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Relationship between otoadmittance and threshold measurements in a TTS paradigm with phonation Andrews, Virginia Anathalie Taylor 1973

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RELATIONSHIP BETWEEN OTOADMITTANCE AND THRESHOLD MEASUREMENTS IN A TTS PARADIGM WITH PHONATION by VIRGINIA ANATHALIE TAYLOR ANDREWS BSc. University of B r i t i s h Columbia, 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF -MASTER OF SCIENCE i n the D i v i s i o n of Audiology and Speech Sciences i n the Department of Paediatrics We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July, 1973 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of PAEDIATRICS, DIVISION OF AUDIOLOGY AND SPEECH SCIENCES The University of British Columbia Vancouver 8, Canada Abstract This i n v e s t i g a t i o n studies the r o l e of the middle ear muscles i n the TTS reduction that occurs when phonation accompanies exposure to a high i n t e n s i t y low frequency pure tone. Changes i n acoustic admittance (taken as a measure of middle ear. muscle a c t i v i t y ) were compared with changes i n TTS, recorded under s i m i l a r experimental conditions. The TTS paradigm consisted of measuring subjects' hearing thresholds before and a f t e r 5 minute exposure to a 500 Hz, 117.5 dB SPL tone, accompanied or not by phonation (humming). The paradigm was repeated with threshold measurement being replaced by otoadmittance measurement; i n t h i s case admittance changes were recorded before, during, and a f t e r the fatigue exposure. The r e s u l t s show that TTS from the exposure tone with phonation was s i g n i f i c a n t l y less than TTS from the exposure tone with no phonation. The e f f e c t of phonation on TTS was most s i g n i f i c a n t at early post-exposure times. No s i g n i f i c a n t TTS differences between males and females were found. Changes i n the two admittance components at the beginning and at the end of exposure were s i g n i f i c a n t l y larger when phonation accompanied the exposure than when not. This f i n d i n g suggests that more middle ear muscle a c t i v i t y occurs when phonation accompanies exposure than when no phonation i s performed. Most admittance measurements did not correlate s i g n i -f i c a n t l y with any of the TTS measurements. The only s i g n i f i c a n t correlations indicated that the smaller the middle ear muscle a c t i v i t y r e s u l t i n g from the fatigue exposure alone, the larger the amount of protection provided by phonation, as measured by differences between TTS values at early post-exposure times between the two conditions. This f i n d i n g suggests that most indivi d u a l s may have middle ear muscles that contract weakly i n response to intense acoustic stimula-t i o n alone but that these muscles contract s i g n i f i c a n t l y when phonation accompanies the acoustic stimulation. Thus, phona-t i o n provides considerable protection of the ear from the 5 0 0 Hz fatigue tone, as shown by the reduced TTS when phona-t i o n accompanies exposure. The r e s u l t s also suggest that the middle ear muscles are a major factor i n reduced TTS with phonation but other mechanisms such as i n e f f i c i e n t stapes v i b r a t i o n and attentional factors may also be involved. More research i s necessary to determine the exact r o l e each mechan-ism plays i n the reduction of TTS with phonation. TABLE OP CONTENTS Page ABSTRACT , . ±± LXST OF TABLiii> . . , . . , « « . . . « vi. LIST OF FIGURES . . . . . . . . . . . . v i i Chapter 1. INTRODUCTION 1 Chapter 2 . REVIEW OF THE LITERATURE . . . . . . . 4 2 .0 Introduction 4 2.1 E f f e c t of Phonation on TTS. . . 4 2 C 11 TTS . . 4 • 2.12 Phonation and TTS . . . . 6 2 .2 E f f e c t of Phonation on Sound Transmission i n the Middle Ear. 9 2.21 The Middle Ear Muscles (MEM) . •. 9 Anatomy 9 MEM A c t i v i t y 11 MEM A c t i v i t y During Phonation 14 E f f e c t of MEM A c t i v i t y on Sound Trans-mission. . . . . . . 15 2„22 Stapes V i b r a t i o n , 0 , , , 18 2.3 Control of Possible Mechanisms Involved i n TTS Reduction.'. 20 2.31 Control of MEM Contrac-t i o n During. Phonation . , 20 2«32 Control of Stapes Vibra-t i o n During Phonation e . 21 2.33 Attention Factors and TTS Reduction . . ... . . '22 - i v --V-Page Chapter 3 . AIMS OF THE INVESTIGATION . . 24 Chapter 4. EXPERIMENTAL APPARATUS AND PROCEDURES, . . 25 4 . 1 Experimental Apparatus. . . 25 4.11 TTS Instrumentation 25 4 .12 Otoadmittance Instrumentation . 27 4 .13 C a l i b r a t i o n 28 4.2 Subjects. . . . . . . . . . JO 4 . 3 Experiments . . . . . . . . . . . . . 51 4 .31 Experimental Design . . . . . . 3-1 4 . 3 2 Procedures. . . . . . . . . . . 34 TTS Sessions. . .' . . . . . . . 54 Otoadmittance Sessions. . . . . 55 . 4 ,33 Data Measurement. . . . . . . . . 56 TTS Measurement 56 Otoadmittance Measurement . . . 59 Chapter 5 . RESULTS. 42 5.1 TTS Data 42 5.2 Otoadmittance Data. . . . . 48 5.3 Comparison of TTS and Otoadmittance Data 49 Chapter 5 . DISCUSSION . . . 57 REFERENCES . . . . . . . . . . . . . . . . 67 APPENDIX . . . . . . . . . . . e . . . . . 71 L i s t of Tables Table Page 1 . Summary of ANOVA. TTS as a function off sex, post-exposure time ( 0 ' \ 7 . 5 " , 15"» 3 0 % 2*, V ) , and condition (N^,. H^) for 7 BKS£1?S. and 7 female subjects. . < > . , . . . . . » e. i« . . . 4-3 2 a . Results of Neuman-Keuls test f o r signiflasaace of differences between TTS f o r 7 p0st--«a'grasure times (7 male and 7 female subjects),™ » « . . . 4 - 5 2 b . Results of Neuman-Keuls te s t for si g n i f i c a n c e of differences between TTS f o r N T and 1^, conditions or . . . . a . . . . . . . , 4-7 2 c . Results of Neuman-Keuls te s t f o r signlliccrmce of differences between TTS f o r conditierres Hm and Hrr, at the s p e c i f i e d post-exposure Wmvs, . . 4-7 3 . Matrix of Results £>f Pearson Product-Moment Correlations between s p e c i f i e d TTS and (fife-admittance values. . . * ». «« i* • . . 51 - v i -L i s t of Figures  Figure Page 2.1 Schematic diagram of the tensor tympani .muscleo . , . . o c . . . . . . . . 10 2.2 Schematic diagram of middle ear ligaments and stapedius muscle. . . . . . . . . . . . . . . . . 1 0 2.3 Movement of the stapes, a) Normal stapes motion i n response to air-borne sound, b) Movement around the long axis of the stapes (possible during phonation) r e s u l t i n g i n reduced displace-ment of cochlear f l u i d , . . . . . . . . . . . . . . o^, 4 . 1 Block diagram of instrumentation for TTS procedures. . . . . . . . . . . . . . . 25 4 . 2 Block diagram of instrumentation for Otoadmittance procedures. . . . . . . . . . . . . . . . . . . 2^ 4 . 3 Experimental design and parameters. A) Design and parameters of TTS procedures. . . ^2 B) Design and parameters of Otoadmittance procedures . . . . . . . . ^2 4 . 4 Record of a representative TTS procedure (condition 4 . 5 Example photograph of otoadmittance pl o t s (condition N Q ) . 40 4 . 6 Representative schematic of otoadmittance p l o t s . . 40 5 .1 Comparison of TTS (measured at 700 Hz) at 6 d i f f e r e n t post-exposure times f o r conditions N T and H T. . „ „ . „ 44 5.2 Comparison of TTS difference values (15" post-exposure with i n i t i a l change i n G^ during exposure . o . . . . . . . . . c,2 5 . 3 Comparison of TTS difference values (15" post-exposure) with f i n a l change in G^ during GXpO SUITS e » o i r o o 0 e t o o » « 0 * e « 6 o « « ^ 5.4 Comparison of TTS difference values (15" post- . exposure) with i n i t i a l change i n during e x p o s u r e . , . . . . . . . . . . . « . * . . . . . ^4 - v i i -- v i i i -Figure Z§££ 5,5 Comparison of TTS difference values (15" post-exposure) with i n i t i a l change .in Y N during exoosure, . • o , , , t , , • • c « . . » • • • • 55 Chapter 1 Introduction Each time a person talks aloud he i s constantly dependent on h i s auditory system to monitor his vocal output. Thus speaking i s influenced by hearing but, as recent e v i -dence suggests, hearing i s also influenced by speaking. I t i s known that i f we speak while l i s t e n i n g to a sound,the sound we hear i s alt e r e d . But how does t h i s occur? What mechanisms come into play during phonation and how do they change the sound that enters the ear? When the human ear i s exposed to any sound the acoustic energy of the sound tra v e l s from the external audi-tory meatus through the tympanic membrane, middle ear struc-tures and cochlear f l u i d to the hair c e l l s of the Organ of C o r t i . At the hair c e l l s the acoustic energy i s transformed into e l e c t r i c a l energy which i s transmitted along the audi-tory nerve and higher order neurons to the cortex. This transmission of sound energy i s influenced by the frequency, i n t e n s i t y , and duration of the sound to which the ear i s exposed. I t has been shown ( B e l l and Fairbanks, 1963) that a 60 second exposure to as low an i n t e n s i t y tone as 10 dB SL at 1, 2 , or 4 kHz can s i g n i f i c a n t l y r a i s e the post-exposure behavioural threshold for a tone of the same frequency. Such a post-exposure temporary threshold s h i f t (henceforth TTS) -1--2-i s a function of the i n t e n s i t y , duration, and frequency of the exposure tone. Thus, a high i n t e n s i t y exposure tone w i l l produce a greater TTS than a low i n t e n s i t y exposure tone. A recent study (McBay, 1971) has shown that TTS produced by a f i v e minute exposure to a 118 dB SPL 5 0 0 Hz tone i s reduced i f the l i s t e n e r phonates (hums) during the exposure. Such r e s u l t s i ndicate that i f the TTS i s decreased when phonation occurs during exposure, then phonation might attenuate trans-mission of the exposure tone to the inner ear. I f l e s s acous-t i c energy reaches the inner ear,less fatigue of the hair c e l l s must occur, thus TTS due to exposure i s reduced. I t i s not c l e a r , however, what factors cause transmission of sound to be attenuated. -,T.he sound may be attenuated by the contraction of middle ear muscles (henceforth MEM), which occurs during or just p r i o r to phonation (Djupesland, 1964, 1967; Salomon and Starr, 1963» Shearer and Simmons, 1965)1 or the sound transmission may be altered by a change i n the mode of stapes v i b r a t i o n , p o s s i b l y r e s u l t i n g from a change i n the d i r e c t i o n of s k u l l v i b r a t i o n that occurs during phonation (Bekesy, I 9 6 0 , p.201; c f . sec. 2 . 