a s e s , t h i s l a t e r a l i z a t i o n i s t o t h e l e f t h e m i s p h e r e , b u t a few p e r c e n t o f t h e p a t i e n t s who have have been o b s e r v e d a r e e i t h e r r i g h t s p e e c h - d o m i n a n t o r b i l a t e r a l l y r e p r e s e n t e d . R i g h t - h a n d e d n e s s and l e f t speech dominance a r e h i g h l y c o r r e l a t e d , but t h e dominance o f l e f t - h a n d e d s u b j e c t s i s much l e s s c e r t a i n . T h i s u n c e r t a i n r e l a t i o n s h i p makes i t i m p o s s i b l e t o s t u d y e i t h e r r i g h t - o r l e f t - h a n d e d s u b j e c t s and a p p l y the r e s u l t s t o both l e f t and r i g h t h e m i s p h e r i c asymmetries o f speech dominance. In normal s u b j e c t s , however, i t i s not p o s s i b l e t o use d e f i n i t i v e t e s t s o f speech dominance, such as t h e c a r o t i d amytal t e s t (Wada, 1949; Wada and Rasmussen, 1 9 6 0 ) , i n w h i c h a b a r b i t u r a t e i s used t o p a r a l y z e t h e s p e e c h - d o m i n a n t h e m i s p h e r e , o r e l e c t r i c a l s t i m u l a t i o n o f t h e exposed c o r t e x ( P e n f i e l d and R o b e r t s , 1 9 5 9 ) . These p r o c e d u r e s a r e o n l y j u s t i f i e d f o r d i a g n o s t i c o r t h e r a p e u t i c p u r p o s e s . In t h i s s t u d y , 3 groups o f s u b j e c t s were i n v e s t i g a t e d t o c i r c u m v e n t the problem o f unknown speech dominance. B a s i c coherence asymmetries were i n v e s t i g a t e d u s i n g r i g h t and l e f t s p e e c h - d o m i n a n t (RSD and LSD) e p i l e p t i c p a t i e n t s , whose dominance had been a s c e r t a i n e d 89 by t h e c a r o t i d amytal t e s t . The r e s u l t s o f the LSD p a t i e n t s were then compared w i t h a group o f n o r m a l , r i g h t - h a n d e d , and hence presumably l e f t s p e e c h - d o m i n a n t s u b j e c t s . Agreement o f t h e s e two groups would i n d i c a t e t h a t asymmetries o f LSD and RSD p a t i e n t s were p r o b a b l y a l s o a p p l i c a b l e t o normal s u b j e c t s . The comparison o f l e f t and r i g h t h e m i s p h e r i c c o h e r e n c e s showed t h a t , i n most s u b j e c t s , c l i c k r e s p o n s e s o r AEPs were more c o h e r e n t on t h e sp eech-domina nt s i d e w h i l e f l a s h r e s p o n s e s o r VEPs were more c o h e r e n t on t h e speech non-dominant s i d e . Not a l l s u b j e c t s showed t h e s e g e n e r a l group t e n d e n c i e s ; however i t was not c l e a r i f t h e e x c e p t i o n s were due t o e x p e r i m e n t a l o r a n a l y t i c t e c h n i q u e d e f i c i e n c e s , i n d i v i d u a l v a r i a t i o n , o r p a t h o l o g i c a l r e s p o n s e s i n t h e p a t i e n t s . I n d i v i d u a l v a r i a t i o n c o u l d a c c o u n t f o r some o f t h e normal s u b j e c t v a r i a t i o n , s i n c e t h e i r l e f t speech dominance was o n l y assumed. For the amytal p a t i e n t s , the s i n g l e e x c e p t i o n t o maximum c l i c k coherence on the s p e e c h - d o m i n a n t s i d e was one o f the t h r e e RSD p a t i e n t s who had shown some b i l a t e r a l i t y w i t h the amytal t e s t . One o f the f l a s h e x c e p t i o n s was a l s o from a b i l a t e r a l p a t i e n t , b u t no f u r t h e r t r e n d s o f b i l a t e r a l r e p r e s e n t a t i o n were e v i d e n t . T a b l e II shows one RSD b i l a t e r a l p a t i e n t i n whom the f l a s h and c l i c k c o h e r e n c e s were v e r y n e a r l y equal between h e m i s p h e r e s . A p o s s i b l y i m p o r t a n t f a c t o r i n coherence v a r i a t i o n s may be a n a l y t i c . The e s t i m a t i o n o f c o h e r e n c e from s h o r t r e c o r d s such as EPs i s s t i l l v e r y much u n e x p l o r e d , s i n c e most a p p l i c a t i o n s o f s p e c t r a l a n a l y s i s to d a t a have been one l o n g d a t a r e c o r d s such as EEG, m e t e o r o l o g i c a l and 90 g e o p h y s i c a l d a t a . Coherences a r e h i g h l y v a r i a b l e u n l e s s e i t h e r l o n g r e c o r d s o r many e s t i m a t e s o f coherences from s h o r t r e c o r d s a r e a v a i l a b l e . A n a l y s e s o f group t e n d e n c i e s w i t h many s u b j e c t s , and c o n s e q u e n t l y many coherence e s t i m a t e s , a r e thus u s e f u l i n d i c a t o r s o f o v e r a l l group b e h a v i o r . For s i n g l e s u b j e c t s , however, t h e methods o f t h i s c h a p t e r have produced o n l y a f e w , h i g h l y v a r i a b l e e s t i m a t e s . I t i s thus n o t s u r p r i s i n g t h a t some s u b j e c t s do n o t seem to f i t t h e o v e r a l l p a t t e r n o f t h e s p e e c h - d o m i n a n t g r o u p . The e s t i m a t i o n o f c o h e r e n c e f o r s i n g l e s u b j e c t s m i g h t p o s s i b l y have been improved by a v e r a g i n g fewer s t i m u l i i n each averaged EP. For e x a m p l e , the 160 s t i m u l i o f each s u b j e c t m i g h t have been averaged i n t o 16 groups o f 10 s t i m u l i , r a t h e r than 4 groups o f 4 0 . T h i s method might not n e c e s s a r i l y g i v e b e t t e r e s t i m a t e s o f c o h e r e n c e , however, s i n c e t h e averaged EP on which t h e y were based would be more v a r i a b l e . The s i t u a t i o n i s c l e a r l y o f a " t r a d e o f f " t y p e i n which t h e i n c r e a s e d number o f c o h e r e n c e s a v a i l a b l e must be b a l a n c e d a g a i n s t t h e i r i n c r e a s e d v a r i a b i l i t y . The m u l t i v a r i a t e a n a l y s i s o f v a r i a n c e was used t o i n v e s t i g a t e i n more d e t a i l the s o u r c e o f the coherence a s y m m e t r i e s . The r e s u l t s i n d i c a t e t h a t the two s i g n i f i c a n t main e f f e c t s on t h e asymmetries a r e the s u b j e c t and the s t i m u l u s m o d a l i t y . In o t h e r w o r d s , d i f f e r e n t s u b j e c t s have d i f f e r e n t d e g r e e s o f c o h e r e n c e asymmetry, and t h i s asymmetry c h a n g e s , depending on t h e s t i m u l u s m o d a l i t y . The a n a l y s i s a l s o showed s e v e r a l s i g n i f i c a n t i n t e r a c t i o n s . The s u b j e c t x m o d a l i t y (SM) i n t e r a c t i o n i n d i c a t e s t h a t not a l l s u b j e c t s 91 respond i n the same way t o t h e same s t i m u l u s . The mean c o h e r e n c e s o f t h i s i n t e r a c t i o n showed t h a t n o t a l l o f t h e r i g h t - h a n d e d s u b j e c t s showed l e f t s p e e c h - d o m i n a n t a s y m m e t r i e s . The mean coherences o f t h e s i g n i f i c a n t s u b j e c t x m o d a l i t y x f r e q u e n c y (SMF) i n t e r a c t i o n s u g g e s t e d t h a t VEP c o h e r e n c e s and asymmetries tended t o be g r e a t e r a t 9 and 12 h z . , w h i l e AEP c o h e r e n c e s tended t o be l a r g e r a t 6 and 9 h z . T h i