Arts, Faculty of
Philosophy, Department of
DSpace
UBCV
Wright, Robert Leslie Douglas
Master of Arts - MA
University of British Columbia
The hypothesis was presented that speed of visual perception is related to degree of harmonic cortical response which can be elicited by intermittent photic stimulation, through some mechanism of CNS sensitivity to afferent stimulation. This hypothesis was based on Hebb's theory of the neuron assembly, and on Mundy-Castle's observations of the cortical effects of photic stimulation. Experimental examination of this hypothesis was made on a sample of fifty young adult volunteer subjects of both sexes. Recognition threshold was measured by tachistoscopic presentation of ten simple geometric patterns at exposures ranging upwards from ten milliseconds, until correct identifications of all patterns were made. The MRT score (recognition threshold) consisted of the average for the ten scores over an ascending and descending series. The EEG measurements were made in the conventional manner, and scored for variations in frequency pattern by measurement of electronic frequency analysis records. Photic stimulation at frequencies of 6 to 16 c/sec. was provided by an electronic stroboscope during the EEG recording. The results obtained from these procedures did not support the original hypothesis. The obtained correlation of 0.27 was not significant at the .05 level of confidence. An after-the-fact inspection of the data showed that the original hypothesis was applicable to roughly half of the subjects tested, and that these subjects could be selected by means of a third independent variable. The group showing this relationship was characterized by relatively high dominant EEG frequencies, and relatively loose organization of frequency pattern with little or no alpha activity In the normal or low frequency ranges. Due to limitations of method and sample size, this relationship could not be conclusively determined. While a generalization of universal validity was not established, one of restricted application was suggested. The possibility was discussed that speed of visual perception may be related to certain EEG criteria of cortical sensitivity to extrinsic stimulation. Further experiments in this field were proposed.
THE RELATIONSHIP BETWEEN SPEED OP VISUAL PERCEPTION AND FLICKER-INDUCED HARMONIC CORTICAL ACTIVITY by Robert L e s l i e Douglas Wright A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS i n the Department o f Ph i l o s o p h y and Psychology We accept t h i s t h e s i s as conforming t o the standard r e q u i r e d from c a n c i d a t e s f o r fefie degree o f MASTER OF ARTS. Members of the Department of P h i l o s o p h y and Psychology THE UNIVERSITY OF BRITISH COLUMBIA September. 1956 ABSTRACT The hypothesis was pres e n t e d t h a t speed of v i s u a l p e r c e p t i o n i s r e l a t e d t o degree of harmonic c o r t i c a l response which can be e l i c i t e d by i n t e r m i t t e n t p h o t i c s t i m u l a t i o n , through some mechanism o f CNS s e n s i t i v i t y t o a f f e r e n t s t i m u l a t i o n * T h i s h y p o t h e s i s was based on Hebb's t h e o r y of the neuron assemW.y, and on Mundy-Castle's o b s e r v a t i o n s o f the c o r t i c a l e f f e c t s o f p h o t i c s t i m u l a t i o n . Experimental examination o f t h i s h y p o t h e s i s was made on a sample of f i f t y young a d u l t v o l u n t e e r s u b j e c t s of both sexes. R e c o g n i t i o n t h r e s h o l d was measured by t a c h l s t o s c o p l c p r e s e n t a t i o n o f t e n simple geometric p a t t e r n s a t exposures ranging upwards from t e n m i l l i s e c o n d s , u n t i l c o r r e c t i d e n t i f i c a t i o n s o f a l l p a t t e r n s were made. The MRT score ( r e c o g n i t i o n t h r e s h o l d ) c o n s i s t e d o f the average f o r the t e n scores over an ascending and descending s e r i e s . The EEG measurements were made i n the c o n v e n t i o n a l manner, and scored f o r v a r i a t i o n s i n frequency p a t t e r n by measurement o f e l e c t r o n i c frequency a n a l y s i s r e c o r d s . P h o t i c s t i m u l a t i o n at f r e q u e n c i e s o f 6 t o 16 c/sec. was p r o v i d e d by an e l e c t r o n i c stroboscope d u r i n g the EEG r e c o r d i n g . The r e s u l t s obtained from these procedures d i d not support the o r i g i n a l h y p o t h e s i s . The obtained c o r r e l a t i o n of 0.27 was not s i g n i f i c a n t at the .05 l e v e l o f c o n f i d e n c e . 2 An a f t e r - t h e - f a c t i n s p e c t i o n o f the d a t a showed that the o r i g i n a l h ypothesis was a p p l i c a b l e t o r o u g h l y h a l f of the s u b j e c t s t e s t e d , and t h a t these s u b j e c t s c o u l d be s e l e c t e d by means of a t h i r d independent v a r i a b l e . The group showing t h i s r e l a t i o n s h i p was c h a r a c t e r i z e d by r e l a t i v e l y h i g h dominant EEG f r e q u e n c i e s , and r e l a t i v e l y l o o s e o r g a n i z a t i o n of frequency p a t t e r n w i t h l i t t l e or no a l p h a a c t i v i t y I n the normal or low frequency ranges. Due t o l i m i t a t i o n s o f method and sample s i z e , t h i s r e l a t i o n s h i p could not be c o n c l u s i v e l y determined. While a g e n e r a l i z a t i o n o f u n i v e r s a l v a l i d i t y was not e s t a b l i s h e d , one of r e s t r i c t e d a p p l i c a t i o n was s u g g e s t e d . The p o s s i b i l i t y was d i s c u s s e d t h a t speed o f v i s u a l p e r c e p t i o n may be r e l a t e d t o c e r t a i n EEG c r i t e r i a of c o r t i c a l s e n s i t i v i t y t o e x t r i n s i c s t i m u l a t i o n . F u r t h e r experiments I n t h i s f i e l d were proposed. ACKNOWLEDGEMENTS T h i s p r o j e c t c o u l d not have been undertaken or completed without much h e l p from many people* I am p a r t i c u l a r l y g r a t e f u l t o my s u p e r v i s o r s , Dr. M. A. Kennard and Dr. D. T. Kenny, f o r t h e i r u n f a i l i n g i n t e r e s t and support which f r e q u e n t l y surpassed the c a l l o f duty. I should l i k e a l s o t o thank Dean S. N. P. Chant, P r o f e s s o r D. C. G. MacKay, Mr. A. E. Cox, and Squadron Leader Glen Bending, RCAF, f o r t h e i r generous c o o p e r a t i o n I n s e c u r i n g v o l u n t e e r s f o r t h i s experiment. To these u n s e l f i s h men and women themselves, who gave so f r e e l y of t h e i r time and comfort, goes my s i n c e r e s t g r a t i t u d e . Dr. W. C. Gibson and the e n t i r e s t a f f o f the Department of N e u r o l o g i c a l Research were completely u n s e l f i s h i n m a t e r i a l support and encouragement, and i t was a p r i v i l e g e t o be a s s o c i a t e d w i t h them. To a l l t h o s e not named here, but who i n any way gave t h e i r time, I n t e r e s t , or e f f o r t t o t h i s p r o j e c t , my most h e a r t f e l t thanks* T h i s r e s e a r c h was i n i t i a t e d under F e d e r a l Mental H e a l t h Grant Number 609-5-111 (EEG and Psyche-pathology S t u d i e s ) , and completed under Defence Research Board Grant Number 8845-02 (Stu d i e s on F l i c k e r and V i s u a l P e r c e p t i o n ) . i i . TABLE OP CONTENTS CHAPTER PAGE I. PROBLEM, ASSUMPTIONS, AND HYPOTHESIS . . . 1 I I . EXPERIMENTAL DESIGN AND METHOD 5 D e f i n i t i o n of experimental v a r i a b l e s • • 5 R e c o g n i t i o n t h r e s h o l d • • 6 Harmonic index 7 Co n s i d e r a t i o n s o f sampling . 8 S e l e c t i o n of s u b j e c t s 10 Method 14 Apparatus, experimental environment. • 15 P r e p a r a t i o n of su b j e c t 20 RT t e s t i n g procedure 22 EEG r e c o r d i n g procedure 27 A n a l y s i s of r e s u l t s 30 V a l i d i t y and R e l i a b i l i t y 34 I I I . RESULTS 38 IV. DISCUSSION AND CONCLUSIONS 45 SUMMARY 50 REFERENCES AND BIBLIOGRAPHY 52 APPENDIX I. A n a l y s i s of experimental sample • . 58 APPENDIX I I . V o l u n t e e r s e l e c t i o n b lank . . . . 60 APPENDIX I I I . I n d i v i d u a l r e c o r d i n g blank . . . 61 APPENDIX IV. Stimulus p a t t e r n s 62 i i i . LIST OF TABLES TABLE PAGE I. C h a r a c t e r i s t i c s of the t e n experimental and f i v e dummy sti m u l u s p a t t e r n s • • . 18 I I * Means and standard d e v i a t i o n s of r e c o g n i t i o n t h r e s h o l d s o , . . . . . . 37 I I I . The c o r r e l a t i o n s between r e c o g n i t i o n t h r e s h o l d and harmonic index f o r t o t a l and sub-groups • 44 i v . LIST OP FIGURES i FIGURE PAGE 1. D i s t r i b u t i o n s of the MRT v a r i a b l e i n the background sample and EEG sample • • • • 12 2. Layout of the MRT and EEG t e s t i n g rooms. • 16 3. E l e c t r o d e placement and p o l a r leads . . . 28 4. C o r r e l a t i o n s c a t t e r g r a m of the t o t a l EEG group w i t h r e s p e c t t o the e x p e r i -mental v a r i a b l e s of r e c o g n i t i o n t h r e s h o l d and harmonic index • 39 5. C o r r e l a t i o n s c a t t e r g r a m of the h i g h -a l p h a , well-organized-EEG group . . . . 41 6. C o r r e l a t i o n s c a t t e r g r a m of the low-a l p h a , less-organized-EEG group • • • • 41 7. C o r r e l a t i o n s c a t t e r g r a m of the high-frequency-dominance group 43 8. C o r r e l a t i o n s c a t t e r g r a m of the low-frequency-dominance group . • • • • 43 CHAPTER I PROBLEM, ASSUMPTIONS, AND HYPOTHESIS The purpose of t h i s r e s e a r c h problem was t o i n v e s t i g a t e the p o s s i b l e e x i s t e n c e of a r e l a t i o n s h i p between speed of v i s u a l p e r c e p t i o n and s e n s i t i v i t y o f c o r t i c a l responsiveness t o i n t e r m i t t e n t p h o t i c stimu-l a t i o n . The s u g g e s t i o n o f a p o s s i b l e r e l a t i o n s h i p between these two p s y c h o l o g i c a l l y and p h y s i o l o g i c a l l y d e f i n e d f u n c t i o n s f o l l o w e d from Hebb's (3) t h e o r y o f b r a i n f u n c t i o n , and Mundy-Castle's ( 7 , 8) d i s c u s s i o n of the s i g n i f i c a n c e o f harmonics i n the EEGs of p h o t i c a l l y s t i m u l a t e d s u b j e c t s . One of the foundations o f Hebb's n e u r o p h y s i o l o g i c a l t h e o r y o f b e h a v i o r i s the premise t h a t the n e u r o l o g i c a l aspects o f a t t e n t i o n , p e r c e p t i o n , p e r c e p t u a l g e n e r a l -i z a t i o n , and memory must be due t o the development and c h a r a c t e r i s t i c s of l a r g e neuron n e t s , or assemblies, which are formed over a p e r i o d o f time by some growth process a t the synapses, which are p a r t i c u l a r t o c e r t a i n a f f e r e n t sensory d a t a , and which serve some e s t a b l i s h e d e f f e r e n t f u n c t i o n . T h i s growth p r o c e s s , whatever i t s n a t u r e , would be produced and a c c e l e r a t e d by usage o f whatever p a t t e r n s of temporal and s p a t i a l summation, o c c l u s i o n , and f a c i l i