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The forward masking effect of 2fl-f2 in normal hearing and noise-induced hearing loss subjects Carson, Arlene Jane 1982

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THE FORWARD MASKING EFFECT OF 2 f l - f 2 IN NORMAL HEARING AND NOISE-INDUCED HEARING LOSS SUBJECTS ARLENE JANE CARSON B . S c , M c G i l l U n i v e r s i t y , 1977 THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES ( School of Audiology and Speech Sciences ) We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1982 *cS A r l e n e Jane Carson, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of A-Q~bl OLOCrV SP<££CH SCieMC&S The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) - i i -ABSTRACT The present study i n v e s t i g a t e d the combination tone 2 f l - f 2 i n normal hearing subjects and subjects w i t h high frequency noise-induced hearing l o s s by measuring the forward masking e f f e c t of 2 f l - f 2 exerted at i t s own frequency. Forward masking was p l o t t e d as the increase i n masked t h r e s h o l d of a b r i e f probe tone l o c a t e d at frequency 2 f l - f 2 as a f u n c t i o n of the i n t e n s i t y of the primary tones, f l and f 2 . The masking e f f e c t of the lower primary alone, f l , was al s o measured to determine the p o s s i b l e c o n t r i b u t i o n of f l toward the t o t a l masking obtained w i t h the f l + f 2 stimulus. Results of t h i s i n v e s t i g a t i o n showed th a t 1) a forward-masking e f f e c t a t t r i b u t a b l e to 2 f l - f 2 was seen f o r each normal-hearing s u b j e c t , u s u a l l y beginning at an f l + f 2 i n t e n s i t y between •^0 and 60 dB SPL. Average slopes of the growth of t h i s masking were 0.8 between 60 and 70 dB SPL and 1.1 between 70 and 80 dB SPL. 2) there was l i t t l e or no masking e f f e c t due to f l + f 2 f o r hearing-l o s s subjects at i n t e n s i t i e s between 60 and 70 dB SPL, most l i k e l y explained by the f a c t that the q u i e t thresholds at the f l , f2,and probe tone frequencies were elevated f o r these s u b j e c t s . This f i n d i n g agreed w i t h published s t u d i e s by showing that the forward masking e f f e c t was s t r o n g l y and i n v e r s e l y c o r r e l a t e d w i t h the quiet thresholds at the f l , f 2 , and probe tone frequencies ( s i g n i f i c a n t to p=.0l). 3) at an f l + f 2 i n t e n s i t y of 80 dB SPL, s e v e r a l h e a r i n g - l o s s subjects d i s p l a y e d a masking e f f e c t a t t r i b u t a b l e to 2 f l - f 2 . I t i s not c l e a r why t h i s occurred f o r some of these s u b j e c t s , because even at 80 dB SPL, the se n s a t i o n l e v e l of the f l + f 2 s t i m u l u s was onl y s l i g h t l y s u p r a - t h r e s h o l d . ^) f o r some normal-hearing and h e a r i n g - l o s s s u b j e c t s a s i g n i f i c a n t masking e f f e c t due to f l alone was noted. T h i s f i n d i n g made i t d i f f i c u l t t o assess the c o n t r i b u t i o n of 2 f l - f 2 toward the t o t a l masking d i s p l a y e d w i t h the f l + f 2 s t i m u l u s f o r these s u b j e c t s . 5) r e s u l t s w i t h the f l and f l + f 2 s t i m u l u s were h i g h l y v a r i a b l e f o r both s u b j e c t groups. 6) the un r e s o l v e d r o l e o f s u p p r e s s i o n i n d i s t o r t i o n product g e n e r a t i o n as w e l l as a l i m i t e d understanding of n o i s e - i n d u c e d h e a r i n g l o s s make i t d i f f i c u l t to apply the present r e s u l t s toward one p a r t i c u l a r model of combination tone p r o d u c t i o n . - i v -TABLE OF CONTENTS T i t l e Page ABSTRACT i i TABLE OF CONTENTS i v L I S T OF TABLES v i L I S T OF FIGURES v i i ACKNOWLEDGEMENT i x 1. LITERATURE REVIEW 1 1.1 The C o m b i n a t i o n Tone Phenomenon 1 1.2 Methods o f S t u d y 3 a) S i m u l t a n e o u s t e c h n i q u e s 4 b) The e f f e c t o f s u p p r e s s i o n 7 c) N o n - s i m u l t a n e o u s t e c h n i q u e s 9 1.3 A m p l i t u d e B e h a v i o r o f 2 f l - f 2 i n N o r m a l -h e a r i n g S u b j e c t s 12 a) 2 f l - f 2 a m p l i t u d e as a f u n c t i o n o f f 2 / f l and a b s o l u t e f r e q u e n c y 12 b) L e v e l s o f p r i m a r y t o n e s n e c e s s a r y t o d e t e c t 2 f l - f 2 14 c) B e h a v i o r o f 2 f l - f 2 w i t h i n c r e a s i n g i n t e n s i t y l e v e l o f f l , f 2 15 1.4 T h e o r i e s o f C o m b i n a t i o n Tone O r i g i n and Mechanism o f A c t i o n 17 1.5 2 f l - f 2 P e r c e p t i o n i n H e a r i n g - i m p a i r e d S u b j e c t s 23 2. OBJECTIVES 29 3. METHOD 31 3.1 S u b j e c t s 31 3.2 E q u i p m e n t 32 3-3 S t i m u l i 35 3.4 P r o c e d u r e 37 - V -4. RESULTS 40 4.1 The Forward Masking E f f e c t of f l Alone 40 4.2 The Forward Masking E f f e c t w i t h f l t f 2 as Stimulus: Masking A t t r i b u t a b l e to the Combination Tone 2 f l - f 2 42 5. DISCUSSION 81 5.1 Masking A t t r i b u t a b l e to f l 81 a) Normal-hearing subjects (N group) 81 b) Hearing-loss subjects (HL group) 82 5.2 Masking A t t r i b u t a b l e to 2 f l - f 2 84 a) Normal-hearing subjects (N group) 84 b) Hearing-loss subjects (HL group) 88 5-3' Summary 94 REFERENCES 97 - v i -LIST OF TABLES Table Page I Number of subjects t e s t e d at each of the three s e t s of frequencies used 38 I I Quiet thresholds (range and mean) at the probe, f l , and f2 frequencies f o r the N subjects Ur6 I I I Quiet thresholds (range and mean) at the probe, f l , and f 2 frequencies f o r the HL subjects 48 IV Slopes of masking f u n c t i o n (range and mean) due to f l and f l + f 2 s t i m u l i f o r N subjects 56 V Slopes of masking f u n c t i o n (range and mean) due t o f l and f l + f 2 s t i m u l i f o r HL subjects 58 - v i i -LIST OF FIGURES Figure Page 1 Block diagram of the experimental apparatus ... 3^ 2 Timing diagram of the s t i m u l i 36 3 Masking of the probe tone by f l as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l f o r N subjects 50 k Masking of the probe tone by f l as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l f o r HL subjects 52 5 Histograms d i s p l a y i n g means and ranges i n the amount of masking of the probe tone by f l f o r the N group and HL group 5k 6 D i f f e r e n c e i n amount of masking of the probe tone between the two-component ( f l + f 2 ) and one-component ( f l ) masker as a f u n c t i o n of masker i n t e n s i t y (dB SPL) f o r N subjects 60 7 D i f f e r e n c e i n amount of masking of the probe tone between the two-component ( f l + f 2 ) and one-component ( f l ) masker as a f u n c t i o n of masker i n t e n s i t y (dB SPL) f o r HL subjects 62 8 Histograms d i s p l a y i n g the d i f f e r e n c e i n masking between the two-component ( f l + f 2 ) and one-component ( f l ) masker f o r the N group and HL group 6k 9 Masking of the probe tone by the f l + f 2 stimulus as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l + f 2 f o r N subjects 66 10 Masking of the probe tone by the f l + f 2 stimulus as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l + f 2 f o r HL subjects 68 11 Histograms d i s p l a y i n g the means and ranges i n the amount of masking by f l + f 2 f o r the N and HL groups . 70 12 Masking of the probe tone by f l + f 2 as a f u n c t i o n of i n t e n s i t y l e v e l f o r f o u r N subjects who d i s p l a y e d minimal or no masking by f l alone 72 - v i i i -13 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #1 7^ 1^ Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #15 76 15 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #19 78 16 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #17 80 - i x -ACKNOWLEDGEMENT I wish to express my thanks to the f o l l o w i n g people f o r t h e i r help i n the p r e p a r a t i o n of t h i s manuscript: Dr. David Chung, f o r h i s h e l p f u l comments and as s i s t a n c e through the Workers' Compensation Board of B.C.; Dr. Donald Greenwood, f o r s h a r i n g h i s i n s i g h t about the phenomenon of combination tones; the many subjects f o r t h e i r time and conc e n t r a t i o n ; B r i a n H i r s t , f o r valuable a s s i s t a n c e i n d r a f t i n g the f i g u r e s f o r t h i s t e x t . F i n a l l y , I wish to express my a p p r e c i a t i o n to many f r i e n d s , near and f a r , who have provided much-needed moral support. These i n c l u d e , of course, my parents, whose love has always been a very supportive and motivat i n g f o r c e . 1 1. L i t e r a t u r e Review 1.1 The Combination Tone Phenomenon Combination tones are not a new discovery. The eighteenth-century v i o l i n i s t T a r t i n i i s c r e d i t e d w i t h one of the f i r s t r e p o r t s of combination tone perception. I t i s now w e l l known that the ear i s a n o n l i n e a r system, that i s , i t s output i s not l i n e a r l y p r o p o r t i o n a l to i t s input. With one pure tone d e l i v e r e d to the ear, the presence of n o n l i n e a r i t y i s manifested i n the appearance of overtones or a u r a l harmonics. When the ear i s s t i m u l a t e d simultaneously by two or more tones, other tones not present i n the o r i g i n a l stimulus are generated i n the ear by not only harmonic but a l s o i n t e r m o d u l a t i o n d i s t o r t i o n products. Such intermodulation d i s t o r t i o n products may take the form of-e i t h e r summation or d i f f e r e n c e tones, w i t h those d i f f e r e n c e tones lower i n frequency than the primaries more p e r c e p t i b l e . Two tones i n p a r t i c u l a r , at frequencies f 2 - f l and 2 f l - f 2 (where f l and f2 represent the frequencies of the lower and higher primary tones, r e s p e c t i v e l y ) are e s p e c i a l l y prominent and hence have r e c e i v e d the most a t t e n t i o n . Extensive data on the p r o p e r t i e s of f 2 - f l and 2 f l - f 2 have been gathered, but the o r i g i n of these two combination tones and the mechanism of d i s t o r t i o n product generation remains mysterious. One c o n s o l i d a t i n g f i n d i n g r e l a t i n g to a l l d i f f e r e n c e tone research has emerged: once generated, d i f f e r e n c e tones lower i n frequency than the primaries behave i n the r e g i o n of t h e i r "places" i n the cochlea as i f they were a c t u a l l y pure tones present i n the a c o u s t i c stimulus. A p i c a l t o the r e g i o n of the primary tones which generate them, d i f f e r e n c e tones are analyzed 2 by the ear s i m i l a r l y to tones of e x t e r n a l o r i g i n and they can i n t e r a c t w i t h other a c o u s t i c s i g n a l s anywhere on the b a s i l a r membrane. The q u a l i t a t i v e p r o p e r t i e s of combination tones o u t l i n e d below d i s p l a y how s i m i l a r they are to tones of e x t e r n a l o r i g i n once they are generated w i t h the cochlea ( G o l d s t e i n , 1967b). 1. P i t c h p e r c e p t i o n : The p e r c e p t i o n of the combination tone can be equated to the p i t c h of a simple tone of p a r t i c u l a r frequency. 2. Removal by c a n c e l l a t i o n tone: The pe r c e p t i o n of the combination tone can be removed by adding an e x t e r n a l tone of appropriate amplitude and phase. 3. Production of beats: The combination tone w i l l beat w i t h an added probe tone of neighbouring frequency. k. Masking of the combination tone: The combination tone can be masked by adding to the stimulus a tone or a narrow band of noise centered at the frequency of the combination tone. 5. Masking by the combination tone: The combination tone i t s e l f can mask other tones of neighbouring frequency (Greenwood, 1 9 7 1 ) . 6. B i n a u r a l masking l e v e l d i f f e r e n c e : The combination tone i n one ear can act w i t h an e x t e r n a l tone i n the ear opposite to produce the phenomenon known as a b i n a u r a l masking l e v e l d i f f e r e n c e ( L u t f i and Yost, 1 9 8 1 ) . 7. P h y s i o l o g i c a l v u l n e r a b i l i t y : Ear canal recordings of the a c o u s t i c d i s t o r t i o n product 2 f l - f 2 have been shown to be s e n s i t i v e t o , and r e v e r s i b l y reduced by, anoxia (Kim, I98O). 3 There i s now n e a r l y f u l l - s c a l e agreement across s t u d i e s that combination tones behave i n important ways l i k e tones o b j e c t i v e l y presented to the ear. This f a c t , besides c a r r y i n g important i m p l i c a t i o n s f o r t h e o r i e s of s u b j e c t i v e tone generation, has provided a means to i n v e s t i g a t e these tones. That i s , p h y s i o l o g i c a l and psych o a c o u s t i c a l techniques have been designed to use combination tones to r e a c t w i t h tones of e x t e r n a l o r i g i n . By p r e c i s e l y s p e c i f y i n g the parameters of e x t e r n a l tones d e l i v e r e d to the ear, these techniques a l l o w the d e t a i l e d q u a n t i t a t i v e a n a l y s i s of combination tones necessary to help develop an adequate model of s u b j e c t i v e tone production. 1.2 Methods of Study Under the appropriate, l i s t e n i n g c o n d i t i o n s , one or more combination tones w i l l be audible to normal-hearing s u b j e c t s . By f i r s t s e t t i n g f 2 = 2 f l and then lowering f 2 w i t h f l f i x e d i n frequency the combination tone 2 f l - f 2 w i l l increase i n frequency (from 0 Hz) and become audible as f 2 approaches f l . I t i s r e l a t i v e l y easy to perceive 2 f l - f 2 because i t s p i t c h w i l l r i s e as f 2 i s lowered. A d d i t i o n of a probe tone s l i g h t l y mistuned from the combination tone frequency w i l l a i d i n d e t e c t i o n of the combination tone, because audible beating between the probe and 2 f l - f 2 w i l l occur. W i t h i n the l a s t 25 years, however, other techniques, both p h y s i o l o g i c a l and p s y c h o a c o u s t i c a l , have per-mitted much more d e t a i l e d q u a n t i t a t i v e study of s u b j e c t i v e tones. These techniques can be d i v i d e d i n t o two categories termed 'simultaneous' and 'non-simultaneous'. 