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Performance of two hemispherectomized subjects on a dichotic binaural frequency fusion test Feick, Elizabeth Anne 1974

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PERFORMANCE OF TWO HEMISPHERECTOMIZED SUBJECTS ON A DICHOTIC BINAURAL FREQUENCY FUSION TEST BY ELIZABETH ANNE FEICK B.Sc, University of Toronto, 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Paediatrics Divison of Audiology and Speech Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June 1974 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements fo r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree t h a t permiss ion for ex tens i ve copying o f t h i s t h e s i s fo r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . Department of OMO/J.^^W /into/ SsomJ-Sc. The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT This study investigates the performance of two hemispherec-tomized subjects and ten normal subjects on a dichotic binaural frequency fusion (DBFF) test and on a competing dichotic message test. The DBFF test was designed to examine whether binaural integration of two complementary frequency segments of the same word, d i c h o t i c a l l y presented, necessitates the presence of two in t a c t hemi-spheres. The competing dichotic message test was presented to provide a measure of the extent of strengthening of i p s i l a t e r a l pathways in the hemispherectomized subjects. The DBFF test consisted of three fifty-word CNC l i s t s which were processed through two band-pass f i l t e r s and recorded on a two--channel magnetic tape. The test consisted of two binaural conditions. In the Dichotic A condition, the high band was delivered to the l e f t ear and the low band to the ri g h t . The Dichotic B condition was the reverse of the f i r s t . For each condition 50 phonetically balanced (PB) words were presented and the subject was required to repeat the word in a 4 second interval between words. The competing dichotic message test consisted of 15 sets of three pairs of words, one of each pair being presented simultan-eously to either ear, using stereophonic head-phones. The subject was required to repeat as many words from each set as possible. The Z scores, measuring the deviation in standard deviation units of the raw scores of the operated subjects from the mean scores i i i of the normals indicated that the removal of a hemisphere did not s i g n i f i c a n t l y decrease the scores of two hemispherectomized subjects on a DBFF test. Removal of a hemisphere, however, decreased the scores of the hemispherectomees on the competing dichotic message test in one of the ears -- s p e c i f i c a l l y the ear contralateral to the removed hemisphere. A comparative analysis of how the central auditory nervous system (CANS) of a hemispherectomized subject might process a complemen-tary dichotic message (exemplified by the DBFF t e s t ) , as opposed to a competing dichotic message, provides an interesting basis for a d i s -cussion on the nature of the "biological detector" of speech elements i n the CANS. i v TABLE OF CONTENTS PAGE ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES vi LIST OF FIGURES v i i ACKNOWLEDGEMENT - . v i i i CHAPTER 1. INTRODUCTION "I 2. REVIEW OF LITERATURE 3 2.0 Introduction 3 2.1 The Auditory Pathways -3 2.2 Detection of Central Auditory Lesions 4 2.2.1 Distinguishing Brainstem Lesions from Temporal Lobe Lesions 4 2.2.2 Dichotic Binaural Frequency Fusion Tests 9 2.3 Functionality of the I p s i l a t e r a l Pathways . . . . 18 2.4 Prepotence of Contralateral Auditory Pathways over I p s i l a t e r a l Pathways 19 2.4.1 Electrophysiological Studies 19 2.4.2 The I n t e l ! i g i b i 1 1 i t y of Distorted Speech 20 2.4.3 Competing Dichotic Message Tests 20 2.5 The Development of Compensatory Mechanisms . . . . 22 2.6 Hemispherectomized Patients 28 CHAPTER v PAGE 3. AIMS OF THE EXPERIMENT 31 3.1 Statement of the Problem 31 3.2 Rationale 32 4. METHOD 35 4.1 General Outline 35 4.2 Preparation of Materials 36 4.3 Stimulus Words 38 4.4 Subjects 39 4.5 Pretest Conditions 41 4.6 Presentation of Materials 43 4.6.1 Calibration 43 4.6.2 Presentation of Pretest Conditions 43 4.6.3 Presentation of Dichotic A and Dichotic B Test Conditions 44 4.7 The Competing Dichotic Message Test 44 5. RESULTS 47 6. DISCUSSION 51 REFERENCES 61 APPENDIX 1 - L i s t 1, L i s t 2, and L i s t 3 from the Northwestern University Test No. 6 64 APPENDIX 2 - Di c h o t i c a l l y Presented Word Pairs 6 5 vi LIST OF TABLES TABLE PAGE 5.1 Scores Obtained on the Pretest, and on the Three Dichotic Listening Tests 48 5.2 Z Scores Indicating the Deviation in Standard Deviation Units of the Raw Scores of the Hemispherectomees from the Mean Scores of the Normals 49 vi i LIST OF FIGURES FIGURE PAGE 2.1 Diagrammatic Representation of Model 1 13 2.2 An Elaboration of Model 1 13 2.3 Diagrammatic Representation of Model 2 15 2.4 Diagrammatic Representation of the Auditory Path-ways U t i l i z e d when Words are Presented to the Left Ear of a Commissurotomized Subject 15 4.1 A Block Diagram of the Equipment Used for the Preparation of the DBFF Test s. 37 4.2 A Block Diagram of the Equipment Used for Presentation of the DBFF Test 45 ACKNOWLEDGEMENTS wish to thank a l l those who had a part in this the members of my thesis committee, Dr. John Delack, Dr. Juhn Wada, and esp e c i a l l y , Dr. John Gil b e r t . a l l the subjects who participated. David Roberts who f i r s t t o l d me about binaural integration. Margaret Roberts, Jim Pearse and Dr. Gannon for t h e i r cooperation in allowing me to use the f a c i l i t i e s of the Audiovestibular Unit at VGH. W i l l , for his w i l l i n g help in transporting equipment and subjects. Sharon, Lyn, Pat, Ingrid and Marilyn for t h e i r encouragement and good s p i r i t s . 1 CHAPTER 1 INTRODUCTION The rationale for a r e l i a b l e audiological diagnostic test must ultimately rest on a sound knowledge of the functional organiza-tion of the auditory system. The understanding of the anatomy and physiology of the peripheral auditory system i s at a r e l a t i v e l y advanced stage. Pathologies accompanying disorders of the middle ear, the cochlea and the eighth nerve in the peripheral auditory system have been well delineated and ef f e c t i v e audiological tests designed, which have proven r e l i a b l y diagnostic in the l o c a l i z a t i o n of the s i t e of the disorder. At a more germinal stage i s the study of disorders of the-central auditory nervous system -- usually defined as that portion of the auditory system beyond the cochlear nuclei in the brainstem. The representation of the acoustic input to each ear in both hemispheres i s realized by the projections of both contralateral and i p s i l a t e r a l auditory pathways beyond the cochlear nuclei. The r e l a t i v e contributions of the i p s i l a t e r a l and contralateral pathways, however, i n the perception and discrimination of acoustic stimuli can be studied only in very special circumstances. Indeed, knowledge of the organization of the central auditory system i s gleaned mainly by studying the defects in the perception of acoustic stim u l i r esulting from a pathological locus at some known level of the auditory system or by the surgical removal of a part of the system. 2 The case of a person who has undergone a complete u n i l a t e r a l hemispherectomy presents a special circumstance in which from each ear there i s only one e f f e c t i v e auditory pathway to the remaining hemisphere, the pathway from the cont ra latera l ear containing a greater number of neuronal f ib res than from the i p s i l a t e r a l ear . Dichot ic l i s t e n i n g tests u t i l i z e the technique of simultaneous presentation of d i f f e r e n t acoustic s t imul i to each ear . The messages in a d ichot ic l i s t e n i n g test can be e i ther (1) competing, as in the condit ion when a d i f fe ren t word i s presented simultaneously to e i ther ear or (2) complementary — as when mutually exclusive frequency seg-ments of the same word are the acoust ic s t i m u l i . In a d ichot i c message presentation to a hemispherectomized subject there i s only one dest inat ion a representation of each acoust ic message must reach before a verbal report of the words i s poss ib le . Thus, the presentation of d ichot ic tests to a person who has undergone surgical removal of an ent i re hemisphere affords a unique opportunity to invest igate the r e l a t i v e contr ibut ions of the i p s i l a t e r a l and cont ra latera l pathways in the processing and t ransmitt ing of complex acoust ic s t imul i to the i n t a c t hemisphere. The dangers inherent in using studies conducted so le l y on brain-damaged ind iv iduals to make general izat ions concerning the funct ional organization of brain structures in the normal population are well recognized. I t i s , however, accepted that to shed some l i g h t on the mechanisms of auditory perception in man, researchers must s t i l l re ly heavi ly on examination of the types of auditory analys is that can or cannot take place in the absence of auditory cortex. 3 CHAPTER 2 REVIEW OF LITERATURE 2.0 Introduction A survey of the l i t e r a t u r e pertinent to this research i s presented in six sections. Section 2.1 describes the peripheral and central auditory pathways beyond the cochlea. Section 2.2 reviews audiological techniques which have been developed to aid in the diagnosis of central auditory lesions. Section 2.3 deals with the f u n c t i o n a l i t y of the i p s i l a t e r a l pathways i n the central auditory system and Section 2.4 reviews some of the research indicating that the contralateral auditory pathways are prepotent to the i p s i l a t e r a l pathways. Section 2.5 discusses the a b i l i t y of the nervous system of brain damaged individuals to develop compensatory mechanisms, while Section 2.6 discusses s p e c i f i c a l l y the a b i l i t i e s of hemispherectomized patients. In t h i s l i t e r a t u r e review and throughout the thesis, the words fusion and integration w i l l be used interchangeably. 2.1 The Auditory Pathways Excitation of the hair c e l l s of the cochlea causes electro-t h chemical stimulation of the afferent endings of the 8 nerve fib r e s whose bipolar c e l l s are grouped together in the s p i r a l ganglion. These eighth nerve fibres enter the pons l a t e r a l l y and s p l i t to synapse with c e l l bodies which comprise the dorsal and ventral cochlear 4 nuc l e i . Beyond the cochlear nuclei in the central auditory pathways to the cortex are the following c e l l stations or location of c e l l bodies where synaptic connections may occur: the superior o l i v e , the nuclei of the l a t e r a l lemniscus, the i n f e r i o r c o l l i c u l u s on the roof of the midbrain, and the medial geniculate body in the thalamus which projects to Heschel's gyrus (the primary auditory cortex) of the temporal lobe. Between cochlear nuclei and the medial geniculate body are at least two neurons, the synapses being located in any of the three intermediate c e l l masses. Approximately 60% of the axonal fibres from the cochlear nuclei cross to the contralateral superior o l i v e , while the remaining 40% project to the i p s i l a t e r a l superior o l i v e . Thus, each cochlea i s represented i n the auditory cortex of each hemisphere. The bundle of fib r e s between the superior o l i v e and the i n f e r i o r c o l l i c u l u s i s the l a t e r a l lemniscus and the f i b r e t r a c t between the nuclei of the i n f e r i o r c o l l i c u l u s and the medial geniculate body i s c a l l e d the brachia of the i n f e r i o r c o l l i c u l u s . Beyond the primary auditory area, further pathways l i n k the auditory cortex with speech and language centres and with association areas for other sensory modalities. 