t a t i o n , which are s p e c i f i c t o the 2 CNS response t o I t s sensory i n p u t ; the c h a r a c t e r i s t i c s of a s t i m u l u s complex would then produce c e r t a i n c h a r a c t e r i s t i c s of n e u r a l c i r c u i t i n g , which, under the i n f l u e n c e of p r e v i o u s l y e s t a b l i s h e d c i r c u i t s or assemblies, would become the n e u r o l o g i c a l e q u i v a l e n t s of what co u l d otherwise be termed p s y c h o l o g i c a l p r o c e s s e s . T h i s complex c i r c u i t i n g i s d e f i n e d as the phase sequence. The a c t u a l e f f e c t of the p o s t u l a t e d s y n a p t i c growth pro c e s s would amount t o a s e l e c t i v e r a i s i n g or l o w e r i n g of the s y n a p t i c r e s i s t a n c e s , and p e r c e p t i o n by t h i s token would be equatable w i t h c o r t i c a l s e n s i t i z a t i o n i n c e r t a i n complex c o n d i t i o n e d p a t t e r n s . I f t h i s i s the case, the ease o r quickness of a p e r c e p t i v e p r o c e s s ought t o be a f u n c t i o n ( i n p a r t a t l e a s t ) o f s y n a p t i c s e n s i t i v i t y . The phenomenon of p h o t i c d r i v i n g has e x c i t e d much r e s e a r c h and s p e c u l a t i o n s i n c e I t s d i s c o v e r y by A d r i a n and Matthews (1) i n 1934. The s i m p l e s t explan-a t i o n , and the one most commonly accepted, i s that each p h o t i c impulse m a s s i v e l y s t i m u l a t e s the v i s u a l c o r t e x causing most, o r a t l e a s t a l a r g e number, o f t h e c o r t i c a l neurons t o d i s c h a r g e s i m u l t a n e o u s l y i n response. T h i s i s noted i f s t r o b o s c o p i c f l i c k e r at v a r i o u s f r e q u e n c i e s i s coupled w i t h EEG r e c o r d i n g In which the c o r t i c a l d i s c h a r g e rhythms are e l e c t r o n i c a l l y a n a l y s e d f o r frequency p a t t e r n . The d r i v i n g e f f e c t l a o f t e n maximal when the f l i c k e r frequency approximates the normal alpha frequency of t e n c y c l e s p e r second. Harder t o account f o r , however, i s the occurrence i n the EEGs of some f l i c k e r - s t i m u l a t e d i n d i v i d u a l s of harmonic a c t i v i t y or f l i c k e r - i n d u c e d d i s c h a r g e rhythms o c c u r r i n g at a frequency which i s a m u l t i p l e of the r a t e o f s t i m u l a t i o n Mundy-Castle (7,8) suggests t h a t harmonics may be due t o h i g h e r than average c o r t i c a l e x c i t a b i l i t y and q u i c k e r recovery time. I n terms of Hebb's t h e o r y t h i s s h ould mean t h a t the c o r t i c a l response t o sensory s t i m u l a t i o n should occur at a lower t h r e s h o l d , and t h a t the neurons and synapses c o n s t i t u t i n g the assembly p a t t e r n s would have r e l a t i v e l y s h o r t r e f r a c t o r y p e r i o d s . T h e r e f o r e i t would be p o s s i b l e f o r the phase sequence t o occur more r a p i d l y and o f t e n . During f l i c k e r s t i m u l a t i o n , i f the v i s u a l c i r c u i t s can r e c y c l e f a s t enough and are s e n s i t i v e enough, they should be a b l e t o d i s c h a r g e a g a i n i n between each p h o t i c s t i m u l u s ; these synchronous "in-between" d i s c h a r g e s would c o n s t i t u t e harmonic a c t i v i t y i n t h e EEGs of persons w i t h the r e q u i s i t e c o r t i c a l s e n s i t i v i t y . The E xperimental Hypothesis t I f an assembly system r e q u i r e s o n l y a r e l a t i v e l y low s t i m u l a t i o n i n t e n s i t y i n order t o r e a c h t h r e s h o l d ( i e . i f I t i s r e l a t i v e l y 4 e x c i t a b l e ) , and can operate a t a f a s t e r r a t e , t h e n i t s p o t e n t i a l i t y s h o u l d be g r e a t e r t o r e c e i v e f a i n t sensory impulses and a m p l i f y them on the h i g h e r i n t e g r a t i v e l e v e l s which would c o n s t i t u t e the p s y c h o l o g i c a l l e v e l of " p e r c e p t i o n " of the agent o f s t i m u l a t i o n . S p e c i f i c a l l y , i n a group of a p p a r e n t l y normal i n d i v i d u a l s t h e r e should e x i s t a c o r r e l a t i o n between speed of r e c o g n i t i o n o f f a i n t and b r i e f v i s u a l s t i m u l i and amount of harmonic response t o p h o t i c s t i m u l a t i o n , as evidenced by EEG r e c o r d i n g and a n a l y s i s . S u b j e c t s w i t h low " r e c o g n i t i o n t h r e s h o l d s " should e x h i b i t a marked harmonic response t o f l i c k e r , w h i l e those w i t h h i g h t h r e s h o l d s should show l i t t l e o r no such response. With r e s p e c t t o t h i s h y p o t h e s i s three assumptions are n e c e s s a r y : (1) t h a t the EEG Is c o r r e l a t e d w i t h n e u r a l f i r i n g , (2) t h a t s t r e n g t h of a v i s u a l s t i m u l u s a t an i n t e n s i t y low enough t o a v o i d after-image f o r m a t i o n i s a f u n c t i o n of exposure time, and (3) t h a t under the experimental c o n d i t i o n s the r e c o g n i t i o n t h r e s h o l d s f o r the s t i m u l i are not a f u n c t i o n of f a c t o r s o p e r a t i n g a t p r e - c o r t i c a l l e v e l s . I t i s not, however, suggested t h a t an experimental c o r r e l a t i o n between two such measurements w i l l n e c e s s a r i l y i n d i c a t e a pure o r simple n e u r o p h y s i o l o g i c a l r e l a t i o n s h i p between them. Since these assumptions seem reasonable, t h i s experiment was under-taken i n an e f f o r t t o t e s t the above-stated h y p o t h e s i s . CHAPTER I I EXPERIMENTAL DESIGN AND METHOD The hypothesis as s t a t e d i m p l i e s a c o r r e l a t i o n between the q u a n t i f i e d r e s u l t s of two t e s t procedures; the d e s i g n o f the experiment f o l l o w e d from the h y p o t h e s i s , a number o f s u b j e c t s b e i n g s e l e c t e d , put through the two procedures, and compared i n a two-way d i s t r i b u t i o n o r scattergram. The I n f o r m a t i o n thus summarized was then s u b j e c t t o a n a l y s i s by c o r r e l a t i o n t e c h n i q u e s . The problem f e l l i n t o three stages: (1) d e f i n i t i o n of the e x p e r i m e n t a l v a r i a b l e s and f o r m u l a t i o n of procedures t o measure them; (2) d e t e r m i n a t i o n of the s t a t i s t i c a l nature of one of these v a r i a b l e s over a f a i r l y l a r g e sample of the experimental p o p u l a t i o n ; and (3) s e l e c t i o n of s u b j e c t s f o r the a c t u a l t e s t i n g of the h y p o t h e s i s . The second stage was c o n s i d e r e d n e c e s s a r y t o ensure t h a t the l a t e r experimental sample be drawn so as t o r e p r e s e n t the f u l l range of the d i s t r i b u t i o n of the v a r i a b l e i n q u e s t i o n , namely speed of p e r c e p t i o n . I . DEFINITION OF EXPERIMENTAL VARIABLES Speed of v i s u a l p e r c e p t i o n , or more o p e r a t i o n a l l y , t h r e s h o l d of r e c o g n i t i o n a l o n g a time a x i s , was s e l e c t e d as an experimental v a r i a b l e f o r s e v e r a l reasons. F i r s t , i n man v i s u a l p e r c e p t i o n r e p r e s e n t s one of the most 6 important e x t e r o c e p t i v e m o d a l i t i e s . Second, speed o f p e r c e p t i o n of a d e f i n e d s t i m u l u s o b j e c t i s a q u a n t i t a t i v e r a t h e r than d e s c r i p t i v e u n i t of i n f o r m a t i o n , and t h e r e -f o r e i s more e a s i l y s u b j e c t t o s t a t i s t i c a l treatment. T h i r d , much work has a l r e a d y been done, and a c e r t a i n amount Is a l r e a d y known, about the i n f l u e n c e of v i s u a l f u n c t i o n s on the EEG. T h i s i s not so t r u e of the other e x t e r o c e p t i v e m o d a l i t i e s . D e f i n i t i o n of R e c o g n i t i o n T h r e s h o l d R e c o g n i t i o n t h r e s h o l d (RT) i s d e f i n e d i n t h i s experiment as the l o g a r i t h m of the s h o r t e s t mean exposure time i n m i l l i s e c o n d s , i n an ascending and descending s e r i e s , a t which the s u b j e c t i s a b l e t o make a c o r r e c t i d e n t i f i c a t i o n of one s t i m u l u s o b j e c t . In p r a c t i c e a number of such o b j e c t s were presented i n the form o f line-drawn geometric p a t t e r n s p r o j e c t e d t a c h i s t o s c o p i c a l l y upon an i l l u m i n a t e d s c r e e n , w i t h the p a t t e r n - s c r e e n r a t i o o f i n t e n s i t y a d j u s t e d t o a constant l e v e l at which exposure time was the c r u c i a l f a c t o r f o r c o r r e c t i d e n t i f i c a t i o n . The Mean R e c o g n i t i o n T h r e s h o l d (MRT) i s the average of the RT's obtained f o r a l l of the p a t t e r n s (ten) d u r i n g one complete t e s t run, and c o n s t i t u t e s the q u a n t i t a t i v e Index of p e r c e p t i o n speed f o r any g i v e n s u b j e c t . 7 D e f i n i t i o n o f Harmonic Index The presence of harmonics i n the EEG re c o r d s of p h o t i c a l l y s t i m u l a t e d s u b j e c t s was taken t o be an index of c o r t i c a l s e n s i t i v i t y and n e u r a l r e c y c l i n g time; and t h e r e f o r e an o b j e c t i v e q u a n t i t a t i v e measure of t h i s f u n c t i o n i n each s u b j e c t . Harmonic a c t i v i t y has a l r e a d y been d e f i n e d (Chap. I ) ; the Harmonic Index (HX) may be d e f i n e d as the a r i t h m e t i c t o t a l maximum i n c r e a s e i n hei g h t ( m i l l i m e t r e s ) of the frequency a n a l y s e r t r a c i n g of EEG f r e q u e n c i e s corresponding t o the second, t h i r d , and f o u r t h harmonics of each s t r o b o s c o p i c f l i c k e r r a t e t o be a p p l i e d t o the s u b j e c t , under c o n t r o l l e d and constant c o n d i t i o n s o f r e c o r d i n g and s t i m u l a t i o n . The b a s e l i n e f o r t h i s measurement i s the he i g h t (mm.) of the frequency a n a l y s e r t r a c i n g obtained i n e x a c t l y the same f a s h i o n but under r e s t i n g c o n d i t i o n s p r i o r t o the commencement of the f l i c k e r . The nu m e r i c a l v a l u e o b t a i n e d as the harmonic index f o r each s u b j e c t was used without f u r t h e r d e r i v a t i o n as the q u a n t i t a t i v e index o r c o r t i c a l s e n s i t i v i t y f o r t h a t s u b j e c t . The d e f i n i t i o n s of these v a r i a b l e s f o l l o w as much from the methods of t h e i r measurement as from t h e i r t h e o r e t i c a l f o u n d a t i o n , and t h e r e f o r e should not be i n t e r p r e t e d a p a r t from the procedures t o be d e s c r i b e d . I I . CONSIDERATIONS OP SAMPLING 8 The sampling problem i n t h i s experiment was t o ensure homogeneity of the t e s t e d group w i t h r e g a r d t o c h a r a c t e r i s t i c s which might a f f e c t the v a l i d i t y o f the measurements and b a s i s