4 a) Simultaneous techniques A number of psychoacoustic s t u d i e s have u t i l i z e d the f o l l o w -i n g idea (introduced by Lewis and Larsen as e a r l y as 1935) s e l i m i n a t i o n of the i n t e r n a l l y produced combination tone by a tone of e x t e r n a l o r i g i n . This technique i s c a l l e d " c a n c e l l a t i o n " (or compensation). I t i s termed a 'simultaneous' technique because i t in v o l v e s p r e s e n t i n g a tone of a frequency equal t o the combination tone under study, at the same time t h a t the primary tones f l and f 2 are presented. When t h i s e x t e r n a l tone i s adjusted to an appropriate phase and amplitude, the combin-a t i o n tone becomes i n a u d i b l e . I t had been assumed by many t h a t , at t h i s p o i n t , the c a n c e l l a t i o n tone i s at the same i n t e n s i t y as the a u r a l l y generated combination tone but 180 degrees out of phase. The c a n c e l l a t i o n technique thus i n t e r p r e t e d attempted to provide an estimate of both combination tone amplitude and phase. Much i n f o r m a t i o n concerning the behavior of 2 f l - f 2 has been obtained by t h i s procedure ( G o l d s t e i n , 1967b; H e l l e , 1969/70; H a l l , 1 9 7 2 ) . Two other methods have a l s o been developed subsequent to the i n i t i a t i o n of the c a n c e l l a t i o n tone method. G o l d s t e i n (1967b) used a procedure of loudness-balancing to check on the v a l i d i t y of the c a n c e l l a t i o n procedure. Smoorenburg (1972b) introduced a technique s i m i l a r to loudness-balancing c a l l e d " m i r r o r i n g " to supplement c a n c e l l a t i o n tone data at low stimulus l e v e l s . Another simultaneous technique used f o r combination tone study i n v o l v e s examining the masking e f f e c t s of tones and noise on both higher and lower frequency s i g n a l s . D e t a i l e d a n a l y s i s 5 of pure-tone masking as a f u n c t i o n of s i g n a l frequency was undertaken by Greenwood (1971) u s i n g a Bekesy t r a c k i n g procedure w i t h a constant masker which v a r i e d i n frequency, and a pulsed f i x e d frequency s i g n a l of various frequencies. The presence of a notch and secondary peak on the high frequency slope of the masked audiogram was revealed. This t h r e s h o l d improvement i n a r e s t r i c t e d frequency r e g i o n above the s i n g l e tone masker was a t t r i b u t a b l e to the d e t e c t i o n of a lower frequency combina-t i o n tone generated by masker-signal i n t e r a c t i o n . Convincing evidence of t h i s was provided by masked audiograms produced by p a i r s of masking tones, or by masking tones s i t u a t e d above the c u t o f f of low-pass noise, which showed no notched r e g i o n above the higher frequency masker. In t h i s case, the s u f f i c i e n t l y intense lower frequency masker; e i t h e r tone or low-pass n o i s e , served to mask the combination tone generated by the higher masking tone and s i g n a l . Further i n v e s t i g a t i o n by Greenwood d i s c l o s e d t h a t not only were combination tones masked by other tones or noise but that they themselves could act as maskers, e l e v a t i n g thresholds f o r s i g n a l s at neighbouring frequencies. Masking by s u b j e c t i v e tones i s now a well-known phenomenon and has been used to detect the presence of combination tones and estimate t h e i r l e v e l s . Analogous to the c a n c e l l a t i o n technique, the i n t e n s i t y of an e x t e r n a l band of noise which s h i f t s s i g n a l t h r e s h o l d by the same amount as the combination noise band has been considered a comparable estimate of the combination component amplitude (Greenwood, 1 9 7 2 ) . Zwicker and F a s t i ( 1 9 7 3 ) t comparing t h i s 'threshold' method ( t h e i r terminology) w i t h the 6 c a n c e l l a t i o n procedure, confirmed close agreement between the two techniques concerning judgments of combination tone l e v e l s . (As w i l l be discussed s h o r t l y , such estimates of combination tone l e v e l s are i n c o r r e c t because they ignore the e f f e c t s of suppression. ) I n addition-, a procedure c a l l e d "tone-on-tone masking" (Clack 1967; E r d r e i c h , 1977) showed that techniques of masking by combination tones could provide phase as w e l l as amplitude information. The review to now has focussed on psychoacoustic accounts of combination tone i n v e s t i g a t i o n where the probe tone ( c a n c e l -l a t i o n tone) occurs simultaneously w i t h the two primary tones (and hence the combination tone). A t t e n t i o n at t h i s time should be d i r e c t e d to important p h y s i o l o g i c a l s t u d i e s of 2 f l - f 2 . These st u d i e s have described the p r o p e r t i e s of 2 f l - f 2 at two l e v e l s of the a u d i t o r y system: the eighth nerve and cochlear nucleus l e v e l . They have a l s o been instrumental i n demonstrating the l i m i t a t i o n s of simultaneous techniques such as c a n c e l l a t i o n i n t h e i r a b i l i t y to estimate combination tone l e v e l . I t i s w e l l known that neuronal elements of the a u d i t o r y system are h i g h l y frequency s e l e c t i v e and primary neurons e x h i b i t phase l o c k i n g i n t h e i r response to t o n a l s t i m u l i below 5kHz. G o l d s t e i n and Kiang (1968) took advantage of these p r o p e r t i e s i n recordings they made from s i n g l e u n i t s of the a u d i t o r y nerve of anesthetized cats. By choosing the two primaries such t h a t 2 f l - f 2 matched the c h a r a c t e r i s t i c frequency of the f i b e r under study, response to the combination tone was noted. E i t h e r primary alone might not cause the f i b e r to f i r e above i t s 7 spontaneous r a t e , since both f l and f2 could be l o c a t e d outside the response area of the neuron. In a d d i t i o n , i t was discovered that the time-locked response of the f i b e r could be c a n c e l l e d by-adding an e x t e r n a l tone of frequency 2 f l - f 2 at an appropriate amplitude and phase. An a l t e r n a t e method of e s t i m a t i n g combination tone l e v e l i n p h y s i o l o g i c a l preparations may be termed "equivalent spike l e v e l " method. This i n v o l v e s equating the l e v e l of the d i s t o r t i o n product w i t h the sound pressure l e v e l of an a c o u s t i c tone at the f i b e r ' s best frequency that e l i c i t s the same spike r a t e as the s u b j e c t i v e tone. Smoorenburg et a l . (1976) adopted t h i s procedure i n studying c o r r e l a t e s of combination tones i n the a n t e r o - v e n t r a l nucleus of the cat. S i m i l a r l y Buunen and Rhode (1978) examined 2 f l - f 2 l e v e l s i n a u d i t o r y nerve f i b e r s of the cat i n t h i s manner. A given spike r a t e was a t t r i b u t e d to 2 f l - f 2 when the response to the two-tone s i g n a l was at l e a s t 20 percent greater than t o f l alone, w i t h the primaries l o c a t e d outside the f i b e r ' s response area. b) The e f f e c t of suppression In recent years, simultaneous techniques, t h a t i s the c a n c e l l a t i o n and simultaneous masking techniques, have been shown not to provide v a l i d estimates of combination tone l e v e l . The problem i n v o l v e s the phenomenon of two-tone suppression and a r i s e s because the c a n c e l l a t i o n tone i s presented at the same time as the primary tones. Two-tone suppression may be seen i n the r e d u c t i o n of the response of an a u d i t o r y neuron at frequency, f l , by the a d d i t i o n of a suppressor tone, at frequency f 2 . 8 A major study of two-tone suppression has "been c a r r i e d out by Sachs and Kiang ( 1 9 6 8 ) . They were able to demonstrate suppression or " i n h i b i t o r y " areas on both the low and high frequency s i d e s of the response area ( t u n i n g curve) of every primary a u d i t o r y neuron s t u d i e d when a sweep frequency pure-tone s i g n a l was super-imposed on a continuous tone at a f i b e r ' s best frequency. Sachs and Kiang defined " i n h i b i t i o n " , or suppression, as a decrease i n f i r i n g r a t e of the best frequency tone by 20fo or more due t o the second tone. I t i s apparent t h a t two tone suppression must be considered i n i n t e r p r e t i n g c a n c e l l a t i o n tone estimates of combination tone l e v e l s . I t cannot be t a c i t l y assumed, as s e v e r a l researchers d i d , t hat there i s l i n e a r s u p e r p o s i t i o n of c a n c e l l a t i o n and combination tone. The c a n c e l l a t i o n tone, d e l i v e r e d simultane-ously with f l and f 2 , i s subject to suppression by these primary tones, because f l and f 2 ( e s p e c i a l l y f l ) may l i e w i t h i n a suppression area f o r the c a n c e l l a t i o n tone frequency ( i . e . c l ose enough to suppress). I f the c a n c e l l a t i o n tone i s suppressed by the primary tones, then the c a n c e l l a t i o n tone l e v e l r e q u i r e d t o e l i m i n a t e the combination tone w i l l overestimate i t s amplitude as "seen" by the r e c e p t o r - n e u r a l elements. The phenomenon of suppression represents another aspect of a u d i t o r y n o n l i n e a r i t y . L i k e the phenomenon of d i s t o r t i o n prod-u c t s , the mechanism of suppression i s as yet unexplained and i s under i n v e s t i g a t i o n . Greenwood et a l . (1976) undertook a study which considered suppression as i t r e l a t e s to c a n c e l l a t i o n tone estimates of combination tone l e v e l s . As discussed i n t h e i r 9 paper, the relevance of two-tone suppression to c a n c e l l a t i o n tone estimates w i l l depend on the mechanisms i n v o l v e d i n supp-r e s s i o n . I f , as Smoorenburg observed, suppression occurs by some mechanical n o n l i n e a r i t y which precedes combination tone generation and works to reduce the amplitude of the c a n c e l l a t i o n tone, then the c a n c e l l a t i o n tone w i l l indeed overestimate the l e v e l of 2 f l - f 2 . I f , on the other hand, suppression succeeds the generation of the combination tone, then the c a n c e l l a t i o n l e v e l may be a true r e f l e c t i o n of the l e v e l at which 2 f l - f 2 i s generated. The question remains, t h e r e f o r e : how does one determine how much suppression i s o c c u r r i n g and at what stage, r e l a t i v e to the generation of the combination tone under study. Zwicker, a long time user of the c a n c e l l a t i o n technique, mentions again i n a recent a r t i c l e that n o n l i n e a r i t i e s must be taken i n t o account i n the i n t e r p r e t a t i o n of r e s u l t s obtained by the c a n c e l l a t i o n method (Zwicker, 1981). The unresolved mechanism of suppression and i t s r o l e i n c a n c e l l a t i o n experiments means that simultaneous techniques cannot, by themselves,accurately assess the l e v e l of combination tones. c) Non-simultaneous techniques Given the i n a b i l i t y of the c a n c e l l a t i o n method to provide i n t r a c o c h l e a r estimates of combination tone l e v e l s , the focus has s h i f t e d to a l t e r n a t e means of i n v e s t i g a t i n g combination tones (which themselves are not without d e f e c t ) , u t i l i z i n g techniques which do not present the primary tones and the probe tone s i m u l t -aneously. Perhaps the best known of these a l t e r n a t i v e s i s the 10 " p u l s a t i o n t h r e s h o l d " method introduced by Houtgast (1972) i n h i s experiments measuring the masking patterns produced by bands of noise. The same technique was employed by Smoorenburg (1972b) f o r s t u d i e s of combination tones. In the p u l s a t i o n t h r e s h o l d technique, the stimulus f l + f 2 i s a l t e r n a t e d w i t h an e x t e r n a l stimulus 2 f l - f 2 (or f 2 - f l ) . When the e x t e r n a l matching tone reaches a c e r t a i n l e v e l , t h i s tone and the combination tone whose frequency i t matches are perceived as a continuous tone. An increase i n the l e v e l of the matching stimulus above t h i s l e v e l produces a ' p u l s a t i o n ' of the tone. The highest l e v e l of the matching tone f o r which the tone at the combination tone frequency sounds continuous i s termed the ' p u l s a t i o n t h r e s h o l d ' . The adjusted matching tone l e v e l becomes an estimate of the combination tone l e v e l . Smoorenburg (1972b) introduced a second non-simultaneous technique known as 'gap-masking', which u t i l i z e s the forward and backward masking e f f e c t of 2 f l - f 2 . I n h i s paradigm, a 150 msec masker i s a l t e r n a t e d w i t h a 50 msec s i l e n t p e r i o d i n which a 10 msec probe tone i s centred. The masker c o n s i s t s of f l + f 2 (or f l alone to determine the masking a t t r i b u t a b l e to the lower frequency primary). Combination tone amplitude i s equated to the l e v e l of an e x t e r n a l reference masker (at a frequency of 2 f l - f 2 when presented alone or w i t h f l added) which provided masking of the s i g n a l comparable t o the masking exerted by the f l , f 2 stimulus ( i . e . by the combination tone 2 f l - f 2 ) . Using an approach s i m i l a r to t h a t adopted f o r the present study, Buunen et a l . (1977) examined the forward-masking e f f e c t 11 of the combination tone 2 f l - f 2 . T h e i r procedure made use of a 115 msec masker (5 msec r i s e - f a l l time included) preceding a 14 msec probe tone of frequency 2 f l - f 2 . The i n t e r v a l between masker and probe tone was one msec. The e l e v a t i o n i n probe tone t h r e s h o l d produced by f l and f 2 i n t e r a c t i o n was measured by the Bekesy t r a c k i n g procedure and compared t o the e f f e c t exerted w i t h f l alone as masker. The t h r e s h o l d d i f f e r e n c e between the one- and two-component stimulus was considered a measure of the masking exerted by the d i s t o r t i o n product. Buune.n et a l . d i d not i n v e s t i g a t e the masking e f f e c t of an e x t e r n a l , r e f erence, 2 f l - f 2 s timulus; hence, no absolute estimate of combination tone l e v e l was p o s s i b l e . A recent study by L u t f i and Yost (1981) circumvents p o t e n t i a l p e r i p h e r a l i n t e r a c t i o n s i n c l u d i n g suppressive e f f e c t s by usi n g both ears i n t e s t i n g and makes use of an a u d i t o r y phenomenon known as the 'binaural masking l e v e l d i f f e r e n c e ' (BMLD). The BMLD r e f e r s to an improvement i n s i g n a l d e t e c t a b i l i t y t h a t r e s u l t s from i n t e r a u r a l d i f f e r e n c e s that e x i s t f o r the s i g n a l or noise. An improvement i n s i g n a l t h r e s h o l d of between 12 and 16 dB i s t y p i c a l when the i n t e r a u r a l phase d i f f e r e n c e of the s i g n a l i s v a r i e d through 180 degrees. P r o p e r t i e s of 2 f l - f 2 were examined by measuring the BMLD f o r an e x t e r n a l s i g n a l t o one ear and a combination tone ( 2 f l - f 2 ) of the same frequency to the other ear. Using a continuous d i o t i c noise, t h r e s h o l d f o r the s i g n a l was-obtained as the phase of t h i s tone v a r i e d through 3^0° r e l a t i v e to harmonically r e l a t e d p r i m a r i e s . The l e v e l of the noise was v a r i e d to e s t a b l i s h t h r e s h o l d i n a t w o - i n t e r v a l same-different 12 adaptive procedure w i t h the primary tones o c c u r r i n g i n both observation i n t e r v a l s . The BMLD procedure i s an a t t r a c t i v e t o o l f o r studying d i s t o r t i o n products because i t avoids p e r i -p h e r a l suppressive e f f e c t s and o f f e r s phase i n f o r m a t i o n about combination tones as w e l l . 