2.2 Detection of Central Auditory Lesions 2.2.1 Distinguishing Brainstem Lesions from Temporal Lobe Lesions The two p r i n c i p a l l o c i of disorder in the central auditory system are the brainstem nuclei and the primary auditory projection 5 area. The development of e f f e c t i v e c l i n i c a l diagnostic techniques for the evaluation of central auditory problems has been inext r i c a b l y linked to research proving that contralateral auditory pathways are functionally superior to i p s i l a t e r a l pathways. Bocca and co-workers in I t a l y investigated the i n t e l l i g i b i l -i t y of speech under a variety of conditions designed to reduce the excess of information contained in the spoken word. In the f i r s t monaural test suggested by Bocca, Calearo, and Cassinari (1954) low frequency f i l t e r e d words were presented to the l e f t , then the r i g h t ears of patients with uni l a t e r a l temporal lobe lesions and the patients were scored on the number of words correctly repeated. In tests devised subsequently by these workers, the redundancy of monaurally presented f i v e word sentences was reduced by temporal inte r r u p t i o n , acceleration, or simultaneous presentation of i n t e r f e r i n g speech applied to the same ear. In a l l of these tests a decrease of the i n t e l l i g i b i l i t y of speech was c l e a r l y observed in the ear contralateral to the pathological temporal lobe although no difference was apparent using undistorted speech. Furthermore, these patients demonstrated v i r t u a l l y normal pure tone s e n s i t i v i t y in both ears. These results seemed to indicate that the contralateral pathway i s the more important route to the brain. Furthermore, upon monaural stimulation of the ear contralateral to the pathological temporal lobe, either the i p s i l a t e r a l pathways are not functionally adequate to process/transmit the speech material to the unimpaired temporal lobe for decoding there, or, the message received by the i p s i l a t e r a l pathway projection area in the unimpaired temporal lobe 6 i s not adequate for correct i d e n t i f i c a t i o n of the word. A further discussion concerning contralateral prepotence over i p s i l a t e r a l pathways follows in a l a t e r section. The following generalizations concerning the nature of central auditory disorders emerged: (1) They did not affect the normal s e n s i t i v i t y for pure tones i n either ear. (2) They were mani-fested by d i f f i c u l t y in understanding speech in the ear contralateral to the pathological s i t e i n the brain. (3) By taxing the central auditory nervous system (CANS) by reducing the redundancy information in speech, an evaluation of central auditory function could best be obtained. Jerger (1970a,b) has developed a central auditory test battery based on the p r i n c i p l e of overloading the system. The rationale for the test rests on the assumption that contralateral pathways are prepotent to i p s i l a t e r a l pathways. I t i s designed to obviate d i f f e r -ences i n ear performances on speech i d e n t i f i c a t i o n tasks under reduced redundancy conditions when there i s absence of ear differences for pure tone s e n s i t i v i t y . The test i s known as the Synthetic Sentence I d e n t i f i c a t i o n Test (SSI) i n conjunction with (1) I p s i l a t e r a l Competing Message (ICM)-- sentences and competing running discourse presented to the same ear and (2) Contralateral Competing Message (CCM) -- presen-tation of sentences and competition to opposite ears. The test sentences are presented at successive message to competition ratios and the patient must ignore the running discourse and pick out the message. Patients with tumours of the brainstem t y p i c a l l y showed a 7 large performance d e f i c i t on the SSI-ICM conditions when the message and the competition were presented to the ear contralateral to the pathology but r e l a t i v e l y normal results when the signals were presented to the i p s i l a t e r a l ear. The e f f e c t for temporal lobe patients on this test i s much less severe than in the case of brain-stem disorder. The CCM test i s especially sensitive to disorders at the temporal lobe l e v e l . There i s a marked performance d e f i c i t when the sentences are presented to the ear contralateral to the lesion and the competition to the i p s i l a t e r a l ear; patients with brainstem disorders usually have l i t t l e trouble with the CCM condition. The observation that patients with brainstem disorders have greater d i f f i c u l t y with a d i f f i c u l t monaural test than with a dichotic presentation would seem to indicate that the information overload i n the contralateral pathway which constitutes the I CM condition i s too great to be processed/filtered at the brainstem level and thus, the s i g n i f i c a n t performance d e f i c i t in the sentence i d e n t i f i c a t i o n task. The fact that temporal lobe patients have r e l a t i v e l y less d i f f i c u l t y . with this test would seem to indicate that t h i s type of information overload presents less trouble for these patients since the i n t a c t brainstem nuclei e f f e c t i v e l y do t h e i r job, perhaps by i n h i b i t i n g the competing message. Generally i t appears that patients with temporal lobe disorders show more d i f f i c u l t y with simultaneous dichotic message tasks than with d i f f i c u l t monaural tasks. The rationale for this observation for the CCM could be that the defective temporal lobe cannot process the sentences received via the contralateral ear 8 due to the i n e f f e c t i v e i n h i b i t i o n of the competing message received v ia the corpus callosum from the in tac t temporal lobe. The d i f f i c u l t y in d i f f e r e n t i a t i n g lesions wi th in the brainstem and les ions at the temporal lobe level by audio logical diagnost ic techniques has long been recognized. Jerger (1960) proposed a general p r i n c i p l e that : "the behavioral manifestations of disorders at d i f f e r e n t s i t e s within the auditory system are determined by the unique function of that s i t e in the chain of events leading u l t imate ly to auditory percept ion." (Jerger, 1973, p. 92). However, a de l ineat ion of the role played by the brainstem nuclei and re lated pathways in the transmission of complex s ignals i s only in the germinal stages. The nuclei of the superior o l i v a r y complex const i tute the f i r s t synaptic connection a f t e r decussation of the auditory f ib res from the cochlear n u c l e i . Converging at t h i s r e l a y , then, are f ib res from both the i p s i l a t e r a l and cont ra latera l ear. A tes t designed by Bocca (1960) and Calearo (1960) which was considered to be diagnost ic of brainstem pathologies consisted of a vocal message which was o s c i l l a t e d p e r i o d i c a l l y between one ear and the other for equal periods of time so that each ear received ha l f of the message. The messages consisted of short , simple sentences and the period of o s c i l l a t i o n could be varied between 2 and 40 a l ternat ions per second. Normal subjects invar iab ly scored 100% in repeating these sentences as the segments were a l te rnate ly switched between l e f t and r ight ear. Likewise pat ients with d i s c r e t e , i so la ted pathologies of the primary auditory cortex had no d isc r iminat ion problems when repeating the sentences. However, in a number of cases of les ions of the brainstem, patients performed poorly on th is tes t . These resu l ts seem to 9 indicate that integration of the two complementary parts of the message occurred at the brainstem level and was dependent on the integration of the message received via the i p s i l a t e r a l pathway from one ear with the segment received via the contralateral pathway from the opposite ear. A discrete, unilateral temporal lobe pathology did not cause a poor discrimination score since at least the intact temporal lobe would receive the message, already integrated at the level of the brainstem after major decussation of the auditory pathways. Based on this research, Jerger (1964) developed the SWAMI Test (speech with alternate masking index). In th i s t e s t , speech i s switched alternately between the ears, four times per second. The other ear receives noise at 20 dB greater i n t e n s i t y than the speech. When the speech i s in one ear, the noise i s present i n the other and vise versa. The SWAMI i s considered to be a type of binaural integra-tion test diagnostic of brainstem dysfunction. 2.2.2 Dichotic Binaural Frequency Fusion Tests Another of the more hopeful diagnostic techniques used to assess the i n t e g r i t y of the brainstem pathways which has alternately been studied, neglected and resurrected i s the usage of dichotic binaural frequency fusion tests. When a word that has been passed through a low frequency band pass f i l t e r i s presented to one ear and the same word but having been passed through a high frequency band pass f i l t e r i s presented simultaneously to the opposite ear, some type of central integration process occurs and a person with normal auditory function can repeat the word. With presentation of only one 10 of the signals either monaurally or binaurally, discrimination i s poor, but the simultaneous presentation of the two semi-spectra of the vocal message allows a normal person to at t a i n a near perfect discrimination score. The test has variously been called (1) a binaural speech  integration t e s t , (2) a binaural summation te s t , (3) a dichotic  integration test of two ear frequency fusion, and more recently, (4) a dichotic binaural fusion tesjt_ (DBF t e s t ) . The designation of dichotic binaural frequency fusion test (DBFF test) i s thought to be the most appropriate inasmuch as both ears are stimulated, but each ear must get a di f f e r e n t frequency segment of the same word from well separated channels of a tape recorder via earphones (a dichotic presentation). In a DBFF t e s t , although the messages to either ear are d i f f e r e n t , they are not "competing" as in the condition when a d i f f e r -ent word i s presented simultaneously to either ear, but rather, the messages in th i s task are complementary, i . e . mutually exclusive frequency segments of the same word. The observation that normal d i s -crimination scores are obtained when complementary frequency segments of the same word are presented simultaneously to either ear, would indicate that at some level of the CANS, fusion (integration or sum-mation) has occurred at a synaptic juncture. Matzker (1959) developed one of the e a r l i e s t DBFF tests. A phonetically balanced l i s t of 41, two s y l l a b l e German words was f i l t e r e d to derive two bands, 500 - 800 Hz for the low band pass f i l t e r e d segment and 1815 - 2500 Hz for the high band. Each band, 11 by i t s e l f , when presented binaurally was too narrow to allow recognition of the test word (an i n t e l l i g i b i l i t y of not more than 26% and 30% respectively). These bands were presented at the int e n s i t y at which a number of f i l t e r e d questions reached maximum i n t e l l i g i b i l i t y . The test was presented in three sections: (1) D i c h o t i c a l l y — a l i s t of words was presented with the high band going to one ear and the low band simultaneously to the other, (2) D i o t i c a l l y -- the same words presented again but with both bands presented simultaneously to each ear, (3) the th i r d l i s t was presented d i c h o t i c a l l y in the same manner as the f i r s t . Matzker noted that normal subjects made few mistakes on each of the three t e s t s , attaining 100% discrimination scores for both the dichotic and d i o t i c conditions. However, patients with brainstem lesions did poorly on the dichotic tests when one frequency segment simultaneously reached either ear, especially in comparison to t h e i r score on the d i o t i c presentation i n which both segments went to both ears. On autopsy of the patients who had d i f f i c u l t y on the DBFF t e s t , Matzker found microhemorrhages with c a p i l l a r y thrombosis, edema and degeneration of ganglion c e l l s throughout the o l i v a r y regions of the brainstem. Matzker maintained that the DBFF test explored s p e c i f i c a l l y the integration of the two frequency segments presumed to take place in the brainstem where i p s i l a t e r a l nerve fib r e s from one ear synapsed with the contralateral nerve fib r e s from the opposite ear. He concluded that normal recognition of the test words appeared to indicate good functioning of the synaptic connections in the brainstem region. 