f o r comparison o f the r e s u l t s . Important i n t h i s r e s p e c t were the f o l l o w i n g f a c t o r s , which served as a b a s i s f o r s e l e c t i o n of the experimental group: 1. Age of s u b j e c t (17 t o 30, i n c l u s i v e ) 2. Sex of s u b j e c t 3. V i s u a l a c u i t y without g l a s s e s , or 4. V i s u a l a c u i t y w i t h g l a s s e s , i f worn h a b i t u a l l y and d u r i n g experiment 5. H i s t o r y of eye t r o u b l e 6. H i s t o r y of severe, i n t r a c t i b l e r e a d i n g t r o u b l e 7. Rough estimate of i n t e l l e c t u a l s t a t u s 8. Handedness ( i n d i c a t i o n of c e r e b r a l dominance) 9. N e u r o l o g i c a l h i s t o r y i f any. Upper and lower age l i m i t s were imposed because o f the changing EEG c h a r a c t e r i s t i c s which become important above and below these l i m i t s (Kennard, 5; Gibbs and Gibbs, 2 ) ; the sex d i s t i n c t i o n was made more as a matter of p r i n c i p l e than f o r any supposed sex d i f f e r e n c e , a l t h o u g h the alpha f r e q u e n c i e s i n women tend t o r u n s l i g h t l y h i g h e r than I n men (Kennard, 5 ) . The t h i r d , f o u r t h , and f i f t h 1 c r i t e r i a above were intended t o c o n t r o l any o p t i c a l f a c t o r s which might a d v e r s e l y a f f e c t the s u b j e c t s ' performance. The l a s t f o u r were intended t o exclude extreme cases of c e r e b r a l a bnormality, a l t h o u g h 9 I n p r a c t i c e no d e c i s i o n s were nec e s s a r y i n t h i s r egard* Other f a c t o r s which were Important d u r i n g r e c o r d i n g were the f o l l o w i n g : 1* Cooperation of s u b j e c t 2* Drowsiness, f a t i g u e , boredom, e t c * 3* Uneasiness or f r i g h t r e g a r d i n g t h e s i t u a t i o n 4 * P h y s i c a l comfort 5* P h a r m a c o l o g i c a l f a c t o r s , as from a s p i r i n , which would i n t r o d u c e b o t h sensory and EEG v a r i a t i o n s 6* M o t i v a t i o n , and r e a l i z a t i o n of the aim and purpose of the t e s t i n g . T h i s l a t t e r f a c t o r was p a r t i c u l a r l y important, and i t was s t r e s s e d t o each su b j e c t thajs t h e r e was no hidden purpose behind the t e s t i n g . I n t h i s way i t was hoped t o ensure maximum v o l u n t a r y performance and some c o n t r o l of p e r s o n a l i t y f a c t o r s which would s p u r i o u s l y a l t e r t h e scores of the s u b j e c t s . The v o l u n t e e r n a t u r e of s e l e c t i o n was a l s o i n s t r u m e n t a l i n e l i m i n a t i n g u n w i l l i n g o r uncooperative i n d i v i d u a l s * The g e n e r a l i t y of r e s u l t s obtained from such a sample i s l i m i t e d t o a p o p u l a t i o n showing the same c h a r a c t e r i s t i c s . No assumption i s made, however, about the " r e p r e s e n t a t i v e n e s s " of t h i s sample t o any o t h e r group. S i n c e the experimental h y p o t h e s i s i s one o f c e r e b r a l f u n c t i o n , t h e r e should be no t h e o r e t i c a l d i f f i c u l t y i n g e n e r a l i z i n g i t from one group t o any o t h e r group o f the same s p e c i e s ; t h i s i s of course open t o experiment i n any number of s e p a r a t e l y d e f i n e d and s e l e c t e d groups* 10 I I I . SELECTION OP SUBJECTS The f i r s t step i n the a c t u a l procedure o f the experiment was t o determine, over as l a r g e a sample as p o s s i b l e , the s t a t i s t i c a l d i s t r i b u t i o n o f the MRT v a r i a b l e . T h i s was done on a group of 102 cases, s e l e c t e d a c c o r d i n g t o the c r i t e r i a g i v e n i n Sec. I I above. The f i r s t - r u n MRT s c o r e s of a l l s u b j e c t s t e s t e d make up t h i s d i s t r i b u t i o n , r e g a r d l e s s o f whether or not the s u b j e c t was g i v e n a second run l a t e r . Twenty-five i n d i v i d u a l s were t e s t e d only once, immediately p r i o r t o t h e i r EEG- measurements; the sco r e s of these were t h e r e f o r e i n c l u d e d I n both the background sample (N=102) and the a c t u a l experimental sample (N=50). Twenty-five i n d i v i d u a l s were t e s t e d t w i c e ; t h e i r f i r s t and second run scores were assigned t o the background and e x p e r i -mental d i s t r i b u t i o n s r e s p e c t i v e l y . F i f t y - t w o i n d i v i d u a l s were t e s t e d only once f o r the background sample, and were not g i v e n EEGs. A t o t a l of f i f t y v o l u n t e e r s u b j e c t s , e q u a l l y d i v i d e d between the sexes, made up the experimental sample. T h i s d i s t r i b u t i o n was made up o f the MRT s c o r e s which were obtained immediately p r i o r t o EEG r u n s . The tw e n t y - f i v e s u b j e c t s who were t e s t e d twice f o r MRT were s e l e c t e d i n t o the experimental group on the b a s i s o f 11 s c o r e s o btained on the f i r s t run; i t was hoped t h a t d e l i b e r a t e s e l e c t i o n of cases a t the extremes of the continuum of MET v a l u e s would ensure a wider range o f t h i s v a r i a b l e and a more r e c t a n g u l a r l o a d i n g o f cases throughout t h i s range i n the experimental group t h a n would be. expected i f s e l e c t i o n had been random. Such a method of s e l e c t i o n meant, however, t h a t i n the l a t e r a n a l y s i s of d a t a techniques of product-moment c o r r e l a t i o n c ould not be employed without r e s u l t i n g i n s p u r i o u s product-moment c o e f f i c i e n t s . F o r t h i s reason, rank o r d e r i n g methods were used i n s t e a d t o reduce but not e l i m i n a t e such b i a s . Twelve s u b j e c t s from the low-t h r e s h o l d end ( i e . MRT ^1.50) and seven from the h i g h -t h r e s h o l d end ( i e . MRT ^2.00) comprised the b i a s e d -s e l e c t i o n group. S i x s u b j e c t s w i t h s c o r e s around the mean ( l e . MRT from 1.50 t o 1.69) were a l s o i n t h i s group. The t w e n t y - f i v e s u b j e c t s who were t e s t e d o n l y once, immediately p r i o r t o t h e i r EEG r e c o r d i n g , were of course s e l e c t e d without r e f e r e n c e to t h e i r MRT, s i n c e t h i s was not known at the time of s e l e c t i o n . F i g u r e 1 shows the r e l a t i o n s h i p between the MRT s c o r e s of the background and EEG (experimental) samples. Subjects were drawn from a number of s o u r c e s . The background sample of 102 cases comprised: (1) s i x t y -s i x students from three f i r s t - y e a r Psychology c l a s s e s , 12 1.1 1.2 1.3 l.W 1.5 1.6 1.7 1.8 1.9. 2.0 2.1 2.2 2.3 2 A 2.5 2.6 2.7 2.8 2.9 MEAN RECOGNITION THRESHOLD F i g u r e 1 . D i s t r i b u t i o n o f the MRT v a r i a b l e i n the background and EEG samples. 13 (2) f i f t e e n e n l i s t e d a i r f o r c e p e r s o n n e l , (3) f o u r t e e n l a b o r a t o r y p e r s o n n e l , and (4) seven m i s c e l l a n e o u s v o l u n t e e r s . Ages ranged from seventeen t o t h i r t y y e a r s , as f o l l o w s : Male Female T o t a l 17 t o 20 i n c l u s i v e 22 37 59 21 t o 25 i n c l u s i v e 22 11 33 26 t o 30 i n c l u s i v e _6 _4 10 T o t a l 50 52 102 The experimental sample of f i f t y was drawn from the above sample as f o l l o w s : (1) eig h t e e n Psychology s t u d e n t s , (2) f i f t e e n a i r f o r c e p e r s o n n e l , (3) t h i r t e e n l a b o r a t o r y p e r s o n n e l , comprising medical s t u d e n t s , t e c h n i c i a n s , c l e r i c a l s t a f f , s e n i o r s t a f f members, and (4) f o u r m i s c e l l a n e o u s . Ages were d i s t r i b u t e d as f o l l o w s : Male Female T o t a l 17 t o 20 i n c l u s i v e 5 12 17 21 t o 25 i n c l u s i v e 15 9 24 26 t o 30 i n c l u s i v e __5 __4 _9 T o t a l 25 25 50 A complete a n a l y s i s of t h i s experimental s ample i s g i v e n i n Appendix I* 14 IV. METHOD The f i n a l t e s t i n g of s u b j e c t s i n the experimental group f e l l i n t o f i v e main s t e p s , the t o t a l of which occupied about one and one-half hours p e r s u b j e c t . 1. C a l i b r a t i o n and adjustment of apparatus 2. P r e p a r a t i o n and i n t r o d u c t i o n of the s u b j e c t t o the t e s t i n g s i t u a t i o n 3. Determination of MRT 4. Recording of " r e s t i n g " EEG 5. Recording of EEG under c o n d i t i o n s of p h o t i c s t i m u l a t i o n . The procedure f o l l o w e d the order g i v e n above; t h i s was done i n o r d e r t o c o n t r o l the p o s s i b i l i t y of experimenter b i a s , which could i n f l u e n c e i n many ways the s u b j e c t s ' performance on the procedure f o r MRT measurement. T h i s was t r u e e q u a l l y f o r a d m i n i s t r a t i o n and s c o r i n g , and foreknowledge of whether the s u b j e c t "should" (by the experimental h y p o t h e s i s ) s c o r e h i g h or low on t h e t h r e s h o l d continuum might w e l l have g r a v e l y a f f e c t e d the v a l i d i t y of the measurement. However, s i n c e the E E G - f l i c k e r procedure f o l l o w e d the a d m i n i s t r a t i o n and s c o r i n g of the MRT s e r i e s , such foreknowledge was o f course p r e c l u d e d . Experimenter b i a s would not of course a f f e c t the E E G - f l i c k e r procedure i n any way t h a t c o u l d not be c o n t r o l l e d by f o l l o w i n g formal r u l e s i n method, I n t e r p r e t a t i o n , and s c o r i n g . 15 Apparatus and E x p e r i m e n t a l Environment The t e s t procedures took p l a c e i n a s m a l l room which was l i g h t p r o o f and reasonably w e l l v e n t i l a t e d ; i l l u m i n a t i o n was constant, c o n s i s t i n g only o f the l i g h t i n c i d e n t upon and r e f l e c t e d from the s c r e e n , and the a i r was c o n s t a n t l y c i r c u l a t e d and renewed by f o r c e d -draught v e n t i l a t i o n . The EEG r e c o r d i n g Instruments were used from an a d j o i n i n g room which communicated by way of a door and s l i d i n g g l a s s window w i t h the t e s t i n g -room. P r i v a c y was a s s u r e d at a l l times, although o u t s i d e n o i s e and d i s t u r b a n c e s were c l e a r l y a u d i b l e through the t h i n w a l l s . The h i g h c e i l i n g and l i g h t w a l l s o f the room were intended to minimize any f e e l i n g o f uneasiness or o p p r e s s i o n from the cramped space; a l s o , d i s t r a c t i n g o b j e c t s or instruments which were p o t e n t i a l l y d i s t u r b i n g were kept t o a minimum. F u r n i t u r e i n c l u d e d a comfort-able c h a i r f o r the s u b j e c t , s t o o l f o r t h e t e s t e r , p r o j e c t i o n s c r e e n , workbench, and t a b l e . The l a y o u t of the t e s t i n g rooms Is shown i n F i g u r e 2. T e s t i n g equipment c o n s i s t e d of the f o l l o w i n g i n s t r u m e n t s : 1. Automatic 2 x 2 i n c h s l i d e p r o j e c t o r ( L a B e l l e "75") equipped w i t h 500 watt lamp and t a c h i s t o s c o p i c s h u t t e r . T h i s was c o n t r o l l a b l e by means o f a cable which changed the s l i d e s and operated the s h u t t e r . Shutter-speed and b r i g h t n e s s c o n t r o l were achieved by l e v e r s on the machine. F i g u r e 2. Layout of the P e r c e p t i o n Speed and EEG t e s t i n g rooms* 17 2* P r o j e c t i o n s c r e e n two f e e t square, c o n s i s t i n g of a non-glossy white s u r f a c e surrounded by a b l a c k b o r d e r . L i g h t i n g on the s c r e e n was p r o v i d e d by two s i x t y - w a t t lamps i n r e f l e c t o r s , which p r o v i d e d an i l l u m i n a t i o n of t e n f o o t -candles a t the centre o f the s c r e e n . Exact even-ness of l i g h t i n g could not be a c h i e v e d , but i t s g r a d i e n t and i n t e n s i t y were kept constant, being checked p e r i o d i c a l l y by means of a p h o t o c e l l meter. A f i x a t i o n - s p o t about two m i l i m e t r e s i n diameter was l o c a t e d i n the cen t r e o f t h e s c r e e n . 