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 presenting this thesis in partial fulfilment of the requirements for
an advanced degree at the University of British Columbia, I agree that
the Library shall make it freely available for reference and study.
I further agree that permission for extensive copying of this thesis
for scholarly purposes may be granted by the Head of my Department or
by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my
written permission.
Department of OMO/J.^^W /into/ SsomJ-Sc.
The University of British 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, dichotically presented, necessitates the presence of two intact hemi• spheres. The competing dichotic message test was presented to provide a measure of the extent of strengthening of ipsilateral pathways in the hemispherectomized subjects.
The DBFF test consisted of three fifty-word CNC lists which were processed through two band-pass filters 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 left ear and the low band to the right. The Dichotic B condition was the reverse of the first. 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 iii of the normals indicated that the removal of a hemisphere did not
significantly 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 -- specifically 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 test), as opposed to a
competing dichotic message, provides an interesting basis for a dis•
cussion on the nature of the "biological detector" of speech elements
in the CANS. i v
TABLE OF CONTENTS
PAGE
ABSTRACT ii
TABLE OF CONTENTS iv
LIST OF TABLES vi
LIST OF FIGURES vii
ACKNOWLEDGEMENT - .viii
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 Ipsilateral Pathways .... 18
2.4 Prepotence of Contralateral Auditory Pathways over Ipsilateral Pathways 19 2.4.1 Electrophysiological Studies 19
2.4.2 The Intel!igibi11ity 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 v
CHAPTER 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 - List 1, List 2, and List 3 from the Northwestern University Test No. 6 64
APPENDIX 2 - Dichotically Presented Word Pairs 65 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 Utilized 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 all those who had a part in this
the members of my thesis committee, Dr. John
Delack, Dr. Juhn Wada, and especially, Dr. John
Gilbert. all the subjects who participated.
David Roberts who first told me about binaural integration.
Margaret Roberts, Jim Pearse and Dr. Gannon for
their cooperation in allowing me to use the
facilities of the Audiovestibular Unit at VGH.
Will, for his willing help in transporting
equipment and subjects.
Sharon, Lyn, Pat, Ingrid and Marilyn for their
encouragement and good spirits. 1
CHAPTER 1
INTRODUCTION
The rationale for a reliable 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 is at a relatively 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 effective audiological tests designed, which have proven reliably diagnostic in the localization of the site of the disorder.
At a more germinal stage is 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 is realized by the projections of both contralateral and ipsilateral auditory pathways beyond the cochlear nuclei. The relative contributions of the ipsilateral and contralateral pathways, however, in 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 is gleaned mainly by studying the defects in the perception of acoustic stimuli resulting 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 unilateral hemispherectomy presents a special circumstance in which from each ear there is only one effective auditory pathway to the remaining hemisphere, the pathway from the contralateral ear containing a greater number of neuronal fibres than from the ipsilateral ear.
Dichotic listening tests utilize the technique of simultaneous presentation of different acoustic stimuli to each ear. The messages in a dichotic listening test can be either (1) competing, as in the condition when a different word is presented simultaneously to either ear or (2) complementary — as when mutually exclusive frequency seg• ments of the same word are the acoustic stimuli. In a dichotic message presentation to a hemispherectomized subject there is only one destination a representation of each acoustic message must reach before a verbal report of the words is possible. Thus, the presentation of dichotic tests to a person who has undergone surgical removal of an entire hemisphere affords a unique opportunity to investigate the relative contributions of the ipsilateral and contralateral pathways in the processing and transmitting of complex acoustic stimuli to the intact hemisphere.
The dangers inherent in using studies conducted solely on brain-damaged individuals to make generalizations concerning the functional organization of brain structures in the normal population are well recognized. It is, however, accepted that to shed some light on the mechanisms of auditory perception in man, researchers must still
rely heavily on examination of the types of auditory analysis 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 literature pertinent to this research is 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 functionality of the ipsilateral pathways in the central auditory system and Section 2.4 reviews some of the research indicating that the contralateral auditory pathways are prepotent to the ipsilateral pathways. Section 2.5 discusses the ability of the nervous system of brain damaged individuals to develop compensatory mechanisms, while
Section 2.6 discusses specifically the abilities of hemispherectomized patients. In this literature review and throughout the thesis, the words fusion and integration will be used interchangeably.
2.1 The Auditory Pathways
Excitation of the hair cells of the cochlea causes electro- t h chemical stimulation of the afferent endings of the 8 nerve fibres whose bipolar cells are grouped together in the spiral ganglion. These eighth nerve fibres enter the pons laterally and split to synapse with cell bodies which comprise the dorsal and ventral cochlear 4 nuclei. Beyond the cochlear nuclei in the central auditory pathways to the cortex are the following cell stations or location of cell bodies where synaptic connections may occur: the superior olive, the nuclei of the lateral lemniscus, the inferior colliculus 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 cell masses.
Approximately 60% of the axonal fibres from the cochlear nuclei cross to the contralateral superior olive, while the remaining
40% project to the ipsilateral superior olive. Thus, each cochlea is represented in the auditory cortex of each hemisphere. The bundle of fibres between the superior olive and the inferior colliculus is the lateral lemniscus and the fibre tract between the nuclei of the inferior colliculus and the medial geniculate body is called the brachia of the inferior colliculus.
Beyond the primary auditory area, further pathways link 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 principal loci of disorder in the central auditory system are the brainstem nuclei and the primary auditory projection 5 area. The development of effective clinical diagnostic techniques for the evaluation of central auditory problems has been inextricably linked to research proving that contralateral auditory pathways are functionally superior to ipsilateral pathways.
Bocca and co-workers in Italy investigated the intelligibil• ity of speech under a variety of conditions designed to reduce the excess of information contained in the spoken word. In the first monaural test suggested by Bocca, Calearo, and Cassinari (1954) low frequency filtered words were presented to the left, then the right ears of patients with unilateral 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 five word sentences was reduced by temporal interruption, acceleration, or simultaneous presentation of interfering speech applied to the same ear. In all of these tests a decrease of the intelligibility of speech was clearly observed in the ear contralateral to the pathological temporal lobe although no difference was apparent using undistorted speech. Furthermore, these patients demonstrated virtually normal pure tone sensitivity in both ears.