12 The neural model, Model 1, (see Figure 2.1) postulated as a rationale for the dichotic presentation, suggests that the signals in the contra-l a t e r a l and i p s i l a t e r a l auditory pathways integrate to form a single encoded message at the brainstem level to be transmitted via the l a t e r a l lemniscus to the higher c o r t i c a l centres. Bocca and Calearo (1963) support the view that DBFF tests must be considered s p e c i f i c to the brainstem, not only because primary decussation occurs at this level but also because many contralateral efferent impulses originate here which activate or i n h i b i t peripheral e x c i t e a b i l i t y . Accordingly, they maintain that binaural integration of complementary frequency segments from either ear should not be discurbed i n cases of discrete temporal lobe pathology; since the i n t a c t hemisphere receives the dichotic message af t e r being integrated in the brainstem, the non-functioning c o r t i c a l area of the primary auditory cortex of the opposite hemisphere should not hinder i t s comprehension. Matzker's research likewise lent credence to the hypothesis that a u n i l a t e r a l temporal lobe lesion should not much aff e c t binaural integration since one temporal lobe at least would apprehend the already integrated message, (see Figure 2.2) Linden (1964) tested the hypothesis that a monaural reduction of i n t e l l i g i b i l i t y for f i l t e r e d speech would necessarily involve a simultaneous reduction of the a b i l i t y of the auditory centre to u t i l i z e the information from the same ear in a DBFF test. With unil a t e r a l temporal lobe damage a decline of i n t e l l i g i b i l i t y in the monaural distorted speech test has repeatedly been shown in the contralateral ear. Figure 2 . 1 Diagrammatic representation of Model 1 hypothesizing that binaural i n t e g r a t i o n of complementary frequency semi-spectra occurs a t the brainstem l e v e l . Figure 2 . 2 An elaboration of Model 1 i l l u s t r a t i n g that a d i s c r e t e tern-oral lobe l e s i o n should not much a f f e c t scores on a DBFF t e s t since the intact_ : temporal lobe aoorehends the alre-idy integrated message. 14 Linden used nine, fifteen-word, word l i s t s f i l t e r e d to obtain two bands: 560 to 715 Hz and 1800 to 2200 Hz. The f i l t e r had a reported rejection rate of 60 dB/octave. In a total of 6 out of 18 cases of patients with expanding i n t r a c r a n i a l lesions, Linden found s i g n i f i c a n t l y low values for the discrimination scores on the DBFF test. However, these six patients also obtained s i g n i f i c a n t l y low discrimination scores for monaurally presented frequency distorted speech. In Matzker 1s binaural resynthesis t e s t , the i n t e l l i g i b i l i t y for frequency distorted speech had been judged in both ears simultan-eously, however, monaural tests for distorted speech had not been performed. Linden's results indicated that because one temporal lobe was not adequately apprehending the information i t received via the contralateral pathway then i t would follow that in a DBFF test the discrimination scores would be lower than those of a group of normal subjects, since the signals received by the two temporal lobes could not be adequately integrated for successful discrimination. Therefore, in the above cases of l o c a l i z e d , u n i l a t e r a l temporal lobe lesions, lower than normal scores obtained on a monaur-a l l y presented distorted speech test would seem to predict concomitantly lower than normal scores also, on a DBFF te s t . Neural Model 2 (see Figure 2.3) could be postulated from these r e s u l t s . The underlying assumption i m p l i c i t in t h i s model i s that binaural integration of the d i c h o t i c a l l y presented, two frequency bands does not occur at the brainstem l e v e l , since i f this were the Figure 2 . 3 Diagrammatic representation of Model 2. hypothesizing that b inaura l in tegra t ion of comolementary frequency semi-soectra requires two i n t a c t temooral lobes . F'irure 2 . 4 Diagrammatic representation of the auditory pathways u t i l i z e d when words are presented to the l e f t ear of a commisrurotomized subject. case the intact temporal lobe would apprehend the integrated message and results on the DBFF test would be near normal. Rather, i t would seem that occlusion of the information carried in the i p s i l a t e r a l pathways occurs at the brainstem l e v e l , and the acoustic message received at the primary auditory cortex of one temporal lobe i s not composed of information from both frequency bands but only from the frequency band presented to the contralateral ear. Another tenable hypothesis i s that a comparison of the two integrated messages received by both primary auditory cortices occurs in the dominant hemisphere, and on the successful comparison hinges the a b i l i t y to recognize the results of fusion in the brainstem as an i d e n t i f i a b l e set of acoustic elements, retrievable from storage as a word. Ohta e_t al_. ( 1 9 6 8 ) produced a Japanese version of Matzker's t e s t , with bands of 3 0 0 to 6 0 0 Hz and 1200 to 2400 Hz from nonsense s y l l a b l e s . The results of t h e i r research were also in disagreement with Matzker's findings, i.e. that poor results on the DBFF test r e f l e c t s dysfunction in the auditory pathways at the level of the brainstem. Their results indicated that the phenomenon of integra-tion or fusion of the frequency segments i s affected not only by the function of the brainstem, but also by the function of the c o r t i c a l or subcortical pathway. Ohta's results suggested p a r t i c u l a r l y that binaural fusion i s poor when i t i s the high band pass f i l t e r e d segment that i s delivered to the ear contralateral to the cerebral l e s i o n . Matzker's theory held that i n cases of temporal lobe l e s i o n , binaural fusion would be i n t a c t , since, regardless of which side the 17 lesion was on, binaural integration having already occurred at the brainstem l e v e l , at least one temporal lobe would be functionally adequate for processing of the acoustic message. In a study e n t i t l e d "Dichotic Listening in Man a f t e r Section of the Neocritical Commissures," Sparks and Geschwind (1968) presented a somewhat d i f f e r e n t binaural fusion test to t h e i r commissurotomized patient involving the following: an object name was presented to one ear at a sound level which allowed for a score of about 50% on preliminary testing. The other ear received the same word simultaneously at SL plus 44 dB but distorted by a low pass f i l t e r . The subject.was asked to repeat the word. Bocca (1955), Calearo (1957) and Jerger (1960) had shown that discrimination i s 100% in normal subjects, but does not exceed the best monaural discrimination i n subjects with auditory cortex pathology. The commissurotomized patient's performance showed no indication that integration of the information from the two ears had occurred and the authors recommended further study of the problem to test the preliminary hypothesis that "summation normally requires an intact c a l l o s a l pathway." (p. 15) More recently, a Dichotic Binaural Fusion Test (DBF t e s t ) , as i t was c a l l e d , was developed at the Washington Hospital Centre by Smith and Resnick (1972). The DBF test was designed primarily to d i f f e r e n t i a t e brainstem from temporal lobe pathologies and i s based on Matzker's o r i g i n a l work. They presented two frequency bands from English PB l i s t s , one from 360 to 890 Hz and the other 1750 to 2200 Hz. Presentation level of the low frequency band was 30 dB SL and that of the high band 10 dB re level of the low frequency band. There were three test condition: (1) Dichotic A - the low frequency band to the l e f t ear and the high to the r i g h t ; (2) Diotic - both bands to both ears; and (3) Dichotic B - the reverse of Dichotic A. Dichotic A and B reflected central fusion and the Diotic condition served as a reference. The test was scored positive i f the d i o t i c score was s i g n i f i c a n t l y better than one or both of the dichotic scores, negative i f the three scores were about the same. The results indicated that for normal subjects there was no s i g n i f i c a n t difference in the test scores among the three test conditions. For subjects with temporal lobe lesions the DBF test was also negative, i . e . the three scores were in close approximation, but for subjects with brainstem lesions the DBF test was positive i . e . the subjects scored from 18 to 34% better on the d i o t i c test than for at least one of the dichotic conditions. The results of these preliminary findings would seem to be i n keeping with Matzker's i n i t i a l premise that binaural integration occurs at the brainstem 1evel. Preliminary to examining the question concerning where in the CANS binaural integration of d i c h o t i c a l l y presented complementary frequency segments i s located, a review of what i s known about the r e l a t i v e contributions of i p s i l a t e r a l and contralateral pathways in speech processing i s necessary. 2.3 Functionality of the I p s i l a t e r a l Pathways The fact that i p s i l a t e r a l pathways alone are p h y s i o l o g i c a l l y adequate to encode and transmit speech material i s demonstrated by 19 the fact that a l l the commissurotomized patients in a study by Milner, Taylor and Sperry (1968) were able to report d i g i t s to thei r l e f t ear when there was no competing input to t h e i r right ear. (Refer to Figure 2.4) The f u n c t i o n a l i t y of the i p s i l a t e r a l auditory pathways was also demonstrated by Goldstein et al_. (1956) in t h e i r paper e n t i t l e d "Hearing and Speech in I n f a n t i l e Hemiplegia Before and After Left Hemispherectomy." The average speech discrimination score f o r the l e f t ear on a p a r t i c u l a r l i s t of words for 4 people after l e f t hemispherectomy was 87% indicating that the i p s i l a t e r a l pathways were adequate for speech processing. 2.4 Prepotence of Contralateral Auditory Pathways over I p s i l a t e r a l  Pathways 2.4.1 Electrophysiological Studies The accumulation of data proving the prepotence of the contra-l a t e r a l pathways i s impressive. Rosenzweig (1951) recorded the e l e c t r i c a l responses in the auditory cortex of the cat when acoustic c l i c k s were delivered to f i r s t one ear and then the other. At each hemisphere, the response to stimulation of the contralateral ear was s i g n i f i c a n t l y larger in amplitude than the response to the stimulation of the i p s i l a t e r a l ear. When both ears were stimulated simultaneously the response was greater than for contralateral stimulation alone, but not as large as the algebraic sum of a contralateral and i p s i l a t e r a l response. The results were interpreted to mean that each ear i s 20 represented by a population of c e l l s at the auditory cortex of both cerebral hemispheres, the population representing the contralateral ear being larger than the population representing the i p s i l a t e r a l ear. The two populations were presumed to overlap. Penfield and Erikson (1941), showed that e l e c t r i c a l stimula-tion of the auditory cortex in man gives the sensation of the ringing of a b e l l , w h i s t l i n g , buzzing, droning or tapping in the ear contralateral to the stimulated hemisphere. 2.4.2 The I n t e l l i g i b i l i t y of Distorted Speech Bocca et a]_. (1954) demonstrated that there was a decrease in the i n t e l l i g i b i l i t y low pass f i l t e r e d speech in the ear contra-l a t e r a l to a pathological temporal lobe. 2.4.3 Competing Dichotic Message Tests The most convincing evidence of the functional prepotency of the contralateral over the i p s i l a t e r a l pathways comes from the results obtained from commissurotomized patients on dichotic l i s t e n i n g tasks. Kimura (1961a), showed that i f pairs of contrasting d i g i t s were presented simultaneously to l e f t and right ears of normal subjects, those presented to the right ear were more accurately reported. She hypothesized that the l a t e r a l i t y e f f e c t could be accounted for by the assumptions of l e f t cerebral dominance for speech and the pre-potence of contralateral auditory pathways to i p s i l a t e r a l pathways (Kimura, 1961b). Thus, the right ear i s better in recognizing d i c h o t i c a l l y presented verbal stimuli since i t i s contralateral to the l e f t cerebral hemisphere which i s dominant for language and speech functions. The right ear message has more rapid access to the l e f t cerebral hemisphere via the di r e c t contralateral pathway in comparison to the l e f t ear input which must travel i t s contralateral pathway to the right hemisphere and then via the corpus callosum to the l e f t "speaking" hemisphere for oral report. Strong supporting evidence for this interpretation comes from the work of Milner, Taylor and Sperry (1968) and Sparks and Geschwind (1968). These investigators studied right-handed patients who were l e f t cerebral dominant for speech and whose corpus callosum l i n k i n g the cerebral hemispheres had been completely sectioned to r e l i e v e epilepsy. Under dichotic stimulation, patients repeated the verbal stimuli presented to the right ear but were unable to report any of the words presented to the l e f t ear. These results suggested the following: 1. That under dichotic stimulation the message t r a v e l l i n g in the contralateral auditory pathway takes precedence over the message t r a v e l l i n g in the i p s i l a t e r a l path-ways . 