s s u g g e s t s t h a t the p r o c e s s e s a r e independent which g e n e r a t e the w i d e s p r e a d AEP a c t i v i t y i n the l e f t h e m i s p h e r e , and t h e w i d e s p r e a d VEP a c t i v i t y i n the r i g h t h e m i s p h e r e . The r e s u l t s o f T a b l e I I I show t h a t the mean asymmetries were s i g n i f i c a n t l y d i f f e r e n t f o r f l a s h and c l i c k . I t i s p o s s i b l e to a p p l y s p e c i f i c t e s t s o f hypotheses t o t h e s e 3 asymmetries t o d e t e r m i n e which i s most s i g n i f i c a n t l y r e l a t e d t o t h e f a c t o r s ( A n d e r s o n , 1 9 5 8 ) . However, t h e s e a r e d i f f i c u l t t o use and no computer programs a r e g e n e r a l l y a v a i l a b l e which i n c l u d e them. The p r e s e n t a n a l y s i s has e s t a b l i s h e d t h a t s i g n i f i c a n t l y d i f f e r e n t asymmetries do e x i s t f o r VEPs and AEPs. Only a q u a l i t a t i v e e v a l u a t i o n o f t h e 3 asymmetries w i l l be d e a l t w i t h f u r t h e r . a t t h i s t i m e . In t h e n e x t e x p e r i m e n t , a n o t h e r t e c h n i q u e w i l l be used t o e v a l u a t e t h e s i g n i f i c a n c e o f each asymmetry and i t s r e l a t i o n s h i p t o h e m i s p h e r i c asymmetry. T a b l e IV shows t h a t t h e two main asymmetries c o n t r i b u t i n g t o the d i f f e r e n c e between VEPs and AEPs a r e t h e o c c i p i t a l t o temporal ( 0 1 : 0 2 - T 3 : T 4 ) and l e f t t o . r i g h t hemisphere ( 0 1 : T 3 - 0 2 : T 4 ) p a i r s . The d i f f e r i n g ways i n which they behave a l l o w some i n f e r e n c e s t o be made about t h e i r meaning . .or r e l a t i o n to c o r t i c a l p r o c e s s e s . 92 The o c c i p i t a l - t o - t e m p o r a l asymmetry ( 0 1 : 0 2 - T 3 : T 4 ) appears to be r e l a t e d t o s t i m u l u s s t r e n g t h , r a t h e r than s t i m u l u s m o d a l i t y . I t s v a l u e i n c r e a s e s f o r c l i c k s t i m u l i , and i n a d d i t i o n , the c l i c k c o h e r e n c e s between each a r e a a r e l e s s than the c o r r e s p o n d i n g f l a s h a s y m m e t r i e s . Lower c o h e r e n c e s c o u l d i n d i c a t e a l o w e r i n t e n s i t y o f r e s p o n s e , hence l e s s i n t e r a c t i o n between t h e / s p e c i f i c p r o j e c t i o n a r e a o f t h e s t i m u l a t e d m o d a l i t y and o t h e r a r e a s . T h i s would m a n i f e s t i t s e l f as the o b s e r v e d low o v e r a l l c o h e r e n c e s but h i g h e r , o c c i p i t a l - t o - t e m p o r a l coherence d i f f e r e n c e s . The r i g h t - t o - l e f t h e m i s p h e r i c asymmetry ( O l : T 3 - 0 2 : T 4 ) r e p r e s e n t s t h e c o n v e n t i o n a l c o n c e p t o f h e m i s p h e r i c asymmetry, i e . d i f f e r e n c e s between homologous a r e a s o f t h e r i g h t and l e f t h e m i s p h e r e s . T a b l e IV shows t h a t i t i s a l s o t h e o n l y coherence asymmetry w h i c h changes i n d i r e c t i o n w i t h s t i m u l u s m o d a l i t y . For A E P s , t h e l e f t h e m i s p h e r i c coherence i s g r e a t e r , w h i l e f o r VEPs, t h e r i g h t h e m i s p h e r i c c o h e r e n c e i s g r e a t e r . T h i s s h i f t i n g o f coherence asymmetries a g r e e s w i t h t h e s i m p l e a n a l y s i s o f t h e average 6-12 h z . c o h e r e n c e s f o r each s u b j e c t . In summary, t h e m u l t i v a r i a t e r e s u l t s i n d i c a t e t h a t s i g n i f i c a n t m o d a l i t y - d e p e n d e n t asymmetries e x i s t b u t o n l y t h e c o n v e n t i o n a l , l e f t - t o - r i g h t asymmetry r e v e r s e s w i t h s t i m u l u s m o d a l i t y . The asymmetries d i f f e r i n degree w i t h d i f f e r e n t s u b j e c t s , and a r e o f s l i g h t l y h i g h e r f r e q u e n c y f o r VEPs than f o r AEPs. The magnitude o f c o h e r e n c e i s s m a l l e r f o r c l i c k than f o r f l a s h , but no p a t t e r n r e l a t e d t o m o d a l i t y i s p r e s e n t i n t h e s p e c i f i c p r o j e c t i o n a r e a o f the s t i m u l a t e d m o d a l i t y . Only i n t h e i n t e r a c t i o n between t h e o c c i p i t a l 93. and temporal a r e a s o f each hemisphere does t h e s t i m u l u s m o d a l i t y appear t o be a c o n t r o l l i n g f a c t o r i n t h e d i r e c t i o n o f asymmetry. The d i r e c t i o n o f c o h e r e n c e asymmetry, e i t h e r t o t h e l e f t o r r i g h t h e m i s p h e r e , was c o n t r o l l e d by s i m p l e f l a s h o r c l i c k s t i m u l i w i t h no v e r b a l c o n t e n t . T h i s i n d i c a t e s t h a t h e m i s p h e r i c asymmetries e x i s t a t a more fundamental l e v e l than v e r b a l p r o c e s s i n g and t h a t v i s u a l and a u d i t o r y p r o c e s s e s a t t h i s l e v e l a r e l a t e r a l i z e d t o o p p o s i t e h e m i s p h e r e s . These c o h e r e n t p r o c e s s e s w i t h i n each hemisphere s u g g e s t s i m i l a r a c t i v i t i e s o c c u r r i n g a t w i d e l y d i f f e r e n t c o r t i c a l l o c a t i o n s . I t i s p o s s i b l e t h a t t h i s o b s e r v e d a c t i v i t y r e p r e s e n t s some w i d e s p r e a d , h i g h - l e v e l c o r t i c a l p r o c e s s a c t i n g o n , o r g e n e r a t e d c o n c u r r e n t l y w i t h a l l s t i m u l u s p a t t e r n s r e a c h i n g t h e c o r t e x , r e g a r d l e s s o f t h e i r u l t i m a t e meaning t o the s u b j e c t . The p a r i e t o - t e m p o r a l a s s o c i a t i o n a r e a s , l o c a t e d between the o c c i p i t a l and temporal r e c o r d i n g s i t e s , may be the common p o i n t o f t h i s a c t i v i t y f o r both v i s u a l and a u d i t o r y s t i m u l i . A s s o c i a t i v e a c t i v i t y between t h e s e a r e a s , as measured by t h e c o h e r e n c e , would be l a t e r a l i z e d t o t h e s peech -domin an t hemisphere f o r a u d i t o r y s t i m u l i , and t o t h e speech non-dominant hemisphere f o r v i s u a l s t i m u l i . P e r c e p t i o n and r e c o g n i t i o n o f v e r b a l s t i m u l i a r e l a t e r a l i z e d t o the s p e e c h - d o m i n a n t h e m i s p h e r e . L e f t : temporal l o b e l e s i o n s are known t o produce many d i s o r d e r s o f language p r o d u c t i o n and comprehension ( M o u n t c a s t l e , 1962; G e s c h w i n d , 1970). P h a r m a c o l o g i c a l p a r a l y s i s (Wada and Rasmussen, 1960) o r e l e c t r i c a l s t i m u l a t i o n o f t h e exposed 9 4 c o r t e x ( P e n f i e l d and R o b e r t s , 1959) i n t h e s e a r e a s r e s u l t s i n t r a n s i e n t impairment o f s p e e c h . P a t i e n t s i n whom the c o r p u s c a l l o s u m has been s e c t i o n e d can d e s c r i b e i n words what t h e y have seen when the l e f t speec h-dom i na nt hemisphere i s shown a p i c t u r e v i a t h e r i g h t v i s u