These results seemed to indicate that the contralateral pathway is the more important route to the brain. Furthermore, upon monaural stimulation of the ear contralateral to the pathological temporal lobe, either the ipsilateral 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 ipsilateral pathway projection area in the unimpaired temporal lobe 6 is not adequate for correct identification of the word.
A further discussion concerning contralateral prepotence over ipsilateral pathways follows in a later section.
The following generalizations concerning the nature of central auditory disorders emerged: (1) They did not affect the normal sensitivity for pure tones in either ear. (2) They were mani• fested by difficulty in understanding speech in the ear contralateral to the pathological site in 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 principle of overloading the system. The rationale for the test rests on the assumption that contralateral pathways are prepotent to ipsilateral pathways. It is designed to obviate differ• ences in ear performances on speech identification tasks under reduced redundancy conditions when there is absence of ear differences for pure tone sensitivity. The test is known as the Synthetic Sentence
Identification Test (SSI) in conjunction with (1) Ipsilateral 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 typically showed a 7 large performance deficit on the SSI-ICM conditions when the message and the competition were presented to the ear contralateral to the pathology but relatively normal results when the signals were presented to the ipsilateral ear. The effect for temporal lobe patients on this test is much less severe than in the case of brain• stem disorder.
The CCM test is especially sensitive to disorders at the temporal lobe level. There is a marked performance deficit when the sentences are presented to the ear contralateral to the lesion and the competition to the ipsilateral ear; patients with brainstem disorders usually have little trouble with the CCM condition.
The observation that patients with brainstem disorders have greater difficulty with a difficult monaural test than with a dichotic presentation would seem to indicate that the information overload in the contralateral pathway which constitutes the I CM condition is too great to be processed/filtered at the brainstem level and thus, the significant performance deficit in the sentence identification task.
The fact that temporal lobe patients have relatively less difficulty . with this test would seem to indicate that this type of information overload presents less trouble for these patients since the intact
brainstem nuclei effectively do their job, perhaps by inhibiting the
competing message.
Generally it appears that patients with temporal lobe
disorders show more difficulty with simultaneous dichotic message
tasks than with difficult 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 ineffective inhibition of the competing message received via the corpus callosum from the intact temporal lobe.
The difficulty in differentiating lesions within the brainstem and lesions at the temporal lobe level by audiological diagnostic techniques has long been recognized. Jerger (1960) proposed a general principle that: "the behavioral manifestations of disorders at different sites within the auditory system are determined by the unique function of that site in the chain of events leading ultimately to auditory perception." (Jerger, 1973, p. 92). However, a delineation of the role played by the brainstem nuclei and related pathways in the transmission of complex signals is only in the germinal stages. The nuclei of the superior olivary complex constitute the first synaptic connection after decussation of the auditory fibres from the cochlear nuclei. Converging at this relay, then, are fibres from both the ipsilateral and contralateral ear.
A test designed by Bocca (1960) and Calearo (1960) which was considered to be diagnostic of brainstem pathologies consisted of a vocal message which was oscillated periodically between one ear and the other for equal periods of time so that each ear received half of the message.
The messages consisted of short, simple sentences and the period of oscillation could be varied between 2 and 40 alternations per second.
Normal subjects invariably scored 100% in repeating these sentences as the segments were alternately switched between left and right ear.
Likewise patients with discrete, isolated pathologies of the primary auditory cortex had no discrimination problems when repeating the
sentences. However, in a number of cases of lesions of the brainstem,
patients performed poorly on this test. These results 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 ipsilateral 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 this test, speech is switched alternately between the ears, four times per second. The other ear receives noise at 20 dB greater intensity than the speech.
When the speech is in one ear, the noise is present in the other and vise versa. The SWAMI is 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 integrity of the brainstem pathways which has alternately been studied, neglected and resurrected is the usage of dichotic binaural frequency fusion tests. When a word that has been passed through a low frequency band pass filter is presented to one ear and the same word but having been passed through a high frequency band pass filter is 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 is poor, but the simultaneous presentation of the two semi-spectra of the vocal message allows a normal person to attain a near perfect discrimination score.
The test has variously been called (1) a binaural speech integration test, (2) a binaural summation test, (3) a dichotic integration test of two ear frequency fusion, and more recently,
(4) a dichotic binaural fusion tesjt_ (DBF test). The designation of dichotic binaural frequency fusion test (DBFF test) is thought to be the most appropriate inasmuch as both ears are stimulated, but each ear must get a different frequency segment of the same word from well separated channels of a tape recorder via earphones (a dichotic presentation).
In a DBFF test, although the messages to either ear are different, they are not "competing" as in the condition when a differ• ent word is presented simultaneously to either ear, but rather, the messages in this task are complementary, i.e. mutually exclusive frequency segments of the same word. The observation that normal dis• 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 earliest DBFF tests.
A phonetically balanced list of 41, two syllable German words was filtered to derive two bands, 500 - 800 Hz for the low band pass filtered segment and 1815 - 2500 Hz for the high band. Each band, 11 by itself, when presented binaurally was too narrow to allow recognition of the test word (an intelligibility of not more than 26% and 30% respectively). These bands were presented at the intensity at which a number of filtered questions reached maximum intelligibility. The test was presented in three sections: (1) Dichotically — a list of words was presented with the high band going to one ear and the low band simultaneously to the other, (2) Diotically -- the same words presented again but with both bands presented simultaneously to each ear, (3) the third list was presented dichotically in the same manner as the first.
Matzker noted that normal subjects made few mistakes on each of the three tests, attaining 100% discrimination scores for both the dichotic and diotic conditions. However, patients with brainstem lesions did poorly on the dichotic tests when one frequency segment simultaneously reached either ear, especially in comparison to their score on the diotic presentation in which both segments went to both ears. On autopsy of the patients who had difficulty on the DBFF test,
Matzker found microhemorrhages with capillary thrombosis, edema and degeneration of ganglion cells throughout the olivary regions of the brainstem.
Matzker maintained that the DBFF test explored specifically the integration of the two frequency segments presumed to take place in the brainstem where ipsilateral nerve fibres from one ear synapsed with the contralateral nerve fibres 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• lateral and ipsilateral auditory pathways integrate to form a single encoded message at the brainstem level to be transmitted via the lateral lemniscus to the higher cortical centres.