2. The right ear input takes precedence over the l e f t ear input by virtue of i t s di r e c t access via the contralater-al pathway to the l e f t "speaking" hemisphere, in comparison to the longer route tra v e l l e d by the l e f t ear input which goes via the right hemisphere and the corpus callosum to the l e f t hemisphere. This extra transcallosal synapse, 22 and the concomitant time delay puts the l e f t ear information at a disadvantage when competing for oral report with the right ear input in the l e f t hemisphere, and thus the oft-reported right ear effect or dominance in competing dichotic l i s t e n i n g tasks in normals; 3. After c a l l o s a l section the l e f t ear input which reaches the right hemisphere no longer has access to the l e f t hemisphere for oral report and thus a 100% extinction of information to the l e f t ear results when dichotic tasks are given to commissurotomized patients. In such patients, only right ear words are reported, since suppression of i p s i l a t e r a l pathways occurs under dichotic conditions and the section of the commissural pathways prevents the transfer of acoustic information from the right temporal lobe (which has arrived via the decussating pathway from the l e f t ear) to the l e f t temporal lobe. 2.5 The Development of Compensatory Mechanisms In 1963, Sparks and Geschwind presented interesting observations on t h e i r patient W.J. who had undergone complete section of the neo-co r t i c a l commissures for epilepsy. As previously mentioned, the patient showed complete extinction of signals received by the l e f t ear both when the competing signals to either ear were d i g i t s and when they were animal names. However, with repeated retesting on the dichotic d i g i t s task, W.J.'s l e f t ear score improved from 0% detection (100% extinction) to 35% detection (65% extinction) which suggested to Sparks and Geshwind that by s p e c i f i c practice the l e f t temporal lobe may eventually begin to separate out messages coming via the weaker i p s i l a t e r a l pathway in dichotic l i s t e n i n g , although i t normally i n h i b i t s them in this competitive s i t u a t i o n . The improved performance implied that simultaneous/coincidental transmission in i p s i l a t e r a l and contralateral pathways of non-ccmplementary speech to the l e f t temporal lobe occurred in this commissurotomized patient. The following questions were f e l t to pertain to these r e s u l t s : 1. Under dichotic l i s t e n i n g conditions in normals, does thi s same simultaneous transmission of information i n the i p s i l a t e r a l and contralateral pathways occur, and therefore, does the suppression of the acoustic message in the i p s i l a t e r a l pathway occur i n the temporal lobe rather than at a subcortical level? 2. In the commissurotomized patient does the improved performance on l e f t ear report after practice sessions in dichotic l i s t e n i n g r e f l e c t the "unlearning" of contralateral suppression of i p s i l a t e r a l encodings in the temporal lobe or in the brainstem nuclei? No comment was made concerning an improvement i n this person's performance on the dichotic animal name l i s t e n i n g task. Kimura (1967) has suggested that contralateral suppression takes place at two di f f e r e n t l e v e l s . F i r s t , the inputs a r r i v i n g along the i p s i l a t e r a l pathway are occluded at a subcortical level by the contralateral inputs to such an extent that very l i t t l e of the information from the i p s i l a t e r a l ear ever reaches the auditory cortex by the i p s i l a t e r a l pathway. Secondly, she suggests a factor of c o r t i c a l competition (or occlusion) also. The question of sub-c o r t i c a l occlusion i s an interesting one in l i g h t of the further investigations by Sparks and Geschwind (1968) with t h e i r patient W.J. In the dichotic t e s t , in which the right ear received distorted speech while the l e f t ear received normal signals both well above thresholds, i t was found that as the r i g h t ear score (ear contra-l a t e r a l to the language dominant hemisphere) improved with decreasing d i s t o r t i o n , the l e f t ear score reflected increasing suppression. This result suggested that suppression of the l e f t ear signal w i l l occur only i f there i s a marked s i m i l a r i t y in the signals being transmitted in the contralateral pathway from the r i g h t ear and the i p s i l a t e r a l pathway from the l e f t ear. The question can then be posed: "At what level i s this 'recognition' process accomplished?" i . e . are the brainstem nuclei mainly transmitting relay stations or do they act as f i l t e r s in such a way as to c l a s s i f y acoustic messages as speech or non-speech in order to "know" whether contralateral suppression of the i p s i l a t e r a l pathways i s i n order? Because white noise and indistinguishable babble produced by multiple voices presented to the right ear did not lead to extinction of the verbal material presented to the l e f t ear, and i f we are to assume the subcortical occlusion theory as tenable, then the brainstem nuclei could be thought of as f i l t e r s which process the messages comparatively -- i f the i p s i l a t e r a l message i s speech-like and the contralateral message also has the essential acoustic characteristics of speech, 25 then the brainstem nuclei would f a c i l i t a t e the suppression of the i p s i l a t e r a l message by the contralateral message. Strengthening of i p s i l a t e r a l pathways would then e n t a i l the "unlearning" at the i n h i b i t o r y synapses at the brainstem level to allow the throughway transmission of messages from the i p s i l a t e r a l ear. Milner, Taylor and Sperry (1968) also reported 100% suppres-sion of the input to the l e f t ear by 2 commissurotomized patients out of a group of 7 under dichotic stimulation, the input once again being d i g i t s . Under monaural conditions, these c a l l o s a l patients correctly reported 87% of the numbers channelled to the l e f t ear and 90% of the numbers channelled to the righ t ear showing that the i p s i l a t e r a l pathway could be u t i l i z e d . The two commissurotomized patients, who showed some success in reporting l e f t ear material, were referred to as "experienced examinees with respect to other modalities" (page 185), once again indicating that the a b i l i t y to use information presented d i c h o t i c a l l y improves with practi.ce a l b e i t by virtue of decreased "subcortical occlusion" or by decreased " c o r t i c a l occlusion." Bryden and Zu r i f (1971) conducted a study on the dichotic l i s t e n i n g performance of a 15 year old boy of normal i n t e l l i g e n c e with congenital agenesis of the corpus callosum. His performance did not d i f f e r appreciably from that of the group of normal control subjects. This result was in sharp contrast to the performance of those patients who underwent surgical section of the corpus callosum after adolescence. This boy's performance would seem to indicate one of two conclusions: either (1) that contralateral suppression of i p s i l a t e r a l pathways had not developed-i.e. that the subcortical occlusion did not occur nor was the suppression at the c o r t i c a l level of s u f f i c i e n t strength to extinguish the i p s i l a t e r a l material, or (2) that i f subcortical occlusion did occur, then the boy must have b i l a t e r a l speech representation. Bryden and Zurif find the former p o s s i b i l i t y more tenable and conclude that the results of t h e i r study, combined with the evidence from other studies, "suggests that the suppression e f f e c t can be overcome with practice, and that the l a t e r a l i t y e f f e c t i n dichotic l i s t e n i n g i s more dependent upon c o r t i c a l competition than on sub-c o r t i c a l occlusion." (page 376) Another study supportive of t h i s view i s Netley's (1972) comparison of the dichotic l i s t e n i n g performance of twelve hemispherec-tomized patients to that of controls matched for age and IQ. In cases where hemispherectomy has been performed there i s only one e f f e c t i v e route from either ear to the remaining hemisphere, which i f the patient has suffered from i n f a n t i l e or congenital hemiplegia, has subsumed the associative, i n t e l l e c t u a l and language functions normally carried out by both hemispheres. By analyzing the dichotic l i s t e n i n g performance of hemispherectomized patients i t i s possible to gauge the r e l a t i v e strength of i p s i l a t e r a l and contralateral pathways. Netley found that the d i g i t material presented to the ear contralateral to the remaining hemisphere was better recalled than material presented to the other ear and that the effect for the congenitally injured group was comparable to the right ear dominance reflected i n the r e c a l l scores of the controls. The e f f e c t , 27 however, although si m i l a r in kind, differed in degree for the i n f a n t i l e injured group inasmuch as the r e c a l l scores for the ear contralateral to the removed hemisphere, were lower than those for the congenitally injured group and of the l e f t ear r e c a l l scores of the controls. Netley's results support the view that i p s i l a t e r a l pathways can become very e f f e c t i v e c a r r i e r s of d i c h o t i c a l l y presented verbal material and that some compensatory mechanism, f a c i l i t a t i n g strengthen-ing of the i p s i l a t e r a l pathway or projection area in the brain, had developed as a resu l t of i n s u l t to the hemisphere which was subsequently removed. The fact that his congenitally injured group had better r e c a l l scores for the ear contralateral to the remaining hemisphere, than did the i n f a n t i l e injured group, further indicated that t h i s compensatory adjustment was more adequately developed, the e a r l i e r the i n s u l t was sustained. Netley postulated that the further along an infant was in the language acquisition process, the more l i k e l y some "hierarchy of functional e f f i c i e n c y " (page 239) would have been established between contralateral and i p s i l a t e r a l pathways or projection areas. Thus, the l a t e r the injury and the greater degree of contralateral pre-potency the poorer would be the r e c a l l score for the ear i p s i l a t e r a l to the remaining hemisphere on a competing dichotic l i s t e n i n g task. I t i s interesting to note that d i g i t s were used as the verbal material for the dichotic l i s t e n i n g tasks in the studies mentioned which might possibly make a difference in the degree of competition offered by the material received via the contralateral pathways at the l i n g u i s t i c processing centre. I t could be 28 speculated that the degree of suppression might be greater i f the material used was less in the "automatic" or "easy to retrieve" c l a s s i f i c a t i o n than one d i g i t numbers. 