1.3 Amplitude Behavior of 2 f l - f 2 i n Normal-hearing Subjects The amplitude of 2 f l - f 2 i s dependent upon three main stimulus v a r i a b l e s : l ) the stimulus l e v e l s of the two primary tones f l and f 2 (symbolized as L I and L2); 2) the frequency s e p a r a t i o n of the two pri m a r i e s - that i s , the r a t i o f 2 / f l and, somewhat l e s s c r i t i c a l , 3) "the absolute frequencies of the prim a r i e s ( r e p r e s e n t i n g more a p i c a l or b a s a l l o c a t i o n s w i t h i n the cochlea). Let us consider b r i e f l y how these three v a r i a b l e s , as w e l l as the experimental paradigm, determine the amplitude of 2 f l - f 2 . a) 2 f l - f 2 amplitude as a f u n c t i o n of f 2 / f l and absolute frequency Results of s e v e r a l s t u d i e s (Zwicker, 1955i G o l d s t e i n , 1967b; H a l l , 1972; S h i p l e y and Matthews, 197^-; Weber and M e l l e r t , 1975) examining 2 f l - f 2 amplitude as a f u n c t i o n of f 2 / f l show that as the frequency s e p a r a t i o n of the two primary tones i n c r e a s e s , the l e v e l of 2 f l - f 2 decreases. Where simultaneous techniques are used i t i s d i f f i c u l t to say how much of t h i s decrease i s due to a re l e a s e from suppression by f l as f 2 / f l i n c reases. I t i s t r u e , however, that the per c e p t i o n of 2 f l - f 2 i s l i m i t e d to a r e s t r i c t e d frequency r e g i o n below f l . The width of t h i s a u d i b i l i t y r e g i o n i s dependent upon the i n t e n s i t y and frequency r e g i o n of the p r i m a r i e s . Smoorenburg (1972a) concentrated on determining the maximum 13 f 2 / f l value f o r which 2 f l - f 2 was p e r c e p t i b l e at d i f f e r e n t primary-i n t e n s i t i e s . Using the d e t e c t i o n procedure, he showed th a t t h i s lower l i m i t of a u d i b i l i t y at very low primary i n t e n s i t i e s i s r e l a t i v e l y constant across the frequency spectrum (up to 5 6 k H z ) and corresponds to an f 2 / f l of 1.1-1.2. As the sens a t i o n l e v e l of the prim a r i e s i s increased, the a u d i b i l i t y r e g i o n widens, but the e f f e c t i s contingent upon frequency. At 5600 Hz, f o r example, i n c r e a s i n g primary l e v e l s expands the a u d i b i l i t y r e g i o n minimally (from an f 2 / f l of 1.1 to 1.3)- At low and mid-frequencies, however, there i s a marked widening of the a u d i b i l i t y r e g i o n w i t h i n c r e a s i n g sensation l e v e l . At 1^00 Hz and primaries of 70 dB SL, f o r instance, d e t e c t i o n of the combination tone i s po s s i b l e f o r an f z / f l as high as 1.75-Results from s t u d i e s t h a t measure c a n c e l l a t i o n tone l e v e l w i t h v a r y i n g f 2 / f l values corroborate Smoorenburg's data (Zwicker, 1955; Goldstein,1967b; H a l l , 1972; S h i p l e y and Matthews, 197^; Weber and M e l l e r t , 1 9 7 5 ) . P u l s a t i o n t h r e s h o l d estimates of 2 f l - f 2 l e v e l as a f u n c t i o n of f 2 / f l show a slower r a t e of decrease than c a n c e l l a t i o n estimates (see Smoorenburg, 197^-t Figure 5; Sachs, 1975)• Results of the forward-masking e x p e r i -ment of Buunen et a l . (1977) do show tha t probe tone t h r e s h o l d w i t h the f l , f 2 masker s t e a d i l y improves as f 2 / f l i ncreases. At a c e r t a i n primary s e p a r a t i o n ( f 2 / f l of approximately 1.3» termed Af max), the masking curves of f l alone and f l , f 2 converge, implying disappearance of the masking e f f e c t due to 2 f l - f 2 . P h y s i o l o g i c a l s t u d i e s a l s o note sm a l l e r decrements i n 2 f l - f 2 l e v e l s w i t h i n c r e a s i n g f 2 / f l . R e s ults from Smoorenburg et a l . (1976, Figure 1 3 ) , f o r example, showed t h a t t h e i r estimated 14 l e v e l of 2 f l - f 2 decreased by only 5 dB as f 2 / f l values increased from 1.38 to 1.58. The l e v e l of 2 f l - f 2 as a f u n c t i o n of f 2 / f l has a l s o been examined w i t h L I as a parameter. Results u s i n g the c a n c e l l a t i o n technique i n d i c a t e t h a t the rate of f a l l - o f f of c a n c e l l a t i o n l e v e l s w i t h i n c r e a s i n g f 2 / f l i s slower at higher primary tone l e v e l s (Zwicker, 1968; H a l l , 1972). Greenwood ( 1 9 7 2 c ) noted a s i m i l a r e f f e c t as determined from the masking produced by combination bands. Humes ( 1 9 7 9 ) . i n h i s gap-masking study of the d i s t o r t i o n product f 2 - f l , also noted t h a t the l e v e l of com-b i n a t i o n tones appears l e s s dependent on f 2 / f l as L I i s i n -creased. I t i s worth n o t i n g that a l l r e s u l t s , r e g a r d l e s s of experimental method, show wide v a r i a b i l i t y among su b j e c t s or p h y s i o l o g i c a l u n i t s . b) L e v e l s of primary tones necessary to detect 2 f l - f 2 The p r i m a r i e s f l and f 2 producing 2 f l - f 2 need not be presented at high i n t e n s i t y f o r normal-hearing s u b j e c t s to detect t h i s d i s t o r t i o n product. Determining the minimum stimulus l e v e l s r e q u i r e d i s d i f f i c u l t , however, due to l i m i t a t i o n s imposed by experimental technique and the l i s t e n e r ' s p e r c e p t i v e a b i l i t y and t r a i n i n g . Smoorenburg (1972a) found the lowest stimulus l e v e l (L1=L2) f o r 2 f l - f 2 d e t e c t i o n , averaged over primary f r e q -uencies and subjects to range from 14 to 24 dB SL. This value was obtained u s i n g an f 2 / f l between 1.1 and 1.2 and three h i g h l y - t r a i n e d s u b j e c t s . I t was Smoorenburg's o p i n i o n t h a t masking of the combination tone by the lower primary p a r t l y determined the t h r e s h o l d l e v e l of the p r i m a r i e s r e q u i r e d to detect 2 f l - f 2 . 15 At a higher f 2 / f l value of 1.25, Plomp (1965, Figure 8 ) a s c e r t a i n e d the mean primary l e v e l f o r 2 f l - f 2 d e t e c t i o n t o he 40 dB SL. Threshold values f o r h i s 18 subjects (having minimum t r a i n i n g ) however, ranged from 1 9 dB below to at l e a s t 2 0 dB above the mean. These higher sensation l e v e l s and the l a r g e v a r i a b i l i t y f o r 2 f l - f 2 p e r c e p t i o n are more i n d i c a t i v e of the r e s u l t s expected w i t h subjects having l i t t l e or no t r a i n i n g i n the psychoacoustic task. I n most s t u d i e s (e.g. H a l l , 1 9 7 2 ) i t appears that i n the frequency area around 1500 Hz subjects are maximally s e n s i t i v e to combination tone p e r c e p t i o n and hence show the lowest thresholds f o r 2 f l - f 2 . c) Behavior of 2 f l - f 2 w i t h i n c r e a s i n g i n t e n s i t y l e v e l of f l , f 2 Most s t u d i e s which have looked at the behavior of 2 f l - f 2 as a f u n c t i o n of i n c r e a s i n g i n t e n s i t y of the primary tones have used the c a n c e l l a t i o n technique ( G o l d s t e i n , 1 9 6 7 b ; Zwicker, 1 9 6 8 5 H e l l e , 1 9 6 9 / 7 0 , Smoorenburg, 1 9 7 ^ ) - The r e s u l t s of these s t u d i e s are q u i t e c o n s i s t e n t i n showing a more or l e s s l i n e a r growth of c a n c e l l a t i o n l e v e l with increases i n L I and L 2 i n the lower range of i n t e n s i t i e s . Up to about 60 dB SPL, a 1 0 dB increase i n L I and L 2 r e q u i r e d a c a n c e l l a t i o n tone 8 to 1 0 dB higher to n u l l i f y the combination tone. Slopes of 0 . 8 t o 1 meant t h a t the estimated amplitude of 2 f l - f 2 r e l a t i v e to the primary tones appeared t o be e s s e n t i a l l y independent of stimulus l e v e l . An important change emerges, however, at higher stimulus i n t e n s i t i e s (above 60 dB SPL), where the curves which p l o t c a n c e l l a t i o n l e v e l as a f u n c t i o n of L I and L 2 become f l a t t e r , and values of slopes are s i g n i f i c a n t l y l e s s than 1 . 16 In absolute values the data f o r f 2 / f l = 1.2 show c a n c e l l a t i o n l e v e l s to be t y p i c a l l y 25 dB below the l e v e l of the pri m a r i e s ( G o l d s t e i n , 1967b); at high values of f 2 / f l (=1.5). c a n c e l l a t i o n l e v e l s f a l l to 40 to 50 dB r e l a t i v e to L I and L2 ( H a l l , 1 9 7 2 a ) . P h y s i o l o g i c a l s t u d i e s give r e s u l t s s i m i l a r to psychoacoustic accounts obtained w i t h the c a n c e l l a t i o n technique, both i n slopes of growth w i t h i n c r e a s i n g stimulus i n t e n s i t y (Smoorenburg et a l . , 1976) and i n absolute estimates of d i s t o r t i o n product amplitude (Buunen and Rhode, 1 9 7 8 ) . In terms of absolute values, physio-l o g i c a l estimates of the l e v e l of 2 f l - f 2 vary from 10 to 60 dB below the l e v e l s of f l and f 2 f o r f 2 / f l values of approximately 1.25 (Kim et a l . , 1 9 7 9 ) . The departures from l i n e a r i t y i n the growth of 2 f l - f 2 w i t h i n c r e a s i n g stimulus l e v e l above moderate l e v e l s as observed i n c a n c e l l a t i o n s t u d i e s have been r e i n f o r c e d by r e s u l t s obtained w i t h techniques other than c a n c e l l a t i o n . Smoorenburg ( 1 9 7 ^ ) , f o r example, found d i s p a r i t i e s between the p u l s a t i o n t h r e s h o l d and c a n c e l l a t i o n techniques i n e s t i m a t i n g the growth of 2 f l - f 2 . The d i f f e r e n c e s emerged f o r sm a l l frequency r a t i o s of the pri m a r i e s and higher i n t e n s i t y l e v e l s . I n such cases p u l s a t i o n t h r e s h o l d estimates were as much as 20 dB below c a n c e l l a t i o n tone estimates. The slope of the p u l s a t i o n t h r e s h o l d f u n c t i o n was a l s o f l a t t e r -of t e n as low as 0.5. Sachs (1975) as w e l l noted p u l s a t i o n t h r e s -hold estimates of 2 f l - f 2 to be 10 to 20 dB below c a n c e l l a t i o n l e v e l s obtained f o r the same s u b j e c t s . Even greater d i f f e r e n c e s i n l e v e l s and slopes were noted by Shannon and Houtgast (1980) i n a comparison they c a r r i e d out between c a n c e l l a t i o n and p u l s a t i o n 17 t h r e s h o l d techniques. The higher c a n c e l l a t i o n tone estimates ( r e l a t i v e to p u l s a t i o n t h r e s h o l d values) evident i n a l l these s t u d i e s most l i k e l y r e f l e c t s the e f f e c t s of suppression, p a r t i -c u l a r l y the suppression of the c a n c e l l a t i o n tone produced at the frequency of 2 f l - f 2 by f l . The above comments have focused on the e f f e c t of primary i n t e n s i t y on 2 f l - f 2 when L1=L2. Without going i n t o d e t a i l on the r e l a t i v e e f f e c t i v e n e s s of each primary tone i t should be s t a t e d that the l e v e l of 2 f l - f 2 appears to be s t r o n g l y dependent upon the i n t e n s i t y of f l . In c o n t r a s t , the amplitude of t h i s d i s t o r t i o n product appears to be a weaker f u n c t i o n of changes i n the i n t e n s i t y of f 2 . Smoorenburg ( 1 9 7 2 a ) , f o r example, as c e r t a i n e d t h a t 2 f l - f 2 could be perceived w i t h f 2 values approaching t h e i r t h r e s h o l d . Zwicker (1955. 1968) noted t h a t , at moderate stimulus l e v e l s , c a n c e l l a t i o n tone l e v e l s reached a maximum when L I exceeded L2 by approximately 10 dB. This f i n d i n g has been confirmed by other researchers (e.g. H e l l e , 1969/70) i n c l u d i n g Sachs (1975)who demonstrated a s i m i l a r p a t t e r n of growth and dependence on L I and L2 l e v e l s w i t h the p u l s a t i o n t h r e s h o l d method as w e l l as the c a n c e l l a t i o n technique. 1.4 Theories of Combination Tone O r i g i n and Mechanism of A c t i o n I t i s c e r t a i n t h a t the combination tone 2 f l - f 2 i s generated at the a u d i t o r y periphery because p h y s i o l o g i c a l s t u d i e s ( c f G o l d s t e i n and Kiang, 1968) have demonstrated f i r i n g i n primary neurons a t t r i b u t a b l e s o l e l y to the combination tone. The middle ear system, f e l t by Helmholtz to be r e s p o n s i b l e f o r combination tone c r e a t i o n , has been abandoned as a candidate f o r the 18 p e r i p h e r a l generation s i t e (at l e a s t of 2 f l - f 2 ) s i n c e i t s behavior has been shown to be l i n e a r over a wide dynamic range. With the middle ear discounted as a source, the inner ear i s thus the remaining candidate as the s i t e of o r i g i n of combination tones. Supporting t h i s i n d i c a t i o n of a cochlear o r i g i n i s the behavior of 2 f l - f 2 , which i n some ways i s very s i m i l a r to t h a t of an e x t e r n a l tone, and which i m p l i e s i t s generation p e r i p h e r a l to the stage at which the a c o u s t i c stimulus i s transduced t o a t r a i n of s p i k e s . In a d d i t i o n , the f a c t t h a t 2 f l - f 2 shows a strong dependence upon primary frequency s e p a r a t i o n f u r t h e r i n d i c a t e s t h a t i t s production i n v o l v e s a process of frequency s e l e c t i v i t y . Both these p o i n t s s t r o n g l y i m p l i c a t e the cochlea as the s i t e of 2 f l - f 2 generation. The major task has thus l a i n i n e l u c i d a t i n g the l o c a t i o n and nature of 2 f l - f 2 production w i t h i n the cochlea. There were i n i t i a l l y two major camps of o p i n i o n regarding the processes generating combination tones: those who b e l i e v e d they were mechanical i n nature and those who f e l t t h e i r o r i g i n was n e u r a l . A p u r e l y n e u r a l theory would hold that i t i s temporal i n f o r m a t i o n , most l i k e l y r e l a t e d t o the t i m i n g of n e u r a l s p i k e s , which leads to the p e r c e p t i o n of combination tone p i t c h . The "beat-tone theory", f o r example, r e l a t e d combination tone a u d i b i l i t y to the p e r c e p t i o n of beats and proposed the existence of a p i t c h e x t r a c t o r operating on the temporal p e r i o d i c i t i e s r e s u l t i n g from the l i n e a r i n t e r a c t i o n of stimulus tones. I n c o n t r a s t , a mechanical theory proposed that a u d i t o r y processing i s n o n l i n e a r i n nature, l e a d i n g to the i n t r o d u c t i o n of new s p e c t r a l components 19 whose subsequent mechanical propagation and a n a l y s i s would give r i s e to combination tone perception. U n l i k e the n e u r a l theory, the mechanical theory s t a t e d t h a t combination p i t c h p e r c e p t i o n could only occur i f there were s p e c t r a l energy at the combination frequency. The a d d i t i o n a l component(s) would a r i s e as a r e s u l t of s p a t i a l , mechanical i n t e r a c t i o n between the two p r i m a r i e s . The s i t e of t h i s i n t e r a c t i o n would be g e n e r a l l y assumed to be the b a s i l a r membrane, or more broadly the cochlear p a r t i t i o n , c o n s i s t -i n g of the s c a l a media and i t s c o n s t i t u e n t s t r u c t u r e s . The strong dependence of 2 f l - f 2 presence and amplitude upon primary frequency s e p a r a t i o n i s c o n s i s t e n t w i t h a theory t h a t in v o l v e s the b a s i l a r membrane i n i t s generation. Smoorenburg (1972b) and others ( c f Plomp, 1 9 6 5 ) propose t h a t 2 f l - f 2 amplitude i s p r o p o r t i o n a l to the amount of i n t e r a c t i o n between the primaries i n t h e i r mechanical s t i m u l a t i o n patterns along the b a s i l a r membrane. At wide A f values, the overlap of these patterns i s minimal. However, as f 2 / f l decreases, the amount of overlap i n c r e a s e s , r e s u l t i n g i n greater 2 f l - f 2 amplitude. G o l d s t e i n p o i n t s t o mechanical a u d i t o r y n o n l i n e a r i t y as the cause of s u b j e c t i v e tones, because, i n h i s words " a l l experimental m a n i f e s t a t i o n (of combination tones) are i d e n t i c a l w i t h the behavior of tones that are mechanical^present i n the ear" ( 1 9 6 7 b ) . This o p i n i o n i s shared by many others (Greenwood, 1971; Smoorenburg et a l . 