3. Grass Model I I I - D e l e c t r o e n c e p h a l o g r a p h machine, connected t o the s u b j e c t on e i g h t r e c o r d i n g channels by means of i n t r a c u t a n e o u s needle e l e c t r o d e s l e a d i n g through f l e x i b l e low-r e s i s t a n c e wires t o an e l e c t r o d e box behind the s u b j e c t ' s head, and thence t o the machine* A m p l i f i c a t i o n of the s i g n a l depended on the c h a r a c t e r i s t i c s o f the i n d i v i d u a l s u b j e c t ' s waves, but g e n e r a l l y i t was such t h a t an ab s o l u t e pen e x c u r s i o n w i t h a f i f t y m i c r o v o l t f o r c e covered about seven m i l i m e t r e s . 4. O f f n e r Type 830 E l e c t r o n i c Frequency A n a l y s e r , connected w i t h the EEG machine i n such a way t h a t an a n a l y s i s c o u l d be obtained from any of the eig h t EEG channels. The a m p l i f i e r g a i n of t h i s machine was constant at a s e t t i n g of "7". 5. Grass P h o t o - S t i m u l a t o r Model PS-1. T h i s instrument i s a s p e c i a l l y designed e l e c t r o n i c stroboscope capable o f d e l i v e r i n g a blue-white f l a s h o f t e n m i l l s e c o n d s d u r a t i o n a t any r a t e between one and one hundred p e r second, at f i v e d i f f e r e n t l e v e l s o f I n t e n s i t y . 6. F i f t e e n p h o t o g r a p h i c a l l y produced stimulus s l i d e s , which when p r o j e c t e d showed as w h i t e - l i n e p a t t e r n s on the s l i g h t l y d a r k e r s c r e e n . F i v e of these were "dummy" p a t t e r n s ; t e n were used i n the a c t u a l measurements. The p a r t i c u l a r s of t h e s e s t i m u l u s p a t t e r n s are g i v e n i n Tab l e I and Appendix IV* 18 TABLE I CHARACTERISTICS OP THE TEN EXPERIMENTAL AND FIVE DUMMY STIMULUS PATTERNS Number Code " P l u s " response Form c h a r a c t e r i s t i c s 1 SQUARE square (complete) 4 s i d e s , 4 a n g l e s , r a d i a l symmetry, l i n e s j o i n e d 2 CRES c r e s c e n t , h a l f moon 2 curves, 2 a n g l e s , acute a n g l e s , b i l a t e r a l symmetry 3 OP SQ square (broken or open) as i n ( 1 ) , w i t h upper l e f t c o r n e r m i s s i n g 4 CIRCLE c i r c l e ( s i n g l e ) s y m m e t r i c a l l y p e r i p h e r a l t o f i x a t i o n p o i n t 5 ARROW arrowhead 4 s i d e s , 4 an g l e s , l i n e s s l a n t e d , angles and l e n g t h s s i m i l a r t o t r i a n g l e ( D - l l ) 6 PLUS p l u s s i g n , cross 2 equal l i n e s v e r t i c a l and h o r i z o n t a l , i n t e r s e c t at f i x a t i o n p o i n t 7 DONUT 2 c i r c l e s , double ( c o n c e n t r i c ) c i r c l e 2 c o n c e n t r i c c i r c l e s , d i a . r a t i o s 1 t o 3, around f i x a t i o n p o i n t 8 OCTA nut (as on b o l t ) , double octagon as i n ( 7 ) , o n l y r a d i a l 8-side symmetry, c o n c e n t r i c around f i x a t i o n p o i n t 9 STAR broken X X i n t e r s e c t . 2 equal l i n e s w i t h p o i n t of i n t e r s e c t m i s s i n g ( at f i x a t i o n p o i n t ) 10 RECTANG r e c t a n g l e complete r e c t a n g l e , l/w r a t i o 4:1, b i l a t e r a l symmetry (continued on next page. • • 19 TABLE I (continued) Number Code " P l u s " response Form c h a r a c t e r i s t i c s D - l l (dummy) t r i a n g l e D-12 (dummy) open c i r c l e D-13 (dummy) j e l l y b e a n D-14 (dummy) diamond D-15 (dummy) complete X equal t r i a n g l e , apex up, d i f f e r s from ARROW (5) onl y i n f l a t base app roximate s CIRCLE ( 4 ) , 30° a r c open at 10 o»clock p o s i t i o n approximates upper a r c of CRES ( 2 ) , bottom convex, b i l a t e r a l symmetry as on p l a y i n g c a r d ; angles and l e n g t h s approximate STAR (9) and ARROW (5) approximates STAR ( 9 ) , 45° s l a n t s , i n t e r s e c t i o n a t f i x a t i o n p o i n t NOTE: The dummy p a t t e r n s D - l l t o D-15 were p r e s e n t e d t o the s u b j e c t s only d u r i n g the p r e l i m i n a r y exposure and naming of the p a t t e r n s , f o r t h e purpose of m i n i m i z i n g c o r r e c t i d e n t i f i c a t i o n from reduced cues. The p a t t e r n s themselves a r e i l l u s t r a t e d In Appendix IV. The importance o f equipment constancy and r e l i a -b i l i t y was not unde r - r a t e d . Standard replacements f o r the instruments (such items as l i g h t b u l b s and vacuum tubes) were used when necessary* Lens s u r f a c e s were cleaned p e r i o d i c a l l y , as were the c o v e r g l a s s e s of the s l i d e s . Each MRT d e t e r m i n a t i o n was s t a r t e d w i t h a c o l d p r o j e c t o r , i n order t o render constant the e f f e c t of temperature on the t a c h i s t o s c o p e speeds. Regular checks were c a r r i e d out on the EEG machinery t o ensure the constancy of i t s response c h a r a c t e r i s t i c s . I t was not found p r a c t i c a l t o employ a v o l t a g e - r e g u l a t o r f o r the p r o j e c t o r and background i l l u m i n a t i o n lamps, but s i n c e b o t h l i g h t sources drew from the same c i r c u i t , and s i n c e i t was u n l i k e l y that any e x t e n s i v e s h i f t i n l i n e -v o l t a g e would p e r s i s t d u r i n g the whole of an MRT s e r i e s , t h i s was not considered n e c e s s a r y . P r e p a r a t i o n of the Subject F o r some of the s u b j e c t s , coming i n t o the t e s t i n g s i t u a t i o n was an a n x i e t y - p r o d u c i n g a f f a i r which had t h e r e f o r e t o be c o n t r o l l e d i n o r d e r t o permit optimum t e s t performance. Since t o t a l c o n c e n t r a t i o n of the su b j e c t s on the MRT procedure was of c r u c i a l importance t o the v a l i d i t y o f t h i s measurement, the m o t i v a t i o n and emotional s t a t e of each s u b j e c t was con s i d e r e d to be at l e a s t as important as the more p e r i p h e r a l i n d i c e s of experimental c o n t r o l . The f i r s t s tep I n p r e p a r a t i o n o f the s u b j e c t was a complete and f r a n k d e s c r i p t i o n o f the procedures, and the aims and purpose of the experiment. In many cases the s u b j e c t s were a l r e a d y g e n e r a l l y f a m i l i a - r w i t h the l a t t e r , from the i n t r o d u c t i o n on the s e l e c t i o n b lanks which they had f i l l e d i n a t the time they v o l u n t e e r e d . T h i s form i s i l l u s t r a t e d I n Appendix I I . The p r e l i m i n a r y d i s c u s s i o n w i t h the s u b j e c t was couched i n a p p r o p r i a t e n o n t e c h n i c a l terms, and re f e r e n c e t o the more d i s t u r b i n g a s p e c t s of the procedure, such as the p o s s i b l e d i s c o m f o r t s o f s t r o b o s c o p i c f l i c k e r , was avoided u n t i l a f t e r the MRT s e r i e s was completed. In most cases the subj e c t r e a c t e d w e l l t o t h i s approach* making a s i n c e r e e f f o r t t o do w e l l on the t e s t s . Care was t a k e n t o p o i n t out the innoccuous n o n - c l i n i c a l nature o f the t e s t i n g , and s p e c i a l r e f e r e n c e was made t o t he f a c t of i n d i v i d u a l s u b j e c t anonymity. G e n e r a l l y , i t i s probable t h a t t h i s method of i n t r o d u c t i o n ensured a h i g h degree o f m o t i v a t i o n and c o o p e r a t i o n from most s u b j e c t s . During o r a f t e r t h i s i n f o r m a l I n t r o d u c t i o n , c e r t a i n i n f o r m a t i o n was requested from each s u b j e c t ( i f such had not been ob t a i n e d i n a p r i o r t e s t s e s s i o n ) . Data thus obtained I n c l u d e d : (1) p e r s o n a l i n f o r m a t i o n , address, telephone number, e t c . f o r use i n r e l o c a t i n g s u b j e c t f o r f u r t h e r t e s t i n g ; (2) i n q u i r y r e g a r d i n g p r e s e n t h e a l t h , f a t i g u e s t a t e , e y e s t r a i n , and other p h y s i o l o g i c a l f a c t o r s which c o u l d e f f e c t t r a n s i t o r y changes i n t e s t performance; (3) p a r t i c u l a r s o f eye-g l a s s e s i f these were worn; and (4) I n q u i r y as t o readin g h a b i t s , w i t h s p e c i a l r e f e r e n c e to any past or pre s e n t f u n c t i o n a l r e a d i n g d i f f i c u l t y . The s u b j e c t ' s v i s u a l a c u i t y was t e s t e d on the S n e l l e n c h a r t , w i t h g l a s s e s i f these were t o be worn during the t e s t i n g . The date and time o f day were noted a l s o . A l l o f t h i s i n f o r m a t i o n , as w e l l as the t e s t s c o r i n g , was entered on a r e c o r d i n g b lank designed f o r the purpose. T h i s b l ank i s shown i n Appendix I I I . Only f o l l o w i n g these above p o i n t s o f procedure were the a c t u a l t e s t i n g procedures commenced. R e c o g n i t i o n T h r e s h o l d T e s t i n g Procedure The s u b j e c t was made comfortable i n the c h a i r at a d i s t a n c e o f e i g h t f e e t from the s c r e e n , and i n s t r u c t e d as f o l l o w s : I n the middle of the scree n you w i l l see a b l a c k d o t . I t i s v e r y important f o r you t o keep your v i s i o n centred on t h i s dot a l l through the experiment, because t h i s w i l l h e l p you t o do b e t t e r on the t e s t . ( P a t t e r n 1—SQUARE p r o j e c t e d on sc r e e n at f u l l i n t e n s i t y , s e t t i n g one on t a c h i s t o s c o p e ) Now you see a square on the screen; t h i s i s one o f the p a t t e r n s 23 which w i l l he f l a s h e d a t you, l i k e t h i s • • • . • (demonstration) • . • • f o r a v e r y b r i e f i n s t a n t , ( p r o j e c t i o n of sample p a t t e r n resumed) During the t e s t , however, i t w i l l not be as i n t e n s e as t h i s , but very dim--perhaps l i k e t h i s . (Reduction of i n t e n s i t y u n t i l p a t t e r n j u s t n o t i c e a b l e a g a i n s t i l l u m i n a t e d s c r e e n ; then r e s t o r e d t o f u l l b r i