a l f i e l d . However, t h e y cannot when t h e p i c t u r e i s shown v i a t h e l e f t v i s u a l f i e l d , presumably because t h e l e f t s p e e c h - d o m i n a n t hemisphere has r e c e i v e d no i n f o r m a t i o n about the p i c t u r e ( S p e r r y , 1964, 1969; G a z z a n i g a , 1970). In c o n t r a s t , v i s u a l p e r c e p t i o n appears t o be more l a t e r a l i z e d to t h e speech non-dominant h e m i s p h e r e . Non-dominant temporal l e s i o n s r e s u l t i n d e c r e a s e d v i s u a l p e r c e p t i o n as measured by d i s c r i m i n a t i o n and s p a t i a l r e l a t i o n s h i p t e s t s , w h i l e dominant temporal l e s i o n s have l i t t l e e f f e c t on t h e s e f u n c t i o n s ( P e n f i e l d and R o b e r t s , 1959; M i l n e r , 1962). In c o r p u s c a l l o s u m - s e c t i o n e d humans, v i s u a l s p a t i a l p e r c e p t i o n seems s t r o n g e r on t h e non-dominant s i d e as measured by a v a r i e t y o f d i s c r i m i n a t i o n t e s t s (Bogen and G a z z a n i g a , 1965, 1970; S p e r r y , 1968). T h i s e v i d e n c e , as w e l l as a l a r g e amount o f d a t a c o l l e c t e d i n normal s u b j e c t s u s i n g d i c h o t i c l i s t e n i n g ( K i m u r a , 1 9 6 1 , 1 9 6 3 ) , t h e r e f o r e s u g g e s t s t h a t t h e s p eec h-dom i nan t hemisphere p o s s e s s e s s u p e r i o r a u d i t o r y p e r c e p t u a l c a p a c i t i e s , a t l e a s t so f a r as speech v e r b a l i z a t i o n and r e c o g n i t i o n are c o n c e r n e d . V i s u a l p e r c e p t i o n , however, i s a more non-dominant hemisphere f u n c t i o n . These a r e t h e same hemispheres i n which t h e most c o h e r e n t a u d i t o r y and v i s u a l evoked p o t e n t i a l s o c c u r r e d . I t i s thus p o s s i b l e t h a t the c o h e r e n t p r o c e s s e s a r e i n some way r e l a t e d t o t h e p e r c e p t i o n o f v i s u a l and a u d i t o r y i n f o r m a t i o n . 95 I t s h o u l d be p o i n t e d out t h a t most o f t h e e v i d e n c e f o r the l a t e r a l i z a t i o n o f a u d i t o r y and v i s u a l f u n c t i o n s has been c o n f i n e d t o r i g h t - h a n d e d , presumably l e f t speech-dominant s u b j e c t s . The e x t e n s i o n to the much s m a l l e r p o p u l a t i o n o f r i g h t s p e e c h - d o m i n a n t persons has been h y p o t h e t i c a l . The r e s u l t s o f the c o h e r e n c e a n a l y s i s , h o w e v e r , s u g g e s t t h a t r i g h t speech-dominant persons may a l s o have t h e i r v i s u a l f u n c t i o n s r e v e r s e d from t h o s e o f l e f t s p e e c h - d o m i n a n t p e r s o n s . Speech dominance would then be one a s p e c t o f a l a r g e r c o n f i g u r a t i o n o f v i s u a l and a u d i t o r y asymmetries which were l a t e r a l i z e d t o o p p o s i t e h e m i s p h e r e s . In most p e r s o n s , t h i s l a t e r a l i z a t i o n would be t o t h e l e f t hemisphere f o r a u d i t o r y f u n c t i o n s and t o t h e r i g h t f o r v i s u a l f u n c t i o n s . A much s m a l l e r group would have both t h e s e asymmetries r e v e r s e d , w i t h a u d i t o r y f u n c t i o n s i n t h e r i g h t h e m i s p h e r e , and v i s u a l f u n c t i o n s i n the l e f t . The p o s s i b i l i t y t h a t c o h e r e n t o r c o r r e l a t e d p r o c e s s e s a r e r e l a t e d t o p e r c e p t i o n i s a l s o s u g g e s t e d by t h e work o f s e v e r a l R u s s i a n i n v e s t i g a t o r s . They r e p o r t e d t h a t t h e c o r r e l a t i o n s o f EEG a c t i v i t y between many s c a l p a r e a s i n c r e a s e d d u r i n g problem s o l v i n g ( A s l a n o v , 1 9 7 0 ) ; d u r i n g c o n d i t i o n e d r e f l e x f o r m a t i o n (Dumenko, 1970); and were a b n o r m a l l y h i g h i n p e r s o n s w i t h a s s o c i a t i v e d i s o r d e r s r e l a t e d t o s c h i z o p h r e n i a and e p i l e p s y ( G a v r i l o v a , 1970). In c o n c l u s i o n , t h e s e r e s u l t s i n d i c a t e t h a t s i g n i f i c a n t h e m i s p h e r i c asymmetries e x i s t f o r s i m p l e , u n s t r u c t u r e d f l a s h and c l i c k s t i m u l i , and t h a t r i g h t and l e f t s p e e c h - d o m i n a n t p a t i e n t s show o p p o s i t e asymmetries between h e m i s p h e r e s . These asymmetries a r e r e l a t e d t o speech dominance, but appear t o be o f a more fundamental n a t u r e than speech p r o c e s s i n g s i n c e 96 they can be e l i c i t e d by s t i m u l i w i t h o u t v e r b a l c o n t e n t . G r e a t e r i n t r a - h e m i s p h e r i c c o h e r e n c e o c c u r s i n t he speech -dom inan t hemi sphere f o r a u d i t o r y evoked p o t e n t i a l s and i n the speech non-dominant hemisphere f o r v i s u a l evoked p o t e n t i a l s . T h i s c o n c e p t o f i n t r a -h e m i s p h e r i c asymmetry o f evoked p o t e n t i a l s , as i n d i c a t e d by the l a t e r a l i z e d c o h e r e n t a c t i v i t i e s , may r e p r e s e n t a s s o c i a t i v e p r o c e s s e s o c c u r r i n g between s e n s o r y and a s s o c i a t i v e c o r t i c a l a r e a s . 97 C. Experiment I I I - D i s c r i m i n a n t a n a l y s i s In t h e t h i r d e x p e r i m e n t , the r e l a t i o n s h i p s o f coherence and power s p e c t r a l asymmetries t o speech dominance and handedness were i n v e s t i g a t e d w i t h d i s c r i m i n a n t a n a l y s i s . The r e s u l t s o f t h e d i s c r i m i n a n t a n a l y s i s showed t h a t asymmetries o f c o h e r e n c e were s i g n i f i c a n t l y r e l a t e d t o speech dominance, but power s p e c t r a l asymmetries were n o t . T h i s was demonstrated w i t h the U - s t a t i s t i c f o r t h e s i g n i f i c a n c e o f t h e d i f f e r e n c e between t h e LSD and RSD means. For t h e coherence a s y m m e t r i e s , t h e U - s t a t i s t i c was s i g n i f i c a n t a t p l e s s than 0 . 0 0 5 , b u t f o r t h e power s p e c t r a l a s y m m e t r i e s , i t was not s i g n i f i c a n t a t t h e v e r y c o n s e r v a t i v e l e v e l o f p l e s s than 0 . 