1976; Kim, S i e g e l and Molnar, 1 9 7 9 ) . S e v e r a l f i n d i n g s of Greenwood (1971) i n h i s s t u d i e s on the high frequency notch of masked audiograms are p e r t i n e n t . The dip i n masked t h r e s h o l d , due to 2 f l - f 2 d e t e c t i o n as a r e s u l t of masker-signal i n t e r a c t i o n , was shown to be e l i m i n a t e d by the a d d i t i o n of a tone 20 or noise very near the 2 f l - f 2 frequency (which masked the com-b i n a t i o n tone). The combination component could i n t u r n mask other s i g n a l s . Not only t h i s , but replacement of one or both pr i m a r i e s by a noise band r e s u l t e d i n the generation of combin-a t i o n bands which s t r e t c h e d downward i n frequency as the primary noise bandwidth widened upwards. The production of combination bands, f o r example by a tone and noise band, i n d i c a t e d to Greenwood that a l l frequency components i n the noise must independ-e n t l y i n t e r a c t w i t h the tone to r e p l i c a t e the band at the lower frequency region. The c r e a t i o n of combination bands i s a str o n g i n d i c a t i o n that combination tones have a p h y s i c a l existence w i t h i n the cochlea i n the form of v i b r a t i n g displacement patterns on the b a s i l a r membrane. Moreover, demonstrations of 2 f l - f 2 masking e f f e c t s i n appropriate remote frequency regions support the opi n i o n (held by Greenwood, 1971; H a l l , 197^; Kim, S i e g e l and Molnar, 1979 and others) that combination components e x i s t as t r a v e l l i n g waves, propagated to t h e i r s u i t a b l e frequency r e g i o n and transduced there l i k e e x t e r n a l components. F i n a l l y , Greenwood's evidence of combination band existence discourages the view of s u b j e c t i v e tones as e x c l u s i v e l y n e u r a l phenomena s i n c e , as he p o i n t s out, combination bands cannot be considered a p u r e l y temporo-neural phenomenon. I f n o n l i n e a r i t y of b a s i l a r membrane motion i s i n v o l v e d i n combination tone generation, then i t should be p o s s i b l e to f i n d evidence of t h i s n o n l i n e a r i t y i n cochlear preparations. Some p h y s i o l o g i c a l , q u a n t i t a t i v e measurements of b a s i l a r membrane v i b r a t i o n i n d i c a t e that i t s motion i s l i n e a r (Bekesy, I960; 21 Wilson and Johnstone, 1 9 7 2 ) ; however, Bekesy's measurements were made using dead preparations and i n the preparations of Wilson and Johnstone the s c a l a tympani was drained i n the "basal r e g i o n of observation. A notably d i f f e r e n t r e s u l t i s the non-l i n e a r i t y reported by Rhode ( 1 9 7 1 ) and Rhode and Robles ( 1 9 7 4 ) . Using the Mossbauer technique, i t was found t h a t the amplitude of v i b r a t i o n of the cochlear p a r t i t i o n (at the mid-membrane l o c a t i o n r e p r e s e n t i n g approx. 7kHz) i n l i v e s q u i r r e l monkeys increased l e s s than l i n e a r l y w i t h stimulus l e v e l . I t now seems probable, as Rhode and others (Kim et a l . 1979) point out, that the use of dead p r e p a r a t i o n as w e l l as p h y s i o l o g i c a l damage to preparations i n c u r r e d by i n v a s i v e observation and measurement techniques have abolished the n o n l i n e a r i t y i n other i n v e s t i g a t i o n s . Going one step f u r t h e r , i t should a l s o be p o s s i b l e to record the presence of 2 f l - f 2 , i f t h i s combination tone does manifest i t s e l f i n a displacement p a t t e r n on the b a s i l a r membrane as a t r a v e l l i n g wave or other v i b r a t i o n a l p a t t e r n . Wilson and Johnstone ( 1 9 7 3 ) and Rhode ( 1 9 7 7 ) have indeed succeeded i n observing the presence of 2 f l - f 2 i n the motion of mammalian b a s i l a r membrane. The l e v e l s of t h i s component were estimated to be from 40 to 60 dB below the p r i m a r i e s . Such low l e v e l s do not appear to match psychoacoustic f i n d i n g s , but do match the t h r e s h o l d l e v e l s of 2 f l - f 2 reported i n p h y s i o l o g i c a l s t u d i e s . As Greenwood ( 1 9 7 7 ) p o i n t s out, these small values can be expected, given the extreme ba s a l l o c a t i o n s and high i n t e n s i t i e s (up to 120 dB SPL) used by Rhode and others as compared w i t h the lower frequencies and l e v e l s i n more a p i c a l regions used i n psycho-a c o u s t i c s t u d i e s , e s p e c i a l l y i f suppression e f f e c t s are a l s o 22 considered. Some researchers have p o s t u l a t e d the need f o r an a d d i t i o n a l , or "second" frequency f i l t e r preceding or at the same stage as the n o n - l i n e a r i t y generating 2 f l - f 2 to e x p l a i n why i t i s so dependent on primary frequency s e p a r a t i o n (e.g. Evans, 1 9 7 ^ ) . I t has "been b e l i e v e d that the tuning of the b a s i l a r membrane i s not sharp enough to account f o r the decreases i n 2 f l - f 2 amplitude w i t h only minimal increases i n f. One p o s t u l a t e d l o c a t i o n of a "second" f i l t e r s i t e has been the cochlear h a i r c e l l - t e c t o r i a l membrane j u n c t i o n , i n v o l v i n g a n o n l i n e a r b i d i r e c t i o n a l c o u p l i n g between these two components ( G o l d s t e i n , 1967b). As measurement techniques have been r e f i n e d , however, evidence i s accumulating to suggest that the n o t i o n of a "second" f i l t e r i s unnecessary - the b a s i l a r membrane i t s e l f provides enough "sharpening" of the s i g n a l . This "sharpening" may be f a c i l i t a t e d by the outer h a i r c e l l s i n which an a c t i v e process may serve to amplify the s i g n a l reaching the inner h a i r c e l l s , from which the great bulk of primary a u d i t o r y responses are measured (Greenwood, personal communication). Over the years a multitude of mathematical and p h y s i c a l models have been put forward to account f o r the existence of 2 f l - f 2 ( P f e i f f e r , 1970; Tonndorf, 197>; D u i f h u i s , 197^; H a l l , 1 9 7 7 ) ' i - s important to the success of any model, however, tha t i t be able to incorporate the f i n d i n g s obtained i n w e l l -defined p a t h o l o g i c a l cases as w e l l as i n normal-hearing s u b j e c t s . Two models put forward w i t h the i n t e n t i o n of i n c o r p o r a t i n g r e s u l t s of st u d i e s on 2 f l - f 2 p e r c e p t i o n i n hearing-impaired subjects are o u t l i n e d i n the next s e c t i o n . 23 1.5 2 f l - f 2 Perception i n Hearing-impaired Subjects There are few published accounts of combination tone per-c e p t i o n i n subjects whose hearing thresholds are elevated. Although i t i s true t h a t the mechanism of 2 f l - f 2 generation i n normal-hearing subjects i s p o o r l y understood, there i s good reason to suppose th a t t h i s mechanism w i l l be a f f e c t e d i n subjects evidencing c e r t a i n kinds of hearing l o s s . Cases of w e l l - d e f i n e d cochlear pathology are v a l u a b l e , because d i f f e r e n c e s i n combin-a t i o n tone generation and/or p e r c e p t i o n i n such subjects may be r e l a t e d to the pathology and may help to e l u c i d a t e the mechanism of combination tone generation. Smoorenburg ( 1 9 7 2 b ) had one subject i n a psychoacoustic task whose presence enabled him to study what e f f e c t a l o c a l i z e d t h r e s h o l d e l e v a t i o n might have on 2 f l - f 2 perception. The s u b j e c t ' s audiogram showed a w e l l - d e f i n e d u n i l a t e r a l hearing l o s s above 2000 Hz w i t h a sharp increase i n t h r e s h o l d to 4-3 dB SPL ( r e l a t i v e to a b a s e l i n e t h r e s h o l d of 0 dB at 1000 Hz) by 2500 Hz. High frequency hearing l e v e l s were w i t h i n normal l i m i t s . The lower primary was f i x e d i n frequency at the lower l i m i t of the notch (f1=2000 Hz) and the upper primary v a r i e d downwards i n frequency from 4000 Hz. The combination tone frequency, t h e r e f o r e , would f a l l i n a r e g i o n of normal t h r e s h o l d . The subject adjusted f 2 to the highest l e v e l f o r which 2 f l - f 2 could be perceived f o r various L2 at each of f i v e L I values (32-72 dB SPL). This process determined the range of frequencies f o r which 2 f l - f 2 was detected. The r e s u l t s of t h i s study showed th a t Smoorenburg's hearing-impaired subject perceived 2 f l - f 2 only when the l e v e l of f 2 (L2) 2k exceeded h i s t h r e s h o l d f o r f2 ( L I was always suprathreshold). The f 2 , L2 r e g i o n f o r which 2 f l - f 2 was audible was q u i t e d i f f e r e n t from r e s u l t s obtained f o r the other (normal e a r ) . For a given L I value, the highest frequency f o r 2 f l - f 2 p e r c e p t i o n i n the normal ear was e s s e n t i a l l y independent of L2 (agreeing w i t h other s t u d i e s ; Smoorenburg, 1972a). In the 'bad' ear, however, there was a trend toward a wider a u d i b i l i t y r e g i o n w i t h increases i n L2. Smoorenburg i n t e r p r e t e d these r e s u l t s by what may be c a l l e d a "gap model", based on a n o n l i n e a r i t y which comes a f t e r the a u d i t o r y defect. This model assumes l ) a complete l a c k of response to stimulus components wherever thresholds are elevated and the components do not exceed t h r e s h o l d , and 2) primaries whose s t i m u l a t i o n patterns have low- and high-frequency slopes matching those of the audiogram f o r the hearing l o s s subject and which are approximately constant (dB/octave) regard l e s s of the l e v e l s of these p r i m a r i e s . Depending on how intense f l and f2 are, t h e i r s t i m u l a t i o n patterns may reach e i t h e r or both sid e s of the gap. Combination components are assumed to be generated only i f both f l and f 2 e f f e c t and are passed through the same no n l i n e a r channel. Thus,-if the amplitude of f l (at 2000 Hz) i s only s u f f i c i e n t to s t i m u l a t e the low frequency side of the gap, f2 must be intense enough to s t i m u l a t e the r e g i o n on the low side of the gap as w e l l i f 2 f l - f 2 i s to be generated. S i m i l a r l y , a component l i k e f 2 t h a t i s below t h r e s h o l d i s not processed, does not reach the s i t e of s u r v i v i n g n o n l i n e a r i t y (on e i t h e r side of the gap), and hence, does not c o n t r i b u t e i t s necessary share to 2 f l - f 2 production. 25 I t has been t a c i t l y assumed th a t l o c a l i z e d hearing l o s s e s r e f l e c t damage at the h a i r c e l l l e v e l . Under t h i s assumption, the r e s u l t s of Smoorenburg's experiment imply that the n o n l i n -e a r i t y r e s p o n s i b l e f o r 2 f l - f 2 perception r e s i d e s at the l e v e l of the h a i r c e l l s or higher. Smoorenburg thus went f u r t h e r and adopted Goldstein's hypothesis of h a i r c e l l - b a s i l a r membrane b i - d i r e c t i o n a l c o u p l i n g to account f o r the subsequent frequency a n a l y s i s of 2 f l - f 2 . Hence, i t i s r e a l l y at t h i s l a t e r stage of 2 f l - f 2 a n a l y s i s that the b a s i l a r membrane, as a propagator of the d i s t o r t i o n products, assumes i t s g r e a t e s t importance. Results from another experiment performed by Sachs and Wightman ( 1 9 7 5 ) corroborate Smoorenburg's data. Using the tech-niques of c a n c e l l a t i o n and p u l s a t i o n t h r e s h o l d w i t h subjects e x h i b i t i n g t h r e s h o l d dips i n the 4000 Hz regio n , 2 f l - f 2 p e r c e p t i o n was s t u d i e d w i t h f 2 placed i n the reg i o n of the notch e x a c t l y as i n Smoorenburg's study. Results showed th a t not one of the subjects could detect the combination tone unless both f l and f 2 were above t h r e s h o l d . The same researchers l a t e r extended t h i s study to t e s t the v a l i d i t y of a model they put forward to e x p l a i n the presence of the combination tone. This model of Sachs ( 1 9 7 5 ) i s patterned a f t e r the proposal of G o l d s t e i n ( 1 9 6 7 b ) and b a s i c a l l y c o n s i s t s of a n o n l i n e a r i t y surrounded by two bandpass f i l t e r s . I t i s unique, however, because the asymmetrical s t i m u l a t i o n patterns of the primaries are modeled such t h a t energy at regions b a s a l to the f 2 place ( i . e . higher frequency l o c a t i o n s ) are i n t e g r a t e d and can co n t r i b u t e s i g n i f i c a n t l y to the r e s u l t i n g l e v e l of 2 f l - f 2 . There-26 f o r e , even i f a l l concerned frequencies ( f l , f 2 and 2 f l - f 2 ) are l o c a t e d i n an area of normal hearing, a t h r e s h o l d e l e v a t i o n at higher frequencies may a l t e r combination tone perception at primary l e v e l s that normally " u t i l i z e " t h i s region. S p e c i f i c a l l y , the model p r e d i c t s a f l a t t e n i n g of the curve d e s c r i b i n g 2 f l - f 2 l e v e l as a f u n c t i o n of L I and L2, known i n normals to d i s p l a y a d i s t i n c t maximum. To t e s t t h i s p r e d i c t i o n of the model, three subjects w i t h s t e e p l y - s l o p i n g ( 150 dB/octave) high frequency hearing l o s s e s of at l e a s t 40 dB were used. Both f l and f2 were placed near the low-frequency edge of the l o s s i n an area of normal or near-normal t h r e s h o l d . Using the p u l s a t i o n t h r e s h o l d technique, 2 f l - f 2 l e v e l was estimated as a f u n c t i o n of changing L I or L2 at an f 2 / f l value of 1.15. The same procedure was c a r r i e d out w i t h the p r i m a r i e s placed i n a r e g i o n remote from the hearing l o s s to serve as a c o n t r o l . P l o t s of 2 f l - f 2 l e v e l versus L I or L2 i n the c o n t r o l c o n d i t i o n showed the same d i s t i n c t peaks at s p e c i f i e d values of L I or L2 as seen i n the normal case. However, w i t h f l and f 2 placed adjacent to the t h r e s h o l d e l e v a t i o n , the amplitude of t h i s same f u n c t i o n was reduced, w i t h the normally d i s t i n c t 2 f l - f 2 maximum e i t h e r absent or diminished. Such f i n d i n g s suggest t h a t areas other than those maximally a f f e c t e d by the primary components ( p a r t i c u l a r l y more b a s a l areas) may c o n t r i b u t e to the amplitude and growth of 2 f l - f 2 . I n p h y s i o l o g i c a l s t u d i e s (Kim et a l . 1 9 7 9 ; Kim, 1980) the e f f e c t s upon combination tone l e v e l of induced r e v e r s i b l e and i r r e v e r s i b l e cochlear damage to anesthetized cats and c h i n c h i l l a s 27 was examined. When c h i n c h i l l a cochleas were i r r e p a r a b l y a l t e r e d i n the b a s a l r e g i o n i n c l u d i n g regions of f l and f 2 f r e q u e n c i e s , there was no evidence of d i s t o r t i o n product presence (Kim et a l . 