g h t -ness) You can see t h a t such a s h o r t exposure a t such a low i n t e n s i t y w i l l make I t q u i t e d i f f i c u l t t o see the p a t t e r n . What I want t o f i n d out i s how s h o r t a f l a s h I can g i v e you, i n which you w i l l s t i l l be a b l e t o r e c o g n i z e the shape of the p a t t e r n . Do you get the idea? ( F u r t h e r a m p l i f i c a t i o n i f n e c e s s a r y ) . The s e r i e s of f i g u r e s was then p r o j e c t e d on the s c r e e n a t f u l l i n t e n s i t y f o r f i v e seconds each, t o permit the s u b j e c t t o see the p a t t e r n s and l e a r n t h e i r names, which were spoken by E d u r i n g the f i v e second exposure. Order of p r e s e n t a t i o n was constant d u r i n g t h i s p r e -exposure f o r a l l Ss. F i v e dummy p a t t e r n s were p r o j e c t e d , a l s o i n constant o r d e r , f o l l o w i n g the t e n e x p e r i m e n t a l ones. N a t u r a l l y the S d i d not know of t h i s d i s t i n c t i o n * Names o f the p a t t e r n s were those most p o p u l a r l y a s s i g n e d t o them d u r i n g the p i l o t study runs. The s u b j e c t was then f u r t h e r i n s t r u c t e d : Now what we w i l l do i s run through t h i s s e r i e s of p a t t e r n s a number of times. Each run w i l l be at a d i f f e r e n t s h u t t e r - s p e e d or exposure time from the run b e f o r e . The f i r s t s e r i e s w i l l be shown at a v e r y f a s t s h u t t e r speed; the p a t t e r n s w i l l be on the s c r e e n f o r a v e r y s h o r t time t o b e g i n w i t h , so « don't be w o r r i e d i f you don't see anything a t f i r s t . S u c c e s s i v e runs w i l l be taken a t l o n g e r and l o n g e r exposure times, u n t i l you c o r r e c t l y I d e n t i f y each -p a t t e r n i n the s e r i e s . Then we w i l l s t a r t making the exposure times s h o r t e r a g a i n u n t i l you are not a b l e to i d e n t i f y any of the p a t t e r n s . I n t h i s way we w i l l be a b l e t o t e l l the s h o r t e s t exposure time you need i n o r d e r t o r e c o g n i z e each p a t t e r n . 24 I t w i l l be hard t o see these p a t t e r n s , as you know* You can he l p y o u r s e l f t o see them b e t t e r i f you l e t your eyes r e l a x and s t a y c e n t r e d on the dot a l l the time. Now b e f o r e each time I f l a s h a p a t t e r n I w i l l say "ready" so t h a t you can concentrate f o r t h a t i n s t a n t on the screen* Do you have any quest i o n s about t h i s procedure? Any questions r a i s e d a t t h i s p o i n t were t h e n d e a l t w i t h , and v a r i o u s p o i n t s I n the i n s t r u c t i o n s were r e i t e r a t e d and perhaps a m p l i f i e d i f necessa r y . The t a c h i s t o s c o p e c o n t r o l was then set t o an I n t e n s i t y s e t t i n g o f "6", a t which the p a t t e r n i n t e n s i t y was twice t h a t o f i t s t h r e s h o l d i n t e n s i t y a g a i n s t the background i l l u m i n a t i o n on the s c r e e n . At t h i s s e t t i n g i t was found t h a t almost no c o r r e c t r e c o g n i t i o n was p o s s i b l e at t e n m i l l i s e c o n d s exposure, w h i l e at f i v e hundred t o one thousand m i l l i s e c o n d s (the oth e r extreme o f the t a c h i s t o s c o p e range), c o r r e c t i d e n t i f i c a t i o n of a l l p a t t e r n s was q u i t e common f o r most o f the p i l o t study sample. I n order t o e l i m i n a t e b a c k l a s h i n the t a c h i s t o -scope diaphragm, t h i s s e t t i n g was always made from the same end of the s c a l e . S t a r t i n g at an exposure time o f t e n m i l l i s e c o n d s , or one-hundredth o f a second, s u c c e s s i v e runs w i t h the t e n s l i d e s were made at exposures o f 10, 20, 40, 100, 200, 500, and 1000 m i l i s e c o n d s d u r a t i o n , u n t i l t h e S managed t o i d e n t i f y a l l of the p a t t e r n s c o r r e c t l y . T h i s u s u a l l y o c c u r r e d b e f o r e the l a t t e r two exposure speeds 25 were reached. Then the s e r i e s was repeated, hut w i t h the s u c c e s s i v e runs a t d e c r e a s i n g exposure times u n t i l no c o r r e c t i d e n t i f i c a t i o n s were made. T h i s gave an ascending and descending t h r e s h o l d v a l u e f o r each p a t t e r n . Order of P a t t e r n P r e s e n t a t i o n . P a t t e r n s were p r e s e n t e d i n prearranged order f o r a l l o f the runs so as t o f a c i l i t a t e s c o r i n g . T h i s was constant f o r a l l Ss, as f o l l o w s : I n o r d e r from 1 t o 10, u n t i l more than two c o r r e c t responses scored on run. (b) I n r e v e r s e o r d e r from 10 t o 1. (c) 1 3 5 7 9 10 8 6 4 2 (d) 10 8 6 4 2 1 3 5 7 9 (e) 9 7 5 3 1 2 4 6 8 10 ( f ) 2 4 6 8 10 9 7 5 3 1 (g) 1 6 2 7 3 8 4 9 5 10 (h) 10 5 9 4 8 3 7 2 6 1 ( i ) I n order from 1 t o 10 and repeat seque S c o r i n g and Response Ambiguity. S u b j e c t s were scored on each response as c o r r e c t ( p l u s ) o r not c o r r e c t (minus) on the r e c o r d i n g blank (Appendix I I I ) . I f the response were c o r r e c t but f o r some e r r o r of d e t a i l (eg. " t r i a n g l e " i n s t e a d o f "arrowhead"), i t would be s c o r e d as "almost 26 c o r r e c t " but not counted as c o r r e c t i n the t a b u l a t i o n . T h r e s h o l d s f o r each p a t t e r n were noted f o r b o t h runs i n the a p p r o p r i a t e columns, and t h e n averaged a c r o s s t o g i v e a mean t h r e s h o l d f o r each p a t t e r n . These were then expressed l o g a r i t h m i c a l l y and averaged, g i v i n g each s u b j e c t one score between the v a l u e s of 1,000 and about 3,000 as an index of t e s t performance, A number of conventions had t o be observed i n the a d m i n i s t r a t i o n and s c o r i n g of c e r t a i n p a t t e r n s . These are l i s t e d as f o l l o w s : 1, SQUARE, I f S simply responds "square", query " i s i t open o r c l o s e d ? " Score p l u s f o r c o r r e c t q u a l i f i c a t i o n . I f S does not know or w i l l not guess, s c o r e p l u s i f the p a t t e r n was the complete SQUARE, minus i f the incomplete OP SQ, 2, STAR, I f S responds "X" or "times s i g n " , query " i s i t broken o r complete?" Score minus f o r i n c o r r e c t q u a l i f i c a t i o n . 3, CIRCLE, Score p l u s f o r any " c i r c l e " response, except "double c i r c l e " or "octagon" response; l e , i f confused w i t h p a t t e r n s 7 or 8, 4, In the case of S b e i n g unable to r e c a l l name of p a t t e r n , but b e i n g a b l e to d e s c r i b e and draw i t c o r r e c t l y , p l u s t o be scored, and the name o f the p a t t e r n repeated t o S, 5, I f S i n t r o d u c e s a new but c o r r e c t l y d e s c r i p t i v e name f o r p a t t e r n , he must draw i t c o r r e c t l y b e f o r e b e i n g scored p l u s . S may s u b s t i t u t e a n o v e l term i f he does so c o n s i s t e n t l y . 27 EEG Recording Procedure Immediately f o l l o w i n g the above procedures, the i n d i v i d u a l was s u b j e c t e d t o the EEG procedure. T h i s was done In the darkened room (a c o n t r o l l e d amount o f i l l u m i n -a t i o n was provided) w i t h s u b j e c t i n a s i t t i n g p o s i t i o n and w i t h the eyes c l o s e d , thus a v o i d i n g any u n n e c e s s a r i l y " c l i n i c a l " aspects which might prove d i s t u r b i n g t o some i n d i v i d u a l s . B i p o l a r l e a d s from twelve e l e c t r o d e s i n s e r t e d i n t o the s c a l p were l e d through an e l e c t r o d e t e r m i n a l box t o the EEG machine. The e l e c t r o d e s were f i n e l y sharpened 30-gauge needles s o l d e r e d t o s i l v e r w i res, and were i n s e r t e d i n t o the s c a l p o n l y f a r enough t o ensure good e l e c t r i c a l c o n t a c t . With one or two e x c e p t i o n s , the Ss responded f a v o r a b l y t o t h i s method o f e l e c t r o d e attachment. As w e l l as the r e c o r d i n g e l e c t r o d e s , a ground e l e c t r o d e was p l a c e d on the s c a l p , and one or b o t h of the ears grounded a c r o s s and t o the machine. The placement of the e l e c t r o d e s i s shown i n F i g u r e 3. From i t s commencement each EEG run was c a r r i e d through w i t h as l i t t l e i n t e r r u p t i o n as p o s s i b l e and with a minimum of d i s t u r b a n c e to the s u b j e c t . Each channel was analysed f o r at l e a s t t h r e e epochs ( t h i r t y seconds), i n o r d e r t o ensure at l e a s t one normal a n a l y s i s , F i g u r e 3 . E l e c t r o d e placement (schematic) and p o l a r leads t o EEG machine* 29 and so as t o p r o v i d e a rough check on t h e s t a b i l i t y o f the frequency p a t t e r n . Channel 6 ( r i g h t o c c i p i t a l ) , which l a t e r was analysed d u r i n g t h e s t r o b o s c o p i c s t i m u l a t i o n , was analysed f o r a t l e a s t s i x epochs i n order t o p r o v i d e f o r a reasonably w e l l s e l e c t e d b a s i s f o r measuring the f l i c k e r response. T h i s p a r t of the run u s u a l l y took up l e s s t han t e n minutes time. F l i c k e r . The stroboscope -lamp was p l a c e d twenty-four inches from the s u b j e c t ' s f a c e , on an a x i s v e r t i c a l t o a p o i n t midway between the eyes. The i n t e n s i t y c o n t r o l was p l a c e d t o a s e t t i n g o f 2 t o g i v e about 190,000 candles peak i n t e n s i t y , which was adequate f o r stimu-l a t i o n through the c l o s e d e y e l i d s , yet not so h i g h as t o b e . p a i n f u l or dangerous. During continuous r e c o r d i n g on a l l channels and a n a l y s i s on channel 6, f l i c k e r was a p p l i e d at f r e q u e n c i e s of 6, 8, 9, 10, 11, 12, 14, and 16 f l a s h e s per second, f o r t h i r t y seconds ( t h r e e epochs) at each frequency. An i n t e r v a l of twenty seconds (two epochs) separated each p e r i o d of f l i c k e r . The s u b j e c t was under i n s t r u c t i o n s t o note any sensory phenomena d u r i n g s t i m u l a t i o n , and t o c a l l a h a l t t o the f l i c k e r should i t become o v e r l y u p s e t t i n g . I n f a c t t h i s was never necessary, and no c l i n i c a l motor m a n i f e s t a t i o n s were r e p o r t e d as a r e s u l t of the f l i c k e r . 