2 2 ) . I f , during phonation, some or a l l of the attenuation of sound transmission i s due to MEM contraction, how i s the action of these muscles controlled? I t i s possible that the MEM are d i r e c t l y influenced by c o r t i c a l control or i t may be that the MEM are activated r e f l e x i v e l y by a c t i v i t y of the larynx during and just p r i o r to phonation. - 5 -The general objective of t h i s study i s to investigate one of the factors that may be responsible for reduction i n TTS i f phonation accompanies exposure to a high i n t e n s i t y tone. 'More s p e c i f i c a l l y , the r o l e of the MEM i n the reduction of TTS w i l l be investigated. Using acoustic admittance as a measure of MEM a c t i v i t y , changes i n admittance and changes i n TTS, recorded under s i m i l a r experimental conditions, w i l l be compared to reveal possible c o r r e l a t i o n s between MEM a c t i v i t y and TTS. Chapter 2 Review of the L i t e r a t u r e 2«0 Introduction Section 2.1 includes a discussion of the TTS phenomenon and a review of experiments that show some eff e c t s of phonation on TTS. Section 2.2 discusses the e f f e c t of phonation on sound transmission i n the middle ear. Included are subsections on a c t i v i t y of the middle ear muscles and on v i b r a t i o n of the stapes. Although only i n d i r e c t l y relevant to t h i s study a f i n a l section, which examines the possible c o n t r o l mechanisms involved i n TTS reduction, i s presented. Included i n Section 2.3 are subsections on control of MEM ac-ti-vi-ty -and stapes v i b r a t i o n and on c e n t r a l factors i n f l u -encing auditory fatigue. 2.1 E f f e c t of Phonation on TTS 2.11 TTS I f a l i s t e n e r i s exposed to any sound of s u f f i c i e n t i n t e n s i t y and duration his ears" post-exposure s e n s i t i v i t y w i l l be a l t e r e d . This change i n s e n s i t i v i t y may be measured i n terms of a temporary s h i f t i n absolute hearing threshold (TTS). This TTS or post-stimulatory auditory fatigue i s "usually, but not always, a decrease i n threshold sensi-t i v i t y . " (Ward, 1963i p.241) Measurement of TTS requires determination of pre-exposure threshold, followed by exposure of the same ear to the f a t i g u i n g stimulus, a f t e r which the post-exposure threshold -4- ' - 5 -of that ear i s measured. TTS i s the dif f e r e n c e , i n dB, bet-ween the post- and pre-exposure thresholds. TTS that follows a pure tone fatigue stimulus generally increases with the duration and frequency of the stimulus u n t i l a l i m i t i n thresh-o l d s h i f t i s reached (Ward, 1963* Botsford, 1971) . I t i s generally accepted that TTS varies d i r e c t l y with i n t e n s i t y of the pure tone fatigue stimulus ( i e . as i n t e n s i t y increases TTS becomes l a r g e r ) . This occurs even for i n t e n s i t i e s below 70 dB SL i f TTS i s measured within 5 seconds of exposure cessation ( B e l l and Fairbanks, 1963)« The frequency of the pre- and post-exposure t e s t tone also influences TTS as does the post-exposure time at which threshold of the tone i s measured. For exposure tones of l e s s than 80 dB SPL the r e s u l t i n g TTS i s maximum at the frequency of the exposure. TTS from such lower i n t e n s i t y s t i m u l i i s of short duration, thus measurement roust be taken s h o r t l y a f t e r cessation of the exposure tone. Higher inten-s i t y stimulation (80 dB SPL and above) produces longer l a s t i n g TTS with a maximum value one-half octave or more above the exposure frequency (Davis, et a l , 1950; Epstein and Schubert, 1957; B e l l and Fairbanks, 1963; Rodda, 1964) . A f t e r cessation of the exposure tone TTS gradually decreases, i n , roughly, an exponential fashion, r a p i d l y i n the f i r s t few seconds then gradually to zero ( i e . the threshold becomes equal to the pre-exposure va l u e ) . This recovery occurs, f o r each subject, at a f a i r l y constant rate which does not -6-seem to depend on the parameters of the fatigue stimulus (Ward, 1 9 6 3 ) . There i s great i n t e r - s u b j e c t v a r i a b i l i t y i n the magnitude of TTS, produced by a given exposure, which seems dependent on i n d i v i d u a l s u s c e p t i b i l i t y to auditory fatigue and not r e l a t e d to differences i n auditory threshold at the exposure frequency (Ward, 1963) . Intra-subject v a r i a t i o n i n TTS magnitude produced by a s p e c i f i c exposure i s , however, i n s i g n i f i c a n t . Riach, et a l . (1964) investigated i n d i v i d u a l s u s c e p t i b i l i t y to auditory fatigue by g i v i n g 12 subjects the same high i n t e n s i t y pure tone exposure 20 times over a period of weeks. No s i g n i f i c a n t change i n the pre-exposure threshold (at 2800 Hz) was noted during the sessions but there was a "trend toward a smaller TTS at the one minute post-exposure point." (Riach, et a l . , 1964, p.1195) Nixon and G l o r i g (1962) also looked at s u s c e p t i b i l i t y to auditory f a t i g u e . They warned that TTS experiments must allow f o r the f a c t that i f the ear does not have recovery time between exposures but i s subjected to repeated f a t i g u i n g s t i m u l a t i o n before recovery has occured, cumulative e f f e c t s may occur and lead to permanent s h i f t s i n hearing threshold. 2 .12 Phonation and TTS Recent inves t i g a t i o n s (Karlovich and Luterman, 1969i 1970; Luterman and Karlovich, 1969; McBay, 1971? Benguerel and KcBay, 1972) have shown that phonation appears to a l t e r sound transmission i n the auditory system. K a r l o v i c h and Luterman (1969) exposed four normal hearing - 7 -subjects to a 4000 Hz 90 dB SL f a t i g u i n g tone for three minutes. The subjects tracked t h e i r thresholds at 5656 Hz f o r two minutes before and three minutes a f t e r exposure. During exposure the subjects either read a set passage aloud or read i t s i l e n t l y . Post-exposure TTS was found to be con-s i s t e n t l y greater when the subjects read aloud than when they read s i l e n t l y during exposure. Results suggest that, during the reading aloud a c t i v i t y , transmission of a 4000 Hz tone i s enhanced. In a l a t e r study, Luterman and Karlovich (1969) found that when subjects, exposed to a 2000 Hz 90 dB SL tone, read aloud during exposure they obtained consistently l e s s TTS than i f they read s i l e n t l y , read s i l e n t l y while ar-t i c u l a t i n g , or engaged i n rev e r i e during exposure. These r e s u l t s suggest that, during the reading aloud a c t i v i t y , , transmission of a 2000 Hz tone to the cochlea i s reduced. A t h i r d study by these experimenters exposed sub-ject s to a 1000 Hz 110 dB SPL tone f o r three minutes during which the subjects either voiced or gestured-only the vowels / a / or / i / (Karlovich and Luterman, 1970). Post-exposure TTS at 1414 Hz was s i g n i f i c a n t l y l e s s i f voiced / a / or / i / accompanied the exposure than i f non-voiced gestures were performed. As with the 2000 Hz tone, these r e s u l t s imply that, i f v o i c i n g accompanies exposure, the transmission of a 1000 Hz tone i s attenuated, Karlovich and Luterman suggest that attenuation of low frequency tones during phonation may r e s u l t from MEM contraction, known to occur during phonation (Salomon and St a r r , 19631 Shearer and Simmons, 19651 Djupesland, -8-1 9 6 7 )i and/or from i n e f f i c i e n t stapes v i b r a t i o n during phona-t i o n (Bekesy, I 9 6 0 ) . Most rec e n t l y McBay and Benguerel (McBay, 1971? Benguerel and McBay, 1972) used TTS studies to investigate more c l o s e l y the e f f e c t of phonation on sound transmission i n the auditory system. They subjected l i s t e n e r s to a 500 Hz 118 dB SPL exposure tone for 5 minutes during which the l i s t e n e r hummed at s p e c i f i e d fundamental frequency and inten-s i t y , approximated the vocal f o l d s without voicing, l i s t e n e d to a recording of humming, performed a c t i v i t i e s to e l i c i t (non-acoustically) the MEM ref l e x e s , or sat q u i e t l y perform-i n g no task. Post-exposure TTS was measured by tracking thresholds at 700 Hz for 4 minutes a f t e r the exposure tone ended. They found that "TTS from the exposure tone accompanied by phonation (humming) was consistently and s i g n i f i c a n t l y l e s s than TTS from the exposure tone without any supplementary a c t i v i t y , " (McBay, 1971. p .107) The most s i g n i f i c a n t differences i n magnitude of TTS occurred when post-exposure threshold was measured 10 to 15 seconds a f t e r exposure cessation. S l i g h t decreases i n TTS were noted when c e r t a i n acoustic r e f l e x e l i c i t i n g movements accompanied exposure ( i e . repeated turning of the head, chewing, smiling f o r c e f u l l y , and swallowing). No s i g n i f i c a n t a l t e r a t i o n s i n TTS occurred when subjects l i s t e n e d to recorded humming or approximated the vocal f o l d s without humming during exposure. In addition, they found that phonation during exposure was - 9 -more e f f e c t i v e i n decreasing TTS f o r females than for males. This suggests that females may have more e f f i c i e n t mechanisms fo r attenuating low-frequency sound transmission to the cochlea during phonation than have males. This also supports the hypothesis that females may have more e f f i c i e n t MEM than males (Ward, 1 ° 6 6 ) . For a more detailed discussion of the e f f e c t of phonation on TTS see McBay (1971) . 