2 5 . In terms o f t h e c h a r a c t e r i s t i c s o f t h e E P s , t h e s e r e s u l t s i n d i c a t e t h a t the d i f f e r e n c e s i n a m p l i t u d e s between hemispheres a r e n o t r e l a t e d t o speech dominance. In c o n t r a s t , t h e d i f f e r e n c e s i n form f o r t h e EPs r e c o r d e d from d i f f e r e n t a r e a s a r e r e l a t e d t o speech dominance. A m p l i t u d e asymmetries o f v a r i o u s t y p e s o f e v e n t - r e l a t e d p o t e n t i a l s have been r e p o r t e d f o r n a t u r a l speech s t i m u l i ( M o r r e l l and Salamy, 1971); d u r i n g l i n g u i s t i c v e r s u s n o n - l i n g u i s t i c t a s k s (Wood and G o f f , 1971); w i t h m e a n i n g f u l v e r s u s n o n - m e a n i n g f u l s t i m u l i ( M a t s u m i y a , T a g l i a s c o , Lombroso and G o o d g l a s s , 1 9 7 2 ) ; f o r t a c h i s t o s c o p i c p r e s e n t a t i o n 9 8 o f words and s i m i l a r l y p a t t e r n e d s t i m u l i (Buchsbaum and F e d i o , 1 9 6 9 , 1 9 7 0 ) ; and f o r motor p o t e n t i a l s p r e c e d i n g a r t i c u l a t i o n (McAdam and W h i t t a k e r , 1971). In a l o w e r f r e q u e n c y r a n g e , M o r r e l l and H u n t i n g t o n (1971) and Low, Wada and Fox (1973) have r e p o r t e d a m p l i t u d e asymmetries o f c e r e b r a l s l o w p o t e n t i a l s p r e c e d i n g s p e e c h . There a r e thus i n d i c a t i o n s from a v a r i e t y o f s o u r c e s t h a t d i f f e r e n c e s e x i s t i n the a m p l i t u d e s o f l e f t and r i g h t h e m i s p h e r i c p r o c e s s e s . The n e g a t i v e r e s u l t s f o r a m p l i t u d e asymmetries r e l a t e d t o speech dominance do n o t n e c e s s a r i l y c o n t r a d i c t t h e s e f i n d i n g s . Only t h e r e p o r t by Low e t a l used s u b j e c t s whose speech dominance was d e f i n i t e l y known. In most o f t h e o t h e r r e p o r t s , o n l y r i g h t - h a n d e d s u b j e c t s were used and i t was s u g g e s t e d o r i n f e r r e d t h a t o b s e r v e d a m p l i t u d e asymmetries might be more g e n e r a l l y r e l a t e d t o speech dominance. The r e s u l t s o f t h e d i s c r i m i n a n t a n a l y s i s , however, show t h a t t h i s e x t e n s i o n i s not l i k e l y . t o be c o r r e c t . The d i s c r i m i n a n t e q u a t i o n s (1) f o r the c o h e r e n c e asymmetries g i v e a q u a n t i t a t i v e comparison o f the c o n t r i b u t i o n s o f VEP and AEP asymmetries t o t h e d e t e r m i n a t i o n o f speech dominance. F i r s t , t h e LSD and RSD f a c t o r s have o p p o s i t e s i g n s f o r b o t h f l a s h and c l i c k . T h i s i n d i c a t e s t h a t f l a s h and c l i c k asymmetries c o n t r i b u t e o p p o s i t e l y t o speech dominance d e t e r m i n a t i o n and t h a t LSD and RSD p a t i e n t s t e n d t o have o p p o s i t e a s y m m e t r i e s . For e x a m p l e , a l a r g e r l e f t h e m i s p h e r i c c l i c k c o h e r e n c e and a l a r g e r r i g h t h e m i s p h e r i c f l a s h c o h e r e n c e w i l l c o n t r i b u t e t o a h i g h e r 99 LSD s c o r e and a l o w e r RSD s c o r e . S e c o n d , t h e a p p r o x i m a t e l y equal magnitudes o f t h e c l i c k d i s c r i m i n a n t f a c t o r s ( 1 4 . 6 7 and 1 1 . 5 9 ) f o r t h e LSD and RSD groups show t h a t c l i c k asymmetries a r e about e q u a l l y i m p o r t a n t i n d e t e r m i n i n g LSD o r RSD membership. The s m a l l e r f l a s h d i s c r i m i n a n t f a c t o r s ( 5 . 9 2 and 1 . 9 3 ) f o r the LSD and RSD groups show t h a t f l a s h asymmetries a r e c o m p a r a t i v e l y l e s s i m p o r t a n t than c l i c k asymmetries i n d e t e r m i n i n g speech dominance. They a l s o show t h a t VEP maximum c o h e r e n c e s a r e more c l e a r l y r e l a t e d t o the r i g h t hemisphere i n LSD p a t i e n t s than t o t h e l e f t hemisphere i n RSD p a t i e n t s . In terms o f p r o b a b i l i t i e s , the d i s c r i m i n a n t e q u a t i o n s show t h a t a h i g h AEP coherence between t h e o c c i p i t a l and temporal a r e a s o f one hemisphere i n c r e a s e s the p r o b a b i l i t y t h a t speech dominance i s l o c a l i z e d w i t h i n t h a t h e m i s p h e r e . C o n v e r s e l y , a h i g h VEP coherence between the o c c i p i t a l and temporal a r e a s o f one hemisphere i n c r e a s e s t h e p r o b a b i l i t y t h a t speech dominance i s l o c a l i z e d w i t h i n the o p p o s i t e h e m i s p h e r e . The a p p l i c a t i o n o f t h e s e f i n d i n g s t o normal s u b j e c t s i s o f g r e a t i n t e r e s t , s i n c e t h e r e i s no way o f u n e q u i v o c a l l y d e t e r m i n i n g speech dominance i n normal s u b j e c t s . D e f i n i t i v e p r o c e d u r e s , such as t h e c a r o t i d amytal t e s t , o r e l e c t r o s t i m u l a t i o n o f t h e exposed c o r t e x a r e c l i n i c a l t e c h n i q u e s w i t h l i t t l e e x p e r i m e n t a l u s e f u l n e s s . D i c h o t i c l i s t e n i n g , which can be a p p l i e d t o normal s u b j e c t s , i s o n l y about 70% a c c u r a t e i n a s s e s s i n g e a r - d o m i n a n c e i n t h e same s u b j e c t s o v e r a one month p e r i o d ( P i z z a m i g l i o , De P a s c a l i s and V i g n a t i , 1974). 100 D i s c r i m i n a n t a n a l y s i s o f EP coherence asymmetries seems t o g i v e v e r y s i m i l a r r e s u l t s t o t h e c a r o t i d amytal t e s t . The computer and amytal p r e d i c t i o n s o f speech dominance agree i n about 90% o f the p a t i e n t s . The amytal t e s t i t s e l f has been s u r g i c a l l y v a l i d a t e d as b e i n g g r e a t e r than 95% a c c u r a t e by B r a n c h , M i l n e r and Rasmussen ( 1 9 6 4 ) . S i n c e o n l y 11 p a t i e n t s have been t e s t e d w i t h t h e computer p r o c e d u r e , i t would be premature t o s t a t e more than a c o r r e s p o n d e n c e seems t o e x i s t between t h e amytal and computer t e s t s . The d i s c r i m i n a n t a n a l y s i s o f t h e normal s u b j e c t s showed s l i g h t but n o t s i g n i f i c a n t d i f f e r e n c e s between t h e r i g h t and l e f t - h a n d e d g r o u p s . U s i n g a s i g n i f i c a n c e l e v e l o f p l e s s than 0 . 0 5 t o d e f i n i t e l y a s s i g n speech dominance, the a n a l y s i s c l a s s i f i e d t h e 12 r i g h t - h a n d e d s u b j e c t s i n t o 8 LSD, 3 RSD and 1 b i l a t e r a l ( more RSD than LSD). The l e f t - h a n d e d s u b j e c t s were c l a s s i f i e d i n t o 5 LSD, 1 RSD and 6 b i l a t e r a l (4 s u b j e c t s more LSD than RSD). T h u s , i f b i l a t e r a l i t y i s i g n o r e d , about 70% o f t h e r i g h t - h a n d e d s u b j e c t s (8/12) and 75% o f t h e l e f t - h a n d e d s u b j e c t s (9/12) were LSD. The a n a l y s i s o f v a r i a n c e o f t h e normal s u b j e c t s v e r y c l e a r l y c o n f i r m s t h e s i g n i f i c a n t s h i f t i n g o f c o h e r e n t a c t i v i t y from the l e f t hemisphere d u r i n g a u d i t o r y s t i m u l a t i o n t o t h e r i g h t hemisphere d u r i n g v i s u a l s t i m u l a t i o n . Handedness had no s i g n i f i c a n t e f f e c t on the coherence asymmetries and t h e r e was no i n t e r a c t i o n between handedness and s t i m u l u s m o d a l i t y . T h i s i n d i c a t e s t h a t both t h e r i g h t and l e f t - h a n d e d s u b j e c t s respond i n t h e same way t o the s t i m u l u s m o d a l i t y , t h a t i s , 101 l a r g e r VEP coherences i n the r i g h t hemisphere and l a r g e r AEP c o h