2.6 Hemispherectomized Patients In attempting to construct any model of the functional organiza-tion . of brain structures, i t i s important to remember that one cannot i n f e r conclusions about the normal population from studies conducted solely on brain-damaged in d i v i d u a l s . The extent to which compensatory mechanisms have developed in people with a history of cerebral mal-function i s not ea s i l y established. Krynauw (1950) presented a series of eleven patients with i n f a n t i l e hemiplegia and epilepsy, ranging in ages from 7 months to 20 years of age, who improved a f t e r hemispherec-tomy. Ten of the patients were l e f t hemispherectomized. In none of these cases was any s i g n i f i c a n t impairment of speech noted postoperatively and in some cases there was considerable improvement. Krynauw concluded that cerebral dominance for speech had adjusted i t s e l f before hemi-spherectomy and that the speech and language functions had in a l l cases become l a t e r a l i z e d to the unimpaired hemisphere. Therefore, i t was the minor hemisphere with respect to speech and language functions that was removed in a l l cases. He also suggested that, with the removal of the pathological hemisphere, the remaining hemisphere was liberated to manifest those functions or a b i l i t i e s i t had acquired but which had been inhibited by the presence of the imperfect "other half." 29 McFie (1961) in his study e n t i t l e d : "The Effects of Hemi-spherectomy on Inte l l e c t u a l Functioning in Cases of I n f a n t i l e Hemiplegia" compared the results of pre and post-operative i n t e l l i g e n c e tests. He found that the mean change for the 34 complete hemispherec-tomized patients studied was nearly +8 points of IQ. In concurrence with Krynauw, McFie concluded that: liberated from the influence of the damaged hemisphere, the remaining hemisphere i s able to function normally, but i t i s doubtful i t can be ca l l e d 'normal function' supernormal might be more appropriate. For not only does i t perform motor and sensory functions for both sides of the body, i t performs the associative and i n t e l l e c t u a l functions normally allocated to two hemispheres." (p. 248) To what age t h i s degree of p l a s t i c i t y i s achievable i s undeter-mined, however, both Krynauw and McFie noted that those patients with i n f a n t i l e hemiplegia resulting from b i r t h injury showed no impairment of speech and greater improvement on i n t e l l i g e n c e tests following hemispherectomy than did those patients who sustained a l a t e r juvenile i n j u r y . The positive change for those injured at b i r t h or in t h e i r f i r s t twelve months of l i f e contrasted with the negative change in the group injured after one year of age. This observation raises the question: "Is there a c r i t i c a l age at which these transfers of function to areas not normally allocated the 'learning' of certain a b i l i t i e s i s possible and beyond which the p o t e n t i a l i t y for taking over i s l o s t ? " A very s i m i l a r question can be asked in reference to the p o t e n t i a l i t i e s of the strengthening i p s i l a t e r a l pathways or the increased 'demand to be heard from' of the i p s i l a t e r a l auditory projection areas in the temporal lobe. The degree to which t h i s phenomenon has occurred, i f at a l l , could be partly gauged by giving pre- and post-operative dichotic l i s t e n i n g tests to patients who w i l l undergo either a hemispherectomy or a c a l l o s a l section. To date, only post-operative studies have been undertaken. 31 CHAPTER 3 AIMS OF THE EXPERIMENT 3.1 Statement of the Problem The uncertainty of the diagnostic v a l i d i t y of the DBFF test in d i f f e r e n t i a t i n g brainstem lesions from temporal lobe lesions in the central auditory system poses the following theoretical question: "Does binaural integration occur at the subcortical (brainstem) level or at the c o r t i c a l (temporal lobe) l e v e l , i .e. does Model 1 as suggested by Matzker's, Smith's and Resnick's results apply, or does Model 2 as suggested by Linden's (1964) and Ohta's (1968) results apply?" Model 1 suggests that the complementary signals in the i p s i l a t e r a l and contralateral pathways in the DBFF test integrate to form a single encoded message at the brainstem l e v e l , whereas Model 2 suggests that the complementary messages relayed in the contralateral pathways must f i r s t reach the primary auditory cortices before they are integrated at a c o r t i c a l level via transmission i n the corpus callosum. In t h i s l a t t e r model, the information transmitted in the i p s i l a t e r a l pathway is assumed to be occluded at either the brainstem or c o r t i c a l l e v e l . A t h i r d p o s s i b i l i t y must also be considered, i . e . that the acoustic messages i n the contralateral and i p s i l a t e r a l pathways are transmitted coincidentally past the brainstem level and are integrated in the intact primary auditory cortex. By comparing the number of words correctly reported by 32 normals with the results obtained from hemispherectomized subjects on a DBFF t e s t , an attempt could be made to determine, very broadly, whether (a) binaural integration of the frequency semi-spectra of a word requires two intact hemispheres, since the contralateral pathways are known to be the major auditory pathways, or (b) integration occurs at some subcortical level or in the intact primary auditory projection area of the remaining hemisphere. 3.2 Rationale I f two intact hemispheres are necessary for binaural integra-tion of d i c h o t i c a l l y presented frequency semi-spectra of the same word, then Model 2 would be more tenable and a hemispherectomized subject should be unable to score well on the DBFF test. This would imply either that the complementary messages are not e f f e c t i v e l y integrated at a subcortical l e v e l , or, barring subcortical integration, that the intact primary auditory projection area does not e f f e c t i v e l y integrate messages received via contralateral projections from one ear and i p s i l a t e r a l projections from the other ear. I f , however, a subject scores well on the DBFF test i t would indicate that contralateral pathway suppression of i p s i l a t e r a l path-ways does not occur and that both messages are relayed beyond the brainstem nuclei and recognized as an integrated 'unit' composed of the essential acoustic elements, retrievable from storage to form a word in the intact hemisphere. Although by these results Model 2 could be eliminated, Model 1 could not be unequivocally accepted 33 since the integration does not necessarily have to occur at the brain-stem level but could occur i n the intact primary auditory projection area with the simultaneous a r r i v a l of messages in the i p s i l a t e r a l and contralateral pathways. It would also be of interest to present subjects with a dichotic l i s t e n i n g task in which the messages to either ear are competing. For subjects who have had pre-operative dichotic l i s t e n i n g tests the comparison of results would give some measure of the extent of stren-thening of the i p s i l a t e r a l pathway. In cases where hemispherectomy has been performed there i s only one eff e c t i v e route from either ear to the remaining hemisphere, the route from the i p s i l a t e r a l ear having approximately 2/3 the number of fibres as the stronger route from the contralateral ear. If individuals who have undergone complete hemispherectomies are used as subjects, surgical reports would concisely outline the pathologies -- a very d i f f e r e n t s i t u a t i o n than that of using subjects with lesions of the temporal lobe since the exact size and extent of the affected area i s rarely delineated in t h i s l a t t e r case, and thus the results of specialized audiological techniques are inconclusive and subject to further interpretation. The major aim of this experiment was to investigate the a b i l i t y of hemispherectomized subjects to fuse binaurally what can be termed "dichotic speech" inasmuch as two di f f e r e n t signals are presented simultaneously to either ear. The messages presented in the DBFF test were complementary, ( i . e . mutually exclusive frequency segments of the same word), as opposed to competing (as in the condition when a different word i s presented simultaneously to either ear). 34 More s p e c i f i c a l l y , the following null hypothesis was tested: "That binaural integration of the frequency semi-spectra of a word requires two intact hemispheres." 35 CHAPTER 4 METHOD 4.1 General Outline A dichotic binaural frequency fusion t e s t , consisting of phonetically balanced l i s t s of English monosyllables, was processed through two band-pass f i l t e r s and recorded on a two channel mangetic tape. Three f i f t y word CNC word l i s t s (Northern University Auditory Test No. 6) were used for the test vocabulary. The test consisted of two binaural conditions. In the Dichotic A condition, the high band was delivered to the l e f t ear and the low band to the right. The Dichotic B condition was the reverse of the f i r s t , so that the high band was in the r i g h t ear and the low band in the l e f t . For each condition, 50 words were presented and the subject was required to repeat the word in a 4 second interva l between words. Half of the normal subjects were presented with the Dichotic A condition followed by Dichotic B and half received the conditions in the reverse order. The test was presented at a sensation level of 30 dB re the pure tone averages in each ear for the control subjects, and 35 dB re the pure tone average for the hemispherectomized subjects. Each subject underwent conventional audiometric assessment to obtain pure tone a i r conduction thresholds and speech reception thresholds (SRT) p r i o r to the DBFF test. 36 The equipment necessary for presentation of the test consisted of a two channel tape recorder (Sony 500 A), a two channel audiometer (Madsen OB 60) and standard subject head-phones (Telephonic TDH 39). Testing was conducted in an IAC booth. 4•2 Preparation of Materials The test vocabulary was recorded on magnetic tape by a male ta l k e r . A Scully two-track stereo recorder was used to record a l l stimulus words; recordings were made in a sound treated room. Ambient noise level in the room was 20 dBA as tested by a Bruel and Kjaer Precision sound level meter. An Altec 681 A microphone was used for recording at approximately 6" mouth to microphone distance. Nine l i s t s of 50 words each were recorded on one tape at a normal conversation level with a 4 second interval between words. The o r i g i n a l tape was played back from the Scully 280 tape-recorder to a Revox two-track stereo tape recorder, (see Figure 4-1) Prior to re-recording each word, the record level on the Revox was adjusted to ensure that each word registered a peak int e n s i t y of 0 VU at the record head of the Revox. Thus a l l stimulus words were matched for peak int e n s i t y at re-recording time. A 1000 Hz c a l i b r a t i o n tone was recorded on the same tape at an in t e n s i t y which also registered 0 VU on the VU meter. The f u l l track recording was then played back from the Scully 280 to two channels of a Tandberg 64X four track recorder with a Simpson model 1349 VU meter connected to i t which registers on two motors the record levels of each channel. The record levels Tape TT I V Re vox Scul ly 280 Stereo Scul ly 280 Tape Tape Recorder \ Recorder PIayback ) Record Playback Mode Mode Mode Intensity adjustment of the test words from the o r i g i n a l tape Tape # 2 Tape # 2 Ch.2 Tandberg 6 4 X 2 channel 4 track Tape Recorder Record Mode Figure 4.1 A block diagram of equipment used for preparation of the DBFF tes t . 38 of channels 1 and 2 were adjusted such that the 1 v o l t , 1000 Hz c a l i b r a t i o n tone peaked at 0 VU for both channels and the words before f i l t e r i n g peaked synchronously between ±1 on the VU meter. Following this procedure, the f u l l track recording on which each word had been adjusted to ensure approximate equivalent peak i n t e n s i t y , was then played back on the Scully and passed through two standard Krohn-hite f i l t e r sets (Model 334 2R) to form the high band (H) and the low band (L). The low band was a one and one t h i r d octave from 360 - 890 Hz and the high band was a narrow band from 1950 - 2050 Hz. The slope of the f i l t e r s was 48 dB/octave. The two outputs of the f i l t e r s were re-recorded onto the two channels of the Tandberg tape recorder with the record knob set at the previously determined l e v e l . 