2 .2 E f f e c t of Phonation on Sound Transmission i n the Middle Ear I t has been established ( c f . Section 2.1) that phonation during exposure to a f a t i g u i n g stimulus a l t e r s the post-exposure TTS and i t i s implied that phonation a l t e r s transmission of the f a t i g u i n g stimulus to the cochlea, the following discussion w i l l consider the possible mechanisms by which t h i s change i n middle ear sound transmission occurs. 2.21 The Middle Ear Muscles (MEM) Anatomy Of the two middle ear muscles, the larger i s the tensor tympani which i s about 25 mm i n length and 2 about 5«85 mm i n cross section. This muscle l i e s within a bony canal p a r a l l e l with and superior to the Eustachian tube. The tendon of the muscle passes through the poste r i o r opening of the canal and i s inserted on the manubrium just below the neck of the malleus, (see F i g . 2.1) On contraction the tensor tympani moves the malleus medially and a n t e r i o r l y , almost at r i g h t angles to the d i r e c t i o n of r o t a t i o n of the o s s i c l e s , thus increasing tension on the tympanic membrane. Innervation i s supplied by a branch of the trigeminal nerve -10-Tendon of tensor tymponi Cochleoriform process Tensor tymponi Septum canalis musculotubarii Auditory tube' Figure 2.1 Schematic diagram of the tensor tympani muscle. (from Zenilin,1968,p.383) Superior ligcment of lateral ligament , h e n } a l , e u s of the malleus Posterior ligament of the incus • Remains of the anterior ligament of the malleus Stapedius muscle Poster ior ligament Annular ligament of the stapes Anterior ligornen.t Figure 2.2 Schematic diagram of middle ear ligaments and stapedius m u s c l e ' (from Zemlin,1968,p.380) -11-(Jepsen, 1963? Djupesland, 1967; Zemlin, 19685 Mjiller, 1972) . The Stapedius muscle, 6 .3 mm i n length, i s the smallest muscle i n the human body. I t occupies a bony canal on the p o s t e r i o r v/all of the tympanic cav i t y and i t s tendon i n s e r t s at the posterior margin of the head of the stapes. (See F i g . 2 .2) Contraction draws the stapes p o s t e r i o r l y , at r i g h t angles to the d i r e c t i o n of movement of the o s s i c u l a r chain,thus a l t e r i n g the movement of the stapes footplate against the oval window. Innervation i s supplied by a branch of the f a c i a l nerve (Jepsen, 19635 Djupesland, 1967? Zemlin, 1 9 6 8 ) . MEM A c t i v i t y The two MEM contract i n opposition to each other but the r e s u l t of contraction i s a dampening of o s s i c u l a r movement and an increase of acoustic impedance at the tympanic membrane. Contraction can be e l i c i t e d i n a number of ways. A small percentage of i n d i v i d u a l s are able to v o l u n t a r i l y contract t h e i r MEM (Metz, 1951) Reger, i960? Jepsen, 1963? Zemlin, 1968) but, f o r most i n d i v i d u a l s , a c t i v i t y u s u a l l y r e s u l t s r e f l e x i v e l y from acoustic or non-acoustic s t i m u l i . Acoustic e l i c i t a t i o n of MEM r e f l e x contraction occurs when the ear i s presented with an acoustic stimulus of an i n t e n s i t y at or above the r e f l e x threshold. This threshold i s normally between 80 dB SL and 90 dB SL f o r pure tones of 125 Hz to 4000 Hz (Jepsen, 1963? M i l l e r , .196lb| Jerger, 1970) . -12-The acoustic r e f l e x (henceforth AR) i s b i l a t e r a l , thus i f a loud tone i s presented to one ear, r e f l e x contraction of MEM w i l l occur i n both ears. The r e f l e x centre i s thought to be the superior o l i v a r y nucleus of the pons, just v e ntral to the motor nucleus of the f a c i a l nerve, where efferent neurons of the r e f l e x are located, while the afferent neurons are located i n the dorsal and ventral cochlear n u c l e i (Jepsen, 1 9 6 3 ) . Higher frequencies appear to e l i c i t the AR at simi-l a r thresholds to lower frequencies (Jerger, 1970; Jepsen, 1963? Porter, 1972) . Complex signals such as random noise, however, are more e f f i c i e n t than pure tones i n e l i c i t i n g the AR. Peterson and Liden (1972) found that narrow bands or f u l l bands of white noise produced AR thresholds approximately 15 dB more s e n s i t i v e than thresholds from pure tones. The AR has a latency of about 45 to 150 msec depending on the frequency and i n t e n s i t y of the stimulus s i g -n a l (Zemlin, 1 9 6 8 ) . As the i n t e n s i t y of the signal i s i n -creased the degree of contraction of the stapedius muscle, i n p a r t i c u l a r , increases to a maximum. I f the sound continues, the contraction of MEM gradually decreases to a r e s t i n g l e v e l . I f a sound of a d i f f e r e n t frequency i s introduced a new con-t r a c t i o n r e s u l t s (Metz, 1951) . This f i n d i n g suggests that r e f l e x adaptation i s due to processes other than muscular f a t i g u e . Karlovich et a l . (1972) have shown that MEM contrac-t i o n , as measured by changes i n acoustic impedance, i s main-tained i f a tone or noise i s presented pulsed 12 dB above - 1 3 -the AR threshold. The same tone or noise, i f presented con-tinuously, was found to r e s u l t i n a decrease i n impedance ( i e . an adaptation of the r e f l e x ) . Such r e s u l t s suggest that a pulsed stimulus i s a more e f f e c t i v e a c t i v a t o r of MEM than i s a continuous stimulus. Frequency of stimulation may a f f e c t r e f l e x adaptation as shown by Brasher et a l (1969) who found that "AR stimulated by the 1000 Hz (octave band) noise did adapt more slowly than that from the 4000 Hz (octave band) noise." (Brasher, et a l , 1969. p.583) Several investigators have shown MEM re f l e x e s to be e l i c i t e d by non-acoustic procedures. Klockhoff and Anderson (1959) e l i c i t e d r e f l e x contraction of the stapedius muscle by pulsed e l e c t r i c a l stimulation of the external audi-tory canal. An a i r b l a s t directed toward the external meatus e l i c i t e d tensor tympani response (Klockhoff and Anderson, i 9 6 0 ) or response of both muscles (Djupesland, 1964) . A s i m i l a r b l a s t of a i r on the eyes resulted i n tensor tympani contraction as part of a s t a r t l e response (Klockhoff, 196l» Djupesland, I 9 6 7 ) . Touching the skin of the a u r i c l e s with a twist of cotton was found to produce stapedius muscle con-t r a c t i o n (Djupesland, 1967). Djupesland (1967) also found that voluntary movements, such as t i g h t closure of the eyes, swallowing, opening of the mouth, and clenching of the teeth, r e s u l t i n contraction of one or both tympanic muscles. L i f t i n g or turning the head also appeared to r e s u l t i n such contractions -14-(Salomon and S t a r r , 1 9 6 3 ) 0 For a more complete discussion of the l i t e r a t u r e dealing with acoustic and non-acoustic e l i c i t a -t i o n of MEM r e f l e x e s see Djupesland, ( 1 9 6 7 . p p . 1 8 - 2 4 ) . MEM A c t i v i t y During Phonation The occurrence of MEM contraction during speech a c t i v i t i e s has been found i n a number of recent i n v e s t i g a t i o n s . Salomon and Starr (1963) studied the electromyographic (EMG) a c t i v i t y of MEM i n two human subjects during various motor a c t i v i t i e s . As one subject began to t a l k or hum increased a c t i v i t y of his tensor tympani was r e g i s t e r e d . Some contractions occurred at onset of phonation while others occured 4 0 msec to 300 msec before onset of phonation. A l l contractions continued f o r up to 300 msec a f t e r cessation of phonation. Increased a c t i v i t y of the stapedius muscle during v o c a l i z a t i o n followed a s i m i l a r temporal pattern. In an EMG study with cats Simmons found that " i n v o c a l i s a t i o n , middle ear muscle contractions begin about 100 msec before actual sound i s produced and considerably outlast the speech ( s i c ) sound? the contractions do not appear to habituate; t h e i r magnitude i s proportional to the i n t e n s i t y of v o c a l i z a t i o n to be a n t i c i -pated." (Simmons, 1 9 6 4 , p . 