Bocca and Calearo (1963) support the view that DBFF tests must be considered specific to the brainstem, not only because primary decussation occurs at this level but also because many contralateral efferent impulses originate here which activate or inhibit peripheral exciteability. Accordingly, they maintain that binaural integration of complementary frequency segments from either ear should not be discurbed in cases of discrete temporal lobe pathology; since the intact hemisphere receives the dichotic message after being integrated in the brainstem, the non-functioning cortical area of the primary auditory cortex of the opposite hemisphere should not hinder its comprehension.
Matzker's research likewise lent credence to the hypothesis that a unilateral temporal lobe lesion should not much affect 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 intelligibility for filtered speech would necessarily involve a simultaneous reduction of the ability of the auditory centre to utilize the information from the same ear in a DBFF test. With unilateral temporal lobe damage a decline of intelligibility 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 integration of complementary frequency semi-spectra occurs at the brainstem level.
Figure 2.2 An elaboration of Model 1 illustrating that a discrete tern-oral lobe lesion should not much affect scores on a DBFF test since the intact_: temporal lobe aoorehends the alre-idy integrated message. 14
Linden used nine, fifteen-word, word lists filtered to obtain two bands: 560 to 715 Hz and 1800 to 2200 Hz. The filter had a reported rejection rate of 60 dB/octave. In a total of 6 out of 18 cases of patients with expanding intracranial lesions, Linden found significantly low values for the discrimination scores on the DBFF test. However, these six patients also obtained significantly low discrimination scores for monaurally presented frequency distorted speech. In Matzker1s binaural resynthesis test, the intelligibility 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 it received via the contralateral pathway then it 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 localized, unilateral temporal lobe lesions, lower than normal scores obtained on a monaur- ally presented distorted speech test would seem to predict concomitantly lower than normal scores also, on a DBFF test. Neural Model 2 (see
Figure 2.3) could be postulated from these results.
The underlying assumption implicit in this model is that binaural integration of the dichotically presented, two frequency bands does not occur at the brainstem level, since if this were the Figure 2.3 Diagrammatic representation of Model 2. hypothesizing that binaural integration of comolementary frequency semi-soectra requires two intact temooral lobes.
F'irure 2.4 Diagrammatic representation of the auditory pathways utilized when words are presented to the left 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, it would seem that occlusion of the information carried in the ipsilateral pathways occurs at the brainstem level, and the acoustic message received at the primary auditory cortex of one temporal lobe is not composed of information from both frequency bands but only from the frequency band presented to the contralateral ear.
Another tenable hypothesis is 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 ability to recognize the results of fusion in the brainstem as an identifiable set of acoustic elements, retrievable from storage as a word.
Ohta e_t al_. (1968) produced a Japanese version of Matzker's test, with bands of 300 to 600 Hz and 1200 to 2400 Hz from nonsense syllables. The results of their research were also in disagreement with Matzker's findings, i.e. that poor results on the DBFF test reflects 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 is affected not only by the
function of the brainstem, but also by the function of the cortical
or subcortical pathway. Ohta's results suggested particularly that
binaural fusion is poor when it is the high band pass filtered segment
that is delivered to the ear contralateral to the cerebral lesion.
Matzker's theory held that in cases of temporal lobe lesion,
binaural fusion would be intact, since, regardless of which side the 17 lesion was on, binaural integration having already occurred at the brainstem level, at least one temporal lobe would be functionally adequate for processing of the acoustic message.
In a study entitled "Dichotic Listening in Man after Section of the Neocritical Commissures," Sparks and Geschwind (1968) presented a somewhat different binaural fusion test to their 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 filter. The subject.was asked to repeat the word. Bocca (1955), Calearo (1957) and Jerger
(1960) had shown that discrimination is 100% in normal subjects, but does not exceed the best monaural discrimination in 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 callosal pathway." (p. 15)
More recently, a Dichotic Binaural Fusion Test (DBF test),
as it was called, was developed at the Washington Hospital Centre by
Smith and Resnick (1972). The DBF test was designed primarily to
differentiate brainstem from temporal lobe pathologies and is based
on Matzker's original work. They presented two frequency bands from
English PB lists, 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 left ear and the high to the right; (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 if the diotic score was significantly better than one or both of the dichotic scores, negative if the three scores were about the same.
The results indicated that for normal subjects there was no significant 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 diotic test than for at least one of the dichotic conditions. The results of these preliminary findings would seem to be in keeping with Matzker's initial premise that binaural integration occurs at the brainstem
1evel.
Preliminary to examining the question concerning where in the CANS binaural integration of dichotically presented complementary frequency segments is located, a review of what is known about the relative contributions of ipsilateral and contralateral pathways in speech processing is necessary.
2.3 Functionality of the Ipsilateral Pathways
The fact that ipsilateral pathways alone are physiologically
adequate to encode and transmit speech material is demonstrated by 19 the fact that all the commissurotomized patients in a study by Milner,
Taylor and Sperry (1968) were able to report digits to their left ear when there was no competing input to their right ear. (Refer to
Figure 2.4)
The functionality of the ipsilateral auditory pathways was also demonstrated by Goldstein et al_. (1956) in their paper entitled
"Hearing and Speech in Infantile Hemiplegia Before and After Left
Hemispherectomy." The average speech discrimination score for the left ear on a particular list of words for 4 people after left hemispherectomy was 87% indicating that the ipsilateral pathways were adequate for speech processing.
2.4 Prepotence of Contralateral Auditory Pathways over Ipsilateral Pathways
2.4.1 Electrophysiological Studies
The accumulation of data proving the prepotence of the contra• lateral pathways is impressive. Rosenzweig (1951) recorded the electrical responses in the auditory cortex of the cat when acoustic clicks were delivered to first one ear and then the other. At each hemisphere, the response to stimulation of the contralateral ear was significantly larger in amplitude than the response to the stimulation of the ipsilateral 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 ipsilateral response. The results were interpreted to mean that each ear is 20 represented by a population of cells at the auditory cortex of both cerebral hemispheres, the population representing the contralateral ear being larger than the population representing the ipsilateral ear. The two populations were presumed to overlap.
Penfield and Erikson (1941), showed that electrical stimula• tion of the auditory cortex in man gives the sensation of the ringing of a bell, whistling, buzzing, droning or tapping in the ear contralateral to the stimulated hemisphere.
2.4.2 The Intelligibility of Distorted Speech
Bocca et a]_. (1954) demonstrated that there was a decrease
in the intelligibility low pass filtered speech in the ear contra•
lateral 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 ipsilateral pathways comes from the results
obtained from commissurotomized patients on dichotic listening tasks.
