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A comparison of threshold and supratheshold measurement of temporal integration in normal and cochlea-impaired… Smith, Faye Evelyn 1979

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A COMPARISON OF THRESHOLD AND SUPRATHRESHOLD MEASUREMENT OF TEMPORAL INTEGRATION IN NORMAL AND COCHLEA-IMPAIRED EARS by FAYE EVELYN SMITH B.A., University of Guelph, 1977 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Faculty of Graduate Studies i n the Department of PAEDIATRICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1979 © Faye Evelyn Smith, 1979 In presenting th is thesis in pa r t i a l fulf i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the Library shal l make i t f reely avai lable for reference and study. I further agree that permission for extensive copying of th is thesis for scholar ly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publ icat ion of th is thesis for f inancia l gain shal l not be allowed without my writ ten permission. . , PAEDIATRICS Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 n a f p MAY 5, 1979 ABSTRACT The present study was undertaken i n order to investigate temporal integration at threshold and suprathreshold l e v e l s i n normal-hearing and cochlea-impaired subjects, and also to examine the e f f e c t of frequency on the amount of integration for each group. Thresholds for 500-and 20-msec tone pulses were established i n a Bekesy tracking procedure by 20 subjects with normal hearing acuity and 20 subjects with noise-induced hearing lo s s . The amount of threshold s h i f t between the long-and short-duration tones, or temporal integration, was examined as a function of subject group and signal frequency (500 Hz and 4000 Hz) both i n quiet and i n the presence of a white noise masker. Results of t h i s study showed that the two groups could be d i f f e r e n t i a t e d by the mean threshold s h i f t between the two tone durations, although overlap between groups was marked. The amount of integration observed for the group with normal hear-ing was frequency-dependent i n both quiet and masked conditions The group of subjects with cochlear impairment exhibited normal integration values at 500 Hz, where no hearing loss was present and s i g n i f i c a n t l y smaller than normal values at 4000 Hz, where they demonstrated cochlear pathology. The masker altered the amount of temporal integration of the hearing-impaired group at both signal frequencies, while the normal subjects were unaffected by the presence of the masker at 4000 Hz. C r i t i c a l r a t i o s which were calculated from the masked thresholds showed - i i i -larger values for the hearing-impaired group than the normal-hearing group. The present re s u l t s support the use of b r i e f -tone audiometry i n the c l i n i c a l assessment of cochlear impair-ment. However, the group overlap i n performance which was demonstrated i n t h i s study indicates that an individual's per-formance using t h i s assessment must be interpreted cautiously. That i s , a subject cannot be r e l i a b l y assigned to either group in t h i s experiment when the amount of integration f a l l s within the region of overlap. - i v -TABLE OF CONTENTS T i t l e Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENT . v i i i 1. INTRODUCTION 1 2. LITERATURE REVIEW 4 2.1 P s y c h o p h y s i c a l p r o c e d u r e s f o r the i n v e s t i g a t i o n o f t empora l i n t e g r a t i o n 4 2.2 S t i m u l u s parameters 7 2.3 The p h y s i o l o g i c a l b a s i s o f t empora l i n t e g r a t i o n 11 2.4 B r i e f - t o n e audiometry i n the c l i n i c a l d i a g n o s i s o f a u d i t o r y p a t h o l o g y 15 2.5 The f requency e f f e c t i n t e m p o r a l i n t e g r a t i o n . . . . 18 2.6 The use o f masking i n the s tudy o f t e m p o r a l i n t e g r a t i o n . 21 2.7 The e f f e c t of, a f a t i g u i n g s t i m u l u s on the t h r e s h o l d - d u r a t i o n f u n c t i o n 23 2.8 S u p r a t h r e s h o l d measurement o f t empora l i n t e g r a t i o n 25 3. OBJECTIVES 2 9 - V -4 . METHOD 31 4.1 Subjects 31 4.2 Apparatus 32 4.3 Stimuli 32 4.4 Procedure 34 5. RESULTS 36 6. IDISCUSSION 47 6.1 Summary of re s u l t s 47 6.2 Diagnostic s e n s i t i v i t y of brief-tone audiometry. 47 6.3 The issue of frequency dependence 49 6.4 Masking and temporal integration 52 6.5 Relationship between hearing loss and threshold s h i f t 54 6.6 Data v a r i a b i l i t y 55 6.7 C r i t i c a l r a t i o 57 6.8 Conclusion 61 - v i -LIST OF TABLES Table Page 1 Mean thresholds and threshold s h i f t by tone duration and frequency i n quiet and masked conditions for each subject group 37 2 Results of the 12 t-tests 39 3 C r i t i c a l r a t i o i n dB by subject group as a function of signal duration .... 46 - v i i -LIST OF FIGURES Figure Page 1 Block diagram of the apparatus 33 2 Histograms showing amount of threshold s h i f t for each group, frequency, and condition. Note: N=normal; P=noise-induced hearing loss; Q=quiet; M=masked; 5=500 Hz; 4=4000 Hz 38 3 Threshold s h i f t as a function of absolute threshold at 4000 Hz for 20 N subjects (•) and 31 NIHL subjects ( + ) 42 4 Histograms displaying number of subjects with corresponding amounts of threshold s h i f t i n the quiet condition for N group (•) and NIHL group (|) 43 5 Histograms displaying number of subjects with corresponding amounts of threshold s h i f t i n the masked condition for N group (•) and NIHL group (|) 44 - v i i i -ACKNOWLEDGEMENT My sincere appreciation i s extended to the following persons for t h e i r contribution to the preparation of t h i s t h e s i s : Dr. D.Y. Chung who supervised the research and made he l p f u l suggestions i n the writing of the thes i s . Dr. D.D. Greenwood who read the manuscript and offered many useful comments i n i t s completion. The employees at the Workers' Compensation Board of B.C. who made available t h e i r f a c i l i t i e s to carry out t h i s research. The persons who w i l l i n g l y gave t h e i r time to act as subjects in the experiment. My friends who have shown great i n t e r e s t i n my work. The members of my family who give generous support to a l l my endeavours. - 1 -CHAPTER 1 1. INTRODUCTION A concept of recent i n t e r e s t i n the d i f f e r e n t i a l diagnosis of hearing impairment i s that involving the r e l a t i o n between threshold of a u d i b i l i t y and stimulus duration. The manner in which the ear sums acoustic energy over time i s known as temporal integration and the c l i n i c a l application of threshold-duration relationships has become known as brief-tone audiometry (Harris, Haines, and Myers, 1958). Theoretical i n t e r e s t i n the concept of the auditory system as an energy-integrator began with the description provided by Hughes ( 1 9 4 6 ) of the time-intensity relationships observed i n normal-hearing persons. Garner (1947b) and Garner and M i l l e r (1947) also provided data on t h i s phenomenon. It was observed that as the duration of an auditory signal i s decreased, i t s i n t e n s i t y must be increased i n order to maintain a u d i b i l i t y . Perfect integration occurs when a 3 dB increase i n signal i n -tensity i s required to maintain a u d i b i l i t y each time signal duration i s halved. This amounts to a 10 dB s h i f t i n threshold for each decade ( i . e . log unit) change i n duration. Observa-ti o n of t h i s phenomenon i s limited to durations less than approximately 200-500 msec. T y p i c a l l y (depending on frequency), at durations longer than 500 msec threshold does not change. While the above threshold-duration r e l a t i o n s h i p i s demon-- 2 -strated i n persons with normal hearing acuity, i t has been noted that cochlear impairment a l t e r s t h i s r e l a t i o n s h i p (Miskolczy-Fodor, 1953; Harris et a l . , 1958; Sanders and Honig, 1967; Wright 1968b; Hattler and Northern, 1970). The 10 dB s h i f t i n threshold per log unit change i n duration which i s t y p i c a l of normal hearing persons i s reduced to less than 6 dB i n persons with cochlear pathology. This observation has shown the diagnostic pot e n t i a l of temporal integration data i n the c l i n i c a l evaluation of cochlear function. Miskolczy-Fodor (1953) was one of the f i r s t to document the reduction i n temporal integration i n impaired ears. He reported that the a t y p i c a l integration demonstrated i n cochlear pathology accompanied the symptom of loudness recruitment, and suggested that d i f f e r e n t pathologies might exhibit d i f f e r e n t integration patterns. The study of Harris et a l . (1958) did not support Miskolczy-Fodor 1s data c o r r e l a t i n g the symptoms of at y p i c a l i n -tegration and recruitment but confirmed the c l i n i c a l p otential of temporal integration data i n the diagnosis of cochlear pathology. Despite the growing body of l i t e r a t u r e concerning temporal i n t e g r a t i o n , i t has not yet been f u l l y incorporated into the audiologic test battery. This i s p a r t i a l l y due to the lack of agreement concerning the s p e c i f i c a t i o n of stimulus parameters for use i n a c l i n i c a l s e tting. These parameters include stimulus duration, stimulus frequency, and psychophysical - 3 -testing procedure. In addition, there i s some c o n f l i c t i n g evidence i n various studies concerning the existence of frequency dependence i n temporal integration. Other variables, such as masking, require further investigation i n order to provide a better understanding of the process involved i n audi-tory temporal integration. The present study provides more data bearing on some of the factors influencing threshold-duration relationships i n both normal and impaired auditory systems. - 4 -CHAPTER 2 2. LITERATURE REVIEW 2.1 PSYCHOPHYSICAL PROCEDURES FOR THE INVESTIGATION  OF TEMPORAL INTEGRATION A v a r i e t y o f p s y c h o p h y s i c a l p r o c e d u r e s are f e a s i b l e f o r the s tudy o f t empora l i n t e g r a t i o n . The e a r l y l i t e r a t u r e c o n c e r n i n g t h i s phenomenon t y p i c a l l y r e p o r t s the use o f the c l a s s i c a l p r o c e d u r e s such as method o f adjustment and method o f l i m i t s (Garner , 1947a, 1947b; Garner and M i l l e r , 1947; H a r r i s e t a l . , 1958; Plomp and Bouman, 1959; E l l i o t t , 1963). In the more r e c e n t c l i n i c a l i n v e s t i g a t i o n s o f t h r e s h o l d - d u r a t i o n r e l a t i o n -s h i p s a c o n v e n t i o n a l p s y c h o p h y s i c a l t r a c k i n g p roced u re has commonly been employed (Wright , 1968, 1969; H a t t l e r and N o r t h e r n , 1970; M a r t i n and W o f f o r d , 1970; S a n d e r s , J o s e y , and Kemker, . 