7 7 3 ) Simmons investigated s i m i l a r phenomena with human subjects; however, he defined stapedius muscle a c t i v i t y as occurring when the subject's acoustic impedance at the ear-drum showed a s p e c i f i e d change, (Recall that when MEM con-t r a c t , they s t i f f e n the. o s s i c u l a r chain, thus increasing the t r a n s f e r impedance of the middle ear.) When Simmons' subjects said "one, two, three" impedance changes consistent with stapedius muscle a c t i v i t y were produced. In one ear, with a stapedius muscle p a r a l y s i s , speech resulted i n no impedance change (Simmons, 1964} Shearer and Simmons, 19&5). Shearer and Simmons (1965) found acoustic impedance changes (MEM a c t i v i t y ) to precede i n i t i a t i o n of phonation by 65 msec to 100 msec or to coincide with onset of phonation. To determine whether head and jaw movements were responsible for some of the change i n impedance the authors asked subjects to a r t i c u -l a t e words without producing voice. The n e g l i g i b l e impedance changes that resulted indicate that such muscle a c t i v i t i e s did not i n t e r f e r e with measurement of MEM a c t i v i t y . Djupesland (I967) recorded EMG a c t i v i t y from various muscles when h i s subjects spoke the words " j a " and "nei". A l l subjects showed increased a c t i v i t y i n the o r b i c u l a r i s o c u l i , stapedius and tensor tympani muscles. "This a c t i v i t y seemed to increase d i r e c t l y with the i n t e n s i t y of the speech." (Ibidem, p.80) A c t i v i t y of the tensor tympani began 30 msec to 450 msec before phonation was recorded and lasted up to 300 msec a f t e r phonation ceased. E f f e c t of MEM A c t i v i t y on Sound Transmission Evidence that MEM contraction has an e f f e c t on sound trans-mission through the middle ear i s found p r i m a r i l y i n animal studies, A number of investigators (Wersall, 1958? Weaver and Vernon, 1955? Simmons, 1959? and others c i t e d i n Jepsen, 1963i p.221) e i t h e r measured MEM a c t i v i t y d i r e c t l y or observed -16-e f f e c t s of the a c t i v i t y on cochlear microphonics. On the "basis of such studies i t i s now accepted that MEM contraction attenuates sounds below 1000 Hz by up to 20 dB (Jepsen, 1963; Brasher et a l , 1969), whereas i t either does not e f f e c t or enhances s l i g h t l y transmission of higher frequency sounds. Simmons (1959). a f t e r c u t t i n g either the stapedius or tensor tympani of cats, measured the cochlear microphonic response of that ear to intense sounds. He found that the microphonic disappeared only when the stapedius had been cut r thus concluded that the stapedius has a more important r o l e than the tensor tympani i n sound transmission. Recently, however, K e v a n i s h v i l i and Gvacharia (1972) measured the e f f e c t of tensor tympani contractions on sound transmission i n cats and found transmission of 5 0 0 Hz, 800 Hz, and 1000 Hz tones to be attenuated. Higher frequencies were not attenuated and conduction of 1800-Hz to 2000-Hz tones may even have been increased. The authors concluded that, although tensor tym-pani contraction does not a l t e r sound transmission to the same extent as does stapedius contraction, both muscles ap-pear to act i n a complementary fashion over the frequency range. Note, however, that findings i n cats may not p e r t a i n to human ears. D i r e c t investigations of transmission changes cannot e a s i l y be accomplished with humans^thus i n d i r e c t methods have been employed. Reger (I960) observed s h i f t s i n absolute hearing threshold before, during, and a f t e r subjects - 1 7 -v o l u n t a r i l y contracted t h e i r MHVI. He found that MEM' contrac-t i o n increased the threshoM of hearing for low frequency sounds (125 Hz to 1000 Hz) and that t h i s threshold s h i f t was greatest at 125 Hz and 250 Hz. Borg (1968) found, i n one subject, that a $00 Hz tone 20 dB above acoustic r e f l e x (AR) threshold was attenuated by 12 to% 15 dB by stapedius contraction while a 1450 Hz tone 16 dB above AR threshold was attenuated by only 0 to 6 dB. The r e f l e x had l i t t l e or no e f f e c t at frequencies above 2000 Hz and at moderate (lower) sound i n t e n s i t i e s . When MEM contract,the mobility of the o s s i c l e s changes, thus changing the acoustic.impedance at the eardrum as well as the transfer impedance (transmission character-i s t i c s ) of the middle ear, ^Measurement of impedance changes can, therefore, as mentioned above, give information about concurrent changes i n sound transmission. Metz (1951) obser-ved that reduced sound transmission during human MEM contrac-t i o n could be measured as a change i n impedance of the exter-n a l auditory canal, p r i n c i p a l l y as a reduction i n the absorption c o e f f i c i e n t with l i t t l e or no. change i n the phase (also c i t e d by Jepsen, 1963? M i l l e r , 1958) . M i l l e r (196lb) investigated impedance changes i n normal ears during acoustic e l i c i t a t i o n of the MEM r e f l e x . He found that a $00 Hz tone produced a greater change i n impedance than a 1500 Hz tone, which supports the hypothesis that MEM contraction attenuates transmission of low frequency -18-sounds more than that of higher frequency sounds. A study, i n which M i l l e r (1965) investigated admittance (inverse of impedance) changes produced by MEM contraction i n cats and r a b b i t s , showed that, on e l i c i t a t i o n of MEM contraction, these animals produced impedance changes s i m i l a r to those produced by humans. 2.22 Stapes V i b r a t i o n In addition to MEM contraction, a change i n the v i b r a t i o n mode of the stapes may also occur during phonation, B£k6sy ( i 9 6 0 ) has shown that when a sub-j e c t i s exposed to moderate i n t e n s i t y air-conducted s t i m u l i h i s stapes rotates around a v e r t i c a l axis (as i n F i g , 2 , 3 a ) . He hypothesized that during phonation the stapes movement changes to a r o t a t i o n around i t s long (horizontal) axis (as i n F i g . 2 . 3 b ) . This change would r e s u l t i n minimal d i s -placement of the cochlear f l u i d which, i n turn, would reduce e f f i c i e n c y of sound transmission to the cochlea (Bekesy, I 9 6 0 , pp.201-202.) As discussed i n Section 2 . 3 2 , MEM contraction may r e s u l t i n such al t e r e d stapes v i b r a t i o n . Since MEM contrac-t i o n occurs during phonation, both MEM contraction and altered stapes v i b r a t i o n may r e s u l t i n decreased e f f i c i e n c y of sound transmission to the cochlea. The r o l e each mechanism plays i n the reduction of sound transmission has, however, not yet been determined. - 1 9 -I M w I Figure 2.3 Movement of the stapes, a) Normal stapes motion in response to air-borne sound, b) Movement around the -long axis of the stapes (possible during phonation) resulting in reduced displacement of cochlear fluid. (from Bekesy,1960,p.202) -20-2 . 3 Control of Possible Mechanisms Involved i n TTS Reduction 2 . 3 1 Control o f MEM Contraction During Phonation The anatomy of the r e f l e x arc through which the MEM are activated i s incompletely known,, One portion of the arc appears to involve a c t i v i t y from the cochlear nerve through the dorsal and v e n t r a l cochlear nuclei to the supe'rior-olivary complex and f i n a l l y to the motor n u c l e i of the f a c i a l and trigeminal nerves which innervate the MEM. A l t e r n a t i v e pathways tlsrough the l a t e r a l lemniscus and the i n f e r i o r c o l l i c u l u s may also be involved ( M i l l e r , 1 9 7 2 ) . Since the MEM often contract just p r i o r to the s t a r t of phonation.^ i t seems l i k e l y that contraction i s neuro-l o g i c a l l y associated with laryngeal a c t i v i t y , such that the MEM are activated concurrently -with the' laryngeal musculature (Shearer and Simmons, 1 9 6 5 ) . McCall and Rabuzzi • ( 1 9 7 0 * 1 9 7 3 ) investigated the p o s s i b i l i t y that the MEM and laryngeal muscles are a c t i v i a t e d as part of a r e f l e x during phonation. Their r e s u l t s with cats demonstrated r e f l e x contractions of both MEM associated with contraction of the c r i c o t h y r o i d muscle of the larynx. • Approximation of the vocal f o l d s has been suggested as the movement necessary to e l i c i t r e f l e x MEM contraction. This a c t i v i t y , however, performed during exposure to a f a t i g u i n g tone, resulted i n TTS not s i g n i f i c a n t l y d i f f e r e n t from TTS a f t e r no a c t i v i t y during exposure (McBay, 1971). Ka r l o v i c h and Luterman (1970) compared TTS r e s u l t s between -21-conditions i n which vowels were 1) voiced or 2) whispered or gestured during the fatigue exposure. They found v o i c i n g to r e s u l t i n l e s s TTS than the whispered or gestured condition. I t , therefore, seems u n l i k e l y that vocal f o l d approximation e l i c i t s MEM contraction hut rather that "vo c a l - f o l d v i b r a t i o n i s the necessary c r i t i c a l f a c t o r f o r e l i c i t i n g the muscle contraction" (Karlovich and Luterman, 1970, p.516) In addition, higher areas of the c e n t r a l nervous system may d i r e c t the MEM to contract at about the same time as the laryngeal musculature but the neural pathways may not be organized as a d i r e c t r e f l e x between MEM and larynx. As Carmel and S t a r r (1963) suggested, MEM contraction i n association with phonation may simply be part of complex motor acts such as swallowing, yawning, etc. that involve many c r a n i a l nerves. Since i t i s known that c e r t a i n i n d i v i -duals can v o l u n t a r i l y contract t h e i r MEM (Metz, 1951t Reger, I960? Jepsen, 1963)1 i t should be remembered that higher c o r t i c a l influence over these muscles i s one of the control mechanisms and must be considered along with any r e f l e x con-t r o l that occurs during phonation. 