Kimura (1961a), showed that if pairs of contrasting digits were
presented simultaneously to left and right ears of normal subjects,
those presented to the right ear were more accurately reported. She
hypothesized that the laterality effect could be accounted for by
the assumptions of left cerebral dominance for speech and the pre-
potence of contralateral auditory pathways to ipsilateral pathways
(Kimura, 1961b). Thus, the right ear is better in recognizing dichotically presented verbal stimuli since it is contralateral to the left cerebral hemisphere which is dominant for language and speech functions. The right ear message has more rapid access to the left cerebral hemisphere via the direct contralateral pathway in comparison to the left ear input which must travel its contralateral pathway to the right hemisphere and then via the corpus callosum to the left
"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
left cerebral dominant for speech and whose corpus callosum linking
the cerebral hemispheres had been completely sectioned to relieve
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 left ear. These results suggested the
following:
1. That under dichotic stimulation the message travelling
in the contralateral auditory pathway takes precedence
over the message travelling in the ipsilateral path•
ways .
2. The right ear input takes precedence over the left ear
input by virtue of its direct access via the contralater•
al pathway to the left "speaking" hemisphere, in comparison
to the longer route travelled by the left ear input which
goes via the right hemisphere and the corpus callosum
to the left hemisphere. This extra transcallosal synapse, 22
and the concomitant time delay puts the left ear
information at a disadvantage when competing for oral
report with the right ear input in the left hemisphere,
and thus the oft-reported right ear effect or dominance
in competing dichotic listening tasks in normals;
3. After callosal section the left ear input which reaches
the right hemisphere no longer has access to the left
hemisphere for oral report and thus a 100% extinction of
information to the left ear results when dichotic tasks
are given to commissurotomized patients. In such patients,
only right ear words are reported, since suppression of
ipsilateral 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
left ear) to the left temporal lobe.
2.5 The Development of Compensatory Mechanisms
In 1963, Sparks and Geschwind presented interesting observations on their patient W.J. who had undergone complete section of the neo- cortical commissures for epilepsy. As previously mentioned, the patient showed complete extinction of signals received by the left ear both when the competing signals to either ear were digits and when
they were animal names. However, with repeated retesting on the dichotic digits task, W.J.'s left ear score improved from 0% detection (100% extinction) to 35% detection (65% extinction) which suggested to
Sparks and Geshwind that by specific practice the left temporal lobe may eventually begin to separate out messages coming via the weaker ipsilateral pathway in dichotic listening, although it normally inhibits them in this competitive situation. The improved performance implied that simultaneous/coincidental transmission in ipsilateral and contralateral pathways of non-ccmplementary speech to the left temporal lobe occurred in this commissurotomized patient. The following questions were felt to pertain to these results:
1. Under dichotic listening conditions in normals, does
this same simultaneous transmission of information in
the ipsilateral and contralateral pathways occur, and
therefore, does the suppression of the acoustic message
in the ipsilateral pathway occur in the temporal lobe
rather than at a subcortical level?
2. In the commissurotomized patient does the improved
performance on left ear report after practice sessions
in dichotic listening reflect the "unlearning" of
contralateral suppression of ipsilateral encodings in
the temporal lobe or in the brainstem nuclei?
No comment was made concerning an improvement in this person's
performance on the dichotic animal name listening task.
Kimura (1967) has suggested that contralateral suppression
takes place at two different levels. First, the inputs arriving along
the ipsilateral pathway are occluded at a subcortical level by the
contralateral inputs to such an extent that very little of the information from the ipsilateral ear ever reaches the auditory cortex by the ipsilateral pathway. Secondly, she suggests a factor of cortical competition (or occlusion) also. The question of sub• cortical occlusion is an interesting one in light of the further investigations by Sparks and Geschwind (1968) with their patient W.J.
In the dichotic test, in which the right ear received distorted speech while the left ear received normal signals both well above thresholds, it was found that as the right ear score (ear contra• lateral to the language dominant hemisphere) improved with decreasing distortion, the left ear score reflected increasing suppression.
This result suggested that suppression of the left ear signal will occur only if there is a marked similarity in the signals being transmitted in the contralateral pathway from the right ear and the ipsilateral pathway from the left ear. The question can then be posed: "At what level is this 'recognition' process accomplished?" i.e. are the brainstem nuclei mainly transmitting relay stations or do they act as filters in such a way as to classify acoustic messages as speech or non-speech in order to "know" whether contralateral suppression of the ipsilateral pathways is in 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 left ear, and if we are to assume the
subcortical occlusion theory as tenable, then the brainstem nuclei
could be thought of as filters which process the messages comparatively
-- if the ipsilateral message is speech-like and the contralateral message also has the essential acoustic characteristics of speech, 25 then the brainstem nuclei would facilitate the suppression of the ipsilateral message by the contralateral message. Strengthening of ipsilateral pathways would then entail the "unlearning" at the inhibitory synapses at the brainstem level to allow the throughway transmission of messages from the ipsilateral ear.
Milner, Taylor and Sperry (1968) also reported 100% suppres• sion of the input to the left ear by 2 commissurotomized patients out of a group of 7 under dichotic stimulation, the input once again being digits. Under monaural conditions, these callosal patients correctly reported 87% of the numbers channelled to the left ear and
90% of the numbers channelled to the right ear showing that the ipsilateral pathway could be utilized. The two commissurotomized patients, who showed some success in reporting left ear material, were referred to as "experienced examinees with respect to other modalities" (page 185), once again indicating that the ability to use information presented dichotically improves with practi.ce albeit by virtue of decreased "subcortical occlusion" or by decreased
"cortical occlusion."
Bryden and Zurif (1971) conducted a study on the dichotic
listening performance of a 15 year old boy of normal intelligence with congenital agenesis of the corpus callosum. His performance did not differ 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 ipsilateral pathways had not developed-i.e. that the subcortical occlusion did not occur nor was the suppression at the cortical level of sufficient strength to extinguish the ipsilateral material, or (2) that if subcortical occlusion did occur, then the boy must have bilateral speech representation.