e r e n c e s i n t h e l e f t . The VEP and AEP asymmetries d i d not change s i g n i f i c a n t l y f o r l e f t and r i g h t - h a n d e d s u b j e c t s , but d i d f o r LSD and RSD p a t i e n t s . T h i s i n d i c a t e s t h a t handedness i s n o t r e l a t e d t o speech dominance. The m a j o r i t y o f the normal p o p u l a t i o n i s thus p r o b a b l y l e f t s p e e c h -d o m i n a n t , r e g a r d l e s s o f t h e i r handedness. L e f t - h a n d e d s u b j e c t s , however, show a much g r e a t e r t r e n d to b i l a t e r a l r e p r e s e n t a t i o n , a l t h o u g h t h e LSD component o f t h i s b i l a t e r a l i t y i s s t i l l l a r g e r than the RSD component. The r e l a t i o n s h i p o f handedness t o speech dominance has been i n v e s t i g a t e d by many a u t h o r s , w i t h o f t e n d i f f e r i n g c o n c l u s i o n s . I t has been s t a t e d t h a t r i g h t - h a n d e d n e s s i s a good i n d i c a t i o n o f l e f t speech dominance, but t h a t t h e speech dominance o f l e f t - h a n d e d s u b j e c t s i s e i t h e r u n p r e d i c t a b l e o r s l i g h t l y f a v o u r e d t o t h e r i g h t hemisphere ( B e n t o n , 1962; Hecaen and A j u r i a g u e r r a , 1964). I t i s i m p o r t a n t t o n o t e , . h o w e v e r , t h a t t h i s r e l a t i o n s h i p has been assumed on the b a s i s o f v e r y l i t t l e d i r e c t e v i d e n c e . A l m o s t a l l has i n v o l v e d p a t i e n t s i n whom t h e speech p r o c e s s has been i n t e r r u p t e d by p a t h o l o g i c a l , s u r g i c a l o r p h a r m a c o l o g i c a l means. I t has a l s o been d i f f i c u l t t o d e t e r m i n e t h e e x t e n t t o w h i c h e a r l y i n j u r i e s o r unknown c o n d i t i o n s have a f f e c t e d m a t u r a t i o n and l a t e r a l i z a t i o n o f t h e speech p r o c e s s ( G e s c h w i n d , 1970). 102 P e n f i e l d a n d R o b e r t s ( 1 9 5 9 ) , u s i n g e l e c t r o s t i m u l a t i o n o f t h e e x p o s e d c o r t e x , c o n c l u d e d t h a t m o s t p e r s o n s w e r e l e f t s p e e c h -d o m i n a n t r e g a r d l e s s o f t h e i r h a n d e d n e s s . T h e y e s t i m a t e d 9 8 % L S D a n d 2% RSD i n t h e r i g h t - h a n d e d p o p u l a t i o n , a n d 9 0 % L S D a n d 10% RSD i n t h e l e f t - h a n d e d p o p u l a t i o n . B i l a t e r a l i t y w a s n o t c o n s i d e r e d . B r a n c h e t a l ( 1 9 6 4 ) , u s i n g t h e c a r o t i d a m y t a l t e s t , e s t i m a t e d 9 0 % L S D a n d 10% R S D i n t h e r i g h t - h a n d e d p o p u l a t i o n , a n d 4 0 % R S D , 5 0 % L S D a n d 10% b i l a t e r a l i n t h e l e f t - h a n d e d p o p u l a t i o n . T h e y a d m i t t e d , h o w e v e r , t h a t t h e q u e s t i o n o f b i l a t e r a l i t y h a d o n l y b e c o m e a p p a r e n t s o m e d i s t a n c e t h r o u g h t h e i r , s e r i e s o f p a t i e n t s . N e i t h e r o f t h e a b o v e s t u d i e s m a d e c l e a r d e f i n i t i o n s o f h a n d e d n e s s , w h i c h r e q u i r e m u c h m o r e s u b t l e d i s t i n c t i o n s t h a n w h i c h h a n d i s u s e d f o r w r i t i n g , o r t h e p e r s o n ' s o w n e v a l u a t i o n o f h i s h a n d e d n e s s . L o w e t a l ( 1 9 7 3 ) h a v e r e p o r t e d C N V - l i k e a s y m m e t r i e s i n a g r o u p o f 8 L S D a n d 3 RSD p a t i e n t s w h o s e s p e e c h d o m i n a n c e h a d b e e n d e t e r m i n e d w i t h t h e c a r o t i d a m y t a l t e s t . T h e y a l s o a p p l i e d t h e i r t e c h n i q u e t o 11 " p u r e " ( A n n e t t , 1 9 6 7 ) r i g h t - h a n d e d a n d 11 " p u r e " l e f t - h a n d e d n o r m a l s u b j e c t s . U s i n g m a x i m u m CNV a m p l i t u d e p r e p a r a t o r y t o s p e e c h a s a n i n d e x o f l a t e r a l i t y , t h e y w e r e a b l e t o c o r r e c t l y p r e d i c t t h e s p e e c h d o m i n a n c e o f 8 / 8 L S D p a t i e n t s a n d 2 / 3 RSD p a t i e n t s . I n t h e n o r m a l p o p u l a t i o n s , t h e y f o u n d a d i s t r i b u t i o n o f CNV a s y m m e t r i e s c o r r e s p o n d i n g t o 8 L S D , 1 R S D a n d 2 b i l a t e r a l i n t h e r i g h t - h a n d e d s u b j e c t s , a n d 3 L S D , 6 RSD a n d 2 b i l a t e r a l i n t h e l e f t - h a n d e d s u b j e c t s . 103 The above d i s t r i b u t i o n f o r r i g h t - h a n d e d s u b j e c t s i s s i m i l a r t o t h a t d e r i v e d from t h e d i s c r i m i n a n t a n a l y s i s . The l e f t - h a n d e d d i s t r i b -u t i o n i s d i f f e r e n t , m a i n l y i n the number o f RSD c l a s s i f i c a t i o n s . However, s i n c e o n l y 3 RSD amytal p a t i e n t s were t e s t e d w i t h t h e CNV p r o c e d u r e and one o f t h e s e was m i s c l a s s i f i e d , t h e i r c o n c l u s i o n s about RSD g r o u p i n g must be weighed c a u t i o u s l y . They a l s o r e p o r t e d c o n s i d e r a b l y more b i l a t e r a l i t y i n t h e normal p o p u l a t i o n than had been p r e v i o u s l y s u s p e c t e d , which a grees w i t h t h e d i s c r i m i n a n t a n a l y s i s . Wada, C l a r k and Hamm (1975) found t h a t s i g n i f i c a n t m o r p h o l o g i c a l asymmetries e x i s t i n t h e c o r t i c a l speech zones o f both i n f a n t and a d u l t b r a i n s . T h i s s u g g e s t s t h e p o s s i b i l i t y t h a t asymmetries r e l a t e d t o speech dominance i n a d u l t s may a l s o be p r e s e n t i n young c h i l d r e n , o r even b a b i e s . M o l f e s e (1973) r e p o r t e d EP asymmetries to speech and m u s i c a l s t i m u l i i n i n f a n t s . An ongoing l o n g i t u d i n a l s t u d y o f n e o n a t a l evoked p o t e n t i a l asymmetries u s i n g s p e c t r a l a n a l y s i s i s c u r r e n t l y underway t o c l a r i f y t h e p o s s i b i l i t y o f e a r l y o r even p r e d e t e r m i n e d speech l a t e r a l i z a t i o n . U s i n g t h e above r e s u l t s , a f a i r l y s i m p l e c o m p u t a t i o n a l p r o c e d u r e f o r the d e t e r m i n a t i o n o f speech dominance can be d e f i n e d . The BMD programs a r e w i d e l y a v a i l a b l e , but any s p e c t r a l a n a l y s i s program c o u l d be used to c a l c u l a t e t h e coherence a s y m m e t r i e s . Speech dominance c l a s s i f i c a t i o n and p r o b a b i l i t i e s can then be c a l c u l a t e d u s i n g e q u a t i o n s (1) and ( 2 ) . 