1971) . Other p r o c e d u r e s which have been l e s s commonly used i n -c l u d e f o r c e d - c h o i c e t r a c k i n g , c o n s t a n t s t i m u l i and c o n f i d e n c e r a t i n g ( B i l g e r and Fe ldman, 1969; Chamber la in and Z w i s l o c k i , 1970) . T r a c k i n g i s an a t t r a c t i v e method o f c h o i c e f o r c l i n i c a l use o f b r i e f - t o n e audiometry s i n c e t h i s p roced u re has a l r e a d y been e s t a b l i s h e d as a p o w e r f u l d i a g n o s t i c t o o l i n a u d i o l o g y . Bekesy audiometry (Bekesy, 1960) i s based on p s y c h o p h y s i c a l t r a c k i n g and has proven to be a r e l i a b l e p roced u re f o r - 5 -determining threshold as well as having a variety of other c l i n i c a l applications. The Bekesy audiometer i s e a s i l y adapted for brief-tone audiometry by the addition of c i r c u i t r y to provide control of stimulus duration. An advantage which the tracking method has over the c l a s s i -c a l methods i s the amount of time required for establishing threshold. The l a t t e r methods, p a r t i c u l a r l y the method of l i m i t s , require repeated measurements for r e l i a b i l i t y and may involve lengthy testing sessions. In a tracking procedure, thresholds can be obtained r e l i a b l y i n a few minutes and the subject i s presented with a r e l a t i v e l y easy task. The success of brief-tone audiometry i n the c l i n i c a l t est battery may be affected by the factors of time and t a s k - d i f f i c u l t y and they deserve important consideration. The method of adjustment and method of l i m i t s have been chosen for both c l i n i c a l and t h e o r e t i c a l investigations of temporal integration. The method of l i m i t s , however, has not been popular i n brief-tone studies and has been used seldom with hearing-impaired subjects. The method of adjustment has received more frequent use and has become the most popular alternative to the tracking method. Gengel and Watson (1971) and Richards and Dunn (1974) have compared subjects' performance under these l a t t e r two procedures. It i s apparent i n the l i t e r a -ture that method of investigation may influence t e s t r e s u l t s . C o n f l i c t i n g data have been reported on frequency dependence i n - 6 -temporal integration and part of t h i s c o n f l i c t has been a t t r i -buted to testing procedures used. Gengel and Watson (1971) com-pared the performance of normal-hearing subjects and hearing-impaired subjects with the tracking procedure and method of adjustment and found d i f f e r e n t r e s u l t s for the normal subjects with each procedure. They discussed the need for a standard c l i n i c a l procedure to avoid the differences i n te s t interpre-tation which would arise on the basis of d i f f e r e n t test procedures. Richards and Dunn (1974) suggested that r e l i a b i l i t y of threshold measurements i s an important consideration i n b r i e f -tone audiometry and that a method of investigation be chosen which could provide a high degree of t e s t - r e t e s t r e l i a b i l i t y . Their study involved a comparison of the r e l i a b i l i t y of Bekesy tracking and method of adjustment. Included i n the investiga-t i o n was a comparison of two attenuation rates of the tracking method. Their results showed no s i g n i f i c a n t change i n thres-hold as a function of measurement procedure or attenuation rate. Although no difference was found, they recommended that tracking be the preferred c l i n i c a l procedure, due to the ease of the task and r e l a t i v e l y short t e s t i n g time involved. Despite the number of psychophysical procedures which are suitable for the investigation of temporal integration, i t i s evident that method has introduced a source of v a r i a t i o n i n the e x i s t i n g data. While these variations are in t e r e s t i n g - 7 -from a th e o r e t i c a l perspective, they have caused some disagree-ment i n the standardization of a c l i n i c a l protocol for b r i e f -tone audiometry. 2.2 STIMULUS PARAMETERS Regardless of the psychophysical procedure used i n the i n -vestigation of temporal integration, certain stimulus parameters must be given consideration. These include r i s e - f a l l time and sp e c i f i c a t i o n of stimulus duration. In the case of tracking, r e p e t i t i o n rate and attenuation rate must also be c a r e f u l l y chosen. A r t i f a c t s are introduced into the experimental paradigm when the duration (and hence r i s e - f a l l time) of the stimulus i s s u f f i c i e n t l y short that, i n the course of turning the tone pulse on and o f f , unwanted transient energy appears. This energy, causing audible c l i c k s , may be detected by the subject and can a l t e r thresholds s i g n i f i c a n t l y (Wright, 1967). Fre-quency spread from tone pulses increases as stimulus duration decreases, making r i s e - f a l l time a very important consideration. There i s a l i m i t , however, to how much increase i n r i s e - f a l l time i s possible when the desired duration of the tone i s very short. Choice of a r i s e - f a l l time i s necessarily related to how duration i s defined. Many d i f f e r e n t r i s e - f a l l times have been employed i n b r i e f -- 8 -tone studies. In order to determine the e f f e c t of shaping the stimulus envelope, Dallos and Johnson (1966) obtained auditory thresholds for eight l i s t e n e r s at 1000 Hz with r i s e - f a l l times which varied from 0-40 msec. They found that when the over-a l l energy content was held constant, changes i n r i s e - f a l l time did not a f f e c t threshold. This allows some choice i n r i s e - f a l l suitable for brief-tone audiometry. Wright (197 8), however, suggests that r i s e - f a l l time should not be less than 5 msec to ensure adequate elimination of transient energy. He reported an experiment i n which normal-hearing subjects tracked the audible a r t i f a c t s of tones where r i s e - f a l l time varied from being instantaneous to 5 msec. It was found that at 5 msec int e n s i t y had to be increased to levels greater than 100 dB before the transients could be detected. For c l i n i c a l t e s t i n g , Wright suggested that a r i s e - f a l l time of 10 msec be adopted. When r i s e - f a l l time i s not instantaneous, duration must be c a r e f u l l y defined. However, there has been no consistency i n the l i t e r a t u r e on the s p e c i f i c a t i o n of signal duration. To overcome the ambiguity i n stating both r i s e - f a l l time and stimulus duration, Dallos and Olsen (1964) suggested the con-cept of equivalent duration. The equation they devised to describe the equivalent duration of a short tone burst i s : t = 2r/3+P where t represents equivalent duration i n milliseconds, r i s the r i s e - f a l l time (where both r i s e and f a l l are equal), and P i s the peak time of the tone burst. The equivalent durations - 9 -computed from t h i s equation m a i n t a i n the same energy content as a tone b u r s t w i t h instantaneous r i s e - f a l l times. In temporal i n t e g r a t i o n s t u d i e s , the c h o i c e of stimulus d u r a t i o n s has been widely spread. Durations as s h o r t as .5 msec and as long as 10 seconds have been used (Plomp and Bouman, 1959). P r a c t i c a l l i m i t a t i o n s have posed some con-s t r a i n t s on what d u r a t i o n s are c l i n i c a l l y f e a s i b l e . Due t o the need f o r r i s e - f a l l ramps t o reduce t r a n s i e n t energy, a minimum d u r a t i o n of 10 msec has been suggested by Wright (1978). A maximum d e s i r a b l e d u r a t i o n i s 500 msec. Beyond 500 msec l i t t l e i n t e g r a t i o n takes p l a c e . However, a number of i n v e s t i g a t o r s have attempted to s p e c i f y the p o i n t at which temporal i n t e g r a t i o n ceases ( H a r r i s e t a l . , 1958; G o l d s t e i n and Kramer, 1960; Sanders and Honig, 1967; Counter, 1974; Y o n o v i t z , M i t c h e l l and C l a r k , 1978). T h i s p o i n t has been c a l l e d the c r i t i c a l d u r a t i o n . The n o t i o n of c r i t i c a l d u r a t i o n was i n t r o d u c e d by H a r r i s e t a l . (1958) to d e s c r i b e the p o i n t on the temporal i n t e g r a t i o n f u n c t i o n where the a b c i s s a i s c r o s s e d by a s t r a i g h t l i n e f i t t e d t o the d a t a . T h i s p o i n t was a measure of where s t a b a l i z a t i o n of t h r e s h o l d o c c u r r e d . Sanders and Honig (1967) a l s o used the term c r i t i c a l d u r a t i o n which they d e s c r i b e d as the d u r a t i o n beyond which no observable t h r e s h o l d change takes p l a c e . For normal-h e a r i n g persons, t h i s p o i n t was e s t a b l i s h e d a t approximately 150 msec. G o l d s t e i n and Kramer (1960) had p r e v i o u s l y r e p o r t e d t h a t , i n normal-hearing persons, i n t e g r a t i o n c o u l d take p l a c e up to 2 seconds. An investigation by Counter (1973) of both normal-hearing and cochlea-impaired subjects showed that no s p e c i f i c c r i t i c a l duration could be defined for the normal group, but the cochlea-impaired group showed a d e f i n i t e term-ination of integration at approximately 100 msec. In a unique procedure devised to provide a d i r e c t measure of c r i t i c a l duration as well as a temporal summation function, Yonovitz et a l . (1978) allowed normal-hearing subjects to track threshold by c o n t r o l l i n g the duration of the tone burst rather than i t s in t e n s i t y . Burst width could be varied between 10 and 800 msec while the experimenter controlled stimulus frequency and inten-s i t y . The c r i t i c a l durations obtained with t h i s procedure were consistent with the values obtained by Sanders and Honig (1967) This procedure also was found to be a feasi b l e method for ob-taining threshold-duration functions. A parameter which must be given consideration when thres-hold-duration functions are obtained with pulsed tones, i s re-p e t i t i o n rate. In c l i n i c a l use of brief-tone audiometry, r e p e t i t i o n rate i s frequently established with respect to the longest test tone to be used i n the procedure as well as the s i l e n t i n t e r v a l which w i l l occur between successive tone pulses The s i l e n t i n t e r v a l must be s u f f i c i e n t l y large to allow i s o l a -t i o n of neural excitation i n i t i a t e d by each tone pulse and should thus be at least 200 msec i n duration (Zwislocki, 1960). This would t h e o r e t i c a l l y allow a maximum r e p e t i t i o n rate of 2.5 per second i f the duration of the longest tone used was 200 msec. Wright (1978) prefers to use a r e p e t i t i o n rate of one per second given that the longest tone i n his i d e a l c l i n i c a l procedure i s 500 msec. Repetition rate should be maintained at a . fixed value for a l l durations sampled i n a brief-tone experiment i n order to eliminate the potential confounding of rate with duration. A f i n a l consideration i n stimulus parameters i s attenua-tion rate. Wright (1978) suggests that i n brief-tone audio-metry, the attenuation should be set at 2.5 dB per second since t h i s i s the conventional rate used i n Bekesy audiometry and by maintaining t h i s rate results from the two procedures may be used together i n diagnosis. Martin and Wofford (1970) had suggested that a lower attenuation rate might increase the precision of the t e s t , although Richards and Dunn (1974) found no difference between a 1 dB per second and 2 dB per second attenuation rate except that the l a t t e r rate appeared more r e l i a b l e . 