2.32 Control of Stapes Vib r a t i o n During Phonation Control of the mode of v i b r a t i o n of the stapes during phona-t i o n should also be considered, Bekesy ( i 9 6 0 ) has shown that during exposure to moderate i n t e n s i t y air-conducted sounds, s k u l l v i b r a t i o n i s maximal i n a d i r e c t i o n p a r a l l e l to the auditory meatus. During phonation, v i b r a t i o n i n t h i s d i r e c t i o n -22-i s minimal and s k u l l v i b r a t i o n becomes maximal i n the v e r t i c a l d i r e c t i o n . This a l t e r a t i o n i n s k u l l v i b r a t i o n may change stapes v i b r a t i o n from that i n Figure 2 .3a to that i n Figure 2.3b (Beklsy, I 9 6 0 ) . A s i m i l a r s h i f t of r o t a t i o n a l axes, r e s u l t i n g i n l i m i t i n g transmission of excessive v i b r a t i o n s to the cochlea, i s known to occur during stimulation by intense low frequency soundSji^. 130 dB SPL or greater (Ward, 1962, 1 9 6 3 ) . I t i s also possible that contraction of MEM might s h i f t the v i b r a t i o n a l axis of the stapes. M i l l e r explained that "The stapes i s assumed to rotate around i t s lower (posterior) ligament because i t i s much s t i f f e r than the anterior ligament." ( M i l l e r , 1961a, p.l '69) Figure 2 .2 shows the anatomical positions of these ligaments. Contraction of the stapedius muscle may, as M i l l e r suggests, a l t e r r o t a t i o n of the stapes around the s t i f f e r , posterior annular ligament to a r o t a t i o n around a l i n e through both anter i o r and p o s t e r i o r annular ligaments. Such a change i n the axis of stapes v i b r a t i o n would, as previously discussed, reduce sound transmission to the cochlea. Thus, both MEM contraction and a l t e r e d s k u l l v i b r a -t i o n during phonation may cause i n e f f i c i e n t stapes v i b r a t i o n which, i n turn, may r e s u l t i n TTS reduction that occurs when phonation accompanies exposure. 2.33 Attention Factors and TTS Reduction A number of inv e s t i g a t o r s have studied the e f f e c t v a r i a t i o n s of mental -23-a c t i v i t y might have on auditory fatigue ( s p e c i f i c a l l y TTS). Studies by experimenters such as C o l l i n s and Capps (1965) and Fr i c k e (1966) suggest that c e r t a i n types of mental a c t i v i t y might reduce or i n t e n s i f y the f a t i g u i n g e f f e c t of an exposure tone, however, at present, i t i s too d i f f i c u l t to delineate the mental task assigned to a l i s t e n e r . Thus, the e f f e c t of attention factors on TTS experiments must be considered,even though the control of such variables i s not yet p o s s i b l e . Chapter 3 Aims of the Investigation The purpose of t h i s research i s to investigate the r o l e of the MEM i n the reduction of TTS that occurs i f phona-t i o n accompanies exposure to a low frequency tone. S p e c i f i c -a l l y , the aims are: 1) To investigate the e f f e c t on TTS ( r e s u l t i n g from a 5 minute exposure to a 500 Hz tone) of phonation (humming) during the exposure, hencet a) to compare changes i n TTS between humming and non-humming conditions. b) to compare the rate of TTS recovery between conditions. c) to determine i f these e f f e c t s are d i f f e r e n t i n males and females. 2) To investigate the e f f e c t on middle ear admittance of a 5 minute 500 Hz exposure tone accompanied or not by a humming a c t i v i t y . 3) To compare and attempt to correlate TTS r e s u l t s with admittance measurements, i n p a r t i c u l a r : a) changes i n TTS and changes i n admittance for each condition. b) rate of TTS recovery and rate of MEM relaxa t i o n (from admittance changes) f o r each condition. c) changes i n TTS and rate of MEM relaxation (from admittance changes) f o r each condition. -24-Chapter 4 Experimental Apparatus and Procedures .1 Experimental Apparatus 4.11 TTS Instrumentation Figure 4.1 shows a block diagram of the equipment used i n signal-generation and res-ponse-recording during TTS procedures. A Grason-Stadier Model E800 Bekesy Audiometer was used f o r the pre- and post-exposure threshold tracking. The attenuation rate, controlled by the s e t t i n g of the motor speed switch, was set at 5 dB/sec f o r sweep frequency audiograms and at 2 ,5 dB/sec f o r fixed frequency threshold tracings. Since a l l subjects had better than average hearing^the 20 dB fixed attenuation switch was used f o r each tracking procedure so the tracking record would remain on the graph. The continuous 500 Hz exposure tone was generated by Channel 1 frequency o s c i l l a t o r of a Madsen Model OB 60 Audiometer. In each experimental session, the 5 dB i n t e r v a l step attenuator was set at the maximum output s e t t i n g of 110 dB (=117.5 dB SPL). Outputs of both the Bekesy and Madsen audiometers were sent to one set of earphones v i a an external switch box that allowed each output to be sent to either the r i g h t or the l e f t earphone. The earphones were Madsen Model TDH-39 i n MX-41/AR cushions, set i n insulated p l a s t i c mountings on a light-weight headband. In addition, a Bruel and Kjaer Type 2203 P r e c i s i o n Sound Level Meter with Type 1613 Octave F i l t e r was set up i n - 2 5 -] THRESHOLD TRACKING EXPOSURE TONE OSCILLATOR INTERRUPTER SPEAKER RECORDING ATTENUATOR 1 SWITCH BOX OSCILLATOR STEP MIXER ATTENUATOR J WINDOW AMPLIFIER LIGHT SUBJECT CONTROL SWITCH SUBJECT MICROPHONE SOUND LEVEL METER I ro I CONTROL ROOM TEST ROOM Figure 4.1 Block diagram of instrumentation for TTS procedures. - 2 7 -the t e s t i-oom to provide subjects with v i s u a l feedback for maintaining constant i n t e n s i t y during the humming condition. As shown i n Figure 4.1, the signal-generating and response-recording equipment was in-the control room ;while the subject, control switch, and sound l e v e l meter were i n the sound treated test room. A window permitted observation of the subject by the experimenter and a switch i n the control room permitted external control of the l i g h t i n the test room. The experimenter could monitor subject a c t i v i t y v i a a micro-phone i n the t e s t room connected to an amplifier and speaker i n the c o n t r o l room. 4.12 Otoadmittance Instrumentation The* Grason-Stadler Model 1720 Otoadmittance Meter was chosen for t h i s part of the study-. (The r e c i p r o c a l of acoustic impedance w i l l hence-f o r t h be r e f e r r e d to as acoustic admittance.) One advantage of the Otoadmittance Meter i s that i t allows separate measure-ment of the two admittance components, the conductance G and the susceptance B. A second advantage i s that i t permits easier computation of ear drum admittance values than i f impedance i s used. For example; i f we look at the equivalent e l e c t r i c a l c i r c u i t of the combination "ear drum + ear canal," the l a t t e r i s represented by a p a r a l l e l branch. Knowing the value of the whole, one wants to obtain that of the ear drum alone. Using admittances, t h i s can be done by a simple subtraction, whereas with impedances the c a l c u l a t i o n i s more cumbersome. For a discussion of the p r i n c i p a l components of the bridge and for a review of impedance and -28-admittance relationships^see Grason-Stadler Otoadmittance Handbook 2 ( 1 9 7 3 ) . The t o t a l G and 3 of the ear were amplified, low pass f i l t e r e d and recorded on a H.P. 3960 Instrumentation Recorder. This was played back v i a an A to D converter into a PDP-12 D i g i t a l Computer for an a l y s i s . The sign a l was sampled at approximately 1 Hz, appropriately scaled, displayed on a Tetronix Type $6kB Oscilloscope, and was photographically recorded with a Polaroid camera (See F i g . 4 . 2 ) , During otoadmittance procedures, the exposure tone was provided by an Interstate E l e c t r o n i c s Function Generator Type F 3 3 . The exposure tone reached the ear v i a a TDH-49 earphone attached to the headband that also held the otoadmit-tance probe (See F i g . 4 . 2 ) . 