104 The d i s c r i m i n a n t a n a l y s i s shows s e v e r a l d i f f e r e n c e s and improvements o v e r t h o s e o f t h e p r e v i o u s e x p e r i m e n t s . F i r s t , t h e r e l a t i o n s h i p o f l e f t and r i g h t hemisphere c o h e r e n c e s t o speech dominance has been e v a l u a t e d s t a t i s t i c a l l y w i t h t h e p a t i e n t s . S e c o n d , i n t h e normal s u b j e c t s , the s h i f t i n g o f h i g h e r c o h e r e n c e t o t h e l e f t hemisphere w i t h a u d i t o r y s t i m u l i and t o t h e r i g h t hemisphere w i t h v i s u a l s t i m u l i has a l s o been shown to be s i g n i f i c a n t . T h i r d , t h e s e r e s u l t s have been e s t a b l i s h e d f o r t h e l a r g e s t a m p l i t u d e component o f the 500 msec. EP, r a t h e r than the m o d a l i t y - d e p e n d e n t , 6-12 hz . component. Of c o u r s e , the f r e q u e n c y ranges o f t h e s e two components a r e a p p r o x i m a t e l y t h e same, b u t the r e s u l t s a r e independent i n t h a t t h e components were chosen on t h e b a s i s o f two d i f f e r e n t c r i t e r i a . 105 CONCLUSIONS A. Summary o f s p e c t r a l a n a l y s i s The power s p e c t r a l a n a l y s i s o f EPs showed t h e e x i s t e n c e o f two major f r e q u e n c y g r o u p s , one from 0 - 5 h z . and the o t h e r from 6-12 h z . T h e i r s p a t i a l d i s t r i b u t i o n s and l a t e n c i e s s u g g e s t e d t h a t the 0 - 5 h z . group was i n v o l v e d i n p r o c e s s e s which were not s p e c i f i c a l l y r e l a t e d t o e i t h e r t h e s t i m u l u s m o d a l i t y o r c o r t i c a l r e c o r d i n g a r e a -such as a t t e n t i o n o r e x t r a c t i o n o f meaning. The 6-12 h z . component, on t h e o t h e r h a n d , was a f u n c t i o n o f b o t h t h e s t i m u l u s m o d a l i t y and r e c o r d i n g area and was o f l a r g e s t a m p l i t u d e o v e r t h e c o r t i c a l p r o j e c t i o n a r e a o f the s t i m u l a t e d m o d a l i t y . I t thus seemed t o be r e l a t e d t o the p r o c e s s i n g o f s e n s o r y i n f o r m a t i o n i n s p e c i f i c c o r t i c a l a r e a s . In t h e second e x p e r i m e n t , t h e c o h e r e n c e o r form s i m i l a r i t y between p a i r s o f E P s , and t h e i r r e l a t i o n s h i p t o s t i m u l u s m o d a l i t y and speech dominance were i n v e s t i g a t e d . In both l e f t and r i g h t s p e e c h - d o m i n a n t e p i l e p t i c p a t i e n t s , and i n n o r m a l , r i g h t - h a n d e d , presumably l e f t s p e e c h - d o m i n a n t s u b j e c t s , the o c c i p i t a l - t o - t e m p o r a l c o h e r e n c e o f EPs was g r e a t e r i n t h e s p e e c h - d o m i n a n t hemisphere f o r c l i c k s t i m u l i , and g r e a t e r i n the speech non-dominant hemisphere f o r f l a s h s t i m u l i . A l t h o u g h o t h e r asymmetries o f c o h e r e n c e were p o s s i b l e between the f o u r r e c o r d i n g a r e a s , o n l y t h e a b o v e , l e f t - m i n u s - r i g h t h e m i s p h e r i c coherence changed i n d i r e c t i o n w i t h d i f f e r e n t s t i m u l u s m o d a l i t i e s . 1 0 6 In the t h i r d e x p e r i m e n t , the asymmetries o f coherence were i n v e s t i g a t e d u s i n g d i s c r i m i n a n t a n a l y s i s . T h i s t e c h n i q u e showed, as was q u a l i t a t i v e l y deduced i n Experiment I I , t h a t o n l y t h e o c c i p i t a l - t o - t e m p o r a l coherence w i t h i n each hemisphere was s i g n i f i c a n t l y r e l a t e d t o speech dominance. A l a r g e AEP c o h e r e n c e w i t h i n one hemisphere i n c r e a s e d t h e p r o b a b i l i t y t h a t speech dominance was l a t e r a l i z e d to t h a t h e m i s p h e r e . C o n v e r s e l y , a l a r g e VEP c o h e r e n c e w i t h i n one hemisphere i n c r e a s e d the p r o b a b i l i t y t h a t speech dominance was l a t e r a l i z e d t o t h e o p p o s i t e h e m i s p h e r e . In c o n t r a s t t o asymmetries o f c o h e r e n c e , the d i s c r i m i n a n t a n a l y s i s showed t h a t power s p e c t r a l o r a m p l i t u d e - r e l a t e d asymmetries between the r i g h t and l e f t hemispheres were not s i g n f i c a n t l y r e l a t e d t o speech dominance. F i n a l l y , the d i s c r i m i n a n t e q u a t i o n s , e x p r e s s i n g t h e r e l a t i o n s h i p s between c o h e r e n c e and speech dominance were a p p l i e d to the EP c o h e r e n c e s o f n o r m a l , r i g h t o r l e f t - h a n d e d s u b j e c t s . T h i s a n a l y s i s showed t h a t the m a j o r i t y o f both groups showed a s i g n i f i c a n t l e f t s p e e c h - d o m i n a n t p a t t e r n . There was no s i g n i f i c a n t d i f f e r e n c e between t h e c o h e r e n c e asymmetries o f t h e two g r o u p s , a l h t o u g h t h e l e f t - h a n d e d s u b j e c t s tended to show more b i l a t e r a l o r ambiguous r e p r e s e n t a t i o n . S i n c e t h e s e coherence asymmetries were s i g n i f i c a n t l y r e l a t e d t o t h e speech dominance o f t h e p a t i e n t s , i t was c o n c l u d e d t h a t speech dominance and handedness were not r e l a t e d , and t h a t t h e m a j o r i t y o f both l e f t and r i g h t - h a n d e d s u b j e c t s were l e f t s p e e c h - d o m i n a n t . 107 B. P h y s i o l o g i c a l s i g n i f i c a n c e o f t h e s p e c t r a l components The s p e c t r a l d e s c r i p t i o n s o f the EP waveforms showed c l e a r r e l a t i o n s h i p s t o p h y s i o l o g i c a l f a c t o r s such as s t i m u l u s m o d a l i t y , c o r t i c a l a r e a , speech dominance and handedness. T h i s was shown f o r both t h e power s p e c t r a l and coherence c h a r a c t e r i s t i c s o f t h e E P s , which t o g e t h e r c o n s t i t u t e a complete d e s c r i p t i o n o f the waveforms and t h e i r i n t e r a c t i o n s w i t h each o t h e r . The power s p e c t r a l o r a m p l i t u d e c h a r a c t e r i s t i c s o f the EPs depend on s t i m u l u s m o d a l i t y , r e c o r d i n g a r e a and f r e q u e n c y component o f t h e EP. The low f r e q u e n c y , 0 - 5 h z . component i s s p a t i a l l y g e n e r a l i z e d , i n d e p e n d e n t o f t h e s t i m u l u s m o d a l i t y , and o f s i m i l a r l a t e n c y t o EP c o r r e l a t e s o f such n o n - s e n s o r y p r o c e s s e s as s e l e c t i v e a t t e n t i o n . In c o n t r a s t , t h e h i g h e r f r e q u e n c y , 6-12 h z . component i s m o d a l i t y -s p e c i f i c , and o f l a r g e s t a m p l i t u d e o v e r t h e c o r t i c a l p r o j e c t i o n a r e a o f t h e s t i m u l a t e d m o d a l i t y . I t s s h o r t e r l a t e n c y and h i g h e r f r e q u e n c y , s u g g e s t s a p o s s i b l e r o l e i n the c o r t i c a l p r o c e s s i n g o f i n c o m i n g i n f o r m a t i o n . The c o h e r e n c e , o r form s i m i l a r i t y o f d i f f e r e n t E P s , i s dependent on s t i m u l u s m o d a l i t y , which hemisphere the r e c o r d i n g , i s made f r o m , and the p e r s o n ' s speech dominance. I t i s not r e l a t e d t o handedness. C f l i c k s t i m u l i produce EPs which a r e more c o h e r e n t w i t h i n t h e s p e e c h - d o m i n a n t h e m i s p h e r e , w h i l e f l a s h s t i m u l i produce-more c o h e r e n t r e s p o n s e s w i t h i n t h e speech non-dominant h e m i s p h e r e . The m a j o r i t y o f both l e f t and r i g h t - h a n d e d , normal s u b j e c t s show t h i s asymmetry, a l t h o u g h t h o s e o f l e f t - h a n d e d s u b j e c t s a r e somewhat l e s s a c c e n t u a t e d . 