2.3 THE PHYSIOLOGICAL BASIS OF TEMPORAL INTEGRATION The physiological basis of temporal summation in the audi-tory system i s not c l e a r l y understood. Many of the early investigations described hypotheses which attempted to account for the manner i n which the ear integrates acoustic energy. Garner and M i l l e r (1947) hypothesized that the process of - 12 -integration i s l i n e a r with time. Inherent i n t h e i r proposal was the notion that the ear i s a perfect integrator above a certain minimum duration. This would r e s u l t i n a threshold decrease of 3 dB for each time that signal duration i s doubled. To test t h e i r hypothesis, they examined the change i n masked threshold for four frequencies over eight signal durations from 12.5 msec to 2 seconds. The data indicated that the ear integrated l i n e a r l y up to a maximum time of 200 msec. That i s , threshold changed by 10 dB for each log unit change i n duration. Above 200 msec integration s t i l l took place but i t was not l i n e a r . Although masked thresholds were used i n t h i s study, Garner and M i l l e r assumed that the observed r e l a t i o n s would also hold for quiet thresholds. In a further study of threshold-duration relationships, Garner (1947b) stated the hypothesis that the rate of temporal integration of energy i s dependent on the frequency band of energy to be integrated. Thus, duration would be proportional to energy only when a l l the energy available for integration would f a l l within a narrow band of frequencies. When the energy f e l l into a wider band of frequencies, integration would s t i l l occur but less change i n threshold would occur as duration changed. Garner's ideas were based on the observation that as the duration of a tone becomes very short the bandwidth of energy increases. Intensity would have to be raised to compen-sate for not only a loss i n t o t a l energy but also for the spread of that energy over a greater bandwidth. Bandwidth of the energy - 1 3 -was defined as being inversely proportional to the duration of the tone. In the course of shortening the duration of a tone, t o t a l energy would decrease while bandwidth increased. Garner examined these ideas by obtaining threshold-duration functions at 2 5 0 , 1 0 0 0 and 4 0 0 0 Hz. The shape of the functions for the 1 0 0 0 and 4 0 0 0 Hz stimuli were sim i l a r and showed l i n e a r integ-rat i o n over the durations tested, but at 2 5 0 Hz the change in threshold was unexpectedly small when stimulus duration decreased. This finding was explained i n terms of the energy splatter associated with the 2 5 0 Hz tone. Garner suspected that energy was available to the more sensitive frequencies i n the normal hearing s e n s i t i v i t y curve and contributed to thres-hold. While early investigations of temporal integration were concerned with describing the r e l a t i o n s h i p between in t e n s i t y and duration, there were few speculations made about the locus of the mechanism involved with integration. Zwislocki ( 1 9 6 0 ) suggested that the process most l i k e l y occurred beyond the cochlea at a higher neural l e v e l . His reasoning for a higher l e v e l was based on the observation that the neural a c t i v i t y which i s recorded from the f i r s t - o r d e r cochlear neurons f a i l s to show gradual build-up as would be expected i n the process of integration. In developing his theory of auditory temporal summation, Zwislocki made the assumption that the decay of ex-c i t a t i o n i n the auditory system i s exponential. At the thres-hold of a u d i b i l i t y , the time constant of the exponential decay - 14 -was e s t i m a t e d to be on the o r d e r o f 200 msec. T h i s v a l u e may be used as a lower l i m i t f o r the s i l e n t i n t e r v a l between s u c c e s -s i v e tone p u l s e s i n the measurement o f t e m p o r a l i n t e g r a t i o n . I t i s u n c l e a r how a h i g h e r l e v e l i n t e g r a t i v e mechanism c o u l d account f o r the p s y c h o p h y s i c a l d a t a showing depres sed t h r e s h o l d -d u r a t i o n f u n c t i o n s a s s o c i a t e d w i t h c o c h l e a r p a t h o l o g y . In an attempt to c l a r i f y t h i s m a t t e r , Wr ight (1968a) proposed t h a t the r e s u l t s which are t y p i c a l o f c o c h l e a r impairment do not r e f l e c t a d i s t u r b a n c e i n the i n t e g r a t i o n p r o c e s s i t s e l f but a r e , r a t h e r , the r e s u l t o f r a p i d s i g n a l a d a p t a t i o n a t the l e v e l o f the c o c h l e a . A d a p t a t i o n would l i m i t the energy i n p u t a v a i l a b l e f o r summation a t a h i g h e r l e v e l . Wr ight suggested t h a t r a p i d a d a p t -a t i o n o c c u r s w i t h l o n g - d u r a t i o n s t i m u l i i n c o c h l e a r p a t h o l o g y . J e r g e r e t a l . (1958) observed an a d a p t a t i o n e f f e c t i n s u b -j e c t s w i t h c o c h l e a r p a t h o l o g y when t h r e s h o l d s were o b t a i n e d i n a Bekesy t r a c k i n g procedure f o r c o n t i n u o u s t o n e s . A comparison •made between t h r e s h o l d s f o r c o n t i n u o u s and i n t e r r u p t e d tones showed t h a t , w h i l e no t h r e s h o l d change o c c u r r e d w i t h the i n t e r -r u p t e d s i g n a l s , a s h i f t o f 8 dB was observed a f t e r t h r e e minutes o f t r a c k i n g the c o n t i n u o u s t o n e . T h r e s h o l d remained s t a b l e a f t e r t h i s t ime i n c o n t r a s t to s u b j e c t s w i t h r e t r o c o c h l e a r p a t h -o l o g y who demonstrated f u r t h e r t h r e s h o l d s h i f t . These o b s e r v a -t i o n s support the n o t i o n t h a t a d a p t a t i o n a t the l e v e l o f the c o c h l e a may be r e l a t e d t o reduced t empora l i n t e g r a t i o n f u n c t i o n s i n s u b j e c t s w i t h c o c h l e a r p a t h o l o g y when the s t i m u l u s has s u f f i c i e n t d u r a t i o n . - 15 -Gersuni (1965) also supports the contention that the locus of temporal integration i s beyond the cochlea. However, he prefers to assume that there i s not just one neural l e v e l involved but, rather, several successive l e v e l s . He also suggests d i f f e r e n t types of summation for b r i e f intense sounds . compared to long weaker stimuli and states that there must be sp a t i a l summation involved as well as temporal summation. Although s p a t i a l summation i s a probable contributing factor i n the integration of acoustic energy, i t has not been systemati-c a l l y evaluated i n the l i t e r a t u r e concerning brief-tone audio-metry and deserves further attention. 2.4 BRIEF-TONE AUDIOMETRY IN THE CLINICAL DIAGNOSIS  OF AUDITORY PATHOLOGY Brief-tone audiometry has gained support as a useful tool i n the d i f f e r e n t i a l diagnosis of auditory pathology. Studies of temporal integration with hearing-impaired subjects have des-cribed the shape of the integration function which t y p i f i e s a variety of auditory pathologies. Together with other audiometric t e s t s , brief-tone audiometry can provide useful diagnostic i n f o r -mation to the c l i n i c i a n . The strength of brief-tone audiometry l i e s i n i t s a b i l i t y to reveal cochlear pathology (as opposed to other sensorineural pathologies). Numerous investigators have observed that the temporal integration function i n a cochlea-impaired l i s t e n e r i s - 16 -s i g n i f i c a n t l y depressed i n comparison to functions obtained with normal l i s t e n e r s (Miskolczy-Fodor, 1953; Harr i s et. a l 1 9 5 8 ; Sanders and Honig, 1967; Wright, 1968b). T y p i c a l l y , a thresho ld-s h i f t of less than 6 dB i s observed i n these subjects when s igna l durat ion i s changed by a log u n i t . Attempts have been made to corre la te aspects of the temporal in tegrat ion funct ion with s p e c i f i c types of cochlear pathology but i t does not appear that br ie f - tone audiometry can d i f f e r -t i a t e between types of cochlear impairment. Sanders and Honig (1967) reported that there was no consistent r e l a t i o n s h i p be-tween e i i o l o g y of the cochlear disturbance and the re su l t s of br ie f - tone t e s t i n g . The temporal in tegrat ion functions of four-teen subjects with diagnosis of Menieres disease , acoust ic trauma, and presbycusis d id not reveal s t r i k i n g d i f f erences . Corre la t ions have been reported, however, between the degree of hearing loss and amount of threshold s h i f t as durat ion i s shortened ( E l l i o t t , 1963; Young and Kanofsky, 1973; Pedersen and Salomon, 1977; Wright, 1978). I t has been reported that as hearing loss increases , amount of temporal in tegrat ion decreases. This f ind ing has led to the suggestion that temporal in tegrat ion may be dependent on the sound pressure l e v e l act ing on the cochlea (Gengel, 1972). While cochlear disturbance i s r e f l e c t e d i n the small amount of threshold s h i f t between long and short durat ion tones, i t has been found that retrocochlear les ions have no e f fec t on the temporal in tegrat ion funct ion (Sanders et a l . , 1971). S i m i l a r l y , i t has been reported that conductive hearing impairment does not a l t e r the funct ion (Wright and Cannel la , 1969). Conductive impairment, a f f ec t ing the middle ear s tructures would not be expected to a l t e r the higher l e v e l in tegrat ion process . To examine t h i s matter, Wright and Cannel la a r t i f i c i a l l y induced conductive hearing loss i n a group of normal-hearing subjects by occ lus ion of the external auditory meatus. Threshold-duration functions were obtained before, dur ing , and af ter s imulat ion of conductive impairment. No change i n the in tegra -t i o n functions was observed at any of these times. Results were also obtained for a subject with a pure conductive loss who demonstrated a normal temporal in tegrat ion funct ion . In the case of a mixed hearing loss where both the middle ear and cochlea are invo lved , Wright and Cannel la found that the i n t e -grat ion funct ion re f l ec t ed only the cochlear component of the l o s s . Although conventional audiometric tests are able to i d e n t i f y a hearing loss as of sensorineural o r i g i n , spec ia l tests are usual ly necessary i n order to determine whether the loss i s sensory (that i s , cochlear) or neural (affect ing the auditory nerve else-where). Br ie f - tone audiometry i s able to make t h i s kind of d i s t i n c t i o n . Sanders et a l . , ( l 9 7 l ) found that subjects who had confirmed les ions a f f ec t ing the eighth nerve could be d i s t inguished from subjects with cochlear impairment by the shape of the in tegrat ion funct ion . The former group showed - 18 -threshold-duration functions which were similar to those obtained from normal-hearing persons. Thus, i f a patient demo-strated a sensorineural loss i n conventional autiometric t e s t i n g , brief-tone audiometry could e s t a b l i s h i f the loss was cochlear or neural by the presence of a normal or depressed integration function. A further possible c l i n i c a l use of brief-tone audiometry i s in the diagnosis of a temporal lobe l e s i o n . Wright (1978) observed that a 15 dB threshold s h i f t may be demonstrated i n patients with temporal lobe lesions i n the ear co n t r a l a t e r a l to the l e s i o n . This finding may not be demonstrated at a l l frequencies, however, and res u l t s should be interpreted cautiously. A 15 dB s h i f t i n threshold may also be shown by a malingering patient. In general, accurate diagnosis can be made when brief-tone audiometry i s used i n conjunction with other audiometric t e s t s . 