4.13 C a l i b r a t i o n Before experimentation began the frequency responses of the earphones were determined using a Briiel and Kjaer Type 2203 P r e c i s i o n Sound Level Meter with Type 1613 Octave F i l t e r , a Briiel and Kjaer Type 4152 A r t i f i c i a l Ear with a standard 6 c.c. NBS Coupler, a Briiel and Kjaer Type 1022 Beat Frequency O s c i l l a t o r , and a Briiel and Kjaer Type 2305 Level Recorder, The acoustic outputs of the exposure tone producing oscillator-earphone units were also determined with the above sound l e v e l meter and a r t i f i -c i a l ear. The i n t e n s i t y of the exposure tone at the earphone was recorded each day of data c o l l e c t i o n . The mean of d a i l y EARPHONE PROBE BEAT FREQUENCY OSCILLATOR ATTENUATOR I J II / i OTOADMITTANCE METER FILTERS and TAPE RECORDER 1 RECORDING INSTRUMENTATION TAPE RECORDER ATTENUATOR A to D CONVERTER PDP-12 COMPUTER OSCILLOSCOPE and CAMERA PLAYBACK and ANALYSIS INSTRUMENTATION Figure 4.2 Block diagram of instrumentation for Otoadmittance procedures. - 3 0 -measurements (3*0 during the TTS sessions was 117.5 dB SPL (SD=0.28j Range=117.1 dB to 118 .0 dB). The exposure tone of the otoadmittance sessions was- manually adjustable thus was r o u t i n e l y set at 117.5 dB SPL. Other d a i l y measurements included: i n t e n s i t y c a l i b r a t i o n of the Bekesy audiometer (500 Hz reference)? c a l i b r a t i o n of the c i r c u i t used to amplify G and B signals; and c a l i b r a t i o n of the Otoadmittance Meter using the 1720-1002 t e s t c a v i t i e s . The means of the d a i l y t e s t c a v i t y measurements, using the 660 Hz probe tone, were: B(mmhos) G(mmhos) Large Cavity 9.25 (SD=0.14) 0.62 (SD=0.07) Small Cavity 1.58 (SD=0.02) 0.80 (SD=0) Background noise i n the te s t rooms, measured with the Briiel and Kjaer equipment before and a f t e r data c o l l e c t i o n , was found to be 29 dBA SPL i n the room used f o r TTS procedures and 27 dBA SPL i n the room used f o r otoadmittance procedures. D a i l y octave band analysis showed that frequencies below 250 Hz were the main components of t h i s noise. The frequencies 500 Hz to 16 ,000 Hz never exceeded 28 dBA SPL and 22 dBASPL i n the TTS t e s t room and the otoadmittance te s t room respec-t i v e l y . 4 . 2 Subjects The subjects were 7 male and 7 female unpaid volun-teers between 19 and 33 years of age. A l l subjects had normal hearing as shown byt no his t o r y of ear pathology; normal middle ear function and acoustic reflexes recorded from a Madsen Model 2070 E l e c t r a c o u s t i c Impedance Bridge; and normal pure tone air-conduction hearing thresholds (better than 25 dB ISO I964) between 250 Hz and 8000 Hz as recorded by Bekesy audio-metry. Only those persons were to be included who, at one minute a f t e r cessation of the exposure tone during the condi-t i o n r e q u i r i n g no a c t i v i t y , showed a TTS at 700 Hz of more than 1 dB ( c r i t e r i a of McBay, 1971)* Two male and two female subjects did not f u l l y meet these TTS c r i t e r i a but were i n -cluded to t y p i f y one extreme of the normal range of TTS and, possibly, otoadmittance r e s u l t s . 4 . 3 Experiments For t h i s i n v e s t i g a t i o n TTS i s defined as the d i f -ference between a subject's mean pre-exposure threshold for a pulsed pure tone and h i s mean post-exposure thresholds f o r the same tone, measured a f t e r exposure cessation. The TTS paradigm was, l a r g e l y , a r e p l i c a t i o n of that used by McBay (1971) . I"t was hypothesized that the otoadmittance values before, during, and a f t e r c o n t r a l a t e r a l exposure to the fatigue stimulus would be a reasonable measure of the admittance values that occur during such a TTS paradigm. 4.31 Experimental Design The basic design and para-meters of the experiments are shown i n Figure 4 . 3 . The TTS paradigm consisted of a 2 minute pre-exposure tracking at 700 Hz, a 5 minute exposure to a 1 1 7 .5 dB SPL 500 Hz fatigue tone, and a 4 minute post-exposure tracking at 700 Hz. The otoadmittance paradigm was s i m i l a r except that the THRESHOLD TRACKING ft=700Hz t]_= 2 min. EXPOSURE: 500Hz SPL= 177.5dB t2= 5 min. ACTIVITY: N T ( 1 of ) Hj THRESHOLD TRACKING ft=700Hz t3= 4 min. PRE--EXPOSURE • PERIOD EXPOSURE • PERIOD -POST--» < EXPOSURE-PERIOD A. Design and parameters of TTS procedures. CONTINUOUS RECORDING OF ADMITTANCE CHANGES PRE-EXPOSURE OTOADMITTANCE t]= 2 min. PRE-•EXPOSURE PERIOD EXPOSURE: fe=500Hz POST-EXPOSURE SPL=117.5dB OTOADMITTANCE t2= 5 min. ACTIVITY: N Q tg= 4 min. C 1 of ) H Q EXPOSURE POST-< PERIOD > <—EXPOSURE » PERIOD B. Design and parameters of Otoadmittance procedures. Figure 4,3 Experimental design and parameters. -33-recording of the tracked threshold during the pre-exposure and post-exposure periods was replaced by a recording of the admittance f o r the entire 11 minutes. During the 5 minute exposure period subjects performed one of the following a c t i v i t i e s i 1. N^/NQJ Subject sat q u i e t l y and l i s t e n e d to the tone. He or she was instructed not to concentrate on anything s p e c i f i c while the exposure tone was on. 2. H T / H Q I Subject hummed at 125 Hz (males) or 250 Hz (females) i n cycles of 7-8 sec of humming and 2-3 sec of r e s t , as cued by the l i g h t i n the tes t room (on=humj off=rest and in h a l e ) . A "moderately loud" humming i n t e n s i t y of 65 dB SPL was monitored by the subject, who watched the sound l e v e l meter (plus f i l t e r corresponding to the frequency of humming) placed 54- 2 inches from his/her mouth. The conditions (non-humming TTS condition), H^ (humming TTS condition), N Q (non-humming otoadmittance condi-tion) , and HQ (humming otoadmittance condition) were manda-tory f o r a l l 14- subjects. No two i n d i v i d u a l s were subjected to the same sequence of conditions as a p a r t i a l control against possible cumulative and/or sequential e f f e c t s . A L a t i n square design was not possible with 14 subjects and 4 conditions thus the sequence of conditions was randomized for each sub-je c t with the r e s t r i c t i o n that a l l subjects were to perform N T or HJ condition during the f i r s t session. This permitted the TTS c r i t e r i a ( c f . Section 4 .2) to be determined at the -34-beginning of experimentation. Data c o l l e c t i o n was scheduled to allow 24 hours or more between sessions for each subject. 4 .32 Procedures After i n d i v i d u a l s were accepted as subjects^ they were informed of the general outline of the experiments,and the purely voluntary nature of t h e i r coopera-t i o n was emphasized. Each subject then decided which ear was to be exposed to the fatigue tone; 8 l e f t and 6 r i g h t ears were chosen. At each session the subject was seated comfortably, then given a set of i n s t r u c t i o n s (See Appendix), TTS Sessions A l l TTS sessions proceded as follows: 1. Sweep-frequency (Bekesy) threshold tracking from 250 Hz to 8000 Hz f o r the non-exposure ear; 2. 2-3 minutes of single-frequency (700 Hz) threshold tracking f o r the non-exposure ear; 3 . Sweep-frequency threshold tracking from 250 Hz to 8000 Hz f o r the exposure ear. 4 . 2 minutes of single-frequency (700 Hz) threshold t r a c k i n g f o r the exposure ear (pre-exposure period of TTS paradigm); 5 . 5 minute exposure to 117.5 dB SPL 500 Hz tone accom-panied by s p e c i f i e d a c t i v i t i e s (exposure period); 6 . 4 minutes of single-frequency (700 Hz) threshold t r a c k i n g f o r the exposure ear (post-exposure period). Procedures 1 to 3 provided subjects with pr a c t i c e i n Bekesy tra c k i n g and served as a check f o r possible threshold f l u c t u -ations due to repeated intense exposure, p r a c t i c e , a t t e n t i v e -ness, state of health, etc. No such fluctuations were - 3 5 -observed i n the course of the experiments. The humming task was explained and practiced at the s t a r t of a session and i n s t r u c t i o n s were repeated just p r i o r to the exposure period. The humming a c t i v i t y was usually learned with 5 to 10 minutes of p r a c t i c e . With additional p r a c t i c e , even those subjects who i n i t i a l l y experienced d i f f i c u l t y were able to hum ac-ceptably. Otoadmittance Sessions The exposure ear, f o r these sessions, was always the one c o n t r a l a t e r a l to the TTS expo-sure ear. The probe tone used was 660 Hz. A l l otoadmittance sessions proceded as followst 1. Headset placed on subject with earphone on exposure ear. Probe placed i n TTS exposure ear and an a i r -t i g h t seal obtained. Using ascending and descending a i r pressure, a tympanogram was obtained and the G (conductance) and B (susceptance) values read from the corresponding d i a l s at 0mraHgO (drum loose) and +400 and -400 mm HgO (drum tight)? 2. Pre-exposure conductance and susceptance were recorded f o r 2 minutes; 3. 5 minute exposure to the 117.5 °3 SPL 500 Hz tone was accompanied by s p e c i f i e d a c t i v i t i e s while r e -cording of conductance and susceptance continued; 4. Post-exposure conductance and susceptance were recorded for 4 minutes. The tympanogram measurements provided a check of G and B v a r i a b i l i t y between sessions, allowed pre-exposure admittance values to be calculated, and provided a measure of the conduc-tance and susearptasiee-of the ear canal to be subtracted i n the f i n a l c a l c u l a t i o n of conductance and susceptance at the eardrum. Since the exposure ear received a 500 Hz tone and the probe ear received a. 660 Hz tone, some subjects found i t somewhat more d i f f i c u l t to hum at the s p e c i f i e d fundamental frequency f o r the H Q condition than for the condition. With p r a c t i c e , however, a l l subjects were able to hum accept-ably. 4.33 Data Measurement TTS Measurement Figure 4 . 4 i s a reproduction of the experimental 'record obtained from the pre- and post-exposure threshold tracking i n the TTS procedures. Pre-exposure threshold was obtained by averaging dB values of the l a s t 20 extrema i n the pre-exposure t r a c i n g . The post-exposure tracking was averaged by marking the midpoint of each peak-to-trough or trough-to-peak excursion and f i t t i n g an average curve through these points. The ordinates of t h i s curve at the post-exposure times of 7 .5 sec, 15 sec* 3© sec, 1 min, 2 min, and 4 min were measured with respect t® the subject's pre-exposure threshold. A series of Frenefe curves were used to extend the post-exposure threshold cunre to time zero and to obtain an extrapolated TTS value at. that time. Since a l l subjects were found to have thresholds that l e v e l l e d o f f between 2 and 4 minutes post-exposure, &B eighth TTS value was determined for t h i s portion of the post-exposure • FORM C F 2 A PftlMTID Nt U.I. A. T R A C E TONE MASKING 20 dB d B / S E C 7 0 0 Hz -— 2.5 COLOR c£) L ® B , dB 0, + , - 1'/.. 