4.3 Stimulus Words The Northwestern University Auditory Test No. 6 i s a c a r e f u l l y prepared and thoroughly examined set of CNC word l i s t s . There are four l i s t s of consonant-nucleus-consonant words which have the phonemic balance of the Lehiste-Peterson revised word l i s t s (1962) and which experimentally have high i n t e r l i s t equivalence and also test-retest r e l i a b i l i t y . The " a r t i c u l a t i o n functions" of a p a r t i c u l a r recording of these l i s t s have been established for normal hearing subjects. The function rises l i n e a r l y from about 8% correct at 4 dB below the SRT to 75% correct at 8 dB above SRT. The slope of this part of the curve i s 5.6% per decibel. The function then bends horizontally to a plateau of 997, correct, attained at 32 dB above SRT. The 39 description of the l i s t s appeared in a technical report of the USAF school of Aerospace Medicine (T.W. Tillman and R. Carhart, SAM-TR-66-55 in June 1966). The f i r s t three l i s t s of words were u t i l i z e d in this test and three randomizations of each l i s t were recorded. Each of the subjects received a d i f f e r e n t l i s t for each of the pretest and test conditions. 4.4 Subjects 1. The hemispherectomized subject, S.M. was a 26 year old male who had undergone a right hemispherectomy at the age of 21. B i r t h was normal, but at f i v e weeks he was found unconscious. At nine months, his mother noticed that he walked with a limp. At 2 years of age he was again found unconscious and diagnosed as having a brain hemorrhage. Seizures commenced at seven and were of a major convulsive v a r i e t y , at times preceded by a t i n g l i n g sensation over the l e f t side of the body. A series of EEG's, dating from his t h i r d year revealed the development of very active epileptogenic abnormality maximal in the right anterior head region. Subsequent investigation revealed the right sided abnormality to be of massive size i n -volving the fronto-central-temporal region. The l e f t carotid amytal speech l o c a l i z a t i o n test (Wada and Rasmussen, 1960) revealed that speech and language functions were l a t e r a l i z e d to the l e f t hemisphere. Another s i g n i f i c a n t finding during the carotid amytal test was that a profound right hemiparesis was induced by l e f t sided i n j e c t i o n but no apparent increase of l e f t hemiparesis was observed by right carotid i n j e c t i o n , nor was any speech d e f i c i t noted following right i n t r a c a r o t i d amytal i n j e c t i o n . A series of neurophysiological investigations were carried out by Professor 0. Spreen of the University of V i c t o r i a . On a competing dichotic l i s t e n i n g test S.M. showed complete extinction of signals presented to the l e f t ear, that i s , he repeated only those words which had been presented to the righ t ear. A r i g h t hemispherectomy was performed when the subject was 21. years. At operation i t was noted that the whole r i g h t posterior t h i r d of the brain consisted of whitish, atrophic cortex in which there were no s u l c i or gyral patterns. The operation was described as "a complete righ t hemispherectomy arm block resection of the whole of the righ t hemisphere exclusive of the thalamus." The septum pellucidum was not perforated and the corpus callosum was divided from rostrum to splenium. Post-operatively S.M.'s physical and mental development has improved and he now experiences no seizures. Pure tone a i r conduction thresholds revealed a moderately severe hearing loss at 2000 and 4000 Hz. The thresholds for 500 and 1000 Hz were borderline normal b i l a t e r a l l y . The pure tone average f o r the right ear was 40 dB and f o r the l e f t ear 50 dB. Speech discrimination scores were normal at 30 dB re the pure tone averages for both ears. 2. The hemispherectomized subject, R.K. was an 18 year old female who had undergone a l e f t hemispherectomy at the age of 7 and 1/2 years. Pre-operative diagnosis was "an atro-phic l e f t hemisphere." The operation was described as a "complete l e f t hemispherectomy down to and including the basal ganglia and the upper part of the l e f t thalamus." R.K. successfully completed grade 12 and i s of normal i n t e l l i g e n c e . Pure tone a i r conduction thresholds were within normal l i m i t s for both ears between the frequencies 250 to 8000 Hz., the thresholds for the l e f t ear being about 15 dB better than those for the rig h t . Speech discrimination scores were also normal at 30 dB re the pure tone average for both ears. 3. The ten right handed subjects i n the normal group ranged i n age from 21 to 29. A l l had normal hearing in both ears between the frequencies 500 to 4000 Hz as determined by pure tone testing. Speech discrimination scores were also normal in both ears when tested at 30 dB re t h e i r pure tone average. 4.5 Pretest Conditions A preliminary p i l o t investigation was conducted to specify the f i l t e r i n g band-widths and to s a t i s f y the following c r i t e r i a : 1. That the discrimination scores obtained from the binaural presentation of the low band and then from the high band should r e f l e c t an equal contribution of information to 42 the central auditory nervous system, i.e . that the obtained discrimination scores for each band be about the same; 2. That in the dichotic mode,i.e. with the high band presented to one ear and the low band to the other, the discrimination score should exceed the arithmetic sum of the scores obtained from the binaural presen-tation of each single band alone. Two 50-word CNC l i s t s were tested on a t o t a l of 10 normal hear-ing p i l o t subjects. Several variations in bandwidth were t r i e d in order to s a t i s f y the above c r i t e r i a . Bandwidths from 360-890 Hz for the low band pass f i l t e r and from 1950-2050 Hz for the high band pass f i l t e r were ultimately selected. Single and combined band i n t e l l i g i b i l i t i e s s a t i s f i e d the above requirements as shown by the obtained mean discrimination scores: (a) Low band 14.2; (b) High band 15.2; and (c) Combined High and Low Band 38.1. The f i l t e r e d l i s t s were presented binaurally to the normal control group at 30 dB re t h e i r pure tone average for the better ear. For the hemispherectomized subject S.M. the l i s t s were presented binaurally at 35 dB re the pure tone average for the better ear and for the subject R.K. the l i s t s were presented monaurally at 35 dB re the pure tone average for each ear. 43 4.6 Presentation of Materials 4.6.1 Calibration The c a l i b r a t i o n of the tape on the Audiometer OB 60 was effected as directed in the OB 60 Operation Instruction Manual. A 1 Volt, 1000 Hz c a l i b r a t i o n tone was recorded to peak at 0 VU on two channels of the Tandberg tape recorder as did the f i r s t l i s t of words which were recorded on the same tape. The tape was played back on the Sony 500 S tape recorder with the playback level for Channel 1 and Channel 2 adjusted so that the VU needle on the audiometer peaked at 0 for the c a l i b r a t i o n tone on both Channels 1 and 2 of the audio-meter. The voltages for the c a l i b r a t i o n tone at the playback head of the recorder were 0.92 and 0.90 for Channels 1 and 2 respectively. The c a l i b r a t i o n tone was played through the Telephonic TDH 39 head-phone and the Bruel and Kjaer a r t i f i c i a l ear type 4152 was placed on the sound level meter. The in t e n s i t y level for the tone at a setting of 60 dB (ISO) on the audiometer was within ±2 dB of the equivalent 82 dB SPL for both channels of the audiometer as measured by the Bruel and Kjaer Precision sound level meter for the l e f t and right head-phones. 4.6.2 Presentation of Pre-test Conditions Presentation of the low band-pass f i l t e r e d words binaurally was accomplished by connecting the Channel 1 output of the Sony 500 S recorder to the Channel 1 input of the audiometer and playing the 44 recording through Channel 1 of the audiometer to the l e f t and right head-phones. The high band-pass f i l t e r e d words were presented in the same manner, excepting that the Channel 2 output of the tape recorder was played through Channel 1 of the audiometer. 4.6.3 Presentation of the Dichotic A and Dichotic B Test Conditions The output of Channel 1 of the tape recorder was connected to the Tape 1 input of the audiometer and the Channel 2 output connected to the Tape 2 input of the audiometer (see Figure 4.2). Thus, the low band-pass f i l t e r e d segments of the words went to Channel 1 of the audiometer and the high-band pass f i l t e r e d segments of the words went to Channel 2 of the audiometer. For the Dichotic A conditions the l e f t head-phone received the high band pass f i l t e r e d segments and the righ t head-phone received the low band-pass f i l t e r e d segments of the words. The Dichotic B condition was the reverse, so that the high band was i n the rig h t ear and the low band in the l e f t . 4.7 The Competing Dichotic Message Test The competitive d i c h o t i c word test was prerecorded on dual channel tape in such a way as to permit the simultaneous presentation of stimuli to the two ears, using stereophonic head-phones. The tape was recorded by Shane P. Haydon of the University of V i c t o r i a and used with his permission. The dichotic word test consisted of 15 sets of three pairs of words, one of each pair being presented simultaneously to either ear. Each set of 6 words was separated by a 5 second i n t e r v a l . After Soundproof Booth Sony 500S Two Channel Tape Recorder Playback Mode Ch.l Ch.2 Madsen OB 60 Two Channel Audiometer Left Figure 4.2 A block diagram of the equipment used for presentation of the DBFF test. hearing one set of words (three p a i r s ) , the subject reported as many of the six words as he could i n any order. Scores were obtained for the l e f t and right ears based on the number of words presented to each ear which were correctly reported. The presentation to either ear was at a comfortable hearing level as determined by the subject. 47 CHAPTER 5 RESULTS A DBFF test and a competing dichotic message test were present-ed to two hemispherectomized subjects and a group of ten normal control subjects to examine whether a performance difference was necessitated by the complete removal of a hemisphere. The results obtained by the subjects on the various dichotic tests are shown in Tables 5.1 and 5.2. The mean scores for the ten normal subjects and the raw scores for the hemispherectomized subjects are presented i n Table 5.1. The Z scores, indicating the deviation in standard deviation units of the raw scores of the operated subjects from the mean scores of the normals in Table 5.2, provide a meaningful comparison between the two. It i s of interest f i r s t to compare the mean scores for the Dichotic A and Dichotic B conditions for the normal subjects; the difference was 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 . A learning e f f e c t was observed, however, inasmuch as the subjects consistently scored better on the second condition presented. This learning effect was controlled by presenting half of the normal subjects with the Dichotic A condition f i r s t and half with Dichotic B as the f i r s t test condition. The learning e f f e c t can be observed f o r both hemispherectomized subjects who scored better on the Dichotic B condition when i t was presented second. TABLE 5.1 Scores Obtained on the Pre-test, and on the Three Dichotic Listening Tests Subject Low Pass F i l t e r e d High Pass F i l t e r e d Dichotic A Dichotic B Competing Dichotic Message Test R L Normal Subjects (10) Mean Score 14.2 15.2 38.0 38.1 28.7 23.0 Left Hemispherectomee R.K. (raw score) R L 7 7 R L 21 13 31 34 0 i 32 Right Hemispherectomee S.M. (raw score) 17 10 30 37 29 1 TABLE 5.2 Z Scores Indicating the Deviation i n Standard Deviation Units of the Raw Scores of the Hemispherectomees from the Mean Scores of the Normals Dichotic A Dichotic B Competing Message Dichotic Test Left Hemi spherotomee R.K. Right Hemi spherectomee S.M. R L * 1.89 1.21 3.. 