2.5 THE FREQUENCY EFFECT IN TEMPORAL INTEGRATION Perhaps the greatest source of c o n f l i c t i n the l i t e r a t u r e concerning temporal integration i s the e f f e c t that frequency has on the threshold-duration function. Generally, the e f f e c t which has been described i s that there i s a decrease i n amount of i n -tegration as stimulus frequency increases. There i s some d i f f i -c u lty i n attempting to resolve the issue l a r g e l y because of the great v a r i a b i l i t y i n the manner i n which temporal integration - 19 -has been examined. There has been no c o n s i s t e n c y i n the c h o i c e o f p s y c h o p h y s i c a l p r o c e d u r e , range o f f r e q u e n c i e s t e s t e d , or number o f d u r a t i o n s sampled. The i n f l u e n c e o f each o f these f a c t o r s on the r e s u l t s i s d i f f i c u l t t o a s c e r t a i n . P s y c h o p h y s i c a l procedure has r e c e i v e d the most s p e c u l a -t i o n as the cause o f the c o n f l i c t i n g r e s u l t s c o n c e r n i n g f r e -quency . However, an e x a m i n a t i o n o f the l i t e r a t u r e which i n -v o l v e s a v a r i e t y o f p s y c h o p h y s i c a l p r o c e d u r e s f o r measur ing t empora l i n t e g r a t i o n f a i l s t o r e v e a l any c o n s i s t e n c y i n t h i s m a t t e r . The m a j o r i t y o f d a t a o b t a i n e d w i t h the c l a s s i c a l p s y c h o p h y s i c a l procedures i s i n suppor t o f f requency dependence, a l t h o u g h t h e r e i s i n c o n s i s t e n c y here as w e l l . C h a m b e r l a i n and Z w i s l o c k i (1970) i n v e s t i g a t e d the i n f l u e n c e o f p roced u re i n a s tudy which used s i x p s y c h o p h y s i c a l methods: a d j u s t m e n t , l i m i t s , c o n s t a n t s t i m u l i , t r a c k i n g , f o r c e d - c h o i c e , and c o n f i -dence r a t i n g . The s l o p e o f i n t e g r a t i o n was found to change w i t h frequency f o r a l l p r o c e d u r e s except t r a c k i n g and f o r c e d c h o i c e . B i l g e r and Feldman (1969) a l s o examined the c o m p a r i -son between t h r e s h o l d - d u r a t i o n f u n c t i o n s o b t a i n e d w i t h t h r e e d i f f e r e n t methods: t r a c k i n g , f o r c e d - c h o i c e , and a "yes-no" p r o c e d u r e . R e s u l t s f o r the f i r s t two p r o c e d u r e s , as i n the s tudy of C h a m b e r l a i n and Z w i s l o c k i , showed no change w i t h f r e q u e n c y , w h i l e the "yes-no" p roced u re r e v e a l e d d i f f e r e n t f u n c t i o n s f o r d i f f e r e n t f r e q u e n c i e s . On the b a s i s o f these two s t u d i e s , i t would seem p o s s i b l e t h a t the t r a c k i n g method l a c k s the s e n s i t i v i t y i n i d e n t i f y i n g frequency dependence which other methods have been able to e s t a b l i s h . It i s also possible that there i s no dependence and the observed e f f e c t i s an a r t i f a c t of the other methods. The studies of Wright (1967, 1968b) which involved a tracking procedure, also f a i l e d to show a frequency e f f e c t . However, not a l l studies which used a tracking procedure reported the absence of frequency depend-ence. Hattler and Northern (1970) found a frequency e f f e c t i n the data for t h e i r normal subjects and Olsen and Carhart (1966) found a difference i n the slope of integration between low and high frequencies. Campbell and Counter (1969) proposed that there i s a lower frequency l i m i t for the observation of temporal integration which i s related to the upper l i m i t of p e r i o d i c i t y p i t c h . They suggested that both temporal integration and p e r i o d i c i t y p i t c h are neural i n nature and are based on a temporal analysis which takes place beyond the cochlea. However, they believed that the two processes were related i n a way which made them mutually exclusive and hence the upper frequency l i m i t for p e r i o d i c i t y pitch would also serve as the lower l i m i t for tem-poral integration. In both a "yes-no" adaptive procedure and tracking procedure the threshold-integration function was i n -vestigated for 125-, 175-, and 1000- Hz tones. No integration of energy was observed at 125 Hz and i t was concluded that 100-200 Hz was the lower frequency l i m i t for temporal integration. Watson and Gengel (1969) suggested that the lack of threshold change with short-duration stimuli at 125 Hz was due to the enhanced detection of energy at other frequencies over which the energy had spread. To test t h i s p o s s i b i l i t y , they incorporated a masker into the paradigm of Campbell and Counter to prevent detection of the signal energy at other frequencies. Their r e s u l t s showed that i t was, indeed, l i k e l y that energy spread had influenced the re s u l t s of Campbell and Counter since s i g n i f i c a n t integration was found at 125 Hz. It would appear, then, that the use of a masker or appropriate r i s e - f a l l time i s desirable i n the study of temporal integra-t i o n where energy spread may be available to contaminate the r e s u l t s . The lack of consistency regarding the influence of f r e -quency i n temporal integration i s an important issue to be resolved i n order to allow brief-tone audiometry to become use-f u l as a c l i n i c a l t o o l . Since no resolution of t h i s issue i s available at the present time, further research i s warranted to elucidate the matter. 2.6 THE USE OF MASKING IN THE STUDY OF TEMPORAL  INTEGRATION Masking noise has been used i n the study of temporal i n t e -gration to serve two general purposes. F i r s t , i t has provided a means to prevent energy spread at short durations from being - 22 -detected by the l i s t e n e r . The second use has been to allow suprathreshold (that i s , supra-quiet) measurement of temporal integration. Hattler and Northern (1970) u t i l i z e d an i p s i l a t e r a l white noise masker with cochlea-impaired subjects to investigate what ef f e c t a minimal amount of masking would have on the temporal integration function. They suggested that masking may be desirable i n some cases where threshold may be influenced by bands of transient energy which appear with short tone pulses. Their masking stimulus was broad-band noise presented at a l e v e l which would s h i f t threshold by only 5 dB from the value obtained i n the quiet. The res u l t s of t h i s investigation showed that threshold had shifted s l i g h t l y i n the presence of the masker although the shape of the threshold-duration func-t i o n was unchanged. It was concluded that i t would be advis-able to use a masker when the occurrence of transient energy i s probable. A closer examination of t h e i r data, however, i n d i -cates that the e f f e c t of the masker i s to increase the amount of threshold s h i f t between the longest and shortest tones even though s l i g h t l y . The use of a higher l e v e l masker might indicate whether t h i s trend i s s i g n i f i c a n t and can also be observed i n normal-hearing persons. A masking paradigm can also be used to study suprathreshold measurement of temporal integration. Gengel (1972) stated the p o s s i b i l i t y that reduced temporal integration i n cochlea-- 23 -impaired ears i s a r e f l e c t i o n of the h i g h e r s i g n a l l e v e l s needed to reach t h r e s h o l d . To examine t h i s , the i n t e g r a t i o n f u n c t i o n s of f o u r normal-hearing s u b j e c t s were obtained i n the presence of a masking n o i s e . The masker was i n t r o d u c e d t o r a i s e t h r e s h o l d to a l e v e l which i s c h a r a c t e r i s t i c of moderate to severe c o c h l e a r h e a r i n g l o s s . The r e s u l t s showed t h r e s h o l d -d u r a t i o n f u n c t i o n s which are t y p i c a l of normal-hearing persons, r a t h e r than co c h l e a - i m p a i r e d persons. T h i s f i n d i n g suggested t h a t i n c r e a s e d stimulus l e v e l i s not the f a c t o r r e s p o n s i b l e f o r the reduced temporal i n t e g r a t i o n observed i n s u b j e c t s w i t h c o c h l e a r pathology. However, Gengel compared h i s masked data to those of Watson and Gengel (1969) which were ob t a i n e d i n q u i e t and t h i s makes i t d i f f i c u l t t o i n t e r p e t the e f f e c t of masking. An i n s p e c t i o n of the data from these two s t u d i e s i n -d i c a t e s t h a t the shape of the i n t e g r a t i o n f u n c t i o n i s l a r g e l y unchanged i n the masked c o n d i t i o n but, s i m i l a r t o the H a t t l e r and Northern study (1970), the masker appears to cause a s l i g h t i n c r e a s e i n the amount of t h r e s h o l d s h i f t between the long-and s h o r t - d u r a t i o n tones. T h i s o b s e r v a t i o n appears t o be worthy of f u r t h e r i n v e s t i g a t i o n . 2.7 THE EFFECT OF A FATIGUING STIMULUS ON THE  THRESHOLD-DURATION FUNCTION Measurement of temporal i n t e g r a t i o n has been made a f t e r exposure to a f a t i g u i n g stimulus i n order to a s c e r t a i n whether - 24 -the pattern of temporal integration t y p i c a l l y observed i n cochlear pathology can be demonstrated i n normal ears during recovery from auditory fatigue. Jerger (1955) obtained thres-hold-duration functions for twelve normal-hearing subjects at quiet threshold with a 4000-Hz s i g n a l . Subjects were then exposed to thermal noise presented at a l e v e l of 110 dB SPL for two minutes. After t h i s time the threshold s h i f t was measured for tones of d i f f e r e n t durations. Pre-exposure integration values were found to demonstrate the normal time-i n t e n s i t y relationship while post-exposure thresholds showed a decrease i n the threshold s h i f t between the tones of long and short duration, which i s c h a r a c t e r i s t i c of persons with coch-lear pathology. Threshold measurements obtained at 1, 3, 9, and 12 minutes post-exposure demonstrated a gradual recovery to pre-exposure threshold lev e l s as post-exposure time i n -creased. The findings of t h i s study i l l u s t r a t e d that the e f f e c t of intense noise stimulation on the cochlea was i d e n t i -c a l to the altered time constant of integration associated with hearing loss of cochlear o r i g i n . This also confirmed that a normal-functioning cochlea i s necessary for normal i n t e -gration of acoustic energy. In a similar type of experiment, Henderson (1969) examined the temporal integration function of c h i n c h i l l a s before and afte r exposure to a fatiguing noise stimulus. Functions obtained before exposure for a 2000 Hz tone were sim i l a r to - 25 -those obtained for human subjects. The c h i n c h i l l a s were then given three hours of exposure to an octave band of noise centered at 2000 Hz at a l e v e l of 105 dB SPL. At in t e r v a l s following the exposure, thresholds were measured. The 12 dB difference between the long- and short- duration tones i n the pre-exposure t e s t i n g was reduced to zero at four minutes a f t e r the three hours of noise. This difference increased as post-exposure time increased. Complete recovery of the threshold-duration function was complete aft e r three weeks. These f i n d -ings concur with'Jerger's r e s u l t s for human subjects and show that the integration of energy i s reduced when noise-induced temporary threshold s h i f t occurs i n the auditory system. 