30 T e s t a f o r EEG S t a b i l i t y * F o l l o w i n g the f l i c k e r stimu-l a t i o n , the s u b j e c t was r e q u i r e d t o h y p e r v e n t i l a t e f o r th r e e minutes i n order t o o b t a i n some index o f EEG response t o r a i s e b l o o d pH. Time was then allowed f o r the EEG t o become s t a b l e a g a i n . Alpha b l o c k i n g was c r u d e l y measured by having the s u b j e c t open and c l o s e the eyes, and o c c a s i o n a l l y by g i v i n g some v i s u a l - t y p e problem t o s o l v e m e n t a l l y . V. ANALYSIS OF RESULTS The EEGs were analysed by the methods used i n t h i s l a b o r a t o r y . The EEG records were "read" by an experienced c l i n i c a l n e u r o p h y s i o l o g i s t * , and the frequency a n a l y s e r records measured and graphed a c c o r d i n g t o the method d e s c r i b e d by Kennard, R a b i n o v i t c h , and F i s t e r (6). T h i s c o n s i s t e d of r u l i n g a b a s e l i n e along each epoch t o be s e l e c t e d f o r measurement, and measuring the v e r t i c a l h e i g h t of each pen d e v i a t i o n i n m i l l i m e t r e s . A s p e c i a l t r a n s p a r e n t p l a s t i c template was employed f o r t h i s measurement. In t h i s way f i g u r e s were obtained f o r the raw h e i g h t s of pen d e v i a t i o n s f o r each frequency between 3 and 30 c/sec. i n each measured epoch; these were transposed t o a graph upon which a number o f frequency p a t t e r n s were superimposed. For each s u b j e c t three such * The a s s i s t a n c e of Dr. M. A. Kennard i s g r a t e f u l l y acknowledged. 31 graphs were c o n s t r u c t e d ; one showing the " r e s t i n g " frequency p a t t e r n s from each r e c o r d i n g channel, the second showing the EEG frequency p r o f i l e s d u r i n g each p e r i o d of s t r o b o s c o p i c f l i c k e r , as recorded on channel 6 ( r i g h t o c c i p i t a l ) , and the t h i r d showing the a r i t h -m e t i c a l l y - d e r i v e d d i f f e r e n c e between th e " r e s t i n g " p r o f i l e of channel 6 and the p r o f i l e f o r each p e r i o d of f l i c k e r . T h i s l a t t e r graph was the one used t o determine the type and nature of the response t o the f l i c k e r . An i n c r e a s e i n a c t i v i t y a t any frequency was i n d i c a t e d by a r i s e from the b a s e l i n e , while a decrease showed as a l o w e r i n g . Q u a n t i t a t i v e measurement o f harmonic response was done by adding a r i t h m e t i c a l l y the h e i g h t s above the b a s e l i n e o f the second, t h i r d , and f o u r t h harmonics of a l l f l i c k e r f r e q u e n c i e s whose m u l t i p l e s were rep r e s e n t e d by a n a l y s e r peaks on t h e r e c o r d s . F o r the purposes of t h i s experiment, primary d r i v i n g and subharmonics were not taken I n t o c o n s i d e r a t i o n . Examination of the experimental h y p o t h e s i s was done by r a n k i n g a l l s u b j e c t s i n o r d e r of b o t h measure-ments, namely the MRT scores l y i n g between the v a l u e s of 1.000 and 3.000, and t h e summated a n a l y s e r h e i g h t s , which l a y between values of minus one hundred and p l u s seven hundred m i l l i m e t r e s , a l t h o u g h the m a j o r i t y of the l a t t e r measurements were In the range from p l u s 32 f i f t y t o p l u s two hundred m i l l i m e t r e s . Spearman rank-order c o e f f i c i e n t s of c o r r e l a t i o n were computed f o r the whole group and f o r v a r i o u s subgroups a c c o r d i n g t o v a r i o u s s e l e c t i o n c r i t e r i a (see D i s c u s s i o n ) . While measurement of frequency p a t t e r n from the v a r i o u s a n a l y s e r epochs was a simple enough mechanical o p e r a t i o n , the s e l e c t i o n of epochs t o be measured was a matter which c a l l e d f o r a combination o f judgement and f o r m a l c o n t r o l , i f an extreme b i a s i n t h e r e s u l t s were t o be avoided. The c r i t e r i a f o r s e l e c t i o n are l i s t e d as f o l l o w s i n o r d e r of precedence: 1. " R e s t i n g " epochs s e l e c t e d on b a s i s o f : (a) h i g h e s t normal a n a l y s e r t r a c i n g s through-out e n t i r e frequency range; (b) i f (a) not a p p l i c a b l e , h i g h e s t a n a l y s e r t r a c i n g s i n a l p h a band of 8 t o 12 c/sec. (c) i f (a) o r (b) not a p p l i c a b l e , the l a s t epoch s e l e c t e d a r b i t r a r i l y . 2. " S t r o b o s c o p i c " epochs s e l e c t e d on b a s i s o f : (a) h i g h e s t a n a l y s e r t r a c i n g s a t harmonics of s t r o b o s c o p i c f l i c k e r f r e q u e n c i e s ; (b) I f (a) not a p p l i c a b l e , h i g h e s t a n a l y s e r t r a c i n g s i n a l p h a band o f 8 t o 12 c/sec. (c) I f (a) or (b) not a p p l i c a b l e , the l a s t epoch of a n a l y s i s i n the f l i c k e r p e r i o d s e l e c t e d a r b i t r a r i l y . While i n t e r - s c o r e r r e p r o d u c i b i l i t y was not checked, these c r i t e r i a were b e l i e v e d t o render an acceptable degree of o b j e c t i v i t y i n t h i s aspect of the procedure. 33 Q u a l i t a t i v e a n a l y s i s o f the r e s t i n g a n a l y s e r p a t t e r n s was c a r r i e d out on the b a s i s o f t h e t e n c r i t e r i a of EEG n o r m a l i t y - a b n o r m a l i t y d e s c r i b e d by Kennard, R a b i n o v i t c h , and F i s t e r ( 6 ) . These, l i s t e d i n o r d e r of t h e i r occurrence i n a group of "normal" s u b j e c t s , are as f o l l o w s : 1. W e l l o r g a n i z e d . A l l p r o f i l e l i n e s running c l o s e t o g e t h e r , w i t h no exceptions to the common l i n e of p r o g r e s s . 2. High a l p h a . The presence of a d i s t i n c t r i s e i n 5 o f the 8 channels of the a l p h a (frequency 8 t o 12) band, w i t h a s i n g l e peak and narrow base. 3. Peak i n frequency 16 t o 20. A sharp upward s h i f t In 3~~of the 8 channeTsTn t h i s frequency range. 4. Peak i n frequency 5 t o 7. A sharp upward s h i f t In 5~~of the 8 channel's i n t h i s frequency range. 5. Peak i n frequency 15 t o 15. A sharp upward s h i f t i n 5~~of the 8 channeTs~Tn t h i s frequency range. 6. Spread. When the 8 l i n e s of the graph run i n a band more than 2 cm. i n width. 7. Poly-peak a l p h a . When 5 of the 8 channels i n the alph a range show more t h a n one peak. 8. Peak i n frequency 22 t o 30. A sharp upward s h i f t i n 5~~of the 8 channeTs~Tn t h i s frequency range. 9. Poor o r g a n i z a t i o n . When an outstanding c h a r a c t e r -ratio of the p r o f i l e i s the f a i l u r e of i t s v a r i o u s l i n e s t o converge i n any way; where t h e y c r i s s - c r o s s f r e q u e n t l y and seem t o b e a r no r e l a t i o n t o each other. 10. L e f t - r i g h t dys-synchrony. Where there i s a . s i g n i f i c a n t l e f t - r i g h t d i f f e r e n c e i n 3 or the 4 channel p a i r s . 34 I n t h i s experiment, no examination was made of i n t e r - s c o r e r r e p r o d u c i b i l i t y of t h i s a n a l y s i s . A l l d a t a , i n c l u d i n g the above, were assembled on punch-cards and coded t o make c r o s s - s o r t i n g p o s s i b l e , w i t h r e f e r e n c e t o b o t h experimental v a r i a b l e s and sub-group s e l e c t i o n c r i t e r i a . T h i s l a t t e r , f o l l o w i n g as i t d i d i n s p e c t i o n o f the t o t a l - g r o u p r e s u l t s , w i l l be d i s c u s s e d i n the next chapter. VI. VALIDITY AND RELIABILITY The t e n u i t y o f the l i n k between t h e p o s t u l a t e d c e n t r a l p h y s i o l o g i c a l processes and t h e experimental method of t h e i r measurement made the problem of v a l i d i t y a c r u c i a l p o i n t i n t h i s r e s e a r c h . T h i s i s t r u e f o r b o t h of the measured v a r i a b l e s , but t o a g r e a t e r extent i n the measurement of r e c o g n i t i o n speed. The other (EEG-f l i c k e r ) v a r i a b l e , i n v o l v i n g a h i g h degree o f o b j e c t i v i t y i n both procedure and a n a l y s i s , comes c l o s e r t o being a d i r e c t measure of p h y s i o l o g i c a l f u n c t i o n . The EEG i s a v a l i d t o o l i n the d e s c r i p t i o n o f b r a i n waves, i f we assume t h a t such e x i s t at a l l . The s t a b i l i t y o f the EEG frequency p a t t e r n over a sh o r t p e r i o d o f time ( i e . the time r e q u i r e d f o r r e c o r d i n g the complete run; about one-half hour) i s shown b y . i n s p e c t i o n of the frequency a n a l y s i s f o r t h a t p e r i o d (see a l s o Kennard, R a b i n o v i t c h , 35 and P l a t e r , 6 ) , and over a l o n g e r p e r i o d of time by the c h a r a c t e r i s t i c p a t t e r n a which t e n d t o r e c u r i n the EEGs of s u b j e c t s r e p e a t e d l y t e a t e d (Kennard, 5 ) . F a c t o r a o f m o t i v a t i o n or w i l l f u l n e a a do not a p p a r e n t l y a f f e c t the EEG r e c o r d , except g r o s s l y enough that such changes (:Le. muscle a r t i f a c t , "unrelaxed a l p h a " , v i s u a l a l p h a b l o c k i n g , vasovagal e f f e c t s , e t c . ) may be d e t e c t e d i n the s u b j e c t o r the r e c o r d (see a l s o Kennard, 4, 5; U l e t t and G l e s e r , 9 ) . The method of s c o r i n g the EEG was not p r i m a r i l y " i n t e r p r e t a t i o n " i n the u s u a l sense, but r a t h e r an o b j e c t i v e q u a n t i t a t i v e measurement of the a n a l y s e r pen e x c u r s i o n s (see p. 30); assuming mechanical i n t e g r i t y of the EEG machines, the s c o r i n g r e l i a b i l i t y of t h e EEG procedure i s p r o b a b l y a c c e p t a b l e . The v a l i d i t y and r e l i a b i l i t y o f the p e r c e p t i o n -speed measurement, however, must almost c e r t a i n l y have been s u b j e c t t o an unknown degree of contamination from u n c o n t r o l l e d i n d i v i d u a l and p r o c e d u r a l v a r i a b l e s . M o t i v a t i o n was an extremely important f a c t o r , and such s m a l l t h i n g s as s l i g h t l y d i r t y e y eglasses would have g r o s s l y a f f e c t e d the s u b j e c t on what pur p o r t e d t o be a measure o f c e r e b r a l f u n c t i o n . Even i n view of the c o n t r o l s which were e x e r c i s e d , the MRT v a l i d i t y had l a r g e l y t o be assumed. R e l i a b i l i t y of the meaaurement o f r e c o g n i t i o n 36 t h r e s h o l d was computed by the s p l i t - h a l f method, from the MRT scores o f the f i r s t t h i r t y s u b j e c t s i n the EEG sample. The t e n RT sco r e s f o r each subject, were ass i g n e d t o two groups of f i v e each, by the method of ran k i n g the mean t h r e s h o l d s f o r each p a t t e r n as computed from the background sample (N=100), and a s s i g n i n g the odd ranks t o one of the halves and the even ranks t o the other. T h i s method ensured an optimum degree of s i m i l a r i t y i n the two h a l v e s . The means and standard d e v i a t i o n s of the mean t h r e s h o l d s f o r each p a t t e r n are shown i n T a b l e I I . The c o e f f i c i e n t of r e l i a b i l i t y thus obtained f o r the MRT measurement came t o 0.87; when c o r r e c t e d f o r l e n g t h by the Spearman-Brown formula, the c o e f f i c i e n t became 0.93. T h i s would i n d i c a t e a h i g h degree of i n t e r n a l r e l i a b i l i t y d u r i n g one t e s t procedure. However, t h i s method of computation g i v e s no i n d i c a t i o n of the s t a b i l