Bryden and Zurif find the former possibility more tenable and conclude that the results of their study, combined with the evidence from other studies, "suggests that the suppression effect can be overcome with practice, and that the laterality effect in dichotic listening is more dependent upon cortical competition than on sub• cortical occlusion." (page 376)
Another study supportive of this view is Netley's (1972) comparison of the dichotic listening performance of twelve hemispherec- tomized patients to that of controls matched for age and IQ. In cases where hemispherectomy has been performed there is only one effective route from either ear to the remaining hemisphere, which if the patient has suffered from infantile or congenital hemiplegia, has subsumed the associative, intellectual and language functions normally carried out by both hemispheres. By analyzing the dichotic
listening performance of hemispherectomized patients it is possible
to gauge the relative strength of ipsilateral and contralateral
pathways. Netley found that the digit 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 in the recall scores of the controls. The effect, 27 however, although similar in kind, differed in degree for the infantile injured group inasmuch as the recall scores for the ear contralateral to the removed hemisphere, were lower than those for the congenitally injured group and of the left ear recall scores of the controls.
Netley's results support the view that ipsilateral pathways can become very effective carriers of dichotically presented verbal material and that some compensatory mechanism, facilitating strengthen• ing of the ipsilateral pathway or projection area in the brain, had developed as a result of insult to the hemisphere which was subsequently removed. The fact that his congenitally injured group had better recall scores for the ear contralateral to the remaining hemisphere, than did the infantile injured group, further indicated that this compensatory adjustment was more adequately developed, the earlier the insult was sustained.
Netley postulated that the further along an infant was in the language acquisition process, the more likely some "hierarchy of functional efficiency" (page 239) would have been established between contralateral and ipsilateral pathways or projection areas. Thus, the later the injury and the greater degree of contralateral pre• potency the poorer would be the recall score for the ear ipsilateral to the remaining hemisphere on a competing dichotic listening task.
It is interesting to note that digits were used as the verbal material for the dichotic listening 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 linguistic processing centre. It could be 28 speculated that the degree of suppression might be greater if the material used was less in the "automatic" or "easy to retrieve" classification than one digit numbers.
2.6 Hemispherectomized Patients
In attempting to construct any model of the functional organiza• tion . of brain structures, it is important to remember that one cannot infer conclusions about the normal population from studies conducted solely on brain-damaged individuals. The extent to which compensatory mechanisms have developed in people with a history of cerebral mal• function is not easily established. Krynauw (1950) presented a series of eleven patients with infantile hemiplegia and epilepsy, ranging in ages from 7 months to 20 years of age, who improved after hemispherec•
tomy. Ten of the patients were left hemispherectomized. In none of
these cases was any significant impairment of speech noted postoperatively and in some cases there was considerable improvement. Krynauw concluded
that cerebral dominance for speech had adjusted itself before hemi• spherectomy and that the speech and language functions had in all
cases become lateralized to the unimpaired hemisphere. Therefore, it was the minor hemisphere with respect to speech and language functions
that was removed in all cases. He also suggested that, with the
removal of the pathological hemisphere, the remaining hemisphere was liberated to manifest those functions or abilities it had acquired
but which had been inhibited by the presence of the imperfect "other
half." 29
McFie (1961) in his study entitled: "The Effects of Hemi• spherectomy on Intellectual Functioning in Cases of Infantile
Hemiplegia" compared the results of pre and post-operative intelligence 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 is able to function normally, but it is doubtful it can be called 'normal function' supernormal might be more appropriate. For not only does it perform motor and sensory functions for both sides of the body, it performs the associative and intellectual functions normally allocated to two hemispheres." (p. 248)
To what age this degree of plasticity is achievable is undeter•
mined, however, both Krynauw and McFie noted that those patients with
infantile hemiplegia resulting from birth injury showed no impairment
of speech and greater improvement on intelligence tests following
hemispherectomy than did those patients who sustained a later juvenile
injury. The positive change for those injured at birth or in their
first twelve months of life contrasted with the negative change in
the group injured after one year of age. This observation raises the
question: "Is there a critical age at which these transfers of
function to areas not normally allocated the 'learning' of certain
abilities is possible and beyond which the potentiality for taking
over is lost?"
A very similar question can be asked in reference to the
potentialities of the strengthening ipsilateral pathways or the
increased 'demand to be heard from' of the ipsilateral auditory projection areas in the temporal lobe. The degree to which this phenomenon has occurred, if at all, could be partly gauged by giving pre- and post-operative dichotic listening tests to patients who will undergo either a hemispherectomy or a callosal 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 validity of the DBFF test in differentiating 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 cortical (temporal lobe) level, 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 ipsilateral and contralateral pathways in the DBFF test integrate to form a single encoded message at the brainstem level, whereas Model 2 suggests that
the complementary messages relayed in the contralateral pathways must first reach the primary auditory cortices before they are integrated
at a cortical level via transmission in the corpus callosum. In this
latter model, the information transmitted in the ipsilateral pathway
is assumed to be occluded at either the brainstem or cortical level.
A third possibility must also be considered, i.e. that the acoustic messages in the contralateral and ipsilateral 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 test, 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
If two intact hemispheres are necessary for binaural integra• tion of dichotically 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 effectively integrated at a subcortical level, or, barring subcortical integration, that the intact primary auditory projection area does not effectively
integrate messages received via contralateral projections from one ear and ipsilateral projections from the other ear.
If, however, a subject scores well on the DBFF test it would
indicate that contralateral pathway suppression of ipsilateral 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 in the intact primary auditory projection area with the simultaneous arrival of messages in the ipsilateral and contralateral pathways.
It would also be of interest to present subjects with a dichotic listening task in which the messages to either ear are competing.
For subjects who have had pre-operative dichotic listening tests the comparison of results would give some measure of the extent of stren- thening of the ipsilateral pathway.
In cases where hemispherectomy has been performed there is only one effective route from either ear to the remaining hemisphere,
the route from the ipsilateral 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 different situation than that of using subjects with lesions of the temporal lobe since the exact size and extent of
the affected area is rarely delineated in this latter 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
ability of hemispherectomized subjects to fuse binaurally what can be
termed "dichotic speech" inasmuch as two different 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 is presented simultaneously to either ear). 34
More specifically, 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 test, consisting of
phonetically balanced lists of English monosyllables, was processed
through two band-pass filters and recorded on a two channel mangetic
tape. Three fifty word CNC word lists (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 left ear and
the low band to the right. The Dichotic B condition was the reverse of
the first, so that the high band was in the right ear and the low
band in the left. For each condition, 50 words were presented and
the subject was required to repeat the word in a 4 second interval
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 air conduction thresholds and speech reception thresholds
(SRT) prior 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 talker. A Scully two-track stereo recorder was used to record all 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 lists
of 50 words each were recorded on one tape at a normal conversation
level with a 4 second interval between words.