2.8 SUPRATHRESHOLD MEASUREMENT OF TEMPORAL  INTEGRATION There are other means to investigate the temporal integra-t i o n function at suprathreshold l e v e l s i n addition to the use of masking. These means include measurements of stapedial r e f l e x thresholds and the use of loudness balancing. The threshold-duration relationships observed at auditory threshold have also been observed with stapedial r e f l e x t e s t i n g . The threshold at which the r e f l e x occurs i s elevated when stimulus duration i s decreased. This observation was reported by Djupesland and Zwislocki ( 1 9 7 1 ), who found a 35 dB s h i f t i n ref l e x threshold for a 2000 Hz tone i n normal l i s t e n e r s when stimulus durat ion was reduced from 1000 msec to 10 msec. This i s considerably steeper than the usual 10 dB per decade shortening. Woodford, Henderson, Hamernik, and Feldman (1975) reported s imi lar f ind ings , extending the data to more frequencies . They invest igated threshold-durat ion functions of the acoust ic re f l ex i n ten normal-hearing subjects and three persons with cochlear impairment, at stimulus frequencies of 500, 1000, 2000, 3000, and 4000 Hz. The in tegrat ion functions of the normal-hearing subjects were found to be much steeper than the functions t y p i c a l l y obtained at auditory threshold , which i s consistent with the f indings of Djupesland and Zwis locki (1971). The re f l ex threshold-durat ion functions obtained for the cochlea v-impaired subjects were f la t tened i n comparison to the normals, a trend which i s a lso observed at auditory threshold . In addi t ion to these f ind ings , Woodford et a l . reported a frequency e f fec t i n t h e i r data . Thresholds for the 500- and 1000- Hz s t i m u l i produced a shallower in tegrat ion funct ion than the higher frequencies . This f ind ing i s i n contrast to the frequency e f fec t observed at auditory threshold where the lower frequencies usual ly produce the steepest funct ion . In th i s study, the 4000 Hz stimulus produced the steepest funct ion for both the normal and sensorineural- impaired subjects . Contrary to the f indings of Woodford et a l . , Stelmachowicz and Seewald (1977) found that the slope of in tegrat ion at re f l ex t h r e s h o l d was s i m i l a r f o r both normal-hearing and c o c h l e a : -impaired persons. T h e i r data were obtained u s i n g 20 normals and 20 s u b j e c t s w i t h c o c h l e a r pathology, numbers which they f e l t may have accounted f o r the d i f f e r e n c e s i n the f i n d i n g s of Woodford e t a l . who used o n l y three c o c h l e a - i m p a i r e d and ten normal s u b j e c t s . At a u d i t o r y t h r e s h o l d , however, Stelmachowicz and Seewald observed d i f f e r e n c e s between the two groups which are c o n s i s t e n t with the l i t e r a t u r e demonstrating f l a t t e r func-t i o n s f o r cochlea-impaired s u b j e c t s . The r e s u l t s of t h i s study suggest the p o s s i b i l i t y t h a t c o c h l e a - i m p a i r e d s u b j e c t s may i n t e g r a t e a c o u s t i c energy d i f f e r e n t l y a t t h r e s h o l d and s u p r a t h r e s h o l d l e v e l s . A l o u d n e s s - b a l a n c i n g procedure was used by Pederson and Poulsen (1973) to o b t a i n measurements of s u p r a t h r e s h o l d tempor-a l i n t e g r a t i o n . Monaural and b i n a u r a l loudness b a l a n c i n g was performed f o r 1000 Hz tone p u l s e s of v a r i o u s d u r a t i o n s by p r e s -b y c u s i c s u b j e c t s a t l e v e l s of 75 and 95 dB SPL. Although i n t e r - s u b j e c t v a r i a t i o n was g r e a t , r e s u l t s showed no r e d u c t i o n i n temporal i n t e g r a t i o n from expected normal v a l u e s . The l o u d -ness balances at 95 dB r e s u l t e d i n a temporal i n t e g r a t i o n func-t i o n s i m i l a r t o t h a t observed f o r normals at a u d i t o r y t h r e s h o l d while at 75 dB, the f u n c t i o n was found t o be steeper than nor-mal. These r e s u l t s are i n c o n t r a s t t o data r e p o r t e d by Pedersen and E l b e r l i n g (1973) f o r p r e s b y c u s i c s u b j e c t s who showed reduced temporal i n t e g r a t i o n when measured a t a u d i t o r y - 28 -threshold. Again i t appears that integration may occur d i f -ferently at suprathreshold l e v e l s . - 29 -CHAPTER 3 3. OBJECTIVES It i s apparent from the l i t e r a t u r e that there are several matters i n the research concerning temporal integration which deserve further attention. Among these are the establishment of a standardized c l i n i c a l procedure for brief-tone audiometry, further investigation of the frequency e f f e c t , and more detailed study of the influence of masking on the threshold-duration function. The c l i n i c a l procedure advocated by Wright (1978) for ex-amining temporal integration d i a g n o s t i c a l l y appears to be a suitable paradigm for use with the hearing-impaired. The tracking procedure can be used to esta b l i s h threshold r e a l i a b l y within a few minutes and the patient can e a s i l y be f a m i l i a r i z e d with the task. A two-duration comparison also appears to be an adequate measure of temporal integration differences. Since few studies have attempted previously to validate t h i s proced-ure, the present study w i l l adopt Wright's paradigm for examin-ing temporal integration with both normal and cochlea-impaired l i s t e n e r s . If more investigations are made with si m i l a r proce-dures, standardized data may become available to warrant the use of brief-tone audiometry as a r e l i a b l e diagnostic t e s t of cochlear function. It w i l l also be the aim of t h i s study to make more data available concerning the frequency e f f e c t i n temporal integra-t i o n . With measurement of both threshold and suprathreshold - 30 -integration i n normal and hearing-impaired subjects, i t i s hoped that the data may reveal information useful i n the ex-planation of the c o n f l i c t i n g data which e x i s t i n the l i t e r a t u r e . A f i n a l purpose of the present research i s to examine the e f f e c t of masking on the temporal integration function. The e x i s t i n g l i t e r a t u r e dealing with masking and integration does not as yet, supply an adequate description of a masker's e f f e c t . The study of Hattler and Northern (1970) used only minimal masking with a sensorineural population and revealed a small increase i n the amount of threshold s h i f t between long- and short-duration tones. Gengel's study (1972) also showed such an increase but these data were obtained for a very small sample of normal l i s t e n e r s and the increase can only be mea-sured with respect to data from another study. Since masking allows a suprathreshold measurement of temporal integration, i t i s the aim of t h i s study to compare the amount of threshold s h i f t observed at both auditory and masked threshold. The data of Stelmachowicz and Seewald (1977), as well as Pederson and Poulsen (1973), suggest that suprathreshold integration of acoustic energy involves a somewhat d i f f e r e n t process than the integration which i s measured at auditory threshold. - 3 1 -CHAPTER 4 4 . METHOD 4 . 1 , SUBJECTS Two groups of subjects were selected to pa r t i c i p a t e i n t h i s experiment, one group of normal-hearing persons (N), and one group of persons with noise-induced hearing loss (NIHL). The N group consisted of 2 0 males between the ages of 2 1 and 3 3 with a mean age of 2 5 . 6 years. Eight of these men were employees i n the building at the Workers' Compensation Board of B r i t i s h Col-umbia (WCB). The other 1 2 were patients at the WCB Rehabilita-tion Centre who were being treated for i n j u r i e s unrelated to hearing. There was no known history of ear pathology or pro-longed exposure to noise i n any of these subjects. For i n c l u -sion i n t h i s study, i t was required that t h e i r pure-tone thresholds at the octave frequencies from 2 5 0 Hz to 8 0 0 0 Hz be 1 5 dB HL (ANSI, 1 9 6 9 ) or better. The NIHL group consisted of 2 0 males between the ages of 4 9 and 6 9 with a mean age of 5 9 . 8 years. A l l were claimants at the Hearing Branch at the WCB. The c r i t e r i a for selection of these subjects included good health, no known incidence of middle ear pathology and a history of prolonged exposure to i n d u s t r i a l noise. Requirements for in c l u s i o n i n the group were thresholds of 2 0 dB HL or less at 5 0 0 Hz and thresholds of 5 0 -7 0 dB HL at 4 0 0 0 Hz, with no i r r e g u l a r i t i e s i n the slope of - 32 -the curve p l o t t i n g loss as a function of frequency. In addition to these 2 groups of subjects, 11 other WCB claimants, whose pure-tone thresholds did not meet the c r i t e r i a s p e c i f i e d above, were used i n the experiment. Thres-holds of these subjects were greater than 20 dB HL at 500 Hz and/or greater than 7 0 dB HL at 4000 Hz. Otherwise they were simi l a r to the NIHL group i n a l l respects. The data from these 11 subjects were combined with those of the 20 NIHL subjects for a certain part of the analysis where noted. 4.2 APPARATUS The apparatus used i n t h i s experiment consisted of a Grason-Stadler audiometer, model 1701, to which an electronic switch was adapted for control of stimulus duration. A block diagram of the apparatus i s shown i n figure 1. The experiment was conducted with the subject seated i n a sound-insulated tes t booth, IAC model 11 -330 , at the Hearing Branch of the WCB in Richmond, B.C. Stimuli were delivered monaurally through a TDH-49 earphone. 