1 9 7 9 ) . A t r a n s i e n t but n o t i c e a b l e r e d u c t i o n of 2 f l - f 2 was observed even when thresholds were only temporally elevated by d e l i v e r i n g an intense stimulus i n the frequency r e g i o n of the p r i m a r i e s . On the other hand, when an intense stimulus of a frequency r e p r e s e n t a t i v e of a l o c a t i o n remote r e l a t i v e to f l and f 2 was presented, nerve f i b e r response was not a f f e c t e d . Kim et a l . concluded from t h i s t h a t d i s t o r t i o n products must be generated i n the r e g i o n of the p r i m a r i e s . The generation of 2 f l - f 2 was a l s o shown to be p h y s i o l o g i c a l l y v u l n e r a b l e . The l e v e l of 2 f l - f 2 was r e v e r s i b l y reduced by anoxia and cochlear p e r f u s i o n of potassium cyanide. Lepage and Johnstone (1980) i n t h e i r i n v e s t i g a t i o n of b a s i l a r membrane v i b r a t i o n i n the guinea p i g u s i n g a capacitance probe a l s o noted that the degree of n o n l i n e a r i t y measured ( i m p l i c a t e d i n combination tone generation) appeared to be d i r e c t l y c o r r e l a t e d w i t h s i n g l e u n i t t h r e s h o l d which was r e f l e c t e d i n the N, a c t i o n p o t e n t i a l and which became poorer w i t h mechanical manipulation of the p r e p a r a t i o n . Returning to psychophysical measurements, one f a c e t l e f t to be discussed i s how 2 f l - f 2 p e r c e p t i o n i s i n f l u e n c e d by hearing t h r e s h o l d at the combination frequency. By p l a c i n g the p r i m a r i e s such that 2 f l - f 2 f e l l i n an area of elevated t h r e s h o l d , Smoorenburg\s (1972b) hearing impaired subject could not perceive 2 f l - f 2 even though f l and f 2 were both a u d i b l e . Smoorenburg c i t e d t h i s f i n d i n g as evidence f o r the mechanical frequency 2 8 a n a l y s i s of 2 f l - f 2 subsequent to i t s generation. A r e p l i c a t i o n of t h i s procedure was undertaken by Sachs and Wightman (1975) but t h e i r r e s u l t s were reported to be much more v a r i a b l e . The f a c t o r s i n v o l v e d i n 2 f l - f 2 p e r c e p t i o n i n cases where hearing t h r e s h o l d i s elevated warrants f u r t h e r study, c o n s i d e r i n g the small pool of hearing impaired subjects t e s t e d to date and the wide v a r i a t i o n i n r e s u l t s obtained. CHAPTER 2 2 9 2.1 Objectives From the preceding l i t e r a t u r e review i t i s evident t h a t many st u d i e s have been c a r r i e d out to i n v e s t i g a t e the combina-t i o n tone 2 f l - f 2 . I t i s a l s o evident, however, that much of the accumulated data on 2 f l - f 2 should be i n t e r p r e t e d w i t h c a u t i o n because such data has been obtained u s i n g the c a n c e l l a t i o n t e c h -nique or other simultaneous technique where the e f f e c t s of suppression cannot be r u l e d out. The present research aims to circumvent t h i s inherent problem of the simultaneous methods of measurement by adopting a forward-masking paradigm. S p e c i f i c a l l y , the o b j e c t i v e i s to i n v e s t i g a t e the presence of the combination tone 2 f l - f 2 by attempting to measure the forward masking e f f e c t of 2 f l - f 2 upon a b r i e f probe tone l o c a t e d at t h i s combination tone frequency. By a l t e r i n g the i n t e n s i t y l e v e l s of the primary tones which produce 2 f l - f 2 , a measure of the growth of t h i s masking e f f e c t as a f u n c t i o n of LI and L 2 may be obtained. The l i t e r a t u r e review a l s o revealed the f a c t that there i s very l i t t l e data p r e s e n t l y a v a i l a b l e concerning the existence and p r o p e r t i e s of 2 f l - f 2 i n subjects whose hearing thresholds are elevated. A second o b j e c t i v e of the present study w i l l be to provide a d d i t i o n a l data as to the e f f e c t s of 2 f l - f 2 i n hearing impaired subjects and thus expand the small data base of i n f o r -mation about such s u b j e c t s . Thus the forward masking paradigm w i l l be employed w i t h a group of subjects e x h i b i t i n g high f r e -quency noise-induced hearing l o s s as w e l l as wi t h a group of normal hearing s u b j e c t s . I n t h i s way, r e s u l t s f o r the two groups 30 may "be e a s i l y compared to analyze d i f f e r e n c e s i n the forward masking "behavior of 2 f l - f 2 which occur as a r e s u l t of the th r e s h -o l d e l e v a t i o n . F i n a l l y , "because the l i t e r a t u r e i s r e p l e t e w i t h experiments that u t i l i z e very few subjects i n the data c o l l e c t i o n , i t i s an o b j e c t i v e of the present research to o b t a i n data on a r e l a t i v e l y l a r g e number of subjects i n both the normal-hearing and hearing-impaired groups. In many studi e s which use only one, two, or three subjects i t i s d i f f i c u l t to p r e d i c t trends or draw con-c l u s i o n s from the r e s u l t s obtained. This i s p a r t i c u l a r l y true when the v a r i a t i o n i n r e s u l t s among subjects i s l a r g e , as i s of t e n the case. CHAPTER 3 ^ 3. METHOD 3.1 Subjects P a r t i c i p a n t s i n t h i s research belonged, to one of two cla s s e s s e l e c t e d f o r i n v e s t i g a t i o n , one group of normal-hearing subjects and one group of subjects w i t h noise-induced hearing l o s s . The normal (N) group c o n s i s t e d of 13 subjects (seven females and s i x males) between the ages of 23 and 6 l w i t h a mean age of 32 years. Ten of these subjects were r e c r u i t e d as volunteers from s t a f f of the Workers' Compensation Board of B r i t i s h Columbia (WCB). The other three were f r i e n d s of the experimenter. A l l were i n good h e a l t h w i t h no known h i s t o r y of middle ear pathology or prolonged exposure to noise. The p r e r e q u i s i t e f o r subjects to belong to the N group was an audio-gram which d i s p l a y e d pure tone thresholds of 15 dB HL (ANSI, 19 6 9 ) or b e t t e r at the octave frequencies from 250 to 8000 Hz and at the s p e c i f i c f 1 , f 2 , and probe tone frequencies used. A t o t a l of 31 males e x h i b i t i n g noise-induced hearing l o s s served as su b j e c t s . Out of t h i s p o o l , data from 20 of the most co n s i s t e n t responders were used i n the f i n a l a n a l y s i s . A l l were claimants at the Hearing Branch C l i n i c of the WCB. They ranged i n age from 39 to 7^ years w i t h a mean age of 56.5 years. A l l s ubjects were i n good h e a l t h ; however, each e x h i b i t e d a background of prolonged exposure to i n d u s t r i a l noise. The c r i t e r i a f o r subject s e l e c t i o n i n t h i s group included a negative h i s t o r y of middle ear pathology and the presence of a high-frequency s e n s o r i n e u r a l hearing l o s s (air-bone gap no greater than 10 dB) wit h no evidence of r e t r o c o c h l e a r involvement. The 32 h e a r i n g - l o s s (HL) group's audiograms d i s p l a y e d s l o p i n g l o s s e s f o r frequencies above 1000 to 2000 Hz- ( i . e . more bas a l than cochlear p o s i t i o n s of about 14 to 18 mm from the apex) f o r a l l but one subject (whose thresholds rose sharply from 500 Hz). In eighty percent of these 20 cases t h i s slope ranged i n value from 50 t o 100 dB/octave. For 15 s u b j e c t s , pure tone audio-metric thresholds f o r frequencies up to the edge of the s l o p i n g l o s s were no worse than 20 dB HL. The remaining f i v e showed a m i l d l o s s (average of 30 dB) at 500 and 1000 Hz. Although subjects were not a l l p r e c i s e l y matched i n terms of audiogram contour, i t w i l l be seen that the nature of t h i s study obviated the need to s p e c i f y and u n i f y s t r i n g e n t l y the audiometric c o n f i g u r a t i o n of a l l s u b j e c t s . 3.2 Equipment The major equipment component, used i n t h i s study t o generate a l l experimental s t i m u l i , was the Starkey Hearing Science Labor-atory. The Starkey Hearing Science Laboratory i s a s e l f - c o n t a i n e d e l e c t r o n i c system i n c o r p o r a t i n g components such as a t t e n u a t o r s , a m p l i f i e r s , f i l t e r s , and mixers i n one u n i t . A d i g i t a l programmer w i t h i n the u n i t c o n t r o l s the temporal sequencing of experimental s t i m u l i v i a programmable e l e c t r o n i c switches (gates). A l l t e s t i n g was c a r r i e d out at the Hearing Branch of the WCB i n Richmond, B.C. w i t h subjects seated i n a double-walled sound-insulated t e s t booth, IAC model 1 1 - 3 3 0 . Test s t i m u l i were d e l i v e r e d monaurally to the subject through a Telex 1^70 earphone mounted i n a MX 41/AR cushion. The ear c o n t r a l a t e r a l to the t e s t ear was a l s o covered w i t h an earphone. A block diagram 33 i l l u s t r a t i n g stimulus generation and experimental setup i s shown i n Figure 1. At the beginning of every t e s t i n g s e s s i o n , a B r u e l and K j a e r (B & K) 2204 p r e c i s i o n sound l e v e l meter, coupled to a B & K type 4l65 condenser microphone, was used w i t h a B & K 6 cc coupler, type 4152,to c a l i b r a t e the sound pressure l e v e l output at the t e s t earphone, ensuring that stimulus i n t e n s i t y was always w i t h i n .5 dB of the r e q u i r e d l e v e l s . Exact stimulus frequencies were set and monitored by a Fluke 1900A frequency counter. I n experimenting on the d e t e c t i o n of i n t e r n a l l y - g e n e r a t e d combination tones, i t i s necessary to s p e c i f y and l i m i t the d i s t o r t i o n products produced by the apparatus i t s e l f . Thus, p r i o r to the onset of any t e s t i n g and again a f t e r the experiment was concluded,the d i s t o r t i o n content of the two-tone stimulus was measured a c o u s t i c a l l y at the output of the earphone. Measure-ments were c a r r i e d out by means of a B & K model 2114 spectro-meter and di s p l a y e d by a B & K graphic l e v e l recorder, model 2300. Readings were taken at two sets of frequencies (1400, 1820 HZ; 2000, 2600 Hz) and i n t e n s i t i e s of 80, 85, and 90 dB SPL. The p r i n t o u t f o r inputs of 80 and 85 dB always showed d i s t o r t i o n products at the combination tone frequency, 2fl-f2, to be at l e a s t 60 dB below the l e v e l of the primaries (L1,L2). For an input of 90 dB SPL however, the d i s t o r t i o n product at 2fl-f2 had increased, by the c o n c l u s i o n of t e s t i n g , to an un-d e s i r a b l e l e v e l of 45 dB r e l a t i v e to the p r i m a r i e s . Therefore, while apparatus d i s t o r t i o n at the onset was acceptable f o r a 90 dB SPL input, t h i s change l e d to the d i s c a r d i n g of r e s u l t s f o r data a n a l y s i s obtained at t h i s stimulus l e v e l . Oscillator Attenuator I Oscillator 2 Attenuator 2 Oscillator 3 2 f , - f 2 Probe Attenuator 3 1 Mixer Programmable Electronic Switch "B" Programmer Programmable Electronic Switch "A" Amplifier (+20 dB) Mixer "B" Amplifier / / / 'A A A A 1 / 1 V / 4 Response Button FIGURE BLOCK DIAGRAM OF THE EXPERIMENTAL APPARATUS 35 3-3 S t i m u l i This study used a forward-masking paradigm to examine the amount of forward masking produced by the combination tone 2 f l -f2 at i t s own frequency. For t h i s , three separate pure tones were re q u i r e d as s t i m u l i . Two of these,,fl and f 2 , represented the lower and upper primary tones r e s p e c t i v e l y , necessary f o r the p roduction of 2 f l - f 2 . The term "masker" or "stimulus" w i l l be used to designate f l and f 2 . The t h i r d tone (probe) of b r i e f d u r a t i o n was placed at frequency 2 f l - f 2 . Measurements of the probe's masked t h r e s h o l d f o r d i f f e r e n t i n t e n s i t i e s of f l and f2 generating 2 f l - f 2 gave estimates of the masking exerted by the combination tone. Measurements were al s o c a r r i e d out u s i n g f l alone as the masker to i n v e s t i g a t e the e f f e c t t h i s lower frequency component may have had on e l e v a t i n g t h r e s h o l d f o r the probe tone. I n t e n s i t y l e v e l of the f l and f l + f 2 s t i m u l i v a r i e d i n 10 dB steps from 60 to 90 dB SPL w i t h L I (the i n t e n s i t y l e v e l of f l ) equal to L2 (the i n t e n s i t y l e v e l of f2) f o r the two-tone masker. In a d d i t i o n , s e v e r a l subjects i n the N group were t e s t e d at • lower masker i n t e n s i t i e s down to 20 dB SPL. Figure 2 o u t l i n e s the stimulus t i m i n g diagram. The masker was 200 msec i n d u r a t i o n , w i t h a 10 msec r i s e - f a l l time shaped by an e l e c t r o n i c s w i t c h , which produces a l i n e a r ramp. The s i g n a l was 20 msec long, s i m i l a r l y shaped. The approximate time between h a l f - p r e s s u r e p o i n t s of the masker and s i g n a l waveforms was 8 msec. The time i n t e r v a l between masker-plus-probe tone s t i m u l i was v a r i e d by the experimenter between approximately three and f i v e seconds (see Figure l ) , dependent upon the response p a t t e r n of the subject. fj + f 2 or fj alone 2fj - f 2 Masker Probe Masker F I G U R E 2- T I M I N G D I A G R A M O F T H E S T I M U L I NOTE: Unless otherwise noted, all numbers refer to milliseconds. O N 37 The sets of frequencies chosen f o r the s t i m u l i are l i s t e d i n Table I. O r i g i n a l l y , only one set (probe frequency = 1400 Hz) was used; however two e x t r a sets were added to match more a c c u r a t e l y the frequency range of hearing l o s s i n the HL group. S p e c i f i c a l l y , the aim was to place f l and f2 i n the r e g i o n of t h r e s h o l d e l e v a t i o n and the probe i n an area of r e l a t i v e l y normal hearing. To provide the necessary primary frequency s e p a r a t i o n from 2 f l - f 2 to achieve t h i s c o n d i t i o n without s a c r i f i c i n g 2 f l - f 2 amplitude, an f 2 / f l value of 1.3 was considered most appropriate. 3.4 Procedure The f i r s t step was the determination of thresholds f o r the d i s c r e t e frequencies, f l and f 2 , used f o r each subject. Next, the quiet threshold, f o r the 20 msec probe tone was obtained against which a l l other masked thresholds would be l a t e r compared. Subjects were then i n s t r u c t e d and f a m i l i a r i z e d w i t h the l i s t e n -i n g task. Already acquainted w i t h the probe tone s t i m u l u s , the subjects were t o l d to depress the response button whenever they detected t h i s b r i e f tone immediately f o l l o w i n g a longer, higher-p i t c h e d tone. I t was mentioned that t h i s short tone may not be present i n each stimulus. Although the word "tone" was used i n the probe d e s c r i p t i o n , the emphasis was placed on d e t e c t i o n of t h i s s i g n a l f o l l o w i n g the masker. No d i s t i n c t i o n was made be-tween the one- and two-component masker unless, upon hearing the two maskers, a subject n o t i c e d the d i f f e r e n c e . An example of each c o n d i t i o n , masker alone and masker-plus-probe, was given to ensure subject understanding of the task. A short p r a c t i c e t r i a l preceded the a c t u a l data c o l l e c t i o n . Group Frequency (Hz) Ratio N HL 2fl-f2(Probe) f l f 2 f l / f 2 20-90dB(N=7) 60-90dB(N=6) N=20 980 1400 1820 1.30 5 4 9 1400 2000 2600 1.30 2 2 8 2000 3000 4000 1.33 — 3 Table I Number of subjects t e s t e d at each of the three sets of frequencies used. 39 For a l l subjects eight masked thresholds were obtained. Two thresholds were obtained at each stimulus l e v e l before i n t e n s i t y was a l t e r e d (from 60 to 90 dB SPL). Thresholds w i t h f l alone as masker were always determined before thresholds w i t h the two component masker. Where lower i n t e n s i t y c o n d i t i o n s were inc l u d e d ( i . e . , N group) subjects were t e s t e d i n two separate sessions. Threshold f o r the probe tone i n qui e t was r e t e s t e d a f t e r each s e s s i o n . For a l l subjects pre- and p o s t - t e s t q u i e t thresholds d i f f e r e d by no more than 4 dB. For a l l t e s t i n g , a modified method of l i m i t s was used. S t a r t i n g from a l e v e l w e l l above t h r e s h o l d , stimulus i n t e n s i t y was decreased i n 10 dB steps u n t i l the experimenter had bracketed t h r e s h o l d to w i t h i n 10 dB. The experimenter subsequently i n -creased or decreased stimulus i n t e n s i t y i n 4 dB steps based upon subject response. For very c o n s i s t e n t responders stimulus i n t e n s i t y was v a r i e d i n 2 dB steps. Four to s i x crossings were re q u i r e d of most subjects to o b t a i n t h r e s h o l d . To ease the task, s e v e r a l c l e a r l y suprathreshold l e v e l s of the probe were d e l i v e r e d . Catch t r i a l s , i n which the probe tone was omitted, were a l s o included p e r i o d i c a l l y as a t e s t of subject a t t e n t i o n and to f a c i l i t a t e the l i s t e n i n g task. Probe tone thresholds f o r the forward-masking paradigm are f e l t to be accurate t o w i t h i n -4 dB f o r most s u b j e c t s . O c c a s i o n a l l y , thresholds could only be bracketed to w i t h i n ±6 dB. There were s e v e r a l very c o n s i s t e n t , responders whose thresholds could be a c c u r a t e l y determined to w i t h i n ±2 dB. T e s t i n g time f o r a t o t a l of ten thresholds took from 20 to 3° minutes. 