108 The d i f f e r e n t c h a r a c t e r i s t i c s o f the power s p e c t r a l and coherence asymmetries s u g g e s t t h a t t h e s e measures r e p r e s e n t d i f f e r e n t a s p e c t s o f c o r t i c a l p r o c e s s i n g . A m p l i t u d e s were l a r g e s t o v e r both hemispheres o f t h e c o r t i c a l p r o j e c t i o n a r e a o f t h e s t i m u l a t e d m o d a l i t y , and were not r e l a t e d t o speech dominance. Coherences were l a r g e s t o v e r t h e hemisphere r e l a t e d t o the p r o c e s s i n g o f the s t i m u l a t e d m o d a l i t y , and were r e l a t e d t o speech dominance. T h i s i n d i c a t e s t h a t l a r g e a m p l i t u d e s a r e c h a r a c -t e r i s t i c o f i n c o m i n g s e n s o r y d a t a , w h i l e l a r g e c o h e r e n c e s r e p r e s e n t t h e o c c u r r e n c e o r s p r e a d o f t h e s e a m p l i t u d e s o v e r l a r g e a r e a s o f one h e m i s p h e r e . The w i d e s p r e a d o c c u r r e n c e f o f s i m i l a r forms o f EPs may r e p r e s e n t a s s o c i a t i v e a c t i v i t i e s w i t h i n the temporal and p a r i e t a l a r e a s . These would be l a t e r a l i z e d t o the speech-dominant hemisphere f o r t h e p e r c e p t i o n and a s s o c i a t i v e p r o c e s s i n g o f a u d i t o r y i n f o r m a t i o n , and t o t h e speech non-dominant hemisphere f o r v i s u a l i n f o r m a t i o n . In o t h e r w o r d s , i n c o m i n g d a t a may produce equal r e s p o n s e s i n both h e m i s p h e r e s , but the p e r c e p t u a l p r o c e s s i n g o f t h a t i n f o r m a t i o n i s l a t e r a l i z e d t o o n l y one hemisphere and i s a s s o c i a t e d w i t h h i g h c o h e r e n c e s . C. V e r b a l and n o n - v e r b a l s t i m u l i I t i s o f g r e a t i m p o r t a n c e t h a t t h e s p e c t r a l asymmetries were produced w i t h s i m p l e f l a s h and c l i c k s t i m u l i . These have v e r y l i t t l e s p a t i a l o r temporal s t r u c t u r e , and y e t g e n e r a t e c l e a r a s y m m e t r i e s . T h i s s u g g e s t s t h a t c o r t i c a l asymmetries o f p r o c e s s i n g e x i s t a t a more 109 fundamental l e v e l than t h a t o f v e r b a l e n c o d i n g o r d e c o d i n g . I f t h e a s y m m e t r i c , c o h e r e n t r e s p o n s e s a r e assumed t o r e p r e s e n t some w i d e s p r e a d , p o s s i b l y a s s o c i a t i v e p r o c e s s i n g o f i n f o r m a t i o n , then t h e coherence r e s u l t s s u g g e s t t h a t o p p o s i t e l a t e r a l i t i e s e x i s t f o r v i s u a l and a u d i t o r y i n f o r m a t i o n . The q u e s t i o n o f " v e r b a l v e r s u s n o n - v e r b a l " i s thus reduced t o one o f " a u d i t o r y v e r s u s v i s u a l " . p r o c e s s i n g . D. I m p l i c a t i o n s f o r h e m i s p h e r i c f u n c t i o n a l asymmetries The o b s e r v a t i o n t h a t s i m p l e f l a s h and c l i c k s t i m u l i produce o p p o s i t e h e m i s p h e r i c asymmetries i s a g r e a t s i m p l i f i c a t i o n from e a r l i e r v e r b a l o r n o n - v e r b a l d i s t i n c t i o n s . However, i t s t i l l does not seem to answer t h e q u e s t i o n : what i s t h e b a s i c d i f f e r e n c e between s t i m u l i p r o c e s s e d i n the l e f t h e m i s p h e r e , and t h o s e p r o c e s s e d i n the r i g h t ? A p o s s i b l e answer t o t h i s q u e s t i o n may l i e i n hidden a s s u m p t i o n s o f t h e q u e s t i o n i t s e l f , w h i c h a r e t h a t , f i r s t t h e r e i s a d i f f e r e n c e between s t i m u l i , and s e c o n d , t h a t somehow t h e b r a i n r e c o g n i z e s t h a t t h e s t i m u l u s i s v e r b a l , n o n - v e r b a l , v i s u a l , a u d i t o r y o r w h a t e v e r . What a r e t h e consequences o f assuming t h a t no d i f f e r e n c e e x i s t s between a v i s u a l and an a u d i t o r y s t i m u l u s ? I t i s o b v i o u s , o f c o u r s e , t h a t t h e y a r e g e n e r a t e d by d i f f e r e n t means and d i f f e r e n t s e n s o r y systems 110 respond t o them. But p h r a s i n g the q u e s t i o n i n t h i s way s u g g e s t s a n o t h e r p o s s i b i l i t y . For t h e r e a r e two p a r t s t o t h e s y s t e m , - t h e s t i m u l u s , and t h e p e r c e i v e r , and i t may be p o s s i b l e t h a t t h e key t o h e m i s p h e r i c asymmetries l i e s w i t h t h e p e r c e i v e r , r a t h e r than w i t h t h e n a t u r e o f the s t i m u l u s . The s i m p l e s t o f v i s u a l and a u d i t o r y s t i m u l i produce h e m i s p h e r i c asymmetries which a r e r e l a t e d t o p e r c e p t u a l a s y m m e t r i e s . I t i s thus p o s s i b l e t h a t a l l v i s u a l s t i m u l i a r e i n i t i a l l y p e r c e i v e d w i t h i n the r i g h t h e m i s p h e r e , and t h a t a l l a u d i t o r y s t i m u l i a r e i n i t i a l l y p e r c e i v e d w i t h i n the l e f t h e m i s p h e r e . Of c o u r s e , a t h i g h e r l e v e l s o f p r o c e s s i n g , as i n d e t e r m i n i n g the l i n g u i s t i c meaning o f words seen on a p a g e , t h i s i n i t i a l l a t e r a l i z a t i o n would not h o l d , but the p o s s i b i l i t y f o r the v e r y f i r s t s t e p s o f p r o c e s s i n g seems r e a s o n a b l e . To make more i n f e r e n c e s about t h e s e a s y m m e t r i e s , t h e n a t u r e o f p e r c e p t i o n must be more c a r e f u l l y d e f i n e d . The d i c t i o n a r y d e f i n e s p e r c e p t i o n as a r e s p o n s e t o a p a t t e r n c o u p l e d w i t h an u n d e r s t a n d i n g o f what t h a t p a t t e r n i s , o r r e p r e s e n t s . There a r e thus two components t o the p r o c e s s ; t h e f i r s t r e s p o n d i n g to t h e c h a r a c t e r i s t i c s o f the s t i m u l u s , and the second e x t r a c t i n g meaning from i t . These two components o f the p e r c e p t u a l p r o c e s s may be r e l a t e d t o t h e power s p e c t r a and c o h e r e n c e s . The power s p e c t r a , o r a m p l i t u d e - r e l a t e d c h a r a c t e r i s t i c s o f t h e EPs were l a r g e s t i n t h e c o r t i c a l p r o j e c t i o n a r e a o f t h e s t i m u l a t e d m o d a l i t y , and were not r e l a t e d to speech dominance. In c o n t r a s t , t h e c o h e r e n c e o r s i m i l a r i t y o f form o f r e s p o n s e s was d i f f e r e n t between h e m i s p h e r e s , and was r e l a t e d t o the p e r s o n ' s speech dominance. m The magnitude