4.3 STIMULI The stimuli consisted of pure tones at the frequencies 500 Hz and 4000 Hz, and a white noise masker. The bandwidth of the masker was limited by the response of the earphone. The spec-trum l e v e l of the noise was calculated to be 52.7 dB with a bandwidth of 5370 Hz. oscillator electronic switch amplifier recording attenuator noise generator amplifier attenuator response bu tton F i g u r e 1. B l o c k d i a g r a m o f t h e a p p a r a t u s . - 34 -Intensity l e v e l of the masker was fixed at 90 dB SPL and i t was presented i p s i l a t e r a l l y to the tonal s t i m u l i . Duration of the pure tones was either 500 msec or 20 msec and was determined i n the manner suggested by Dallos and Olsen (1964), where t=2r/3+P. In t h i s equation, t refers to time, r represents the r i s e - f a l l time and P i s the peak time. R i s e - f a l l time for both the 500 msec and 20 msec tones was six msec. 4.4 PROCEDURE Threshold measurements were made with a psychophysical tracking procedure. Each subject tracked a t o t a l of eight thresholds, with each threshold being determined for a d i f f e r e n t condition of stimulus frequency, duration, and masking. The stimulus was pulsed monaurally at a r e p e t i t i o n rate of approxi-mately one per second. Attenuation rate of the signal was 2.5 dB per second, and the d i r e c t i o n of change was controlled by the subject with a hand switch. A l l subjects tracked four thresholds i n quiet followed by four thresholds i n the presence of the continuous white noise masker. Presentation order of the four combinations of stimulus frequency and duration was randomly determined within each of the quiet and masking condi-tions . Subjects were f a m i l i a r i z e d with the task i n a practice t r i a l p r i o r to the beginning of the experiment. Both the f r e -quency and duration of the stimulus tone used i n t h i s practice t r i a l were d i f f e r e n t from those used i n the experiment. In each of the experimental t r i a l s , subjects tracked the stimulus u n t i l a t o t a l of 12 threshold crossings were made. The f i r s t two crossings were discarded i n the determination of threshold. The threshold was defined as the average of the 10 points at which the attenuator made a reversal. Total testing was com-pleted i n an average of 15 minutes. - 36 -CHAPTER 5 5. RESULTS The mean thresholds for the 500- and 20- msec tones were calculated for each subject group at the 2 stimulus frequen-cies i n both the quiet and masked conditions. These values are presented i n Table 1. The amount of threshold s h i f t between the long- and short-duration tones, or temporal integration, i s depicted i n a histogram i n Figure 2 with bars representing each of the experimental conditions. Preliminary examination of the data indicated that there were differences i n the amount of temporal integration between groups, frequencies and masking conditions. A series of t- t e s t s were computed to de-termine the significance of these differences. These re s u l t s are shown i n Table 2. The amount of temporal integration i n the N group and NIHL group were s i g n i f i c a n t l y d i f f e r e n t at 4000 Hz, as expected, with threshold s h i f t between the long- and short-duration tones for the N group being 7.3 dB, and 4.1 dB for the NIHL group. At 500 Hz, where thresholds for the NIHL group f a l l i n the normal hearing range, there was no difference between groups (8.6 dB for the N group and 8.0 dB for the NIHL group). - 37 -TABLE 1 Mean thresholds and threshold s h i f t by tone duration and frequency i n quiet and masked conditions for each subject group. N Quiet Condition Frequency (Hz) 5 0 0 4 0 0 0 Duration (Msec) Mean Difference (dB) (dB) 20 5 0 0 20 5 0 0 1 1 . 3 0 2 . 7 0 1 0 . 0 0 2 . 7 0 8 . 6 0 7 . 3 0 N Masked1 Condition 5 0 0 4 0 0 0 20 5 0 0 2 0 5 0 0 7 2 . 1 0 6 0 . 8 0 7 4 . 6 0 6 7 . 5 0 1 1 . 3 0 7 . 1 0 NIHL Quiet-Condition 5 0 0 4 0 0 0 2 0 5 0 0 20 5 0 0 1 8 . 0 0 1 0 . 0 0 6 4 . 4 0 6 0 . 3 0 8 . 0 0 4 . 1 0 NIHL Masked Condition 5 0 0 4 0 0 0 2 0 5 0 0 2 0 5 0 0 7 4 . 0 0 6 4 . 3 0 7 8 . 5 0 7 2 . 7 0 9 . 7 0 5 . 8 0 - 38 -NQ5 NM5 NQ4 NM4 P©5 PM5 PQ4 PM4 Figure 2. Histograms showing amount of thres-hold s h i f t for each group, frequency, and condition. Note: N=normal; P=noise-induced hearing loss; Q=quiet; M=masked; 5=500 Hz; 4=4000 Hz. - 39 -TABLE 2 R e s u l t s o f the 12 t - t e s t s t - t e s t s c o n d i t i o n s t d f p<. 05 Normal 500 H z , q u i e t 0. 97 38 N v s . 500 H z , masked 2.69 38 Y NIHL 4000 H z , q u i e t 5.23 38 Y 4000 H z , masked 1.86 38 Y 500 Hz n o r m a l , q u i e t 2.17 19 Y v s . n o r m a l , masked 7.14 19 Y 4000 Hz N I H L , q u i e t 7.56 19 Y NIHL, masked 5.60 19 Y q u i e t n o r m a l , 500 Hz 3.87 19 Y v s . n o r m a l , 4000 Hz 0.14 19 N masked NIHL, 500 Hz 2.82 19 Y NIHL, 4000 Hz 1.98 19 Y Note : Y = • s i g n i f i c a n t , N = not s i g n i f i c a n c e based on one-s i g n i f i c a n t ; • t a i l e d t e s t . l e v e l o f - 4 0 -In the masked condition the differences between groups were s i g n i f i c a n t at both test frequencies. The masking ap-pears to have affected the 2 groups d i f f e r e n t l y at 5 0 0 Hz i n -creasing the . 6 dB difference observed i n the unmasked condi-tion to a difference of 1 . 6 dB when masking was introduced. At 4 0 0 0^Hz the difference between groups decreases from 3 . 2 dB to 1 . 3 dB i n the presence of the masker although the difference i s s t i l l s i g n i f i c a n t . A frequency e f f e c t was observed i n the normal group for both unmasked and masked conditions. The amount of threshold s h i f t between the 5 0 0 - and 20-msec tones was s i g n i f i c a n t l y d i f f e r e n t between the 5 0 0 Hz and 4 0 0 0 Hz signals. As seen i n Figure 2 , t h i s difference i s more apparent when masking i s i n -troduced and i n both conditions shows greater threshold s h i f t for the 5 0 0 Hz tone. In the case of the NIHL group, a freq-uency e f f e c t i s also apparent. This difference i s i n the d i r -ection expected on the basis of the hearing loss at 4 0 0 0 Hz. The presence of cochlear pathology at t h i s frequency r e s u l t s i n a threshold s h i f t of 4 . 1 dB (in the quiet condition) com-pared with 8 . 0 dB at 5 0 0 Hz where hearing i s within normal l i m i t s . The presence of the white noise masker s i g n i f i c a n t l y af-fected the amount of threshold s h i f t between the tones of long and short duration for a l l conditions except 4 0 0 0 Hz for the N group. In each of these cases, threshold s h i f t became greater when masking was introduced. However, at 4000 Hz the mean threshold s h i f t s for the normal-hearing subjects were 7.1 dB and 7.3 dB under quiet and masked condit ions respect-i v e l y . On the basis of contentions made by various inves t igators about the r e l a t i o n s h i p between amount of hearing loss and ab-solute hearing threshold ( E l l i o t t , 1963; Pederson, 1977; Wright, 1978) , an inves t iga t ion was made of such a_,relationship i n the present data . For t h i s a n a l y s i s , the data of the 11 subjects described e a r l i e r , who d id not meet the experimental c r i t e r i a , are included to provide a wider d i s t r i b u t i o n of hear-ing l e v e l s . The data for 500 Hz are not included since hearing thresholds at th i s frequency are normal or near normal for the 32 subjects . In Figure 3 absolute thresholds for the 4000 Hz tone are p lo t ted as a funct ion of the threshold s h i f t which occurs when durat ion i s decreased from 500 msec to 20 msec. A s i g n i f i c a n t c o r r e l a t i o n of negative - .63 was found between these 2 v a r i a b l e s . .TRe-grgup/ov-erlap i n the in tegrat ion data i s re f l ec ted i n Figures 3: to 5> which d i sp lay histograms showing the number of subjects who demonstrated various amounts of threshold s h i f t between the long- and short -durat ion tones. The amount of overlap between groups i s greater at 500 Hz than 4000 Hz, as expected, due to the absence of hearing loss at t h i s frequency. t i l l 1 1 1 1 - • • • • . • • • - • — • • — — — • • • • • • • • • • A . ^ i -— • — 1 1 1 1 1 1 i i 0 10 20 30 40 50 60 70 ABSOLUTE THRESHOLD (dB) F i g u r e 3 . T h r e s h o l d s h i f t a s a f u n c t i o n o f a b s o l u t e t h r e s h o l d a t 4000 Hz f o r 20 N s u b j e c t s ( • ) a n d 31 N I H L s u b j e c t s ( + ) . NUMBER OF SUBJECTS H-cQ d H (D — CO £ ffi • tr H - H -— Hi £f rt rt 3 H-OJ 3 O O K 3 r t K CD 13 Ch CD Cfl CO vQ H O d H-CO ~ o M 3 o d H- H- I—1 O 3 di r t i Q H-O P) 3 3 cQ O • ft lo- d 3 d 3 CD o Hi 3 rt H- to O 3 O Hi Hi O rt H 3" H co a CD d co tr &<->• H O CD O M O d Oi r t CO o * 0> I I I I I I I CO "T~T CO 1 to ui oi 0) Ol o o X N I 1 I ' ' » '— I—L o o o X N I I I I I I I I - e*. -1 I I i 1 r T 1 f -i r o UJ -> m ce ui CD is z 8 6 4 2 0 8 6 4 b J I L 4000 Hz 500 Hz JQ 10 II 12 13 14 15 THRESHOLD SHIFT (dB) Figure 5. Histograms displaying number of subjects with corresponding amounts of threshold s h i f t i n the masked condition for N group (Q) and NIHL group (fl) . - 45' ~ However, i t i s apparent that there i s no one frequency or masking condition i n which the 2 groups are e a s i l y separated on the basis of threshold s h i f t . That i s , one can expect to see a normal-hearing person with a threshold s h i f t " t y p i c a l " of (i. e . near the mean for) cochlear pathology and vice versa. A f i n a l analysis of the data was made with measurement of the c r i t i c a l r a t i o at masked threshold. These values appear i n Table 3. It can be seen that an increase i n signal duration has a corresponding increase i n c r i t i c a l r a t i o for both groups. In a l l cases, the values are larger for 4000 Hz than for 500 Hz. The, NIHL group has larger c r i t i c a l r a t i o s than the N group for each duration and each frequency. Also the difference between groups i s smaller at 20 msec than at 500 msec. It appears that there i s a difference i n the way i n which both groups pro-cess signals of long duration compared to short duration. - 46 -TABLE 3 C r i t i c a l r a t i o i n dB by s u b j e c t group as a f u n c t i o n of s i g n a l d u r a t i o n . S i g n a l D u r a t i o n  Subject Group Frequency 20 500 500 Hz 30.9 19.6 Normal 4000 Hz 31.4 24.3 Noise-induced 500 Hz 32.8 23.1 4000 Hz 35.3 29.5 CHAPTER 6 6. DISCUSSION 6.1 SUMMARY OF RESULTS The re s u l t s of t h i s study showed that: (1) groups of normal-hearing persons could be d i s t i n -guished from groups of persons with noise-induced hearing loss on the basis of temporal integration. (2) there i s great v a r i a b i l i t y i n the data which results i n overlap between the two subject groups (3) there i s greater threshold s h i f t between the tones of long and short duration at 500 Hz than at 4000 Hz (4) the masker d i f f e r e n t i a l l y a ffects the amount of thres-hold s h i f t at the two t e s t frequencies (5) there i s a negative c o r r e l a t i o n between absolute threshold and threshold s h i f t (6) the c r i t i c a l r a t i o s obtained from the masked thres-hold data show larger values for the group with coch-lear pathology than for the normal-hearing group 6.2 DIAGNOSTIC SENSITIVITY OF BRIEF-TONE AUDIOMETRY The finding that the normal and cochlea-impaired group means could be d i f f e r e n t i a t e d by measurement of temporal integration i s consistent with reports i n the l i t e r a t u r e that brief-tone audiometry might be s u f f i c i e n t l y sensitive to allow i d e n t i f i -cation of cochlear lesions (Miskolczy-Fodor, 1953; Harris et a l , 1958; Sanders and Honig, 1967; Wright, 1968b). However, i t - 48 -does not support t h i s p o s s i b i l i t y i n that group overlap i s so extensive, despite the d i f ference i n means, that no attempted diagnost ic use i n i n d i v i d u a l cases i s j u s t i f i e d as yet . The study leaves open the p o s s i b i l i t y that by t r a i n i n g subjects , the separation of groups might be large enough to allow d i a g -nost ic use i n i n d i v i d u a l cases. The f indings that the NIHL group had normal temporal in tegrat ion at 500 Hz where they demonstrated no hearing l o s s , but reduced in tegrat ion at 4000 Hz where they demonstrated cochlear pathology, also shows the diagnost ic p o t e n t i a l of br ie f - tone audiometry. The re su l t s of t h i s study also show that a 2-duration com-parison as suggested by Wright (1978) has s u f f i c i e n t s e n s i t i v -i t y to d i f f e r e n t i a t e the group means of normal and cochlear-l e s ion subjects . Wright previous ly tested 50 normal-hearing subjects at s ix durations from 10 to 500 msec and compared the threshold s h i f t to that obtained when only two durations (20 and 500 msec) were used. The 6-duration and 2-duration proce-dures y ie lded the same mean values and standard deviat ions i n -d i c a t i n g that two durations are su i table for c l i n i c a l use. A 2-duration comparison also allows br ie f - tone audiometry to be performed i n less time than a sampling of several durations would r e q u i r e . This increases i t s appeal i n c l i n i c a l a p p l i c a -t i o n s . Wright (1978) cautioned about the c l i n i c a l use of only two durations u n t i l data i s ava i lab le concerning the d i f f e r -ences of hearing-impaired persons on two versus s ix durat ions . - 49 -While the present study did not purport to examine such a difference, i t showed that two durations were s u f f i c i e n t to d i f f e r e n t i a t e between groups of normal-hearing subjects and subjects with cochlear impairment. 