2Vi, 5 -10 —I •jj O UJ Q 10 20 30 0 0 — ' l i J UJ < 40 - 1 > Q o o £ a H 60 DC 70 80 90 100 SISI FREQ. % NAME_ SEX I G.A. AGE 2 3 DATE 20/6/73TIMF BY M.R. P r e - exposure TO C O N V E R T ISO R E A D I N G S TO A S A R E A D I N G S S U B T R A C T A P P R O P R I A T E " D I F F E R E N C E IN ( IB" A T E A C H F R E Q U E N C Y . 7-5 15 30 A l l I. 2 - F R E Q U E N C Y -xposure V, 1 2 MINUTES 9*. ^ BEKESY AUDIOMETER ]H GRAS0M - STADLER COMPANY, INC. £j MODEL NO SERIAL NO. FIXED FREQUENCY NO. -10 10 20 30 40 50 60 70 80 90 100 DIFFERENCE IN dB (ISO VS. ASA) 125 250 500 750 1000 1500 2000 3000 4000 6000 8000 9 15 14 (12) 10 10 8.5 8.5 6 93 113 I Figure 4.H Record of a representative TTS procedure (condition N™). -3-8-curve by averaging values the curve intersected at 2 min, 2 , 5 min, 3 min, 3 . 5 min, and 4 min, TTS, i n dB, at each of •the eight post-exposure times (0 sec, 7.5 sec, 15 sec, 30 sec, 1 min, 2 min, 4 min, and 2-4 min) was then obtained by subtract-ing the pre-exposure threshold. j^Note: T T S N 0 „ and TTS H 1^„ r e f e r to the value of TTS at the s p e c i f i e d times f o r condition N,p and H T r e s p e c t i v e l y . TTS - ^ n * however, r e f e r s to the value of TTS at the s p e c i f i e d time for either condition or H TYj Post-exposure threshold tracings revealed, for a l l subjects, a rapid i n i t i a l TTS recovery i n the f i r s t 1 to 2 minutes post-exposure followed by a slower secondary TTS recovery as evidenced, i n part, by the threshold plateau between 2 and 4 minutes post-exposure. . (This secondary TTS recovery may take several hours before the pre-exposure threshold i s reached.) To investigate the rate of i n i t i a l recovery a ninth measurement (it) was taken: (sec) = h a l f - l i f e of the i n i t i a l TTS recovery period = time, measured from the end of exposure, at which the TTS value has decreased by 50%. TTS^ (dB) = TTS 0„(dB) •+ T T S 2,_ v(dB) TTSQ„(dB) = TTS value at zero sec post-exposure TTSg ,_^,.(dB) = average TTS value between 2 and 4 minutes post-exposure, [N o t e j ^ j j = f f o r condition N,j,j -£ ^  = for condition Ten TTS values from the threshold tracings of 5 randomly chosen subjects were remeasured to determine measurement - 3 9 -r e l i a b i l i t y . Standard deviation f o r the 50 p a i r s of values was 0.85 dB. Otoadmittance Measurement The recorded conductance and susceptance values were fed from the A to D converter into the computer and there the (typed in) ear canal conduct-ance and susceptance values were subtracted. The r e s u l t i n g values of conductance and susceptance at the eardrum were then plotted over time f o r each subject. ( i e* G a t the drum = G t o t a l " G c a n a l s B — r> — B ) at the drum t o t a l c a n a l * ; Figure 4,5 i s an example of photographs obtained f o r each p l o t . A s p e c i a l l y constructed transparency was used to measure each photograph. The measurements (as i d e n t i f i e d i n the representative schematic of the pl o t s , F i g . 4.6) were: 1. G^:pre-exposure conductance ( i n mmhos) measured 15 sec before the s t a r t of exposure. 2, A G^: change i n conductance ( i n mmhos) as the exposure tone came on. 3» J1G|,I change i n conductance ( i n mmhos) as the exposure tone ceased. 4. G^ .: post-exposure conductance ( i n mmhos) measured 225 sec. a f t e r the end of exposure, ( i e . 15 sec before the end of the 4 minute post-exposure period.) Four s i m i l a r measurements (B^, AB^, AB f, and B f) were taken from each susceptance p l o t . Although AG^ f AB^,AG f, and A B„ values were much larger on the H n p l o t s than on the N n -40-Figure M.f, Representative schematic of otoadmittance 'plots. -4-1-p l o t s , the same methods of measurement were used f o r both conditions. [Note: AG N^, B H f e ^ c * r e f e r ^° ^ n e s p e c i f i e d measurements i n conditions N Q and H Q r e s p e c t i v e l y . AG^, B^, etc. r e f e r to the s p e c i f i e d measurements i n either condition NQ or HQ. AG and AB r e f e r to changes i n G and B, res p e c t i v e l y , at the beginning and/or end of exposure f o r either condition otoadmittance p l o t photographs to determine measurement r e l i a b i l i t y . Standard deviation f o r the 20 p a i r s of values was 0,05 mmhos. Four otoadmittance values were remeasured from 5 Chapter 5 Results 5.1 TTS Data A three factor analysis of variance (ANOVA) f o r repeated measures (Winer, 1 9 6 2 , p p . 2 9 8 - 3 4 9 ) was used to inves-t i g a t e possible systematic e f f e c t on TTS of the a c t i v i t y per-formed during exposure and to indicate i f t h i s e f f e c t was a function of time of TTS measurement and/or sex of the subject. This ANOVA followed a SEX (male, female) by TIME ( 0 " , 7 . 5 " , 15", 30", 1 ' , 2 ' , 4 ' ) by CONDITION (N T, H T).design i n which 1 4 subjects ( 7 males, 7 females) were tested f o r each time and each condition. Table 1 i s a summary of t h i s analysis. Since TTS from a given exposure i s known to vary greatly among i n d i -v i d u a l s , the considerable variance associated with subjects was expected. Although SEX had a non-significant e f f e c t on TTS, CONDITION and TIME each had a highly s i g n i f i c a n t e f f e c t , beyond the 0 . 0 1 l e v e l . The ANOVA revealed a highly s i g n i f i c a n t i n t e r -a ction between TIME and CONDITION, however, no s i g n i f i c a n t i n t e r a c t i o n s between SEX and CONDITION, or SEX and TIME and no o v e r a l l i n t e r a c t i o n between the three factors was found. Figure 5 . 1 shows the mean values of TTS f o r conditions N^ , and H,j at s i x post-exposure times f o r a l l subjects. The Neuman-Keuls method (Winer, 1 9 6 2 , p p . 7 7 - 8 5 ) was used to probe the nature of the differences between treatment 'totals following a s i g n i f i c a n t F i n the ANOVA. A probe of the time e f f e c t was made to determine the s i g n i f i c a n c e of differences -42--4-3-SEX x TIME x CONDITION Table 1: Summary of ANOVA. TTS as a function of sex, post-exposure time (0",7.5",15",30", l',2',4'), and condition (N^, H^ ) for 7 male and 7 female subjects. Source of Variation df MS F Between Subjects 13 Sex 1 154.89 0.32 Subjects 12 487.02 Within Subjects 182 Time 6 964.28 101.62*** Sex x Subjects .1 31.75 3.35 Time x Subjects 72 9.49 Condition 1 2,099.86 37.03*** Sex x Condition 1 26.97 0.48 Condition x Subjects 12 56.70 Time x Condition - 6 37.73 10.51*** Sex x Time x Condition 6 4.76 1.33 Time x Condition x Subjects 72 3.59 * p <0.05 ** p <0.01 *** p <0.001 TTS Fig.5.1 Comparison of TTS(measured at 700 Hz) at six different post-exposure times for conditions N T and IL^ ,. - 4 5 -between a l l 7 post-exposure times for both conditions and a l l subjects. A summary i s given i n Table 2 a . The r e s u l t s i n d i -cate that time had a highly s i g n i f i c a n t e f f e c t on TTS, i n par-t i c u l a r ? that TTS values for 0 " , 7 . 5 " , 15", and 3 0 " times were s i g n i f i c a n t l y d i f f e r e n t from a l l other times except that TTS fo r 0" was not s i g n i f i c a n t l y d i f f e r e n t from TTS f o r 7 . 5 " , and that TTS values f o r 1 ' , 2 ' , and 4 ' times were, however, not s i g n i f i c a n t l y d i f f e r e n t from each other. A probe of the condition e f f e c t confirmed that, as shown by the ANOVA, had a s i g n i f i c a n t l y d i f f e r e n t e f f e c t on TTS from N T, f o r a l l times and subjects (see Table 2 b ) . A f i n a l probe of the TIME X CONDITION e f f e c t was made to obtain more information on the e f f e c t of condition at s p e c i f i c post-exposure times. The e f f e c t of TIME X CONDITION on TTS f o r a l l subjects was shown to be s i g n i f i c a n t at the 0.01 l e v e l by the ANOVA but the Neuman-Keuls probe revealed that the e f f e c t of condition was d i f f e r e n t at d i f f e r e n t times (See Table 2 c ) . S p e c i f i c a l l y , condition had a s i g n i f i c a n t l y d i f f e r e n t e f f e c t on TTS from condition N T at 0 " , 7 .5"f 15" and 30" f o r a l l sub-jects but the e f f e c t of condition was not s i g n i f i c a n t l y d i f f e r e n t at 1 ' , 2 ' , and 4 ' . A t - t e s t f o r related measures (Bruning and Kintz, 1968, pp.12-15) was used to determine the r e l a t i o n s h i p between conditions N T and f o r the values of TTSg,^, (cf. Sec.4.2) I t was shown that r e s u l t s i n a s i g n i f i c a n t l y smaller average TTS value between 2 ' and 4 ' (0 .01 l e v e l ) than N™, Table 2a: Results of Neuman-Keuls test for significance of differences between TTS for 7 post-exposure times ( 7 male and 7 female subjects ). Time 4' 1' 2' 30" 15" 7.5" 0" T.T. 202.35 234.64 240.55 317.55 428.12 550.57 615.10 4' 202.25 32.39 38.30 115.30** 225.87** 348.32** 412.85** 1' 234.64 5.91 82.91** 193.48** 315.93** 380.46** V 240.55 77.00** 187.57** 310.02** 374.55** 30" 317.55 110.57** 233.02** 297.55** 15" 428.12 122.45** 186.98** 7. 