40 4. RESULTS 4.1 The Forward-Masking E f f e c t of f l Alone Before any experimental t e s t i n g was begun, thresholds f o r the probe tone, f l , and f2 pure tone s i g n a l s were determined. Tables I I and IHpresent these thresholds f o r the N and HL group subjects r e s p e c t i v e l y . In terms of t h r e s h o l d values, i t i s evident that subjects of the N group c o n s t i t u t e a much more homogenous sample than subjects i n the HL group. Figures 3 and 4 d i s p l a y the r e s u l t s obtained f o r the N and HL groups r e s p e c t i v e l y when f l alone was the masking stimulus. These graphs p l o t the amount of masking of the probe tone due to f l as a f u n c t i o n of f l i n t e n s i t y (SPL). I t should be remem-bered that the abscissas (dB SPL) on these two graphs represent d i f f e r e n t ranges r e l a t i v e to t h r e s h o l d f o r the two groups. Hearing thresholds f o r f l as high as 65 dB HL ( 7 4 dB SPL) f o r the HL group and as low as -10 dB HL (-1 dB SPL) f o r the N,group mean that the d i f f e r e n c e i n sensation l e v e l (SL) between sub-j e c t s at these two extremes could be as much as 75 dB. Despite the reduced sensation l e v e l s f o r HL s u b j e c t s , s e v e r a l of them showed greater masking by f l than subjects i n the N group. For such s u b j e c t s , p l o t s of masking by f l e x h i b i t steeper slopes than those f o r normals. The histograms of Figure 5 compare the means and ranges i n the amount of masking by f l at l e v e l s of 60, 70, and 80 dB SPL. The mean masked thresholds are s i m i l a r f o r the two groups, but i t i s evident that there i s gr e a t e r v a r i a b i l i t y i n the amount of f l masking f o r the HL group. There were subjects i n both groups who f a i l e d to e x h i b i t a t h r e s h o l d s h i f t due to f l at any l e v e l . On the other hand, 41 i t i s apparent that some HL subjects showed s i g n i f i c a n t l y more masking by f l than any subject i n the N group. The growth of masking by f l f o r stimulus l e v e l s from 60 to 80 dB SPL i s represented i n the l e f t hand side of Tables IV and V where slopes f o r each N and HL subject are l i s t e d . Slopes of masking by f l are s i m i l a r f o r the two groups between 60 and 70 dB; however, between 70 and 80 dB the mean slope f o r the HL group i s 0.6, compared w i t h a slope of close to h a l f t h i s value f o r the N group. Because these slopes were derived from only a few data p o i n t s , however, one must be cautious i n t h e i r i n t e r -p r e t a t i o n . C o r r e l a t i o n c o e f f i c i e n t s of the amount of f l masking as i t r e l a t e s to probe-tone and f l quiet thresholds were determined f o r each group sep a r a t e l y . No s i g n i f i c a n t c o r r e l a t i o n emerged f o r the N group between the amount of masking by f l and e i t h e r the q u i e t t h r e s h o l d of the probe tone or f l . For the HL group, however, c o r r e l a t i o n c o e f f i c i e n t s of between -.51 to -.65 emerged r e l a t i n g amount of masking to the t h r e s h o l d of the probe tone i n q u i e t , and c o r r e l a t i o n c o e f f i c i e n t s of between -.54 and -.69 were obtained r e l a t i n g amount of masking to the qu i e t t h r e s h o l d f o r f l . A l l c o r r e l a t i o n c o e f f i c i e n t s were found to be s i g n i -f i c a n t at the .01 l e v e l (p=.01). For both sets of c o e f f i c i e n t s , the c o r r e l a t i o n s increased w i t h i n c r e a s i n g f l i n t e n s i t y . Thus, these r e s u l t s i n d i c a t e that those HL subjects w i t h the lowest (or most s e n s i t i v e ) thresholds f o r the probe tone and f l f r e -quency (e.g. subjects 15 and 16 i n Table 3) are those most l i k e l y to show the greatest masking due to f l . Furthermore, t h i s e f f e c t becomes stronger with i n c r e a s i n g stimulus l e v e l . 42 4.2 The Forward-Masking E f f e c t w i t h f l + f 2 as Stimulus: Masking A t t r i b u t a b l e to the Combination Tone 2 f l - f 2 The frequency of the f 2 masker i s s u f f i c i e n t l y removed from the probe tone frequency so as to preclude any noteworthy masking e f f e c t due to t h i s upper primary component. The degree of masking at the probe tone frequency by the f l + f 2 s t i m u l u s , t h e r e f o r e , c o n s i s t s of some combination of two masking e f f e c t s : 1) the masking due to f l , and 2) the masking at the probe tone frequency due to the combination tone generated by f l + f 2 . I f a d i f f e r e n c e i n masking can be demonstrated between the one-and two-component masker, t h i s would provide proof of the ex-i s t e n c e of 2 f l - f 2 . Such a d i f f e r e n c e does occur and i s repre-sented by Figures 6 and 7- These graphs show the d i f f e r e n c e i n masking of the probe tone between the f l and f l + f 2 stimulus as a f u n c t i o n of stimulus i n t e n s i t y f o r N and HL subjects respect-i v e l y . Figure 8 d i s p l a y s the mean and ranges i n t h i s d i f f e r e n c e and allows easy comparison between the HL and N groups. I t can be seen from these graphs and histogram th a t a l l subjects of the N group show a d i f f e r e n c e i n masking between the two s t i m u l i at some i n t e n s i t y of the p r i m a r i e s , u s u a l l y between 40 and 60 dB SPL. On the other hand, most HL subjects ( 1 6 out of 20) d i d not show any masking d i f f e r e n c e at 60 and 70 dB SPL. The r e s u l t s at 80 dB SPL show wide v a r i a t i o n , w i t h s e v e r a l HL subjects c o n t i n u i n g to show no masking e f f e c t by the f l + f 2 s t i m u l u s , and t h e r e f o r e no combination tone. The other way to p l o t the data obtained u s i n g the two-tone stimulus i s d i s p l a y e d i n Figures 9 and 1 0 , where the t o t a l masking by f l + f 2 i s graphed as a f u n c t i o n of stimulus i n t e n s i t y 43 f o r the N and HL groups r e s p e c t i v e l y . As w i l l be explained i n the d i s c u s s i o n s e c t i o n to f o l l o w , i t i s f e l t t h a t t h i s way of p r e s e n t i n g data may provide a more r e a l i s t i c estimate of the masking a t t r i b u t a b l e to the combination tone 2 f l - f 2 . The mean and ranges i n the amount of masking at f l + f 2 l e v e l s of 60, 70, and 80 dB SPL are di s p l a y e d i n the histograms i n Figure 1 1 . The data f o r the N group showed a s i g n i f i c a n t l y g r eater amount of masking w i t h use of the two-tone stimulus than was observed wi t h f l alone. U n l i k e the e f f e c t of the f l masker, every sub-j e c t i n the N group e x h i b i t e d masking by the f l t f 2 s t i m u l u s . There was al s o a wider range of probe tone masked t h r e s h o l d values w i t h the f l + f 2 stimulus than w i t h f l alone. The growth of masking w i t h i n c r e a s i n g stimulus l e v e l , however, d i s p l a y e d a very s i m i l a r p a t t e r n f o r many subjects i n the N group. At 70 dB SPL, f o r example, 9 of the 13 subjects showed between 19 and 27 dB of masking. At 80 dB SPL, 7 N subjects d i s p l a y e d masking w i t h i n the 7 dB range between 33 and 39 dB. These values repre-sent a 14 and 23 dB increase i n the mean amount of masking be-tween the f l and f l + f 2 maskers f o r 70 and 80 dB SPL stimulus l e v e l s r e s p e c t i v e l y . Results f o r the HL group r e v e a l t h a t , w i t h the exception of one subject (#15) 1 there was no s i g n i f i c a n t d i f f e r e n c e i n the amount of masking between the one- and two-tone s t i m u l i f o r l e v e l s of 60 and 70 dB SPL. Above 70 dB, however, masking by the f l + f 2 stimulus showed much greater v a r i a b i l i t y and s e v e r a l HL subjects e x h i b i t e d a marked increase i n the amount of masking exerted by e i t h e r the f l stimulus alone or the f l + f 2 stimulus 44 at lower l e v e l s . This r e s u l t i s r e f l e c t e d i n Table 5 where the mean slopes f o r each masking f u n c t i o n are l i s t e d . The mean slope f o r the f l + f 2 stimulus between 70 and 80 dB (slope = 1.12) was s i g n i f i c a n t l y higher than the mean slope between the other i n t e n s i t i e s . Only at . 8 0 dB d i d an appreciable d i f f e r e n c e i n masking ( i . e . 15 dB) between the one- and two-tone stimulus occur. T-tests d i s c l o s e d a s i g n i f i c a n t d i f f e r e n c e i n the amount of masking between the N and HL groups at each l e v e l of the f l + f 2 stimulus ( p = . 0 l ) . A comparison of Figures 9 and 10 re v e a l s that masking f o r a l l but one of the HL subjects f e l l at l e a s t 10 dB below values f o r the N su b j e c t s . Figure 12 shows the growth of masking wi t h the f l + f 2 masker w i t h i n c r e a s i n g stimulus l e v e l of the primaries f o r f o u r subjects i n the N group f o r whom very l i t t l e or no masking was noted when f l alone was used as the st i m u l u s . The s i g n i f i c a n c e of these r e s u l t s w i l l be elaborated upon i n the f o l l o w i n g chapter. The next chapter as w e l l w i l l focus on the r e s u l t s d i s p l a y e d i n Figures 13 to 16. These graphs d i s p l a y i n d i v i d u a l r e s u l t s w i t h the one- and two-component masker f o r f o u r HL s u b j e c t s , along w i t h t h e i r audiograms (converted from the HL s c a l e to the SPL s c a l e ) . Given the wide v a r i a b i l i t y i n r e s u l t s f o r the HL group, i t i s h e l p f u l to analyze data from a few i n d i v i d u a l subjects f o r whom r e s u l t s are r e p r e s e n t a t i v e of the major trends seen. As w e l l , a n a l y s i s of t h i s data i n terms of i n d i v i d u a l hearing l o s s p r o f i l e s may provide clues to the mechanism of generation of combination tones. 45 Table I I Quiet thresholds (range and mean) at the probe, f l , and f 2 frequencies f o r the N s u b j e c t s . Subject Quiet Threshold (dB HL) Probe f l f 2 46 A 7 0 5 B 10 -5 -5 C 3 -5 -10 D 11 0 -5 E 13 0 0 F 9 -5 0 G 6 -5 0 H 7 0 0 I 5 -10 -5 J 7 0 0 K 10 5 0 L 9 5 5 M 15 5 5 range (dB) 3 to 15 -10 to +5 -10 to +5 mean (dB) 8.6 -1.2 -0.8 47 Table I I I Quiet thresholds (range and mean) at the probe, f l , and f 2 frequencies f o r the HL s u b j e c t s . Subject Quiet Threshold (dB HL) Probe f l f2 48 1 48 65 70 2 52 60 65 3 43 65 80 4 31 60 60 5 28 35 55 6 48 55 60 7 21 40 55 8 46 50 60 9 27 50 60 10 36 30 60 11 23 30 35 12 14 20 40 13 21 40 55 14 24 45 45 15 11 25 4o 16 11 30 40 17 16 55 50 18 9 4o 50 19 20 25 50 20 6 35 45 range (dB) 6 to 52 20 to 65 35 to 80 mean (dB) 26.8 42.8 53.8 V D Figure 3 Masking of the probe tone by f l as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l f o r N subjects. Masking of probe by f, Stimulus (dB) H Figure k Masking of the probe tone by f l as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l f o r HL subjects. ro o i Ol Masking of probe by f. Stimulus (dB) Oi O oi ro o ro Ol 01 o Ol 4* O Ol Ol o 01 o to o i Ol Ol Ol ro o ro Ol 01 o Ol o Ol Ol o 53 Figure 5 Histograms d i s p l a y i n g means and ranges i n the amount of masking of the probe tone by f l f o r the N group and HL group. | | denotes N group denotes HL group denotes the mean amount of masking f o r each group at each i n t e n s i t y Intensity of f, ( dB SPL) 55 Table IV Slopes of masking f u n c t i o n (range and mean) due to f l and f l + f 2 s t i m u l i f o r N s u b j e c t s . 56 Slope of Masking Function Slope of Masking Function due to f l due to f l + f 2 Subject 60-70 dB 70-80 dB 60-70 dB 70-80 dB A 0.2 0.1 0.9 1.3 B 0.3 0.2 0.1 1.5 C 0.3 0.2 0.7 0.9 D 0.7 0.6 0.1 1.2 E 0 0.4 1.7 0.9 F 0.1 -0.2 0.7 1.2 G 1.0 0.5 1.9 1.2 H 0.5 0.9 1.6 1.0 I 0.8 0.1 0.3 1.7 J 0.3 0.8 1.2 1.6 K -0.3 0.1 0.5 0.6 L 0 0.2 0.2 1.0 M 0 0.5 1.0 0.9 range (dB) -0.3 to 1.0 -0.2 to 0.9 0.1 to 1.9 0.6 to 1.7 mean (dB) 0.30 0.34 0.84 1.16 57 Table V Slopes of masking f u n c t i o n (range and mean) due to f l and f l + f 2 ' s t i m u l i f o r HL s u b j e c t s . 58 Slope of Masking Function Slope of Masking Function due to f l due to. f l + f 2 Subject 60-70 dB 70-80 dB 60-70 dB 70-80 dB 1 -0.1 0.2 0 0.5 2 0.5 0.1 0.7 0 3 0.1 -0.1 -0.1 0 4 0 0.1 -0.5 0.2 5 0 0.4 0.1 0.2 6 0.3 0.7 0.1 0.7 7 0.6 -0.2 0.6 -0.2 8 -0.1 -0.3 -0.2 0 9 0.7 0.3 0.7 0.7 10 1.2 1.0 0.2 1.4 11 0.4 0.2 0.3 0.9 12 0.9 0.6 0.8 1.6 13 0.1 2.0 1.1 2.3 14 0.3 1.2 0.8 1.6 15 1.5 0.8 0.8 0.8 16 0.2 2.2 0.7 3-5 17 0.2 0.6 0.4 1.6 18 0.5 0.8 0.6 2.5 19 0.4 0.5 0.5 0.5 20 0.3 0.8 0.6 3-5 range(dB) -0.1 to 1.5 -0.3 to 2.2 -0.5 to 1.1 -0.2 to 3-5 mean (dB) 0.40 0.60 0.41 1.12 VO Figure 6 Di f f e r e n c e i n amount of masking of the probe tone between the two component ( f l + f 2 ) and one component ( f l ) masker as a f u n c t i o n of masker i n t e n s i t y (dB SPL) f o r N subjec t s . 09 O N H Figure 7 D i f f e r e n c e i n amount of masking of the probe tone between the two component ( f l + f 2 ) and one component ( f l ) masker as a f u n c t i o n of masker i n t e n s i t y (dB SPL) f o r HL subjects. ro Ol Difference in masking of probe between f( ond f, + f2 (dB) O Ol o oi g ro 01 o 01 Ol o Ol Ol o OJ o 29 63 Figure 8 Histograms d i s p l a y i n g the d i f f e r e n c e i n masking between the two-component ( f l + f 2 ) and one-component ( f l ) masker f o r the N group and HL group. \ | denotes N group denotes HL group denotes the mean amount of masking f o r each group at each i n t e n s i t y 50 designates N group Intensity of f| , 1% (dB S P L ) ON Figure 9 Masking of the probe tone by the f l + f 2 stimulus as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l + f 2 f o r N subjects. 99 ON -0 Figure 10 Masking of the probe tone by the f l + f 2 stimulus as a f u n c t i o n of i n t e n s i t y l e v e l (dB SPL) of f l + f 2 f o r HL subjects. 