o f the r e s p o n s e thus depends on t h e n a t u r e o f t h e s t i m u l u s , t h a t i s , whether i t i s v i s u a l o r a u d i t o r y , but not on t h e s u b j e c t , t h a t i s , h i s speech dominance. In c o n t r a s t , t h e c o h e r e n c e o f r e s p o n s e s depends both on t h e s t i m u l u s m o d a l i t y and on t h e s u b j e c t ' s p e r c e p t u a l a s y m m e t r i e s , o f w h i c h speech dominance i s p r o b a b l y o n l y one a s p e c t . These o b s e r v a t i o n s s u g g e s t t h a t t h e a m p l i t u d e o f r e s p o n s e s , which i s m o d a l i t y - d e p e n d e n t , may r e p r e s e n t the b i l a t e r a l , r e s p o n s e component o f the p e r c e p t u a l p r o c e s s . S i m i l a r l y , t h e component o f u n d e r s t a n d i n g , o r c o g n i t i v e mechanism w h i c h a t t a c h e s meaning t o what i s seen o r h e a r d , may be r e p r e s e n t e d by t h e l a t e r a l i z e d , w i d e s p r e a d c o h e r e n t a c t i v i t i e s o f t h e o c c i p i t a l , temporal and p a r i e t a l a r e a s . R e t u r n i n g now t o the q u e s t i o n o f which type o f s t i m u l i a r e p r o c e s s e d w i t h i n each h e m i s p h e r e , a s i m p l e answer a p p e a r s . S i n c e asymmetries a r e produced f o r v e r y e l e m e n t a r y s t i m u l i , i t seems l i k e l y t h a t a l l a u d i t o r y i n f o r m a t i o n i s i n i t i a l l y p r o c e s s e d w i t h i n t h e l e f t h e m i s p h e r e , and a l l v i s u a l i n f o r m a t i o n w i t h i n t h e r i g h t . I f i t i s assumed t h a t t h e a m p l i t u d e o f r e s p o n s e s r e p r e s e n t s t h e s e n s o r y r e s p o n s e , and t h e c o h e r e n c e s , the p r o c e s s o f u n d e r s t a n d i n g the meaning o f t h e s t i m u l u s r e s p o n s e , then a u d i t o r y c o g n i t i o n i s p r i m a r i l y a l e f t h e m i s p h e r i c f u n c t i o n , and v i s u a l c o g n i t i o n , a r i g h t h e m i s p h e r i c f u n c t i o n . C o n s e q u e n t l y , t h e hemisphere chosen f o r p r o c e s s i n g does not depend on t h e n a t u r e o f t h e s e n s o r y i n f o r m a t i o n , b u t on w h i c h s e n s o r y pathway i t e n t e r s the c o r t e x . The problem o f s t i m u l u s i d e n t i f i c a t i o n , o r t h e d i f f e r e n c e between v e r b a l and n o n - v e r b a l s t i m u l i , i s thus e l i m i n a t e d . 112 Why would such a l a t e r a l i z a t i o n o f v i s u a l and a u d i t o r y f u n c t i o n s o c c u r ? The r e s u l t s do n o t answer t h i s q u e s t i o n , but t h e y do s u g g e s t a g e n e r a l i z a t i o n o f h e m i s p h e r i c modes o f p r o c e s s i n g w h i c h would r e q u i r e such a s e p a r a t i o n . T h i s n e c e s s i t y may l i e i n t h e d i f f e r e n t ways i n which a u d i t o r y and v i s u a l i n f o r m a t i o n a r e p r o c e s s e d . I t i s p r o b a b l e t h a t , i n i t i a l l y a t l e a s t , a u d i t o r y i n f o r m a t i o n i s s e q u e n t i a l l y a n a l y z e d i n i t s temporal r e l a t i o n s h i p t o sounds t h a t o c c u r r e d e a r l i e r , as i n s p e e c h . In c o n t r a s t , v i s u a l i n f o r m a t i o n a t any i n s t a n t i s s i m u l t a n e o u s l y a n a l y z e d as a c o m p l e t e , s p a t i a l p a t t e r n . These two modes o f p r o c e s s i n g a r e a n t a g o n i s t i c i n t h a t s i m u l t a n e o u s p r o c e s s i n g i n v o l v e s the a n a l y s i s o f s e t s o f d a t a a t one t i m e , w h i l e s e q u e n t i a l p r o c e s s i n g i n v o l v e s t h e a n a l y s i s o f s e t s o f d a t a a t d i f f e r e n t t i m e s . T h i s i s s u p p o r t e d by t h e o b s e r v a t i o n t h a t s p l i t - b r a i n humans a r e more adept than normal s u b j e c t s a t s i m u l t a n e o u s l y p e r f o r m i n g two c o n f l i c t i n g , v e r b a l and n o n - v e r b a l t a s k s . I t i s c o n s e q u e n t l y u n d e r s t a n d a b l e t h a t t h e two d i f f e r e n t modes o f p r o c e s s i n g , s e q u e n t i a l f o r a u d i t o r y i n f o r m a t i o n , and s i m u l t a n e o u s f o r v i s u a l i n f o r m a t i o n , must be s e p a r a t e d t o o p p o s i t e h e m i s p h e r e s . E. F i n a l c o n c l u s i o n s The purposes o f t h i s i n v e s t i g a t i o n were t o a p p l y s p e c t r a l a n a l y s i s t o EP waveforms, t o m e a n i n g f u l l y r e l a t e • ! t h e r e s u l t s to p h y s i o l o g i c a l f a c t o r s , t o d e t e r m i n e i f s i m p l e f l a s h and c l i c k s t i m u l i c o u l d g e n e r a t e h e m i s p h e r i c d i f f e r e n c e s i n E P s , and i f s o , t o s t u d y v a r i o u s a s p e c t s o f t h e s e d i f f e r e n c e s . I t was h y p o t h e s i z e d t h a t , f i r s t , s p e c t r a l a n a l y s i s might d e t e c t - d i f f e r e n c e s t h a t were not o b s e r v a b l e by peak a m p l i t u d e and l a t e n c y measurements, and s e c o n d , t h a t asymmetric r e s p o n s e s c o u l d be g e n e r a t e d by s i m p l e s t i m u l i . I f such were t h e c a s e , t h i s would i n d i c a t e t h a t d i f f e r e n c e s between t h e s o - c a l l e d v e r b a l and n o n - v e r b a l p r o c e s s i n g modes .were o n l y s u b c l a s s e s o f a l a r g e r c l a s s i f i c a t i o n . The s p e c t r a l a n a l y s i s o f EPs has f u l f i l l e d a l l o f t h e above p u r p o s e s . I n t e r p r e t a t i o n o f the EP waveform as a s e t o f f r e q u e n c y components has shown t h a t t h e b e h a v i o r o f t h e s e components can be r e l a t e d t o s t i m u l u s m o d a l i t y , c o r t i c a l a r e a , speech dominance and handedness. The meaning o f the components can thus be extended f a r beyond t h e i r o r i g i n a l m a t h e m a t i c a l d e f i n i t i o n s . The b e h a v i o r o f t h e a m p l i t u d e and form c h a r a c t e r i s t i c s o f the EPs has s u g g e s t e d e l e c t r o p h y s i o l o g i c a l c o r r e l a t e s o f t h e s e n s o r y and c o g n i t i v e a s p e c t s o f p e r c e p t i o n . F i n a l l y , t h e q u e s t i o n o f what t y p e o f s t i m u l u s produces h e m i s p h e r i c asymmetries has been shown t o be m i s l e a d i n g . P e r c e p t i o n i n v o l v e s t h e i n t e r a c t i o n o f both t h e o b j e c t and s u b j e c t , o r p e r c e i v e r o f t h a t o b j e c t , and n e i t h e r can be i g n o r e d i n s t u d y i n g t h a t i n t e r a c t i o n . 1.14 BIBLIOGRAPHY A F I F I , A . A . and AZEN, S . P . S t a t i s t i c a l A n a l y s i s : A Computer O r i e n t e d  A p p r o a c h . Academic P r e s s , New Y o r k , 1972, 366 p . ANDERSON, T.W. An I n t r o d u c t i o n t o M u l t i v a r i a t e S t a t i s t i c a l A n a l y s i s . 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