6.3 THE ISSUE OF FREQUENCY DEPENDENCE The s h i f t i n threshold between the tones of long and short duration was found to be s l i g h t l y greater for the N group at 500 Hz than at 4000 Hz. While t h i s difference was very small in quiet, i t was accentuated i n the masked condition. The ob-servation of a frequency e f f e c t i n temporal integration has been reported many times i n the l i t e r a t u r e and has been d i s -cussed primarily as being a possible r e s u l t of the psychophy-s i c a l procedure used (although no suggestions have been offered to explain why one method should reveal or create the e f f e c t and not another). The tracking procedure generally has f a i l e d to r e f l e c t frequency-dependent r e s u l t s (Wright, 1968b; Bilger and Feldman, 1969; Chamberlain and Zwislocki, 1970). It i s un-certai n whether the decrease i n temporal integration which i s noted with increase i n frequency i s related to certa i n other factors instead of, or i n addition to, being related to the method of investigation. It has been suggested for instance, that the decreased temporal integration for high frequencies might be related to the presence of s u b c l i n i c a l cochlear lesions i n the normal-hearing subjects used (Campbell and - 50 -C o u n t e r , 1 9 6 9 ; B a r r y a n d L a r s o n , 1 9 7 4 ; W r i g h t , 1 9 7 8 ) . B a r r y a n d L a r s o n (1974 ) p r e s e n t some d a t a w h i c h may b e r e -l e v a n t t o t h i s s u g g e s t i o n . T h e t e m p o r a l i n t e g r a t i o n f u n c t i o n s o f c h i l d r e n , b o t h n o r m a l - h e a r i n g a n d c o c h l e a - i m p a i r e d , w e r e o b t a i n e d u s i n g a m o d i f i e d m e t h o d o f l i m i t s f o r t h e o c t a v e f r e q u e n c i e s f r o m 500 Hz t o 4000 H z . T h e a m o u n t o f t h r e s h o l d s h i f t m e a s u r e d f o r t h e n o r m a l c h i l d r e n was s i m i l a r a t a l l f r e -q u e n c i e s . T h a t i s , t h e f r e q u e n c y e f f e c t r e p o r t e d i n m o s t s t u d i e s u s i n g a p r o c e d u r e o t h e r t h a n t r a c k i n g was a b s e n t i n t h e c h i l d r e n e x a m i n e d i n t h i s s t u d y . T h e s e r e s u l t s g a v e i n c r e a s e d p l a u s i b i l i t y t o t h e p r o p o s a l o f C a m p b e l l a n d C o u n t e r w h i c h p r e -s e n t e d t h e p o s s i b i l i t y t h a t t h e f r e q u e n c y e f f e c t may b e d u e t o s u b c l i n i c a l c o c h l e a r p a t h o l o g y i n a d u l t s u b j e c t s . S i m i l a r s p e c u l a t i o n s h a v e b e e n made b y S a n d e r s a n d H o n i g (1967 ) a n d W r i g h t ( 1 9 7 8 ) . I t c o u l d b e r e a s o n e d t h a t t h e c h i l d r e n e x h i b i t no f r e q u e n c y e f f e c t b e c a u s e t h e i r c o c h l e a s a r e r e l a t i v e l y u n - ; i m p a i r e d . S i n c e i t h a s b e e n r e p o r t e d t h a t t h e h i g h e r f r e q u e n -c i e s show t h e l e a s t amoun t o f i n t e g r a t i o n , i t i s n o t u n r e a s o n -a b l e t o s u p p o s e t h a t m o s t a p p a r e n t l y n o r m a l - h e a r i n g a d u l t s may h a v e s u b c l i n i c a l d e t e r i o r a t i o n o f t h e b a s a l e n d o f t h e c o c h l e a . A l t h o u g h t h e s u b j e c t s i n t h e p r e s e n t s t u d y e x h i b i t e d h e a r i n g t h r e s h o l d s l e s s t h a n 15 dB HL a n d w e r e g e n e r a l l y l e s s t h a n 30 y e a r s o f a g e , i t i s , n e v e r t h e l e s s , p o s s i b l e t h a t t h e y h a d some c o c h l e a r d e t e r i o r a t i o n n o t m e a s u r a b l e i n s t a n d a r d p u r e - t o n e a u d i o m e t r y . I t i s f e a s i b l e t o make s u c h a s p e c u l a t i o n s i n c e p e r s o n s a r e o f t e n e x p o s e d t o e x c e s s i v e n o i s e - 51 -i n the course of d a i l y a c t i v i t i e s . Bredberg (1968) also has provided data based on histopathologic studies which show the presence of degeneration i n some of the sensory c e l l s i n the cochlea which i s believed to occur near the t h i r d decade of l i f e . While such reasoning i s speculative i n describing the e f f e c t of frequency i n temporal integration, i t could, never-theless-^ be a contributing factor. Further studies with children may provide more data i n support of such a p o s s i b i l -i t y . In any case, more experiments are needed to convert these speculations into proposals with some p r a c t i c a l u t i l i t y . Further i n d i c a t i o n that cochlear dysfunction may be i d e n t i -f i e d by brief-tone audiometry when hearing thresholds are within normal l i m i t s i s given by Hans, Henderson, and . Hamernik, (1975). These investigators induced permanent threshold s h i f t of various degrees i n c h i n c h i l l a s and compared pre- and post-exposure threshold s h i f t between 500- and 20-msec tones. The pre-exposure values were comparable to those of normal-hearing persons, while the post-exposure threshold s h i f t s were de-pressed. However, the amount of decrease i n the post-exposure condition appeared to be unrelated to amount of permanent threshold s h i f t . Histopathologic study of the c h i n c h i l l a s used i n t h e i r study showed that there was a c o r r e l a t i o n between the amount of permanent threshold s h i f t and degenerated inner hair c e l l s , while the threshold s h i f t between long and short dura-tion tones was correlated with reduction of outer hair c e l l s . - 52 -These findings are consistent with the p o s s i b i l i t y that an atyp i c a l temporal integration function observed for a person with normal hearing threshold le v e l s may be an in d i c a t i o n that there i s degeneration of outer hair c e l l s i n the cochlea. Again, t h i s i s merely a speculative interpretation but certa i n investigators such as Wright (1978) are i n favour of such an explanation to account for the c o n f l i c t i n g r e s u l t s which ex i s t concerning frequency. 6.4 MASKING AND TEMPORAL INTEGRATION The use of a masker i n the present study enabled a compar-ison to be made of both threshold and suprathreshold measure-ment of temporal integration. The amount of threshold s h i f t between the long- and short-duration tones wasrfound, to vary be-tween masked and quiet conditions at the two t e s t frequencies. This appeared to a f f e c t the two groups d i f f e r e n t l y . In the case of the normal-hearing group, the threshold s h i f t increased by almost 3 dB at 500 Hz i n the presence of the masker, while no s i g n i f i c a n t change was observed at 4000 Hz. The hearing-impaired group showed an increase i n threshold s h i f t at both test frequencies i n the presence of the masker. However, the threshold s h i f t remains smaller for the NIHL group than the N group at suprathreshold l e v e l s . The trend observed i n the data for the hearing-impaired group suggests that the amount of integration i s approaching normal values. With the excep-ti o n of the 4000 Hz data i n the case of the N group, i t ap-pears that suprathreshold integration may generally y i e l d larger amounts of threshold s h i f t between tones of long and short duration. This observation i s consistent with data ob-tained by other investigators at suprathreshold l e v e l s . Djupesland and Zwislocki (1971) found as much as a 35 dB s h i f t i n r e f l e x threshold as duration was varied from 1000 msec to 10 msec. Thet- data of Hattler and Northern ( 1970) as well as those of Gengel (1972) also show indications of larger integ-r a t i o n values at masked threshold. There i s no consistency, however, i n the manner i n which suprathreshold measurement of integration affects the normal versus hearing-impaired groups. Stelmachowicz and Seewald ( 1977) found that the amount of threshold s h i f t was equal for the normal and hearing-impaired groups at suprathreshold lev e l s and d i f f e r e d only at auditory threshold. In the present study, the NIHL group produced the same amount of threshold s h i f t with masking both for the 500 Hz s i g n a l , where t h e i r hearing i s normal, and also for 4000 Hz, where they show evidence of cochlear pathology. It remains unclear why no change i n temporal integration took place at 4000 Hz for the N group when the masker was present. It i s proposed that future investigation of integration at masked threshold should include more masker leve l s and signal f r e -quencies to determine whether the masker d i f f e r e n t i a l l y a f f e c t s - 54 -integration at various frequencies and/or whether i t s a f f e c t i s l e v e l dependent. The general increase i n threshold s h i f t which the cochlea-impaired group shows i n the present study as well as i n previous investigations of suprathreshold temporal integration may involve an a l t e r a t i o n of the adaptation e f f e c t which Wright (1968) has proposed to be responsible for the reduced integration i n cochlea-impaired ears. It has been hypothe-sized for diagnostic purposes that cochlear pathology i s more r e a l i a b l y i d e n t i f i e d at threshold rather than suprathreshold levels (Feldman, 1976). If more rapid adaptation than normal i s responsible for the reduced a b i l i t y of the cochlea-impaired ear to integrate the energy of a long-duration s i g n a l , then i t i s possible, judging from the evidence, that at supra-threshold l e v e l s such unduly rapid adaptation does not occur. This would account for the reports i n the l i t e r a t u r e which show a normal slope of integration for cochlea-impaired per-sons when measured at suprathreshold l e v e l s . 