32 .77 2.16 * .32 .07 3.18 *p < .025 50 In comparing the raw scores of the hemispherectomized subjects with the mean score of the normal subjects for the Dichotic A condition, the Z scores indicate that the hemispherectomized subjects did not perform s t a t i s t i c a l l y s i g n i f i c a n t l y d i f f e r e n t l y from the normals. Likewise, for the Dichotic B condition the Z scores for the hemispherec-tomees do not indicate a s i g n i f i c a n t l y poorer performance on this test than for the normal subjects. On the competing dichotic message t e s t , the Z scores indicate that the score for the ear contralateral to the int a c t hemisphere (the "strong ear") in the hemispherectomees did not d i f f e r s i g n i f i c a n t l y from the mean right ear ("strong ear") score for the normal subjects. The s t r i k i n g result on this test was the almost complete extinction of the verbal material presented to the ear i p s i l a t e r a l to the intact hemisphere in the subject S.M. and the complete extinction for the subject R.K. The Z scores for the ear i p s i l a t e r a l to the in t a c t hemisphere were calculated with respect to the mean l e f t ear score f o r the normals. The difference i n performance was s t a t i s t i c a l l y s i g n i f i c a n t . In summary, these results indicate that the removal of a hemisphere did not markedly decrease the scores of two hemispherec-tomized subjects on a D B F F test. Removal of a hemisphere, however, s i g n i f i c a n t l y decreased the scores of the hemispherectomees on the competing dichotic message test in one of the ears -- s p e c i f i c a l l y the ear contralateral to the removed hemisphere. In the case of the patient S . M . , a comparison of his performance on the competing dichotic message test pre-operatively and 6 years post-operatively revealed no improvement in the report of material presented to the ear contra-l a t e r a l to the removed hemisphere. 51 CHAPTER 6 DISCUSSION Two types of dichotic l i s t e n i n g tests were presented to two hemispherectomized subjects and a group of ten normal control subjects. The DBFF test was designed to examine whether binaural integration of two complementary frequency segments of the same word necessitates the presence of two intact hemispheres. The results on a competing dichotic message test provide a measure of the extent of strengthening of the i p s i l a t e r a l pathway. The findings are i n t r i g u i n g since they provide an interesting comparison between the f u n c t i o n a l i t y of the i p s i l a t e r a l and contralateral pathways and projections to the i n t a c t hemisphere in the hemispherectomized subjects. The results indicated that the scores of the hemispherectomized subjects on the DBFF test (which i s a complementary dichotic message test) were not s i g n i f i c a n t l y d i f f e r e n t from those of normal subjects. These findings suggest one of two p o s s i b i l i t i e s : either (1) that the complementary signals presented in a DBFF test (consisting of two semi-spectra of the same word and transmitted separately in the i p s i l a t e r a l pathway from one ear and the contralateral pathway from the opposite ear), integrate to form a single encoded message at the brainstem level to be transmitted via the l a t e r a l lemniscus to the intact hemisphere to allow for verbal report of the word (Model 1): or (2) that the complementary acoustic messages in the contralateral and i p s i l a t e r a l pathways are relayed coincidentally beyond the brain-stem level and integrated i n the intact primary auditory cortex. This l a t t e r suggestion w i l l be discussed more f u l l y with reference to certain hypotheses advanced in a paper by Studdert-Kennedy and Shankweiler (1970). The r e l a t i v e value of the above p o s s i b i l i t i e s i s d i f f i c u l t to assess; however, i t appears d e f i n i t i v e l y indicated by the scores on the DBFF test that integration of the two semi-spectra occurred at some l e v e l . This indication would i n f e r that contralateral suppression of i p s i l a t e r a l pathways did not occur, since the acoustic message at the primary auditory cortex level of the temporal lobe must have been composed of information from both frequency bands to f a c i l i t a t e verbal report of the word. On the basis of these r e s u l t s , Model 2, suggest-ing contralateral suppression of i p s i l a t e r a l pathways and the necessity of two i n t a c t hemispheres and an i n t a c t corpus callosum to mediate binaural integration of the two semi-spectra, would appear untenable. Furthermore, the null hypothesis stating: "That binaural integration of the frequency semi-spectra of a word requires two intact hemi-spheres," can be rejected. Although the results of the hemispherectomized patients did not d i f f e r s i g n i f i c a n t l y from those of normal subjects, they were, however, s t i l l produced by individuals whose central nervous system may have undergone functional changes as a res u l t of hemiplegia and subsequent hemispherectomy and this point must be taken into considera-tion when reviewing these r e s u l t s . Results on the competing dichotic message test show that almost complete extinction of the verbal input to the ear contra-l a t e r a l to the removed hemisphere occurred for both subjects. For subject S.M., who had received a competing dichotic message test p r i o r to surgery, results were the same as on the pre-operative test. These findings indicate that contralateral suppression of i p s i l a t e r a l path-ways occurred at some l e v e l , whether at the subcortical level or in the primary auditory projection area i s not clear. That both subjects mentioned an awareness of "somebody ta l k i n g in the other ear" might suggest that the message in the ear i p s i l a t e r a l to the removed hemisphere did reach the primary auditory cortex but was suppressed by the stronger contralateral contribution at the c o r t i c a l l e v e l . Although i t i s known that neither subject suffered from congenital hemiplegia, the exact age at which both subjects incurred i n f a n t i l e hemiplegia i s not known. Netley (1972) has suggested that there i s a c r i t i c a l age in the development of the normal nervous system at which time a "hierarchy of functional e f f i c i e n c y " between contra-l a t e r a l and i p s i l a t e r a l pathways i s established. He hypothesized that i f injury occurred before t h i s c r i t i c a l age, contralateral pathways did not develop the same degree of prepotence over the i p s i l a t e r a l pathways as when injury occurred l a t e r . Because neither of the subjects in t h i s experiment repeated a s i g n i f i c a n t number of words presented to the ear i p s i l a t e r a l to the i n t a c t hemisphere, i t i s suggested that the c r i t i c a l age at which contralateral prepotency i s established had been reached by the time of injury and that a compensatory mechanism allowing the strengthening of i p s i l a t e r a l pathways, or projection areas i n the primary auditory cortex, did not adequately develop. An interesting observation to consider i s that contralateral suppression of the i p s i l a t e r a l pathways which occurred in the competing dichotic message did not manifest i t s e l f in the complementary dichotic message test. In both tests there was a simultaneous presentation of an acoustic message to both ears. The signals presented had the time, i n t e n s i t y , and frequency c h a r a c t e r i s t i c s that resemble products of the human vocal t r a c t . The c r i t i c a l difference between the inputs for both tests was that for the DBFF test each ear was presented with a signal composed of a narrow band of frequencies which were mutually exclusive; whereas, in the competing dichotic message t e s t , the messages presented to either ear were words such as "port" and "pack," or "zeal" and "zest," with each member of the pair being composed of approximately the same range of frequencies in the complex sound wave. Much i s known about the response of the auditory system to pure tone signals, especially in laboratory animals. The place of maximum displacement of the b a s i l a r membrane when stimulated by a pure tone i s related to i t s frequency and the arrangement i s said to be tonotopic. This tonotopic arrangement was subsequently demon-strated by the use of recording electrodes i n the cochlear nuclei of the cat (Rose, Galambos and Hughes, 1959); and by Tsuchitani and Boudreau (1966) in the l a t e r a l o l i v a r y n u c l e i . Regular tonotopic organization of neurones has also been demonstrated in the cat by Rose, Greenwood, Goldberg and Hind (1963). Whether there i s a tonotopic 55 arrangement of c e l l s in the primary auditory cortex has long been a subject of debate but i t recently has been reported that a tonotopic d i s t r i b u t i o n of best frequencies of neurones could be mapped on the primary auditory cortex of the monkey (Merzenich and Brugge, 1973) and on the primary auditory cortex of the cat (Merzenich, Knight and Roth, 1974). The q u a l i t a t i v e leap from research using pure tone signals as the auditory stimulus in laboratory animals, to research delineating the nature of the biological detector of speech elements in humans, presents a formidable gap. Despite the awesome task ahead, specula-tions concerning at what level in the auditory system the neural machinery possesses a "speech i d e n t i f i c a t i o n " a b i l i t y are especially relevant to t h i s discussion. Studdert-Kennedy and Shankweiler (1970) in t h e i r paper e n t i t l e d "Hemispheric Specialization for Speech Perception" hypothesize that: the auditory system common to both hemispheres i s probably equipped to track formants, regi s t e r temporal i n t e r v a l s , and i n general extract the auditory parameters of speech. But to the domin-ant hemisphere may be largely reserved the tasks of l i n g u i s t i c interpretation: for example, selecting from a formant t r a n s i t i o n the relevant overlapping cues to consonantal place of a r t i c u l a -tion and to neighbouring vowel, or selecting from the i n f i n i t y of temporal int e r v a l s automatically registered in the auditory stream the one interval relevant to the perception of voicing, (p. 579) 56 They suggest that the right ear ef f e c t demonstrated by normals on competing dichotic message tests i s due to the di r e c t a c c e s s i b i l i t y of the right ear input to the l i n g u i s t i c processing device i n the l e f t hemisphere via the contralateral route from the right ear, in comparison to the l e f t ear input which i s transmitted via the rig h t hemisphere and corpus callosum to the l e f t hemisphere. They at t r i b u t e to both hemispheres, however, the function of extracting auditory parameters of speech from the acoustic message, the result of which, must in turn, undergo " l i n g u i s t i c interpretation" in the l e f t hemisphere. If we are to assume these hypotheses as tenable we can extend the theory to examine the special circumstance of a hemispherectomized subject. In t h i s l a t t e r case the l i n g u i s t i c processing device had obviously l a t e r a l i z e d to the unimpaired hemisphere in early development. Consequently the intact hemisphere not only has sole p r i o r i t y on the l i n g u i s t i c processing of auditory parameters, but on the extracting of the auditory parameters of speech from the acoustic message received at the primary auditory cortex; a function usually subserved by both hemispheres in normals. In a competing dichotic message test where word pairs such as " f l e e t " and " f l i g h t " have very s i m i l a r vocal tr a c t analogs and the acoustic messages are likewise s i m i l a r , the two words' i f they reach the primary auditory cortex of the hemispherectom-ized subject, must simultaneously compete for the "auditory parameter extractor" and subsequently the l i n g u i s t i c processing device. In normals, although there i s competition for the l i n g u i s t i c processing device, the a r r i v a l of inputs i s not simultaneous due to the extra synapse across the corpus callosum. However, in hemispherectomized 57 subjects where the acoustic messages arrive d i r e c t l y via the contra-l a t e r a l route from one ear and the i p s i l a t e r a l route from the other ear, there i s no c a l l o s a l synapse and thus the a r r i v a l of messages would not be as staggered. If there i s simultaneous competition for the "auditory parameter extractor" as well as for the l i n g u i s t i c analyzer, and the dichotic messages presented are so s i m i l a r , i t i s reasonable to assume that the input from the contralateral ear w i l l suppress the i p s i l a t e r a l ear input due to the greater number of projection f i b r e s . Supportive of t