The original 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 intensity of
0 VU at the record head of the Revox. Thus all stimulus words were
matched for peak intensity at re-recording time. A 1000 Hz calibration
tone was recorded on the same tape at an intensity which also
registered 0 VU on the VU meter.
The full 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 it which registers
on two motors the record levels of each channel. The record levels V Re vox Tandberg 64X Scully 280 Stereo Scully 280 2 channel Tape Tape 4 track
Recorder \ Recorder ) Tape Ch.2 Recorder PIayback Record Playback Mode Mode Mode Record Mode
Tape TT I Intensity adjustment of Tape # 2 the test words from the original tape Tape # 2
Figure 4.1 A block diagram of equipment used for preparation of the DBFF test. 38 of channels 1 and 2 were adjusted such that the 1 volt, 1000 Hz calibration tone peaked at 0 VU for both channels and the words before
filtering peaked synchronously between ±1 on the VU meter.
Following this procedure, the full track recording on which each word had been adjusted to ensure approximate equivalent peak
intensity, was then played back on the Scully and passed through two
standard Krohn-hite filter sets (Model 334 2R) to form the high band
(H) and the low band (L). The low band was a one and one third
octave from 360 - 890 Hz and the high band was a narrow band from
1950 - 2050 Hz. The slope of the filters was 48 dB/octave. The two
outputs of the filters were re-recorded onto the two channels of the
Tandberg tape recorder with the record knob set at the previously
determined level.
4.3 Stimulus Words
The Northwestern University Auditory Test No. 6 is a carefully
prepared and thoroughly examined set of CNC word lists. There are
four lists of consonant-nucleus-consonant words which have the phonemic
balance of the Lehiste-Peterson revised word lists (1962) and which
experimentally have high interlist equivalence and also test-retest
reliability. The "articulation functions" of a particular recording
of these lists have been established for normal hearing subjects.
The function rises linearly 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 is 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 lists 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 first three lists of words were utilized in this test and three randomizations of each list were recorded. Each of the subjects received a different list 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. Birth
was normal, but at five 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 variety, at times preceded by a tingling
sensation over the left side of the body.
A series of EEG's, dating from his third 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 in•
volving the fronto-central-temporal region.
The left carotid amytal speech localization test (Wada
and Rasmussen, 1960) revealed that speech and language functions
were lateralized to the left hemisphere. Another significant
finding during the carotid amytal test was that a profound right hemiparesis was induced by left sided injection but no apparent increase of left hemiparesis was observed by right carotid injection, nor was any speech deficit noted following right intracarotid amytal injection.
A series of neurophysiological investigations were carried out by Professor 0. Spreen of the University of Victoria. On a competing dichotic listening test S.M. showed complete extinction of signals presented to the left ear, that is, he
repeated only those words which had been presented to the right ear.
A right hemispherectomy was performed when the subject was
21. years. At operation it was noted that the whole right posterior third of the brain consisted of whitish, atrophic
cortex in which there were no sulci or gyral patterns. The
operation was described as "a complete right hemispherectomy arm
block resection of the whole of the right 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 air conduction thresholds revealed a moderately
severe hearing loss at 2000 and 4000 Hz. The thresholds for
500 and 1000 Hz were borderline normal bilaterally. The pure
tone average for the right ear was 40 dB and for the left
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 left hemispherectomy at the age
of 7 and 1/2 years. Pre-operative diagnosis was "an atro•
phic left hemisphere." The operation was described as
a "complete left hemispherectomy down to and including the
basal ganglia and the upper part of the left thalamus."
R.K. successfully completed grade 12 and is of normal
intelligence.
Pure tone air conduction thresholds were within normal
limits for both ears between the frequencies 250 to 8000 Hz.,
the thresholds for the left ear being about 15 dB better than
those for the right. Speech discrimination scores were also
normal at 30 dB re the pure tone average for both ears.
3. The ten right handed subjects in the normal group ranged in
age from 21 to 29. All 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 their pure tone average.
4.5 Pretest Conditions
A preliminary pilot investigation was conducted to specify the filtering band-widths and to satisfy the following criteria:
1. That the discrimination scores obtained from the binaural
presentation of the low band and then from the high band
should reflect 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 lists were tested on a total of 10 normal hear• ing pilot subjects. Several variations in bandwidth were tried in order to satisfy the above criteria. Bandwidths from 360-890 Hz for the low band pass filter and from 1950-2050 Hz for the high band pass filter were ultimately selected.
Single and combined band intelligibilities satisfied 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 filtered lists were presented binaurally to the normal control group at 30 dB re their pure tone average for the better ear.
For the hemispherectomized subject S.M. the lists were presented binaurally at 35 dB re the pure tone average for the better ear and for the subject R.K. the lists 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 calibration of the tape on the Audiometer OB 60 was effected as directed in the OB 60 Operation Instruction Manual. A
1 Volt, 1000 Hz calibration tone was recorded to peak at 0 VU on two channels of the Tandberg tape recorder as did the first list 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 calibration tone on both Channels 1 and 2 of the audio• meter. The voltages for the calibration tone at the playback head of the recorder were 0.92 and 0.90 for Channels 1 and 2 respectively.
The calibration tone was played through the Telephonic TDH 39 head• phone and the Bruel and Kjaer artificial ear type 4152 was placed on the sound level meter. The intensity 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 left and
right head-phones.
4.6.2 Presentation of Pre-test Conditions
Presentation of the low band-pass filtered 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 left and right head-phones. The high band-pass filtered 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 filtered segments of the words went to Channel 1 of the audiometer and the high-band pass filtered segments of the words went to Channel 2 of the audiometer. For the Dichotic A conditions the left head-phone received the high band pass filtered segments and the right head-phone received the low band-pass filtered segments of the words. The Dichotic B condition was the reverse, so that the high band was in the right ear and the low band in the left.