i t y o f the MRT f u n c t i o n over a l o n g e r p e r i o d of time. 37 TABLE I I MEANS AND STANDARD DEVIATIONS OP RECOGNITION THRESHOLDS (MILLISECONDS) N = 100 P a t t e r n Code Mean S. D. Rank 1 SQUARE 1.574 0.269 3 2 ORES 1.582 0.359 4 3 OP SQ 1.793 0.374 8 4 CIRCLE 1.569 0.338 2 5 ARROW 1.613 0.482 5 6 PLUS 1.439 0.393 1 7 DONUT 1.811 0.417 9 8 OCTA 1.622 0.352 6 9 STAR 2.187 0.558 10 10 RECTANG 1.789 0.402 7 Mean MRT 1.697 0.327 CHAPTER I I I RESULTS The c o r r e l a t i o n between the two experimental v a r i a b l e s of r e c o g n i t i o n t h r e s h o l d and harmonic response f o r the t o t a l sample of f i f t y s u b j e c t s was found t o be + 0.27, as computed by the Spearman rank-order method. T h i s was not s i g n i f i c a n t a t the .05 l e v e l of co n f i d e n c e ; the h y p o t h e s i s o f a monotonic r e l a t i o n s h i p between the two v a r i a b l e s was t h e r e f o r e not supported. I n s p e c t i o n of the obtained d a t a , however, does i n d i c a t e t h a t c e r t a i n r e l a t i o n s h i p s may n e v e r t h e l e s s e x i s t between r e c o g n i t i o n t h r e s h o l d and harmonic response t o f l i c k e r . The s c a t t e r p a t t e r n i n the c o r r e l a t i o n chart ( F i g u r e 4) does not appear t o be completely random, but r a t h e r t o f o l l o w a t r i a n g u l a r d i s t r i b u t i o n . Examination of the scattergram I n d i c a t e s the p o s s i b i l i t y of two dumpings of cases, one o f them b e i n g f a i r l y l i n e a r with r e s p e c t t o the experimental h y p o t h e s i s , and the o t h e r b e i n g a c l u s t e r i n g o f cases i n t h e category o f low r e c o g n i t i o n t h r e s h o l d and l i t t l e o r no harmonic response. Attempts were t h e r e f o r e made to d i s c o v e r some c r i t e r i o n by which these two "groups" of cases might be d i f f e r e n t i -a t e d . T h i s was done on c e r t a i n t h e o r e t i c a l grounds (see D i s c u s s i o n ) by a s y s t e m a t i c c r o s s - i n d e x i n g on the 700 600 ' • • a a 1.3 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.It 2.5 MEAN RECOGNITION THRESHOLD F i g u r e 4. C o r r e l a t i o n scattergrara of the t o t a l EEG group, w i t h r e s p e c t t o the experimental v a r i a b l e s o f MRT and Harmonic Index. N = 50 • \ 40 b a s i s of r e s t i n g EEG- c h a r a c t e r i s t i c s and c r i t e r i a o f EEG n o r m a l i t y , which are l i s t e d i n the p r e c e d i n g c h a p t e r . Two f a c t o r s were found which gave a h i g h degree of s e p a r a t i o n t o the two c l u s t e r s of cases. The f i r s t of t h e s e was c o o r d i n a t i o n o f frequency p a t t e r n among the d i f f e r e n t r e c o r d i n g areas ( n o r m a l i t y c r i t e r i o n 1, page 33), and amount of a l p h a c h a r a c t e r i s t i c o f the EEG p a t t e r n ( c r i t e r i o n 2, page 33). T h i s gave q u a l i t a t i v e , though not s t a t i s t i c a l l y s i g n i f i c a n t , d i f f e r e n c e s w i t h regard t o c o r r e l a t i o n between r e c o g n i t i o n t h r e s h o l d and harmonic index. The records o f s u b j e c t s whose EEGs were not t i g h t l y o rganized around a v e r y h i g h a l p h a index tended t o show a b e t t e r c o r r e l a t i o n between MRT and harmonics, w h i l e those w i t h h i g h a l p h a i n d i c e s and we11-organized frequency p a t t e r n s showed a g e n e r a l l y low s e n s i t i v i t y t o the f l i c k e r throughout the e n t i r e range of r e c o g n i t i o n t h r e s h o l d s . The c o r r e l a t i o n scattergrams of the two subgroups are shown In F i g u r e s 5 and 6; the c o r r e l a t i o n d a t a i s summarized i n T a b l e I I I . The second f a c t o r served t o d i s t i n g u i s h the two c l u s t e r s of cases much more d e f i n i t e l y . When the group of f i f t y was d i v i d e d i n t o two on the b a s i s of frequency dominance, i t was found that the subgroup w i t h h i g h ( l e . above 10 c/sec) dominant EEG f r e q u e n c i e s showed a c o r r e l a t i o n of 0.66 between the two v a r i a b l e s 700 500 a a as o- 300 E i SB 200 100 0 1.3 I.1* 1.$ 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2."< 2.5 MEAN RECOGNITION THRESHOLD F i g u r e 5 , C o r r e l a t i o n s c a t t e r g r a m o f the h i g h - a l p h a - w e l l - o r g a n i z e d EEG group. N - : uoo u 300 1.3 l.<* I.? 1.6 1.7 l.S 1.9 2.0 2.1 2.2 2.3 2.>t 2.5 MEAN RECOGNITION THRESHOLD F i g u r e 6. C o r r e l a t i o n s c a t t e r g r a m o f the l o w - a l p h a - l e s s - o r g a n i z e d EEG group. N - 26 42 of r e c o g n i t i o n speed and harmonics. The s i g n i f i c a n c e of t h i s c o e f f i c i e n t f o r the group of 23 s u b j e c t s reaches the .01 l e v e l of c o n f i d e n c e . Those s u b j e c t s (N=27) whose dominant EEG f r e q u e n c i e s were 10 c/sec. or lower showed a n o n - s i g n i f i c a n t c o r r e l a t i o n of 0.35. The scattergrams f o r these two subgroups are i l l u s t r a t e d i n F i g u r e s 7 and 8; i t w i l l be noted t h a t the two c l u s t e r s of cases i n the t o t a l - g r o u p d i s t r i b u t i o n ( F i g u r e 4) are almost completely separated. The s i g n i f i c a n c e of the d i f f e r e n c e s between the c o r r e l a t i o n s obtained f o r b o t h p a i r s o f each sub-group was computed by c o n v e r t i n g the Spearman c o e f f i c i e n t s t o t h e i r Z - v a l u e s and d e t e r m i n i n g the standard e r r o r of the d i f f e r e n c e between the c o r r e l a t i o n s . I n n e i t h e r case was t h i s d i f f e r e n c e s t a t i s t i c a l l y s i g n i f i c a n t . The c o r r e l a t i o n d a t a f o r the t o t a l group and b o t h p a i r s of subgroups are summarized i n T a b l e I I I * 700 70O 1.3 l."t 1.5 1.6 1.7 1.8 1.9 5.0 2.1 2.2 2.3 2.>t 2.5 MEAN RECOGNITION THRESHOLD 1.3 l.W 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.U 2.5 MEAN RECOGNITION THRESHOLD F i g u r e 7. C o r r e l a t i o n s c a t t e r g r a m of the h i g h frequency dominance group, N = 23 F i g u r e 8. C o r r e l a t i o n s c a t t e r g r a m of the low frequency dominance group. N = 27 44 TABLE I I I THE CORRELATIONS BETWEEN RECOGNITION THRESHOLD AND HARMONIC INDEX FOR TOTAL AND SUB-GROUPS Group N r s i g . * Z S.E. d i f f . T o t a l 50 .27 < .05 High a l p h a , 24 .18 <.05 0.1872 w e l l organized 0.075 <.05 p=.47 n. Low a l p h a , 26 .38 0.4001 l e s s organized Low frequency 27 .35 <.05 0.3700 dominant 1.410 >.01 p=.07 n. High frequency 23 .66 0.7946 dominant * I t i s re c o g n i z e d t h a t these p r o b a b i l i t y v a l u e s are p o s s i b l y not v a l i d . CHAPTER IV DISCUSSION AND CONCLUSIONS The f i n d i n g s o f t h i s experiment are suggestive but not c o n c l u s i v e . Since the r e l a t i o n s h i p between r e c o g n i t i o n speed and harmonic response t o f l i c k e r d i d not reach s t a t i s t i c a l s i g n i f i c a n c e i n terms of the hypothesis as o r i g i n a l l y p r e s e n t e d , the q u a l i t a t i v e f i n d i n g s must be considered as hypotheses r a t h e r than r e s u l t s . The manipulations of d a t a which produced the d i f f e r e n t i a t i o n between the suspected "dumpings" of cases i n the scat t e r g r a m ( F i g u r e 4) f o l l o w e d from a number o f t h e o r e t i c a l bases. Kennard (4) summarizes evidence which suggests that lowered t h r e s h o l d c o r t i c a l e x c i t a b i l i t y i s a s s o c i a t e d w i t h a paroxysmal EEG and low a l p h a index. The converse, which should f o l l o w , that good o r g a n i z a t i o n and h i g h a l p h a index a r e a s s o c i a t e d w i t h a r e l a t i v e l y h i g h c o r t i c a l t h r e s h o l d t o environmental s t i m u l a t i o n , was observed t o be t r u e i n t h i s experiment. Subjects w i t h evenly r e g u l a t e d h i g h a l p h a EEGs showed l i t t l e response t o e i t h e r f l i c k e r or h y p e r v e n t i l a t i o n . When these s u b j e c t s were compared w i t h those having l e s s a l p h a and l e s s o r g a n i z a t i o n , (l_e. s e p a r a t i o n a c c o r d i n g t o c r i t e r i a 1 and 2, p. 33), i t was noted t h a t 46 the l a t t e r group b e t t e r i l l u s t r a t e d the experimental hypothesis ( F i g u r e s 5 and 6 ) . The second f a c t o r which produced even b e t t e r s e p a r a t i o n of the suspected " c l u s t e r s " f o l l o w e d from Mundy-Castle !s s u g g e s t i o n (7) t h a t i n d i v i d u a l s w i t h dominant a l p h a f r e q u e n c i e s above the mean of 10.3 c/sec. show " h i g h e r e x c i t a b i l i t y c h a r a c t e r i s t i c s . " When the group of f i f t y cases i n t h i s experiment was d i v i d e d a c c o r d i n g t o frequency of the most dominant EEG rhythms, i t was found t h a t the group w i t h h i g h dominant f r e q u e n c i e s showed a s i g n i f i c a n t c o r r e l a t i o n between r e c o g n i t i o n -t h r e s h o l d and harmonics. I t i s I n t e r e s t i n g t h a t b o t h of these " s e p a r a t i o n " f a c t o r s a r e r e l a t e d t o p o s t u l a t e d i n d i c e s of c o r t i c a l s t a b i l i t y . The q u e s t i o n of the subgroup d i f f e r e n c e s obtained by i n s p e c t i o n o f the d a t a i s one which cannot be s e t t l e d except by f u r t h e r i n v e s t i g a t i o n . Both ad hoc method-ology and l i m i t a t i o n s of d a t a p r e c l u d e p o s i t i v e g e n e r a l c o n c l u s i o n s from t h i s study. The f o l l o w i n g hypotheses, however, would seem to be p a r t i c u l a r l y warranted by the. r e s u l t s of t h i s experiment. (1) I t i s probable t h a t some f a c t o r of c o r t i c a l s t a b i l i t y p r e c l u d e s a marked harmonic response t o f l i c k e r i n s u b j e c t s whose EEGs are t i g h t l y o r g a n i z e d around a dominant a l p h a rhythm. A p o s s i b l e mechanism 47 o f t h i s s t a b i l i t y would be o v e r i n f l u e n c e of the i n t r i n s i c a l p h a pacemaker, which would tend t o r e s i s t e x t r i n s i c sensory e f f e c t s on the a l p h a frequency. The p r e s e n t f i n d i n g s would suggest t h a t a r e l a t i o n s h i p between r e c o g n i t i o n speed and harmonic response might become c l e a r e r i n s u b j e c t s whose EEGs are not so r i g i d l y s e l f - r e g u l a t e d . (2) I f these two f u n c t i o n s are r e l a t e d as hypothesized i n s u b j e c t s whose EEGs are c h a r a c t e r i s t i c a l l y more s e n s i t i v e t o e x t r i n s i c i n f l u e n c e s , t h e n i t should f o l l o w t h a t f a c t o r s which i n c r e a s e EEG s e n s i t i v i t y (eg. m e t r a z o l , a d r e n a l i n e , and c e r t a i n h a l l u c i n o g e n i c drugs) and those which decrease i t (eg. CNS depressants and a n a e s t h e t i c s ) would a l s o s e