89 69 Figure 11 Histograms d i s p l a y i n g the means and ranges i n the amount of masking by tl+£2 f o r the N and HL groups. | | denotes N group denotes HL group denotes the mean amount of masking f o r each group at each i n t e n s i t y 70 Intensity of f, + f 2 ( dB SPL) Masking of the probe tone by f l t f 2 as a f u n c t i o n of i n t e n s i t y l e v e l f o r four N subjects who d i s p l a y e d minimal or no masking by f l alone. Masking of probe by f, + f2 Stimulus (dB) i — _ ro ro OJ OJ cn Figure 13 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #1. Note: V e r t i c a l bars on audiogram denote probe, f l , and f2 frequencies. -o Figure 14 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #15. Note: V e r t i c a l bars on audiogram denote probe, f l , and f2 frequencies. Figure 15 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #19. Note:- V e r t i c a l bars on audiogram denote probe, f l , and f2 frequencies. Figure 16 Masking data and audiometric c o n f i g u r a t i o n f o r HL subject #17. Note: V e r t i c a l bars on audiogram denote probe, f l , and f2 frequencies. Masking of probe (dB) 08 81 CHAPTER 5 5. DISCUSSION 5.1 Masking A t t r i b u t a b l e to f l a) Normal-Hearing Subjects (N Group) There e x i s t s a f a i r l y l a r g e pool of data d e s c r i b i n g forward masking i n normal hearing sub j e c t s . Such data has been c o l l e c t e d u sing a v a r i e t y of masker and probe s i g n a l parameters. These include s p e c t r a l c h a r a c t e r i s t i c s (e.g. n o i s e , tone, envelope shape), d u r a t i o n , absolute frequency, frequency s e p a r a t i o n of the masker from the probe, and i n t e r s t i m u l u s i n t e r v a l . Without going i n t o d e t a i l , i t can be s t a t e d that each of these parameters independently and a d d i t i v e l y exert an e f f e c t on the amount of masking of a probe s i g n a l by the masker (e.g. Wilson and Carhart, 1 9 7 1 ; B e a t t i e et a l , 1974; Smiarowski, 1 9 7 5 ) . Such a v a r i e t y i n the parameters s t u d i e d makes i t d i f f i c u l t to compare r e s u l t s across s t u d i e s . Research conducted by Widin and Viemeister ( 1 9 7 9 ) made use of t e s t parameters roughly comparable to those used i n the present study. Adopting a t w o - i n t e r v a l forced-choice procedure, they i n v e s t i g a t e d tone-on-tone forward masking f o r stimulus l e v e l s from 20 to 80 dB SPL. A probe tone of frequency lkH2 and an i n t e r - s t i m u l u s i n t e r v a l of 5 msec was used. The d u r a t i o n of the probe tone and masker were 20 msec and 280 msec respect-i v e l y . Average slopes of the masking f u n c t i o n r e l a t i n g the amount of masking to masker l e v e l were 0.44. This represented t h r e s h o l d s h i f t s of 10 to 25 dB over the stimulus range of 60 to 80 dB SPL. For Widin and Viemeister's three subjects a f o r -ward masking e f f e c t was f i r s t noted between 40 and 60 dB SPL. 82 The r e s u l t s of the present study, as p l o t t e d i n Figure 3 and Table IV, corroborate the p a t t e r n of r e s u l t s from Widin and Viemeister i n both the stimulus l e v e l s r e q u i r e d to f i r s t e l i c i t a forward masking e f f e c t and i n the slope of the masking f u n c t i o n . The slopes and ranges of masking are reduced i n the present study, probably due to the f a c t that f l i n t h i s study was remote from the probe tone frequency, whereas Widin and Viemeister's r e s u l t s are f o r tone-on-tone masking. They used only three subjects who were very experienced i n the psychoa-c o u s t i c l i s t e n i n g task. Nevertheless, wide v a r i a t i o n i n the amount of forward masking was noted among t h e i r three s u b j e c t s , a f i n d i n g that was a l s o apparent with the r e s u l t s from the 13 normal hearing subjects of the present study. b) Hearing-Loss Subjects (HL Group) In c ontrast to the research conducted on forward masking wi t h normal hearing subjects., there i s r e l a t i v e l y l i t t l e i n f o r -mation a v a i l a b l e about t h i s phenomenon i n hearing-impaired l i s t e n e r s . The r e s u l t s of the few published accounts a v a i l a b l e are again not e a s i l y compared to the present r e s u l t s , at l e a s t q u a n t i t a t i v e l y , because parameters of the s t i m u l i i n v e s t i g a t e d and experimental paradigm used were very d i f f e r e n t ( K e i t h , 1969; E l l i o t t , 1975;Danaher,e.t a l , 1978) The major f i n d i n g of these s t u d i e s appears to be the wide v a r i a t i o n i n r e s u l t s among sub-j e c t s . Danaher et a l ( 1 9 7 8 ) , f o r example, noted l a r g e d i f f e r -ences among t h e i r hearing impaired subjects i n the amount of forward masking of a 20 msec probe tone by the noise masker. ( i n t e r s t i m u l u s i n t e r v a l = 10 msec) A mean t h r e s h o l d s h i f t of 83 11 dB was noted f o r moderately to se v e r e l y hearing-impaired su b j e c t s ; some subjects showed no masking while others d i s -played 15 to 20 dB of masking. The i n f o r m a t i o n obtained from the present study with hearing-impaired subjects serves to expand the r a t h e r meagre base of data p r e s e n t l y a v a i l a b l e on forward masking i n t h i s p o p u l a t i o n . The c o r r e l a t i o n c o e f f i c i e n t s , r e l a t i n g the amount of forward masking to the quiet thresholds of the probe tone and f l were p r e d i c t a b l e - i n general, the b e t t e r the quie t t h r e s h -o l d f o r the probe or f l , the greater the masked t h r e s h o l d of the probe tone. These f i n d i n g s are capable of e x p l a i n i n g r e s u l t s f o r many HL subjects on the ba s i s of sensation l e v e l of f l and the probe. Figure 3» f o r example, showed that f l produced a forward masking e f f e c t f o r some N subjects at 50 dB SPL, or approximately 40 dB SL. Since some HL subjects d i s p l a y e d q u i e t thresholds f o r f l which were only 20 dB higher than f o r N sub-j e c t s , i t i s not s u r p r i s i n g t h a t f l exerted a forward masking e f f e c t at 60 or 70 dB SPL. The f i n d i n g s from other s u b j e c t s , however, are d i f f i c u l t to e x p l a i n : some HL subjects showed masked thresholds of the probe tone that were equal t o , or even greater than, those of t h e i r normal-hearing counterparts, despite a reduced sensation l e v e l at both the masker and probe tone frequencies. Why should t h i s occur? I t has been g e n e r a l l y assumed that forward masking r e s u l t s from one of two events (Moore, 1 9 7 8 ) . There may be a p e r s i s -tence (Moore's terminology) of the neur a l a c t i v i t y evoked by the masker at some l e v e l i n the a u d i t o r y system (see al s o 84 Smiarowski and Carhart, 1 9 7 5 ) ' A l t e r n a t e l y , adaptation or f a t i g u e i n h a i r c e l l s and/or nerve c e l l s , probably a s s o c i a t e d w i t h a d e p l e t i o n of chemical synaptic t r a n s m i t t e r substance, may lead to a reduced tendency f o r h a i r c e l l s to re l e a s e t r a n s -m i t t e r or f o r nerve c e l l s to f i r e . I t i s p o s s i b l e that meta-b o l i c processes d i s r u p t e d by p h y s i o l o g i c a l damage to h a i r c e l l s (which occurs i n noise induced hearing l o s s ) may lead to a u d i t o r y adaptation or f a t i g u e which i s increased above and beyond th a t d i s p l a y e d by normal cochleas. A l t e r n a t e l y , or i n a d d i t i o n to the above, damage at the p e r i p h e r a l l e v e l may be r e f l e c t e d i n c e n t r a l nervous system changes which a l t e r the process of temporal processing of s i g n a l s i n cases of hearing l o s s . There i s another p o s s i b l e e x p l a n a t i o n f o r the increased masked t h r e s h o l d of the probe tone noted f o r some HL sub j e c t s . One problem f a c i n g the l i s t e n e r i n any masking s i t u a t i o n i s tha t of d i s t i n g u i s h i n g the neural a c t i v i t y evoked by the probe from that evoked by the masker. Subjects may have d i f f e r e d i n t h e i r i n t e r p r e t a t i o n of what 'detection' of the probe tone i n t h i s experiment involved. Those subjects who showed more apparent masking by f l may simply have had more ' s t r i n g e n t ' c r i t e r i a i n responding. The point to be made i s that d i f f e r e n c e s i n l i s t e n -i n g s t r a t e g i e s among subjects may be at l e a s t p a r t l y r e s p o n s i b l e f o r d i f f e r e n t r e s u l t s and the wide v a r i a b i l i t y among sub j e c t s . 5.2 Masking A t t r i b u t a b l e to 2 f l - f 2 a) Normal-Hearing Subjects (N Group) Before proceeding w i t h an a n a l y s i s of the data obtained w i t h f l + f 2 as sti m u l u s , i t i s important to h i g h l i g h t two aspects 85 of the present study which should be remembered i n any i n t e r -p r e t a t i o n of r e s u l t s . F i r s t l y , as o u t l i n e d p r e v i o u s l y , the experimental paradigm used i n t h i s study does not permit any estimates of the amplitude of 2 f l - f 2 but only permits a measure of the masking e f f e c t due to t h i s combination tone. An e x t e r n a l reference masker i f i t had been used could have provided an equivalent amount of masking, so that the l e v e l of the reference tone could then have been considered to be an estimate of the combination tone l e v e l . A second aspect of the present study t h a t r e q u i r e s c a u t i o n i n i t s i n t e r p r e t a t i o n a r i s e s i n attempting to e x p l a i n the masking e f f e c t of the f l + f 2 stimulus. How much of t h i s t o t a l masking can one a t t r i b u t e to 2 f l - f 2 , produced by the i n t e r a c t i o n of f l and f2? How much of the t o t a l masking e f f e c t may be due to the f l component? The r e s u l t s as p l o t t e d i n Figures 6 and 7 provide strong evidence that 2 f l - f 2 i s indeed present at a l e v e l s u f f i -c i e n t to provide masking f o r each subject of the N group f o r at l e a s t one stimulus l e v e l of f l and f 2 . I t i s qu i t e p l a u s i b l e that the c o n t r i b u t i o n of f l toward the t o t a l masking may be very minimal, since the combination tone produced by f l + f 2 i s at the same frequency as the probe tone and f l i s l o c a t e d i n an area remote from the probe tone. Results from f o u r of the 13 N group subjects gave a c l e a r i n d i c a t i o n that the masking e f f e c t of f l + f 2 f o r them was due e n t i r e l y to 2 f l - f 2 . These were subjects f o r whom the f l masker when used alone exerted very minimal (5 dB or l e s s ) or no masking e f f e c t at any stimulus l e v e l . R e s u l t s from these subjects are 86 portrayed i n Figure 12. In l o o k i n g at these f o u r subjects alone i t i s evident that there was wide v a r i a t i o n i n t h e i r r e s u l t s : one subject showed masking a t t r i b u t a b l e to 2 f l - f 2 at a stimulus l e v e l above 30 dB SPL while another r e q u i r e d a l e v e l above 70 dB to show a masking e f f e c t . The growth of masking w i t h i n c r e a s i n g stimulus l e v e l a l s o v a r i e d anywhere from a slope of 0.4 to a maximum of 1.0. When data from these f o u r subjects are compared to the other N subjects (see Figure 9) i t i s apparent at a glance t h a t , f o r stimulus i n t e n s i t i e s up to and i n c l u d i n g 70 dB SPL, r e s u l t s from these four subjects are not s i g n i f i c a n t l y d i f f e r e n t from the r e s t of the group. At 80 dB SPL, however, a s i g n i f i c a n t d i f f e r e n c e emerges, which separates the four subjects who show no masking due to f l alone and places them i n the lower h a l f of the graph. Those two subjects who d i s p l a y e d the greatest e l e -v a t i o n of probe tone t h r e s h o l d at 80 dB SPL (46 dB and 49 dB) are also the two subjects who demonstrated the greatest e l e v a t i o n of probe tone t h r e s h o l d when f l was used alone. One cannot conclude from data on the nine subjects who show s i g n i f i c a n t masking by f l alone how much of the t o t a l masking by the f l + f 2 stimulus i s due to 2 f l - f 2 . I t could be contended that f o r a l l s u b j e c t s , r e g a r d l e s s of the masking e f f e c t w i t h f l used alone, the masking e f f e c t of f l + f 2 was due s o l e l y to 2 f l - f 2 . A f t e r a l l , i f f l at a frequency remote from the probe tone exerts such a strong e f f e c t , why shouldn't 2 f l - f 2 , of the same frequency as the probe, exert a great masking e f f e c t ? The wide d i f f e r e n c e s i n r e s u l t s , then, would simply r e f l e c t 87 i n d i v i d u a l v a r i a b i l i t y i n forward masking by a tone, whether e x t e r n a l or i n t e r n a l . :• St i l l others could argue that the masking e f f e c t of f l i s s t i l l present to some degree f o r some subjects who, f o r whatever reason, are more s t r o n g l y a f f e c t e d than other subjects by maskers at remote frequencies. Is i t p o s s i b l e to compare the present r e s u l t s w i t h the f l + f 2 stimulus i n N subjects w i t h f i n d i n g s i n the l i t e r a t u r e ? I t i s d i f f i c u l t because, as mentioned p r e v i o u s l y , few st u d i e s to date have used a comparable forward masking paradigm and those that have made use of a very l i m i t e d number of su b j e c t s . S e v e r a l aspects of Buunen et a l ' s ( 1 9 7 7 ) study, n e v e r t h e l e s s , approximate the present study. With an f 2 / f l = 1 . 3 , L l = 5 0 dB SPL, L2=44 dB SPL, two of t h e i r three subjects showed no evidence of forward masking by 2 f l - f 2 , as defined by the authors as the d i f f e r e n c e i n masking of a probe tone between the one component and two component stimulus. T h e i r t h i r d subject showed a 12 dB masking e f f e c t a t t r i b u t a b l e to 2 f l - f 2 . Of the 8 subjects t e s t e d i n the present study at 50 dB SPL, a masking e f f e c t of anywhere from 0 to 15 dB was noted. This i s not at variance with Buunen et a l ' s psychophysical r e s u l t s , nor wit h t h e i r r e s u l t s obtained v i a e l e c t r o p h y s i o l o g i c a l means (reported i n the same paper) which were q u a l i t a t i v e l y s i m i l a r to psychoacoustic r e s u l t s . The range of slopes obtained i n t h i s study a l s o agree reasonably w e l l with p u l s a t i o n t h r e s h o l d r e s u l t s reported i n the l i t e r a t u r e review. The present study o f f e r s valuable data on the masking e f f e c t of 2 f l - f 2 , not only because the pool of subjects used 88 i s l a r g e compared to other s t u d i e s , hut a l s o because there appears to be very l i t t l e , i f any, i n f o r m a t i o n on the growth of the masking e f f e c t of f l + f 2 w i t h i n c r e a s i n g stimulus l e v e l . I n t h i s regard the graphs and t a b l e s of Chapter 3 provide new data. b) Hearing-Loss Subjects (HL Group) By reference to Figure 10 i t i s apparent that there i s great v a r i a t i o n i n the masking e f f e c t of the two tone stimulus f o r HL s u b j e c t s . Looking at the growth of masking w i t h i n t e n s i t y l e v e l of the p r i m a r i e s , some subjects showed no, or very minimal, e l e v a t i o n i n probe tone masked t h r e s h o l d regardless of the stimulus l e v e l of f l and f 2 . Others showed a growth i n masking approximately equal to that of N subjects ( i . e . approximately equal slopes) but the absolute values of t h i s masking were r e -duced from those of the N s u b j e c t s . S t i l l a few others evidenced a phenomenal increase i n the masking e f f e c t of f l + f 2 between 70 and 80 dB SPL, l e a d i n g to a masked t h r e s h o l d f o r the probe tone which r i v a l l e d any value obtained w i t h N s u b j e c t s . Results obtained at 60 dB were as would be p r e d i c t e d by any model which s t i p u l a t e s that f l (and p r e f e r a b l y f 2 as w e l l ) be supra-threshold f o r combination tone perception. Since q u i e t thresholds f o r f l and f 2 were u n i f o r m l y at l e a s t 20 dB poorer f o r HL subjects than N s u b j e c t s , and since N subjects di d not d i s p l a y any masking e f f e c t by f l + f 2 u n t i l the i n t e n s i t y l e v e l of these tones were at l e a s t 40 dB SPL, i t i s not sur-p r i s i n g that no (or at most very minimal) masking was noted at t h i s l e v e l f o r HL s u b j e c t s . 