4.7 The Competing Dichotic Message Test
The competitive dichotic 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 Victoria 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 interval. After Soundproof Booth
Sony 500S Two Channel Tape Madsen Ch.l Recorder OB 60 Two Channel Playback Audiometer Mode Ch.2
Left
Figure 4.2 A block diagram of the equipment used for presentation of the DBFF test. hearing one set of words (three pairs), the subject reported as many of the six words as he could in any order. Scores were obtained for the left 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 in 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 is of interest first to compare the mean scores for the
Dichotic A and Dichotic B conditions for the normal subjects; the
difference was not statistically significant. A learning effect 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
first and half with Dichotic B as the first test condition. The
learning effect can be observed for both hemispherectomized subjects
who scored better on the Dichotic B condition when it was presented
second. TABLE 5.1
Scores Obtained on the Pre-test, and on the Three Dichotic Listening Tests
Competing Dichotic Low Pass High Pass Dichotic Dichotic Subject Message Test Filtered Filtered A B R L
Normal Subjects (10) 14.2 15.2 38.0 38.1 28.7 23.0 Mean Score
i Left R L R L Hemispherectomee R.K. 7 7 21 13 31 34 0 32 (raw score)
Right Hemispherectomee 17 10 30 37 29 1 S.M. (raw score) TABLE 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
Competing Dichotic Dichotic A Dichotic B Message Test
Left R L * Hemi spherotomee 1.89 1.21 3.. 32 .77 R.K.
Right * Hemi spherectomee 2.16 .32 .07 3.18 S.M.
*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 statistically significantly differently from the normals.
Likewise, for the Dichotic B condition the Z scores for the hemispherec• tomees do not indicate a significantly poorer performance on this test than for the normal subjects.
On the competing dichotic message test, the Z scores indicate
that the score for the ear contralateral to the intact hemisphere (the
"strong ear") in the hemispherectomees did not differ significantly
from the mean right ear ("strong ear") score for the normal subjects.
The striking result on this test was the almost complete extinction of
the verbal material presented to the ear ipsilateral to the intact
hemisphere in the subject S.M. and the complete extinction for the
subject R.K. The Z scores for the ear ipsilateral to the intact
hemisphere were calculated with respect to the mean left ear score for
the normals. The difference in performance was statistically
significant.
In summary, these results indicate that the removal of a
hemisphere did not markedly decrease the scores of two hemispherec-
tomized subjects on a DBFF test. Removal of a hemisphere, however,
significantly decreased the scores of the hemispherectomees on the
competing dichotic message test in one of the ears -- specifically
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•
lateral to the removed hemisphere. 51
CHAPTER 6
DISCUSSION
Two types of dichotic listening 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 ipsilateral pathway. The findings are intriguing since they
provide an interesting comparison between the functionality of the
ipsilateral and contralateral pathways and projections to the intact
hemisphere in the hemispherectomized subjects.
The results indicated that the scores of the hemispherectomized
subjects on the DBFF test (which is a complementary dichotic message
test) were not significantly different from those of normal subjects.
These findings suggest one of two possibilities: 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
ipsilateral 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 lateral 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 ipsilateral pathways are relayed coincidentally beyond the brain- stem level and integrated in the intact primary auditory cortex.
This latter suggestion will be discussed more fully with reference to certain hypotheses advanced in a paper by Studdert-Kennedy and
Shankweiler (1970).
The relative value of the above possibilities is difficult to
assess; however, it appears definitively indicated by the scores on the
DBFF test that integration of the two semi-spectra occurred at some
level. This indication would infer that contralateral suppression
of ipsilateral 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 facilitate verbal
report of the word. On the basis of these results, Model 2, suggest•
ing contralateral suppression of ipsilateral pathways and the necessity
of two intact hemispheres and an intact 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 differ significantly from those of normal subjects, they were,
however, still produced by individuals whose central nervous system
may have undergone functional changes as a result of hemiplegia and
subsequent hemispherectomy and this point must be taken into considera•
tion when reviewing these results.
Results on the competing dichotic message test show that
almost complete extinction of the verbal input to the ear contra- lateral to the removed hemisphere occurred for both subjects. For subject S.M., who had received a competing dichotic message test prior to surgery, results were the same as on the pre-operative test. These findings indicate that contralateral suppression of ipsilateral path• ways occurred at some level, whether at the subcortical level or in the primary auditory projection area is not clear. That both subjects mentioned an awareness of "somebody talking in the other ear" might suggest that the message in the ear ipsilateral to the removed
hemisphere did reach the primary auditory cortex but was suppressed by
the stronger contralateral contribution at the cortical level.
Although it is known that neither subject suffered from
congenital hemiplegia, the exact age at which both subjects incurred
infantile hemiplegia is not known. Netley (1972) has suggested that
there is a critical age in the development of the normal nervous system
at which time a "hierarchy of functional efficiency" between contra•
lateral and ipsilateral pathways is established. He hypothesized that
if injury occurred before this critical age, contralateral pathways
did not develop the same degree of prepotence over the ipsilateral
pathways as when injury occurred later. Because neither of the subjects
in this experiment repeated a significant number of words presented to
the ear ipsilateral to the intact hemisphere, it is suggested that the
critical age at which contralateral prepotency is established had been
reached by the time of injury and that a compensatory mechanism allowing
the strengthening of ipsilateral pathways, or projection areas in the
primary auditory cortex, did not adequately develop.
An interesting observation to consider is that contralateral
suppression of the ipsilateral pathways which occurred in the competing dichotic message did not manifest itself 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, intensity, and frequency characteristics that resemble products of the human vocal tract. The critical 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 test, 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 is known about the response of the auditory system to
pure tone signals, especially in laboratory animals. The place of
maximum displacement of the basilar membrane when stimulated by a
pure tone is related to its frequency and the arrangement is said to
be tonotopic. This tonotopic arrangement was subsequently demon•
strated by the use of recording electrodes in the cochlear nuclei
of the cat (Rose, Galambos and Hughes, 1959); and by Tsuchitani
and Boudreau (1966) in the lateral olivary nuclei. Regular tonotopic
organization of neurones has also been demonstrated in the cat by Rose,
Greenwood, Goldberg and Hind (1963). Whether there is a tonotopic 55 arrangement of cells in the primary auditory cortex has long been a subject of debate but it recently has been reported that a tonotopic distribution 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 qualitative 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 identification" ability are especially
relevant to this discussion.