6.5 RELATIONSHIP BETWEEN HEARING LOSS AND THRESHOLD SHIFT The c o r r e l a t i o n between absolute threshold and threshold s h i f t which was observed i n t h i s study at 4000 Hz i s consistent with reports of other investigators ( E l l i o t t , 1963; Young and Kanofsky, 1973; Pedersen and Salomon, 1977; Wright, 197 8) . E l l i o t t (1963) found correlations of -.74 and -.79 for the - 55 -500- and 4000-Hz tones r e s p e c t i v e l y . Young and Kanofsky (1973) found that the amount of threshold s h i f t for t h e i r subjects with sensorineural pathology was corre lated with absolute threshold . At 4000 Hz the c o r r e l a t i o n obtained i n t h e i r study was - . 8 7 . Wright (1978), however, reported a weak c o r r e l a t i o n for a group of subjects with noise-induced hearing loss when absolute threshold was compared to the threshold d i f ference between tones of 500 msec and 20 msec. The c h i n c h i l l a study discussed e a r l i e r (Hans et a l . , 1975) d id not reveal a cor -r e l a t i o n between the absolute threshold which resul ted from permanent damage and amount of temporal i n t e g r a t i o n . These c o n f l i c t i n g reports i n the l i t e r a t u r e make i t d i f f i c u l t to as-c e r t a i n whether the degree of hearing loss i s re f l ec ted i n the amount of threshold s h i f t . 6.6 DATA VARIABILITY An important aspect of the temporal in tegrat ion data which must be considered i n the c l i n i c a l app l i ca t ion of br ie f - tone audiometry i s the v a r i a b i l i t y which has been observed for both normal-hearing subjects and those with cochlear pathology. In the present study the overlap between amount of threshold s h i f t exhibi ted by the two groups under a l l condit ions of frequency and masking, as shown i n Figures 4 and 5, i s noteworthy. At 500 Hz the overlap i s expected since the NIHL group has normal - 56 -hearing lev e l s at t h i s frequency. However, 40% of the NIHL subjects had threshold s h i f t s which f e l l below the l e v e l s for the N group at 500 Hz (7dB or greater). The overlap between groups at 4000 Hz i s greater than would be expected on the basis of absolute thresholds. A higher percentage of the normal-hearing subjects had threshold s h i f t s which f e l l within 2 dB of the NIHL mean than the cochlea;-impaired group had threshold s h i f t s which f e l l within 2 dB of the N mean (40% vs. 5%). In the masked condition, the amount of overlap i s simi-l a r between the two groups. The v a r i a b i l i t y found i n the data i s consistent with the contention that the normal-hearing subjects i n t h i s study may have s u b c l i n i c a l cochlear disturbances. Barry and Larson (1974) showed that there was v i r t u a l l y no overlap between the normal-hearing and the deaf children at 1000 and 2000 Hz and that there was very minimal overlap at 500 and 4000 Hz. Thus the separation between normal and impaired subjects i s more apparent i n the data for children as compared to adults. This d e f i n i t e separation of scores may also be related to the fact that the children had normal hearing while the deaf children i had hearing losses generally greater than 70 dB HL across the entire frequency range. In contrast to t h i s , the hearing-impaired subjects used i n the present study had more variable l e v e l s of hearing lo s s . Sanders, Josey and Kemker (1971) found a great deal of - 57 -overlap i n the data of t h e i r three groups of subjects who re-presented normal-hearing, cochlear pathology, and eighth nerve lesions, respectively. While such re s u l t s are discouraging, they point out the necessity to consider t h i s v a r i a b i l i t y and overlap of the scores i n any e f f o r t to use brief-tone audio-metry as a t e s t for the diagnosis of cochlear-hearing l o s s . To make diagnostic use of brief-tone measurement of integra-t i o n i t i s more important that the integration scores of the impaired subjects f a l l away from the normal mean than i t i s that the scores of the normal-hearing subjects may f a l l well up i n the range of cochlear impairment. Such appears to be indicated by the data i n the present study, which supports the potential diagnostic value of brief-tone audiometry and also suggests that many normal-hearing persons give evidence of s u b c l i n i c a l cochlear disturbances. 6.7 CRITICAL RATIO Measurement of the c r i t i c a l r a t i o was made i n t h i s study i n order to compare the data with those reported by Gengel (1972) and Simon (1963). Gengel suggested that examination of the c r i t i c a l r a t i o for signals at d i f f e r e n t durations may aid i n understanding the r e l a t i o n s h i p between slope of the integ-rat i o n function and the manner in which long-duration signals are processed i n the auditory system. On the basis of Wright's - 58 -model of rapid physiological adaptation which affects the s i g -nals of long duration, Gengel suspected that the c r i t i c a l r a t i o s of the hearing-impaired subjects might be normal at short durations but not at long signal duration. Alternately, he speculated that the c r i t i c a l r a t i o s might be larger than normal at both long and short durations. In his study, he found that for subjects with normal hearing and tones of 200 msec or greater, the size of the c r i t i c a l r a t i o increased with frequency. This change with frequency was removed, how-ever, when the data for each frequency at 10 msec were com-pared. At t h i s short duration, the c r i t i c a l r a t i o s appeared to be independent of frequency. Gengel interpreted t h i s as an i n d i c a t i o n that the integration function i s related to the processing of long-duration signals as Wright (1978) suggested. He also speculated that the c r i t i c a l r a t i o s of persons with cochlear pathology might also be s i m i l a r across frequencies at short duration i f i t were the case that the reduced temporal integration function of these persons was due to rapid physio-l o g i c a l adaptation. However, Gengel did not investigate t h i s matter with hearing-impaired subjects. Simon (1963) speculated that the decreased temporal integ-ration observed i n r e c r u i t i n g ears was correlated with an i n -crease i n bandwidth within which integration takes place. He examined t h i s p o s s i b i l i t y by c a l c u l a t i n g the c r i t i c a l r a t i o obtained at masked threshold for normal and r e c r u i t i n g ears. At the frequency where recruitment was present, he found a larger c r i t i c a l r a t i o than he found for normal ears at that frequency. The s i m i l a r i t y of c r i t i c a l r a t i o s at short dura-t i o n , across frequency, as Gengel reported, was not found i n Simon's study. This matter i s d i f f i c u l t to interpret from the present study, since the NIHL group had normal hearing at 500 Hz and evidence of cochlear pathology only at 4000 Hz. However, 'the findings of the present study are consistent with Gengel's data showing that the 4000 Hz signal consistently yielded larger c r i t i c a l r a t i o s than the 500 Hz si g n a l . The largest values were obtained by the NIHL group at 4000 Hz, which i s consistent with Simon's suggestion concerning enlarged bandwidth i n cochlea-impaired ears (which he did not d i r e c t l y t e s t ) . The c r i t i c a l r a t i o s of the N group are a l l larger than those obtained by Gengel. There i s , however, a better correspondence with Simon's data at 4000 Hz for the normal-hearing and cochlea-impaired groups i n his and the present study. The data from the present study support Gengel's speculation of enlarged c r i t i c a l r a t i o for cochlea-impaired subjects at both long and short duration. The c r i t i c a l r a t i o s of the NIHL group were larger than for normals at a l l frequen-cies and a l l durations. Even at 500 Hz where no pathology i s ref l e c t e d i n the audiograms of the NIHL group the c r i t i c a l r a t i o s were larger than normal. However, i t i s noteworthy that the difference between groups i s larger for the long duration - 60 -than for the short duration signals. The differences which have been noted between long- and short-duration tones for normal versus cochlea-impaired sub-jects suggest that the two groups are more al i k e i n how short-er tones are processed i n the cochlea. F a s t i (1977) presents some int e r e s t i n g data i n regard to t h i s . In his investigation of temporal integration, F a s t i used a noise masker to simulate a hearing loss i n a normal-hearing subject. The simulated loss was achieved with a bandstop f i l t e r and matched the audiometric configuration of a cochlea;-impaired subject with an abrupt drop at 3000 Hz. A comparison of the amount of integration was made for the two subjects. The threshold s h i f t between long- and short-duration tones at 3000 Hz showed a s i g n i f i c a n t difference between the ear with simulated loss and the cochlea-impaired ear. As Gengel (1972) found, the ear with the simulated loss produced a slope of integration simi-l a r to what would be expected i n a person with normal hearing while the impaired subject showed very l i t t l e integration. At frequencies where the cochlea-impaired l i s t e n e r had l i t t l e hear-ing loss, functions for both subjects were s i m i l a r . As i n d i -cated e a r l i e r , the two subjects were matched i n audiometric configuration. Although t h i s match was achieved with 300 msec tones, a s t r i k i n g difference was found between the subjects when threshold comparisons were made across the frequency range for tones of 3 msec duration. That i s , an audiogram - 61 -obtained with 3 msec tones revealed a configuration quite un-l i k e that obtained with a 300 msec tone. These r e s u l t s suggest that the reduced integration t y p i c a l l y reported for cochlea-impaired subjects may be d i r e c t l y related to an altered mode of processing for either the signals of long duration or those of short duration. 6.8 CONCLUSION :lit fappea^sict'hati brriedi-tonet'audiometf yttifs?a~.potent i a l l y useful diagnostic test of disturbance at the l e v e l of the cochlea. The two durations examined i n t h i s study are s u f f i -cient to disclose cochlear pathology. The frequency depen-dence of the temporal integration function, however, i s a matter which deserves further attention. It appears to be not so much an e f f e c t of psychophysical procedure but also a possible "early warning sign" of cochlear dysfunction unde-tectable i n conventional pure-tone audiometry. Suprathreshold measurement of temporal integration by use of masking appears to y i e l d r e s u l t s unlike those obtained at quiet threshold l e v e l s , and masking affects the normal and cochlea-impaired subjects d i f f e r e n t l y . Further studies u t i l i z i n g masking with both normal and cochlea-impaired subjects i s warranted i n order to provide a more comprehensive set of data to show the re-l a t i o n between masking and the intensity-duration function. Such data may enable a better understanding of the mechanism underlying temporal integration of acoustic energy. - 62 -REFERENCES American National Standards I n s t i t u t e . 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