l e c t i v e l y i n c r e a s e or decrease b o t h speed of p e r c e p t i o n and harmonic response t o f l i c k e r . T h i s h y p o t h e s i s c o u l d be examined by c o r r e -l a t i o n s of these two measurements repeated a number of times on one i n d i v i d u a l under v a r y i n g p h a r m a c o l o g i c a l c o n d i t i o n s . (3) There was s o m e . i n d i c a t i o n t h a t i n d i v i d u a l s w i t h dominant c o r t i c a l d i s c h a r g e f r e q u e n c i e s h i g h e r t h a n the 10 c/sec. of the normal alpha do show a monotonic r e l a t i o n s h i p between r e c o g n i t i o n speed and harmonics; furthermore, there may w e l l be a d i f f e r e n c e i n t h i s regard 48 between the high-frequency group and the low-frequency group ( w i t h dominant f r e q u e n c i e s o f 10 c/sec. or l e s s ) * F u r t h e r r e s e a r c h on l a r g e r samples s e l e c t e d w i t h r e f e r e n c e t o these c r i t e r i a should c l a r i f y t h i s p o s s i b i l i t y . Such a r e l a t i o n s h i p , i f found, would support the hypothesis t h a t s y n a p t i c r e c y c l i n g time i s a f a c t o r I n b o t h speed of p e r c e p t i o n and harmonic f l i c k e r response. The p r a c t i c a l aspects o f these r e s e a r c h f i n d i n g s are o n l y s p e c u l a t i v e . I n the f i e l d o f pe r s o n n e l s e l e c t i o n , p a r t i c u l a r l y f o r c e r t a i n m i l i t a r y o c c u p a t i o n s , the use of a p h y s i o l o g i c a l index f o r the assessment o f p o t e n t i a l speed of p e r c e p t i o n would be of great v a l u e , i n t h a t the n e u r o p h y s i o l o g i c a l events c o r r e l a t e d w i t h t h i s psycho-l o g i c a l v a r i a b l e c o u l d be measured d i r e c t l y u s i n g methods and instruments a l r e a d y common t o many medic a l s e l e c t i o n procedures. Of course, I f a n e u r o p h y s i o l o g i c a l measure-ment such as the EEG were t o be s u b s t i t u t e d f o r a p e r c e p t u a l measurement, the c o r r e l a t i o n between the two would have t o be extremely h i g h , p r o b a b l y at l e a s t 0.9. I n the f i e l d of mental h e a l t h , the d i s c o v e r y o f p h y s i o -l o g i c a l c o r r e l a t e s of p s y c h o l o g i c a l f u n c t i o n s i s becoming i n c r e a s i n g l y important. D i s o r d e r s i n p e r c e p t i o n are c h a r a c t e r i s t i c of the more severe mental i l l n e s s e s , as are c e r t a i n types o f EEG abnormality. T h i s i s d i s c u s s e d and reviewed by Kennard ( 4 ) . The i n d i c a t i o n s of t h i s 49 experiment are t h a t f u r t h e r i n v e s t i g a t i o n of the r e l a t i o n s h i p s between EEG c h a r a c t e r i s t i c s and speed of v i s u a l p e r c e p t i o n would be j u s t i f i e d . C o nclusions (1) The experimental hypothesis as p r e s e n t e d and t e s t e d was not supported by the r e s u l t s . ( 2 ) I n s p e c t i o n and m a n i p u l a t i o n of t h e o b t a i n e d data r e s u l t e d i n s e v e r a l i m p l i c a t i o n s f o r f u r t h e r r e s e a r c h . Most Important i n t h i s regard i s the p o s s i b i l i t y that speed o f v i s u a l p e r c e p t i o n may be r e l a t e d t o c e r t a i n p h y s i o l o g i c a l EEG c r i t e r i a of c o r t i c a l s t a b i l i t y t o e x t r i n s i c s t i m u l a t i o n . SUMMARY (1) The hypothesis was presented t h a t speed o f v i s u a l p e r c e p t i o n I s r e l a t e d t o degree o f harmonic c o r t i c a l response which can be e l i c i t e d by i n t e r m i t t e n t p h o t i c s t i m u l a t i o n , through some mechanism of CNS s e n s i t i v i t y t o a f f e r e n t s t i m u l a t i o n . T h i s h y p o t h e s i s was based on Hebb's t h e o r y o f the neuron assembly, and on Mundy-Castle's o b s e r v a t i o n s of the c o r t i c a l e f f e c t s o f p h o t i c s t i m u l a t i o n . (2) Experimental examination of t h i s h y p o t h e s i s was made on a sample of f i f t y young a d u l t v o l u n t e e r s u b j e c t s o f b o t h sexes. R e c o g n i t i o n t h r e s h o l d was measured by 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 o f t e n simple geometric p a t t e r n s a t exposures ranging upwards from t e n m i l l i s e c o n d s , u n t i l c o r r e c t i d e n t i f i c a t i o n s o f a l l p a t t e r n s were made. The MRT score c o n s i s t e d of the average f o r the t e n p a t t e r n scores over an ascending and descending s e r i e s . The EEG measurements were made i n the c o n v e n t i o n a l manner, and sc o r e d f o r v a r i a t i o n s i n frequency p a t t e r n by measure-ment of e l e c t r o n i c frequency a n a l y s i s r e c o r d s . P h o t i c s t i m u l a t i o n at f r e q u e n c i e s of s i x t o s i x t e e n f l a s h e s p e r second was p r o v i d e d by an e l e c t r o n i c stroboscope d u r i n g the EEG r e c o r d i n g . 51 (3) The r e s u l t s obtained from these procedures d i d not support the o r i g i n a l h y p o t h e s i s . The obtained c o r r e l a t i o n of 0.27 was not s i g n i f i c a n t a t the .05 l e v e l of c o n f i d e n c e . (4) An a f t e r - t h e - f a c t i n s p e c t i o n of the d a t a showed t h a t the o r i g i n a l h ypothesis was a p p l i c a b l e t o roughly h a l f o f the s u b j e c t s t e s t e d , and t h a t these s u b j e c t s c o u l d be s e l e c t e d by means of a t h i r d independent v a r i a b l e . The group showing t h i s r e l a t i o n s h i p was c h a r a c t e r i z e d by r e l a t i v e l y h i g h dominant EEG f r e q u e n c i e s , and r e l a t i v e l y l o o s e o r g a n i z a t i o n o f frequency p a t t e r n with l i t t l e o r no alpha a c t i v i t y i n the normal or low frequency ranges. Due t o l i m i t a t i o n s of method and d a t a , t h i s r e l a t i o n s h i p c o u l d not be c o n c l u s i v e l y determined. 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APPENDICES APPENDIX I ANALYSIS OP EXPERIMENTAL SAMPLE ( N - 50 ) Subject EEG Harmonic Dominant EEG A n a l y s i s No. No. Age Sex MRT Index Preq./sec. 1 2 3 4 5 6 7 8 9 10 59 513 30 P 1.4112 130 l o v o l t . f a s t # # # 46 554 27 P 2.3513 40 11 # # 47 558 23 F 1.9792 85 11 # 1 559 22 M 1.4452 85 9 # # 44 567 27 M 2.2216 19 10 # # 23 573 18 P 2.3656 -27 11 # # 33 574 19 M 2.0349 12 10 # # 43 580 22 F 1.3605 285 11 # 36 584 19 P 2.0376 28 10 # # # 42 588 25 P 1.5242 219 11 # # # 16 597 18 P 1.6036 -1 10 # # # # 76 598 18 P 1.5985 469 15 # # # # 77 601 27 M 1.7057 -62 9 # # # 80 612 23 M 2.2066 3 16-18 # # # 7 613 22 M 1.5907 123 16-20 # # # # 81 616 24 M 1.8735 -31 10 # # 37 617 18 P 2.5958 50 11 # # # 64 618 19 P 2.1534 135 15-30 # 56 621 22 M 2.8264 -25 11 # 70 622 20 F 1.2862 64 5-7 # # 82 623 29 M 1.9036 19 11 # # 68 628 18 M 1.5353 708 15-30 # # # # 13 629 18 F 1.5860 20 10 # # # 65 630 19 F 1.4868 356 11 # # # 51 632 18 F 2.1832 32 10 # # # APPENDIX I (continued) Subject EEG Harmonic Dominant EEG A n a l y s i s No. No. Age Sex MRT Index Freq./sec. 1 2 3 4 5 6 7 8 9 10 11 633 18 P 1.7924 -35 10 # # # 58 636 20 M 1.4133 45 10 # # 12 643 18 M 1.5614 45 5-7 # # 6 646 30 M 1.2811 ;;32 9 # # # 84 655 25 M 1.4112 -24 10 # # 87 657 24 P 1.8295 139 11 # 88 658 26 P 1.3964 103 9 # 89 659 23 P 2.1121 126 none # 90 660 21 F 1.6626 28 10 # 91 661 25 P 1.5066 -54 10 # # # 92 662 30 P 1.5957 10 11 # 93 663 21 F 1.6964 263 20-30 # 94 664 20 P 1.7036 -33 11 # # 95 665 20 M 1.7230 31 10 # # # 97 666 23 M 1.6712 10 10 # # 96 667 24 M 1.6404 47 10 # # # 98 668 21 M 2.0133 52 13 # 99 669 21 M 1.4668 30 9 # # 100 670 23 M 1.6689 165 20-30 # 101 671 24 M 1.4334 228 10 # # 102 672 25 P 2.0203 67 10 # # # 103 673 22 M 1.4832 0 10 # # 104 674 29 M 1.6395 260 11 # # 86 675 21 M 1.5149 118 10 # # # 85 676 21 M 1.6230 35 10 # # APPENDIX I I INTRODUCTION AND SELECTION BLANK FOR SECURING VOLUNTEER SUBJECTS TO ALL PSYCHOLOGY 100 STUDENTS: A research project Is being undertaken In which we are attempting to relate speed of perceDtion of vi s u a l patterns with certain physiological measurements. We are asking the students In Psychology 100 to assist as subjects by giving about one hour of their time to be tested for reaction tine to such patterns as squares, c i r c l e s , etc. when they are flashed on a screen. The test procedures are not related to intelligence, personality, or university standing, and the results w i l l be confidential and not made known to any instructors i n thi s course. We f e e l that many Introductory Psychology students would be glad to participate i n this project, since i t w i l l give them a chance to view first-hand one area of research i n the psychological sciences — which w i l l certainly be of interest to them even i f they do not plan to go on i n Psychology. I f you are interested i n being a subject i n th i s project, we would appreciate your f i l l i n g i n thi s form and returning i t at the end of this hour. NAME: AGE: SEX:. UNIVERSITY COURSE YEAR: ADDRESS: '... PHONE: WHEN COULD YOU 3E AVAILABLE? (please check as many as possible) Day 12:30 1:30 2:30 3:30 V:30 evening Monday Tuesday Wednesday Thursday Friday ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) C ) ( > ) ( ) ( ) ) ( ) ( ) ) ( ) ( ) Saturday mornings (time) Sat. afternoons. Other times: Your help i n this investigation w i l l be very much appreciated. I f you are w i l l i n g to Darticipate you w i l l probably be contacted during the next few days for a definite appointment. I f you should decide la t e r that you are interested i n being a subject, l e t us know and we w i l l t r y our best to f i t you i n . You may have questions about thi s project; we w i l l c e r t a i n l y t r y to answer them to your satisfaction before we use you as a subject. We hope that you a l l participate i n th i s experiment, which w i l l be of value to us and should be of great interest to you. DEPARTMENT OF NEUROLOGICAL RESEARCH HUT B - 5, UBC CAMPUS. Leslie Wright. 61 APPENDIX I I I INDIVIDUAL RECORDING BLANK FOR PERSONAL INFORMATION SUMMARY AND TEST SCORE TABULATION NAME ADDR. • PK. SEMES A # I EXP I\TCR 01 0 2 * .1.2.51.0 r t EXP DECR 1. .5.? .1 dtO? CD. r t mrt 1 SQUARE 2 ORES 3 OP SQ It CIRCLE 5 ARROW e PLUS 7 DOHUT 8 OCTA 9 STAR 10 RECTANG SERIES B # EXP INCR 01 02 OVJ .2 .5 LO r t EXP DECR L .5 2 .10^02 01 r t mrt 1 SQUARE 1 2 CRES 3 OP SQ it CIRCLE 5 ARROW e PLUS 7 DONUT 8 OCTA 9 STAR 10 RECTANG a t t i t u d e drugs glasses reading date time eej SUBJECT # EEG # APPENDIX IV 62 STIMULUS PATTERNS USED IN DETERMINATION OP MRT 1 SQUARE 2 CRES 3 OP SQ *f CIRCLE 5 ARROW 6 PLUS \ / 7 DONUT 8 OCTA 9 STAR 10 RECTANG D- l l TRIANGLE D-12 OP CIR D-13 JELLYBEAN D-l*+ DIAMOND D-15 COMPLETE X
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Philosophy
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The relationship between speed of visual perception and flicker-induced harmonic cortical activity
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