Studdert-Kennedy and Shankweiler (1970) in their paper entitled "Hemispheric Specialization for Speech Perception" hypothesize
that:
the auditory system common to both hemispheres is probably equipped to track formants, register temporal intervals, and in general extract the auditory parameters of speech. But to the domin• ant hemisphere may be largely reserved the tasks of linguistic interpretation: for example, selecting from a formant transition the relevant overlapping cues to consonantal place of articula• tion and to neighbouring vowel, or selecting from the infinity of temporal intervals automatically registered in the auditory stream the one interval relevant to the perception of voicing, (p. 579) 56
They suggest that the right ear effect demonstrated by normals on competing dichotic message tests is due to the direct accessibility of
the right ear input to the linguistic processing device in the left
hemisphere via the contralateral route from the right ear, in comparison
to the left ear input which is transmitted via the right hemisphere and corpus callosum to the left hemisphere. They attribute to both
hemispheres, however, the function of extracting auditory parameters of speech from the acoustic message, the result of which, must in turn,
undergo "linguistic interpretation" in the left 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 this latter case the linguistic processing device had obviously lateralized to the unimpaired hemisphere in early development.
Consequently the intact hemisphere not only has sole priority on the
linguistic 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 "fleet" and "flight" have very similar vocal
tract analogs and the acoustic messages are likewise similar, the two words' if they reach the primary auditory cortex of the hemispherectom-
ized subject, must simultaneously compete for the "auditory parameter
extractor" and subsequently the linguistic processing device. In
normals, although there is competition for the linguistic processing
device, the arrival of inputs is not simultaneous due to the extra
synapse across the corpus callosum. However, in hemispherectomized 57 subjects where the acoustic messages arrive directly via the contra• lateral route from one ear and the ipsilateral route from the other ear, there is no callosal synapse and thus the arrival of messages would not be as staggered.
If there is simultaneous competition for the "auditory parameter extractor" as well as for the linguistic analyzer, and the dichotic messages presented are so similar, it is reasonable to assume that the input from the contralateral ear will suppress the ipsilateral ear input due to the greater number of projection fibres.
Supportive of this view are the findings of Berlin et al.
(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 left ear in normals, and the ear ipsilateral to the intact hemisphere in temporal lobectomees and hemispherectomees. In order to gauge their relative suppressive effect on the ipsilateral message the following "speech•
like" acoustic signals were presented simultaneously to the contra•
lateral ear as "challenges": vowels, "bleats" (isolated second and
third formants of CV syllables) and CV syllables. In general it was
found that the vowels had the least suppressive effect while the
bleats and CV syllables produced a greater degree of suppression.
Furthermore, all the challanges produced much more suppression of the
ipsilateral or "weak ear" message in the patients than in the normals.
To explain the effect it was suggested by these investigators that
"the signals produce their suppressive effect not because they actually
compete for the linguistic processor but rather, because they appear
to the nervous system to be likely '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 left temporal lobec- tomee and hemispherectomee respectively. This observation could be explained in the light of the postulation of a single "auditory parameter extractor" for patients who have had at least one temporal lobe removed whether the excision was in the language dominant hemis• phere or not. Therefore, the more similar the acoustic messages received via the ipsilateral and contralateral pathways to the intact temporal lobe, the greater the competition for the preliminary acoustic analysis before reaching the "linguistic processor."
In comparison, it could be hypothesized, that the dichotic messages presented in the DBFF test, which do not share the same fre• quency characteristics, would thus not have to compete for the same signal detectors at the primary auditory projection area. The messages received via the contralateral and ipsilateral projections would thus make a simultaneous but non-overlapping impression, of which the additive effect could be transmitted to the linguistic processing centre for the sorting of the combined auditory parameters into phonological features.
To explain the ability 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 if injury occurred early enough, the intact primary auditory cortex developed in such a way as to accommodate the simultaneous arrival of acoustic messages via contralateral and ipsilateral pathways. 59
This experiment, although indicating that binaural integra• tion does not necessitate the presence of two intact hemispheres, does not give a sufficiently definitive 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) is congruent with both possibilities; since injury would either (1) disallow effective integration if it occurs at that level, or (2) simply occlude the throughway transmission of information in both the ipsilateral and contralateral pathways, thus reducing the information content of the acoustic messages received by the primary auditory cortex thus making its task of integration more difficult.
It is interesting to note that neither in Matzker's report (1959) or in Smith's and Resnick's report (1970) is mention made of whether the brainstem disorders are lateralized to either side and we can only assume that because of the proximity of brainstem nuclei in such a small area this type of localized delineation is difficult to make, thus lending credence to the hypothesis that a brainstem lesion would affect the transmission of dichotically presented frequency semi- spectra of the same word.
In determining the diagnostic validity of the DBFF test, the results of this experiment suggest: (1) that both sides of the brain have the ability to integrate dichotically 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 intrinsic redundancy of the message has already been reduced by filtering, and
(3) that a unilateral temporal lobe lesion should not much affect a subject's performance on a DBFF test since both sides of the brain have the ability to integrate the frequency segments.
Given the limitations discussed earlier, the present investi• gation yields the following observations: The DBFF test could be an effective tool in the diagnosis of brainstem lesions by virtue of the fact that only bilateral temporal lesions or brainstem lesions should significantly reduce the scores. Although both sides of the brain appear to have the integrating function, and although the DBFF test could be used to differentiate brainstem lesions from unilateral temporal lobe lesions, whether binaural integration of dichotically presented frequency semi-spectra of the same word occurs at the brainstem level or at the cortical level, remains a moot point. 61
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APPENDIX 1 List 1, List 2, and List 3 from the Northwestern University Test No. 6
List 1 List 2 List 3 bean boat met bi te merge bar mouse burn mode book mill 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 fall rag dead rain cool rat fat raid deep read date ring gap raise fail room di tch road goose reach far rot dodge rush hash sell gaze said five search home shout gin shack germ seize hurl size goal shawl good shall jail sub hate soap gun sheep jar sure haze south half soup keen take hush thought hire talk king third juice ton hit team kite tip keep tool jug tell knock tough keg turn late thin laud vine learn voice lid void 1 imb week 1 i ve wag life wal k lot which loaf whi te 1 uck when love whip lore which mess wire yes match young mop youth APPENDIX 2
Dichotically 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 trai 1 nose pride track 6. corn fleet sunk coast flight sake 7. bell deed game bowl damp good 8. sheep vast zeal shine vent zest 9. mill nail pace mass nine pin 10. torn clock fresh tin cloth faith 11. speak bark need spit 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. flag send blown fault sand brain