h i s view are the findings of B e r l i n et a l . (1973) who tested a sample of temporal lobectomees, hemispherectomees and normals for dichotic signs of the recognition of speech elements. CV stimuli were presented to the "weak ear," i . e . the l e f t ear i n normals, and the ear i p s i l a t e r a l to the intact hemisphere i n temporal lobectomees and hemispherectomees. In order to gauge t h e i r r e l a t i v e suppressive e f f e c t on the i p s i l a t e r a l message the following "speech-l i k e " acoustic signals were presented simultaneously to the contra-l a t e r a l ear as "challenges": vowels, "bleats" (isolated second and t h i r d formants of CV s y l l a b l e s ) and CV s y l l a b l e s . In general i t was found that the vowels had the least suppressive ef f e c t while the bleats and CV s y l l a b l e s produced a greater degree of suppression. Furthermore, a l l the challanges produced much more suppression of the i p s i l a t e r a l or "weak ear" message in the patients than in the normals. To explain the ef f e c t i t was suggested by these investigators that "the signals produce t h e i r suppressive effect not because they actually compete for the l i n g u i s t i c processor but rather, because they appear to the nervous system to be l i k e l y 'candidates' for the special processing." (page 11) 58 Of special interest was the observation that the suppressive effects of the "strong ear" message were about equivalent for the two patient groups although there were two right and one l e f t temporal lobec-tomee and hemispherectomee respectively. This observation could be explained in the l i g h t of the postulation of a single "auditory parameter extractor" for patients who have had at least one temporal lobe removed whether the excision was i n the language dominant hemis-phere or not. Therefore, the more s i m i l a r the acoustic messages received via the i p s i l a t e r a l and contralateral pathways to the intact temporal lobe, the greater the competition for the preliminary acoustic analysis before reaching the " l i n g u i s t i c processor." In comparison, i t could be hypothesized, that the dichotic messages presented in the DBFF t e s t , which do not share the same f r e -quency c h a r a c t e r i s t i c s , would thus not have to compete for the same signal detectors at the primary auditory projection area. The messages received via the contralateral and i p s i l a t e r a l projections would thus make a simultaneous but non-overlapping impression, of which the additive ef f e c t could be transmitted to the l i n g u i s t i c processing centre for the sorting of the combined auditory parameters into phonological features. To explain the a b i l i t y of Netley's (1972) hemispherectomized patients to report words presented to both ears in a competing dichotic message test on the basis of the above hypothesis, one must assume that i f injury occurred early enough, the intact primary auditory cortex developed in such a way as to accommodate the simultaneous a r r i v a l of acoustic messages via contralateral and i p s i l a t e r a l pathways. 59 This experiment, although indicating that binaural integra-tion does not necessitate the presence of two in t a c t hemispheres, does not give a s u f f i c i e n t l y d e f i n i t i v e answer to the question of whether binaural integration occurs at the level of the brainstem or at the level of the intact primary auditory cortex. The fact that subjects with brainstem lesions did poorly on the task (Matzker, 1959; Smith and Resnick, 1970) i s congruent with both p o s s i b i l i t i e s ; since injury would either (1) disallow e f f e c t i v e integration i f i t occurs at that l e v e l , or (2) simply occlude the throughway transmission of information in both the i p s i l a t e r a l and contralateral pathways, thus reducing the information content of the acoustic messages received by the primary auditory cortex thus making i t s task of integration more d i f f i c u l t . I t i s interesting to note that neither i n Matzker's report (1959) or in Smith's and Resnick's report (1970) i s mention made of whether the brainstem disorders are l a t e r a l i z e d to either side and we can only assume that because of the proximity of brainstem nuclei in such a small area t h i s type of l o c a l i z e d delineation i s d i f f i c u l t to make, thus lending credence to the hypothesis that a brainstem lesion would affect the transmission of d i c h o t i c a l l y presented frequency semi-spectra of the same word. In determining the diagnostic v a l i d i t y of the DBFF t e s t , the results of this experiment suggest: (1) that both sides of the brain have the a b i l i t y to integrate d i c h o t i c a l l y presented frequency semi-spectra of the same word, (2) that a non-localized brainstem lesion would probably reduce the information content of the acoustic messages received at either cortex especially since the i n t r i n s i c redundancy of the message has already been reduced by f i l t e r i n g , and (3) that a un i l a t e r a l temporal lobe lesion should not much affect a subject's performance on a DBFF test since both sides of the brain have the a b i l i t y to integrate the frequency segments. Given the lim i t a t i o n s discussed e a r l i e r , the present i n v e s t i -gation y i e l d s the following observations: The DBFF test could be an ef f e c t i v e tool in the diagnosis of brainstem lesions by virtue of the fact that only b i l a t e r a l temporal lesions or brainstem lesions should s i g n i f i c a n t l y reduce the scores. Although both sides of the brain appear to have the integrating function, and although the DBFF test could be used to d i f f e r e n t i a t e brainstem lesions from uni l a t e r a l temporal lobe lesions, whether binaural integration of d i c h o t i c a l l y presented frequency semi-spectra of the same word occurs at the brainstem level or at the c o r t i c a l l e v e l , remains a moot point. 61 REFERENCES BERLIN, C , LOWE-BELL, S., PORTER, J . , BERLIN, H., and THOMPSON, C. (1973). "Dichotic Signs of the Recognition, of Speech Elements in Normals, Temporal Lobectomees, and Hemispherec-tomees," IEEE Group on Audio and Electroacoustics Transactions. BOCCA, E. (1955). "Binaural Hearing: Another Approach," Laryngo-scope 65: 1964-1175. BOCCA, E. (1960). Proc. 5th Congress Intern. Soc. Audiol. Bonn 1960, p. 63. BOCCA, E., and CALEARO, C. (1963). "Central Hearing Processes," in Jerger, J . , Modern Developments in Audiology, (Academic Press, New York and LondonTT 337-370. BOCCA, E., CALEARO, C., and CASSINARI, V. (1954). "A New Method for Testing Hearing in Temporal Lobe Tumours," Acta Oto-Laryngol. 4_4: 219-221. BRYDEN, M., and ZURIF, E. (1970). "Dichotic Listening Performance in a Case of Agensis of the Corpus Callosum," Neurophysio-l o g y 8: 371-377. CALEARO, C. (1957). "Binaural Summation in Lesions of the Temporal Lobe," Acta Oto-Laryngol. 47: 393-395. CALEARO, C. (1960). Proc. 5th Congress Intern. Soc. Audiol. Bonn 1960, p. 67. GOLDSTEIN, R., GOODMAN, A., and KING, R. (1956). "Hearing and Speech in I n f a n t i l e Hemiplegia Before and After Hemispherectomy," Neurology 6: 869-875. JERGER, J. (1960). "Audiological Manifestations of Lesions in the Auditory Nervous System," Laryngoscope 7_0: 417-425. JERGER, J. (1964). "Auditory Tests for Disorders of the Central Auditory Mechanisms," in Fi e l d s , W. and Al f o r d , B. (Editors), Neurological Aspects of Auditory and Vestibular Disorders, ~(C." Thomas, New York), 105-119: JERGER, J. (1970a). "Development of Synthetic Sentence I d e n t i f i c a t i o n (SSI) as a Tool for Speech Audiometry," in Rojskjaer, C. (Ed i t o r ) , Speech Audiometry, (Second Danovox Symposium, Odense, Denmark), 44-66. JERGER, J. (1970b). "Diagnostic Significance of SSI Test Procedures: Retrocochlear S i t e , " in Rojskjaer, C. (Edito r ) , Speech Audiometry, (Second Danovox Symposium, Odense, Denmark), 163-175". JERGER, J. (1973). "Diagnostic Audiometry," in Jerger, J. (Edito r ) , Modern Developments in Audiology, (Academic Press, New Y6rkTTT5-115. KIMURA, D. (1961a). "Some Effects of Temporal Lobe Damage on Auditory Perception," Canad. J. Psychol. J_5: 156-165. KIMURA, D. (1961b). "Cerebral Dominance and the Perception of Verbal St i m u l i , " Canada, J. Psychol. 15_: 166-171. KIMURA, D. (1967). "Functional Assymetry of the Brain in Dichotic Listening," Cortex _3: 163-178. KRYNAUW, R. (1950). " I n f a n t i l e Hemiplegia Treated by Removing One Cerebral Hemisphere," J. Neurol. Neurosurg. Psychiat. 13: 243-267. LEHISTE, I., and PETERSON, G. (1962). "Revised CNC L i s t s for Auditory Tests," Journal of Speech and Hearing Disorfers 27: 62-70. LINDEN, A. (1964). "Distorted Speech and Binaural Speech Resynthesis Tests," Acta Oto-Laryngol. 58: 32-48. MATZKER, J. (1959). "Two New Methods for the Assessment of Central Auditory Function in Cases of Brain Disease," Ann. Otol. (St. Louis), 68: 1185-1197. McFIE, J. (1961). "The Effects of Hemispherectomy on Intel l e c t u a l Functioning in Cases of I n f a n t i l e Hemiplegia," J. Neurol. Neurosurg. Psychiat. 24: 240-249. MERZENICH, M. and BRUGGE, J. (1973). "Representation of the Cochlear P a r t i t i o n on the Superior Temporal Plane of the Macaque Monkey," Brain Research 50: 275-296. MERZENICH, M., KNIGHT, P. and ROTH, G. (1974). "Orderly Represen-tation of the Cochlea within the Primary Auditory Cortex of the Cat," JASA, Vo. 55, Supplement: 86-87. MILNER, B., TAYLOR, L., and SPERRY, R. (1968). "Lateralized Suppression of D i c h o t i c a l l y Presented Digits after Commissural Section in Man," Science 161: 184-186. NETLEY, C. (1972). "Dichotic Listening Performance of Hemispherec-tomi zed Patients," Neurophysiology. V0: 233-240. 63 OHTA, F., HAYASHI, R., and MORIMOTO, M. (1968). " D i f f e r e n t i a l Diagnosis of Retrocochlear Deafness: Binaural Fusion Test and Binaural Separation Test," Int. Aud. 6_: 58-62. 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"Dichotic Listening in Man After Section of Neocortical Commissures," Cortex 4^  3-16. STUDDERT-KENNEDY, M., and SHANKWEILER, D. (1970). "Hemispheric Specialization for Speech Perception," J. Acoust. Soc. Amer. 48: 579-594. TILLMAN, T., and CARHART, R. (1966). "An Expanded Test f o r Speech Discrimination U t i l i z i n g CMC Monosyllabic Words from Northwestern University Test No. 6," Technical Report No. SAM-TR-66-55, (USAF School of Aerospace Medicine, Brooks A i r Force Base, Texas). TSUCHITANI, C , and BOUDREAU, J. (1966). "Single Unit Analysis of Cat Superior Olive S-Segment with Tonal Stimuli," J. Neurophysiol. 29: 684-697. WADA, J . , and RASMUSSEN, T. (1960). "Intracarotid Injection of Sodium Amytal for the L a t e r a l i z a t i o n of Cerebral Speech Dominance: Experimental and C l i n i c a l Observations," J. Neurosurg. 17: 266-282. 64 APPENDIX 1 List 1, L is t 2, and L ist 3 from the Northwestern University Test No. 6 List 1 L is t 2 List 3 bean boat met bi te merge bar mouse burn mode book mi l l base name chalk moon bought nice beg note choice nag calm numb cab pain dearth page chai r pad cause pearl dime pool chief pick chat phone door puff dab pi ke cheek pole f a l l rag dead rain cool rat fat raid deep read date ring gap raise f a i l room di tch road goose reach far rot dodge rush hash sel l gaze said f ive search home shout gin shack germ seize hurl size goal shawl good shall j a i l sub hate soap gun sheep jar sure haze south half soup keen take hush thought hire talk king third juice ton hi t team kite t ip keep tool jug t e l l knock tough keg turn late thin laud vine learn voice l i d void 1 imb week 1 i ve wag l i f e wal k lot which loaf whi te 1 uck when love whip lore which mess wire yes match young mop youth APPENDIX 2 Dic h o t i c a l l y Presented Word Pairs (15 pairs of 3) Right Ear Left Ear 1. port tea cow pack tent cat 2. fur sale bee fame sum bond 3. deck shoe gun duck ship gas 4. vane zoo meal vine zone mob 5. name plate t r a i 1 nose pride track 6. corn f l e e t sunk coast f l i g h t sake 7. bel l deed game bowl damp good 8. sheep vast zeal shine vent zest 9. m i l l n a i l pace mass nine pin 10. torn clock fresh t i n cloth f a i t h 11. speak bark need s p i t belt night 12. shore guest vaul t shell guard volt 13. though map note there mad nick 14. pal tongue cream pig teeth crust 15. fl a g send blown f a u l t sand brain 

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