89 At 70 dB r e s u l t s are s u r p r i s i n g l y uniform, w i t h about one h a l f of HL subjects c o n t i n u i n g to show no evidence of any masking due to the f l + f 2 s t i m u l u s , and the other h a l f evidencing an approximate 15 dB masking e f f e c t . This represents a slope of masking of approximately 0.6 to 0.7 "between 60 and 70 dB. I t i s at the stimulus l e v e l of 80 dB that the most i n t e r -e s t i n g r e s u l t s were obtained. At l e a s t one t h i r d of HL subjects showed a sharp increase i n the slope of masking f u n c t i o n between 70 and 80 dB. Re s u l t s from such subjects l e d to the greatest 'spread' of probe tone masked thresholds of e i t h e r group at any stimulus l e v e l . A comparison of Figures 9 and 10 r e v e a l s an i n t e r e s t i n g f i n d i n g regarding the amount of masking due to the f l + f 2 stimulus when presented at moderate i n t e n s i t i e s . Roughly equivalent ranges of the amount of masking occur f o r the two groups when-ever the sound pressure l e v e l of the f l , f 2 stimulus i s 20 dB greater f o r the HL group versus the N group. Thus, the range of masking seen at 60 dB i n the HL group approximates that seen at 40 dB f o r the N group; the amount of masking at 70 dB i n the HL group approximates the amount f o r the N group at 50 dB. At moderate i n t e n s i t i e s , t h e r e f o r e , a 20 dB increase i s a l l that i s needed f o r s e v e r a l HL subjects to give the same masking e f f e c t by f l + f 2 . From Table I I i t can be seen t h a t , f o r s e v e r a l HL s u b j e c t s , t h i s 20 dB represents the d i f f e r e n c e i n t h r e s h o l d between the N subjects and these HL subjects f o r the probe tone and sometimes f l as w e l l . Above 70 dB, however, things are more complicated, and such a r e l a t i v e l y simple e x p l a n a t i o n f a i l s to e x p l a i n the wide d i v e r s i t y of r e s u l t s . 90 To get a b e t t e r grasp of the kinds of r e s u l t s obtained f o r HL s u b j e c t s , i t i s probably best to consider the r e s u l t s of a few r e p r e s e n t a t i v e subjects s e p a r a t e l y , a n a l y z i n g t h e i r data i n terms of t h e i r audiogram and t h e i r q u i e t thresholds f o r the probe, f l , and f 2 frequencies. Subject 1 (see Figure 13) gave r e s u l t s which were t y p i c a l of 8 out of the t o t a l 20 HL subjects of the sample, the l a r g e s t sub-group. For such subjects no s i g n i f i c a n t increase i n probe tone t h r e s h o l d emerged when e i t h e r the one or two component masker was used. Even though r e s u l t s f o r the stimulus l e v e l of 90 dB were not used i n data a n a l y s i s , they are included i n Figure 13 to show t h a t , even at 90 dB SPL, there was no masking a t t r i b u t a b l e to 2 f l - f 2 . These r e s u l t s were v e r i f i e d by the strong negative c o r r e l a t i o n s obtained r e l a t i n g the amount of masking f o r both f l alone and f l + f 2 w i t h the q u i e t thresholds f o r the probe tone, f l and f 2 frequencies. For 6 of the 8 subjects f i t t i n g i n t o t h i s category, q u i e t thresholds f o r the probe tone, f l and f 2 s i g n a l s were s i g n i f i c a n t l y elevated. These r e s u l t s match f i n d i n g s of Smoorenburg (1972a) and Sachs and Wightman (1975)» who reported no evidence of combination tone pe r c e p t i o n i n t h e i r hearing-impaired subjects unless the i n t e n s i t y of the f l and f 2 s i g n a l s surpassed the s u b j e c t s ' quiet thresholds at these frequencies. The remaining 2 subjects who f i t i n t o t h i s category had b e t t e r q u i e t thresholds f o r f 1 , f 2 , and the probe tone, although they by no means approximated those of normals. I t i s not c l e a r why these two subjects showed no evidence of masking a t t r i b u t a b l e to e i t h e r f l alone or f l + f 2 . 91 Subject 15 (see Figure 14) represents the other extreme of Figure 7 > where the masked t h r e s h o l d of the probe tone was 26, 34, and 42 dB at stimulus l e v e l s of 60, 70, and 80 dB SPL r e s p e c t i v e l y . This subject showed an extremely l a r g e amount of masking when f l was presented alone, more masking at each l e v e l than f o r any subject of the N group. The increase i n masking when f l + f 2 was used i s not tha t much above that due to f l alone. Although the quiet thresholds f o r f l and the probe tone are among the best of any HL su b j e c t , t h i s does not e x p l a i n why t h i s subject should evidence such a strong masking e f f e c t due to f l alone. The e f f e c t of l i s t e n i n g s t r a t e g y , as explained previous-l y , may be a f a c t o r here. The r e s u l t s of t h i s subject when usin g the one component masker make i t very d i f f i c u l t to assess how much of the masking due to the f l + f 2 stimulus may be a t t r i b u t a b l e to 2 f l - f 2 . Subject 19 (see Figure 1 5) represents a case where no masking d i f f e r e n c e between the one and two component masker i s noted at l e v e l s up to and i n c l u d i n g 80 dB. The masking e f f e c t w ith f l used alone approximates that obtained f o r N s u b j e c t s . Perhaps the s l i g h t r e d u c t i o n i n quiet probe tone t h r e s h o l d r e l a t i v e to some of the other HL subjects i s enough to prevent an e f f e c t due to 2 f l - f 2 . Another p o s s i b i l i t y , however, may be that the quiet t h r e s h o l d f o r f 2 (50 dB) i s too poor to provide a 2 f l - f 2 l e v e l s i g n i f i c a n t to produce masking at the probe tone frequency. Again, the r e s u l t s at 90 dB are included to i n d i c a t e the p o s s i b i l i t y t h a t the added 10 dB i n f 2 l e v e l i s the boost needed to b r i n g about a masking e f f e c t due to 2 f l - f 2 . 92 Subject 17 (see Figure 16) i s r e p r e s e n t a t i v e of 5 out of the 20 HL subjects i n showing the f o l l o w i n g masking p a t t e r n s : 1) a masking e f f e c t due t o f l alone which was s i m i l a r t o t h a t of N s u b j e c t s ; 2) no d i f f e r e n c e i n masking between the one and two component masker ( a t t r i b u t a b l e to 2 f l - f 2 ) f o r f l , f 2 l e v e l s of 60 or 70 dB SPL; 3) an emergence of a masking d i f f e r e n c e between f l alone and f l + f 2 above 70 dB SPL. These 5 subjects were a l l c h a r a c t e r i z e d by a notched-shaped audiogram that d i s -played a good qui e t probe tone t h r e s h o l d and e x c e l l e n t high frequency hearing i n the t e s t ear. The q u i e t t h r e s h o l d of f l was r e l a t i v e l y poor. The r e s u l t s from these subjects are p a r t i c u -l a r l y i n t e r e s t i n g i n l i g h t of Sachs and Wightman's ( 1 9 7 5 ) model which p r e d i c t s a r o l e of frequency l o c a t i o n s more basal to that of the primary tones i n c o n t r i b u t i n g s p e c t r a l energy and pro-ducing the combination tone. Results f o r these f i v e s u b j e c t s , whose thresholds at 6 and 8 kHz approximate the range of normal hearing s e n s i t i v i t y , lend greater credence to Sachs and Wightman's model of combination tone production. There are s e v e r a l f a c t o r s which make i t very d i f f i c u l t to r e c o n c i l e the r e s u l t s of the present experiment w i t h any one model of combination tone production. A s i g n i f i c a n t aspect i s the f a c t that no one i s sure of the exact morphological changes which occur even i n cases of l o c a l i z e d noise-induced hearing l o s s (as u t i l i z e d here) and the e f f e c t s such morpholo-g i c a l damage may have i n psychoacoustic tasks which i n v o l v e every a u d i t o r y l e v e l c e n t r a l to the damaged p e r i p h e r a l l o c u s . I t i s agreed that noise-induced hearing l o s s r e s u l t s , i n outer 93 h a i r c e l l r eceptor damage. Yet, what i s the e f f e c t upon the inner h a i r c e l l s , from which 95 per cent of a l l h a i r c e l l r e c o r d -ings are made? What e f f e c t does damage at the periphery have upon more c e n t r a l p o r t i o n s of the a u d i t o r y system? In s h o r t , "because noise-induced hearing,,loss i s s t i l l not w e l l understood, i t i s unwarranted to o f f e r any s p e c i f i c hypothesis as to how combination tone generation may occur i n hearing-impaired s u b j e c t s . Another important aspect i n v o l v e s a f a c t that has been discussed i n great d e t a i l i n Chapter 1 , that i s , combination tone generation i s s t i l l p o o r l y understood i n normal hearing s u b j e c t s . The mechanism behind other n o n - l i n e a r processes such as suppression, which i s probably i n t r i c a t e l y i n v o l v e d i n d i s -t o r t i o n product generation, has not been adequately modelled or explained to date. Thus i t becomes even more complicated to e x p l a i n how such a phenomenon as suppression works i n p a t h o l o g i c a l cases and what t h i s means to the production of combination tones. These l i m i t a t i o n s were recognized before and during the course of the present study. Nevertheless, i t i s b e l i e v e d that the data gathered i n t h i s experiment are valuable i n broadening the base of knowledge about 2 f l - f 2 as w e l l as forward masking, both i n normal hearing and hearing-impaired s u b j e c t s . The number of subjects t e s t e d was very l a r g e i n comparison to almost a l l p u blished data to date. As w e l l , the present study gave, f o r these s u b j e c t s , estimates of the growth of masking f u n c t i o n w i t h i n c r e a s i n g stimulus l e v e l of the p r i m a r i e s , i n f o r m a t i o n that has been l a c k i n g i n published accounts of the behavior of 2 f l - f 2 . 9 4 The f a c t that the present study demonstrated the presence of 2 f l - f 2 i n hearing-impaired subjects and was able to document i t s growth i n r e l a t i o n to the l e v e l of f l and f2 and i t s depen-dence upon audiometric c o n f i g u r a t i o n suggest the need to pursue the study of 2 f l - f 2 i n cases of noise-induced hearing l o s s . An expansion of the present study i s warranted, one f o r example, i n which a reference masker could be used to estimate the l e v e l of 2 f l - f 2 ; Continued i n v e s t i g a t i o n of combination tone-percep-t i o n i n hearing-impaired subjects may e v e n t u a l l y lead to an e l u c i d a t i o n of not only the mechanism of d i s t o r t i o n product generation but may a l s o help us to b e t t e r understand the physio-l o g i c and psychoacoustic c o r r e l a t e s of noise-induced hearing l o s s . 5-3 Summary The major r e s u l t s of t h i s study may be summarized as f o l l o w s : 1. The p a t t e r n of forward masking produced by f l alone f o r N sub j e c t s was c o n s i s t e n t w i t h r e s u l t s reported i n the l i t e r a t u r e . Most N subjects f i r s t evidenced masking due to f l at a l e v e l between 40 and 60 dB SPL ( r e p r e s e n t i n g 30 to 50 dB SPL). Average slopes of the growth of masking w i t h f l i n t e n s i t y were approximately 0.3-2. Two trends emerged i n the forward masking e f f e c t of f l f o r HL su b j e c t s . Some subjects showed a reduced or minimal forward masking e f f e c t w i t h f l , most probably due to elevated t h r e s h o l d f o r f l and/or the probe tone, as i n d i c a t e d by c o r r e l -a t i o n c o e f f i c i e n t s which showed a s i g n i f i c a n t negative c o r r e l a t i o n between the masking by f l and probe and f 1 quiet t h r e s h o l d s . Some HL s u b j e c t s , however, showed masking equivalent to or 95 greater than N su b j e c t s . This may be due to d i f f e r e n c e s i n temporal processing as a r e s u l t of the pathology. 3- The presence of the combination tone was evidenced f o r a l l N subjects f o r at l e a s t one i n t e n s i t y of the f l + f 2 s t i m u l u s , as i n d i c a t e d by the increased masking e f f e c t of f l + f 2 over f l presented alone. Average slopes of the growth of masking ( 0 . 8 f o r 60-70 dB; 1.1 f o r 70-80 dB) match those reported i n the l i t e r a t u r e u s i n g the c a n c e l l a t i o n technique r a t h e r than non-simultaneous (e.g. p u l s a t i o n threshold) methods. The comparison of these slopes may not be v a l i d , however, since the present study measured combination tone masking e f f e c t s and d i d not estimate l e v e l s . 4. The r e s u l t s f o r lower f l + f 2 i n t e n s i t i e s w i t h the HL subjects matched those reported i n the l i t e r a t u r e (Smoorenburg, 197 2 ; Sachs, 1 9 7 5 ) : there was l i t t l e or no masking e f f e c t due to f l + f 2 because quiet thresholds at the f l , f 2 , as w e l l as probe tone frequencies were elevated. At an f l + f 2 l e v e l of 80 dB, however, s e v e r a l HL subjects showed a masking e f f e c t due to the f l + f 2 s timulus. P o s s i b l e explanations f o r t h i s are d i s -cussed by l o o k i n g at data from i n d i v i d u a l s u b j e c t s . 5- R e s u l t s obtained f o r each of the two experimental c o n d i t i o n s ( f l alone and f l + f 2 ) f o r the two subject groups showed wide v a r i a b i l i t y among s u b j e c t s . Such wide v a r i a t i o n i n r e s u l t s has a l s o been reported i n many other s t u d i e s of com-b i n a t i o n tones. 6. The unresolved r o l e of suppression and i t s mechanism of generation as w e l l as a l i m i t e d understanding of the e f f e c t s 96 of noise-induced hearing l o s s make i t d i f f i c u l t to use the present r e s u l t s i n any p a r t i c u l a r model attempting to e x p l a i n combination tone production. The present results.- point to the need f o r f u r t h e r i n v e s t i g a t i o n of the phenomenon of combination tones and the forward masking e f f e c t s of such tones i n hearing-impaired as w e l l as normal hearing s u b j e c t s . 97 REFERENCES B e a t t i e , R.C., Pappas, N.J., and S m i a r o w s k i , R.A. Forward masking: I n t e r a c t i o n of probe d u r a t i o n and mask d u r a t i o n . J o u r n a l of A u d i t o r y Research, 1974, '14, 263-272. Bekesy, G von Experiments i n H e a r i n g . New York: McGraw H i l l Book Company, I960. Buunen, T.J.F., t e n Kat e , J.H., Raatgever, J . , and B i l s e n , F.A. 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L e w i s , D., and L a r s e n , M.J. The c a n c e l l a t i o n , r e i n f o r c e m e n t and measurement of s u b j e c t i v e t o n e s . P r o c e e d i n g s of the N a t i o n a l Academy of S c i e n c e s , 1937, 23, 415-421. L u t f i , R.A.-, and Y o s t , W.A. Measurement of the 2 f l - f 2 c u b i c d i f f e r e n c e tone w i t h the b i n a u r a l masking l e v e l d i f f e r e n c e . J ^ A c o u s t , Soc. Am., 1981, 69, 2.16-222. 99 Moore, B.C.J. P s y c h o p h y s i c a l t u n i n g c u r v e s measured i n si m u l t a n e o u s and f o r w a r d masking. "' J . A c o u s t . Soc. Am. 1978, 63, 524-532. P f e i f f e r , R.R. A model f o r two-tone i n h i b i t i o n of s i n g l e c o c h l e a r -nerve f i b r e s . J . A c o u s t . Soc. Am. 1970, .48, 1373-1378. Plomp, R. D e t e c t a b i l i t y t h r e s h o l d f o r c o m b i n a t i o n t o n e s . ' J . A c o u s t . Soc. Am. 1965, 37, 1110-1123. Rhode, W.S. 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" C a p a c i t i v e probe measures of b a s i l a r membrane v i b r a t i o n . In H e a r i n g Theory, e d i t e d B. Cardozo. Eindhoven, The N e t h e r l a n d s : I n s t i t u t e f o r P e r c e p t i o n R e s e a r c h , 1972. W i l s o n , R.H., and J o h n s t o n e , J.R. B a s i l a r membrane c o r r e l a t e s of the c o m b i n a t i o n tone 2 f l - f 2 . N a t u r e , 1973, 241, 206-207. Z w i c k e r , E. Der ungewohnliche Amplitudengang der n i c h t l i n e a r e n V e r z e r r u n g e n des o h r e s . A c u s t i c a , 1955, 5., 67-74-Z w i c k e r , E. Der k u b i s c h e D i f f e r e n z t o n und d i e Erregungen des Gehors. A c u s t i c a , , 1968, 20, 206-209. Z w i c k e r , E., and F a s t i , H. Cubic d i f f e r e n c e sounds measured by t h r e s h o l d - a n d compensation-method. A c u s t i c a , 1973, 29, 336-343. Z w i c k e r , E. Dependence of l e v e l and phase of the ( 2 f l - f 2 ) -c a n c e l l a t i o n tone on f r e q u e n c y range, f r e q u e n c y d i f f e r e n c e , l e v e l of p r i m a r i e s , and s u b j e c t . J . A c o u s t . Soc. Am., 1981, 70, 1277-1288. 

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