5" 550.57 64.53 q 0.99(r,72) 3.75 4.27. 4.59 4.81 4.98 5.12 q 0.99(r,72) x^MSerror 61.13 69.60 74.82 78.40 81.17 83.46 ** p <0.01 Table 2b: Results of Neuman-Keuls test for significance of differences between TTS for N T and lip conditions. Table 2c: Results of Neuman-Keuls test for significance of differences between TTS for conditions and Hj, at the specified post-exposure times. Time t=0" t=7.5" t=15" t= 30" t= 1* t= 2' t= 4' Condition V lij, N T NT N T N T N T T.T. 245.29 369.81 216.84 333.73 151.38 276.74 108.84 207.71 82.84 151.80 89.84 150.71 77.64 124.71 % 124.52** 116.89** 125.38** 97.87** 68.96 60.87 47.07 q 0.99(r,72) 3.75 3.75 3.75 3.75 3.75 3.75 3.75 q 0.99(r,72) x>lMSerror 70.35 70.35 70.35 70.35 70.35 70.35 70.35 Condition «T N T T.T. 973.67 1615.21 641.54** q 0.99(r,12) 4.32 q 0.99(r,12) xJMSerror 322.01 ** p<0.01 -48-Another analysis was done to investigate the ef f e c t of condition on the i n i t i a l rate of TTS recovery (cf. Sec. 4.32). A t - t e s t f o r r e l a t e d measures, performed on the h a l f - l i f e values •£N and f H revealed that the H^ , condition r e s u l t s i n a s i g n i f i -c antly shorter i n i t i a l recovery rate (0.01 l e v e l ) , as defined by the h a l f - l i f e , than the condition. 5.2 Otoadmittance Data Analysis of the otoadmittance data investigated the e f f e c t of condition on middle ear admittance before, during, and a f t e r the 5 minute exposure to a 117.5 dB SPL 500 Hz tone. The otoadmittance measurements (G^, A G^, AG^, G^, B^, A'B.^, ^B^., B^; c f . Sec, 4*3~) for-condition NQ were each compared with the corresponding measurement fo r condition HQ. t - t e s t s for r e l a t e d measures -vwsre ;p:erf.orined o.n the eight paired sets of values. I t was found that condition had no s i g n i f i c a n t e f f e c t on i n i t i a l and f i n a l G and B values, as shown by non-significant t values (non-significant at the 0.10 l e v e l ) . The other four t - t e s t s , which were s i g n i f i c a n t beyond the 0.001 l e v e l , re-vealed that changes i n G and B at the beginning and end of exposure were s i g n i f i c a n t l y larger during condition HQ than during condition NQ. The changes i n G and B at the beginning and end of exposure were then compared. When no phonation accompanied exposure the change i n G at the beginning of exposure was s i g n i f i c a n t l y l a r g e r (0.01 l e v e l ) than the change i n B ( i e . 6 G M. > <4BN.) as shown'by a t - t e s t for related measures. - 4 9 -At the end of exposure the change i n G was s t i l l larger than the corresponding change i n B but not s i g n i f i c a n t l y l a r g e r . When phonation accompanied exposure, however, the change i n B both at the beginning and at the end of exposure was s i g n i f i -cantly larger (0,01 l e v e l ) than the corresponding change i n G ( i e . ^  S H i > A G H i A B H f > A G H f ^ * T n * s f i n d i n g suggests that the humming condition affected the susceptance s i g n i f i -cantly more than i t affected the conductance, 5.3 Comparison of TTS and Otoadmittance Data In an attempt to f i n d some r e l a t i o n s h i p between TTS and otoadmittance data, a number of Pearson product-moment co r r e l a t i o n s were calculated. Four of the NQ condition measure-ments (^GN^, AG N ; f, AB^i' i l B ^ j ) were f i r s t compared with TTS measurements at times 0", 7.5", 15"t 30", 1', 2 ' , 4', and 2'-4' and with the h a l f l i f e t f o r N T and H T conditions. The 72 Pearson r values obtained from these comparisons were a l l small and non-significant, not even at the 0.10 l e v e l . The two l a r g e s t c o r r e l a t i o n c o e f f i c i e n t s werei A G N f X TTS N 2, r=-0.31 p=0.10 i f r=0.46 A G N f X TTS N 1, r=-0.30 The difference between TTS values f o r conditions N^ , and H T at 0", 7.5", 15", 30", and 1' were calculated as a measure of the protection given the ear by the humming condi-t i o n . These 5 differences were compared with four N Q measure-ments 0*G N i, ^ G N f ' ^ B N i * A B N f ^ * T h e 0 n l y s ' t a ' t i s ' t i c a l l y -50-s i g n i f i c a n t Pearson r c o r r e l a t i o n c o e f f i c i e n t was: ( T T S N 7 < 5 H - T T S H 7 t 5 „ ) X 4 G N i r=0.53* *p=0.05 (See Table 3) The 5 TTS differences were then compared with the fo 11 owing r a t i os» A G^i/GN ^ ; A G N f/G N f; A B_N ±/B N i ; A B N f / B N f ^  A l l Pearson r correlations between the A G r a t i o s and the TTS di f f e r e n c e values were larger than the corresponding non-ratio c o r r e l a t i o n s , and, as shown i n Table 3, four of the c o r r e l a -t i o n c o e f f i c i e n t s were s i g n i f i c a n t beyond the 0.05 l e v e l . Points corresponding to three of these correlations are graphed i n Figures 5.2, 5.3, and 5.4. The following admittance values from condition N Q were then calculated f o r each subject; 4 Y N i = 1^ GNi> 2 + ( A B N i > 2 = change i n admittance at the beginning of exposure. 4Y, (AG _ ) 2 + ( A B ^ ) 2 Nf " V V T v "Nf = change i n admittance at the end of exposure. These values correlated weakly and negatively with the 5 TTS di f f e r e n c e values. The l a r g e s t c o r r e l a t i o n c o e f f i c i e n t was; ( T T S N 7 > 5 „ - T T S H ? > ^ J x A Y N i r=-0.51 p<0.10 (See Table 3) The c o r r e l a t i o n c o e f f i c i e n t s were s i m i l a r i n magnitude to c o e f f i c i e n t s of the comparisons between the 5 TTS difference values and the values AG^, AG^, ^ B N i * a n d ^ BNf * Table 3: Matrix of Results of Pearson Product-Moment Correlations between specified TTS and Otoadmittance values. ^ \ A \ * % f / G N f ^ ^ i / G N i \ ^ ^ X H l f ^ f TTSN0',"TrSH0" > v -0.35 -0.38^\ X. -0.16 -0 . 3 o \ . \ . -0.44 \ v -0.11 -0 . 2 o \ . S V -0.31 -0.37\ TrSN7.5"~TrSH7.5" \ -0.44 -0.53^S. \ -0.29 - 0 . 4 o \ ^ \ -0.48 -0.57*\. \w -0.20 -0.22^^ \ . -0.42 - 0 . 5 l \ T r SN15 , l" T r SH15" \ ^ -0.43 - 0 . 4 l \ ^ -0.22 -0.32 \ w \ ^ -0.56* -0.63*\. \ -0.15 -0.0y \ v \ . -0.39 -0 . 4 o \ v TTSN30"~TrSH30" \ v -0.36 -0.35\. \ . -0.20 -0.l8 \ v -0.50 -0.57*Nw \ -0.18 +0.15\v \ . -0.34 -0.33^\^ T T SN1'~ T T SH1' > v -0.39 -0.37 \ v \ v -0.40 -0.32 \ v -0.40 -0.47 \ w \ -0.35 +0.02\v. \ -0.42 -0.36^s^ p<0.10 r=0.46 p<0.02 r=0.61 *p<0.05 r=0.53 **p<0.01 r=0.66 - 5 2 -T T S . N 15 T T S H | 5 » Pearson r = - 0.63 Regression Line Fig.5.2 Comparison of TTS difference values (15" post-exposure) with i n i t i a l change in 6^  during exposure (in percentage), * p <0.05 -53-T T S N 15 - T T S H 15 dB 184 Pearson r = - 0 . 5 6 Regression Line Fig.5.3 Comparison of TTS difference values (15" post-exposure) with final change in during exposure (in percentage) * p< 0.05 - 5 4 -• n f TTS difference values (15" post-exposure) Fig 5.4 Comparison of TTS dirreie • « durins exposure- (in percentage) with i n i t i a l change va B during expo -55-T T S N 15' T T S H 15 Pearson r R e g r e s s i o n Line Fig.5.5 Comparison of TTS difference values (15" post-exposure) with i n i t i a l change in Y^ . during exposure (in percentage) -56-The admittance values A Y N ^ and h Y^ .^ were also conver-ted into r a t i o s , ^ Y ^ i ^ N i ^ N f ^ N f 2 1 1 1 ( 3 c o m P a r e d with the 5 TTS difference values. The largest of the negative, non-s i g n i f i c a n t Pearson r c o r r e l a t i o n c o e f f i c i e n t s was: ( T T S N 1 5 „ - T T S H 1 5 „ ) X A Y N i / Y N i r=-0.51 P<0.10 (See F i g . 5.5) A comparison of the rate of i n i t i a l TTS recovery f o r condition (estimated by 1^) and the rate of MEM relax-a t i o n (estimated by A G N^ - 4G N f) was then undertaken. This d i f f e r e n c e between the i n i t i a l and f i n a l change i n G^ during exposure was chosen as a measure of MEM r e l a x a t i o n since the change i n G N ( 4 Gj^) decreased s i g n i f i c a n t l y more, with respect _to . i t s ...baseline .(G^), than A G^, A , or A B^ decreased, r e l a -t i v e to t h e i r respective baselines (G^^, B N^, and G H^), during exposure. The Pearson r c o r r e l a t i o n c o e f f i c i e n t between ( 4G N^ - AG w f) and "TN was small and non-significant ( r s 0 . 1 5 ) . This same measure of the rate of MEM-relaxation was also com-pared with TTS values for condition N T at times 0 " , 30", 1*, 2*, and 2*-4' but no s i g n i f i c a n t Pearson r correlations were found. Chapter 6 Discussion Most of the TTS r e s u l t s from the procedures that investigated the e f f e c t of phonation on TTS were i n agreement with r e s u l t s of Karlovich and Luterman (1970), who used a 1000 Hz fatigue tone, and of McBay (1971)f who used a 5 0 0 H z tone. A l l these investigations showed thats 1) phonation dur-ing fatigue exposure consistently r e s u l t s i n a s i g n i f i c a n t l y smaller post-exposure TTS than i f no phonation accompanies exposure, f o r a l l post-exposure times and f o r both males and females? and 2) TTS i s a function of the post-exposure time at which i t i s measured such that the TTS difference between phonation and non-phonation conditions i s most s i g n i f i c a n t at early post-exposure times. A minimal tendency, noted by Kar l o v i c h and Luterman and by McBay, f o r females to exhibit greater TTS differences than males between phonation and non-phonation conditions was not found i n t h i s study. Differences between procedures used i n t h i s study and those used by Karlovich and Luterman (1970) ( i e , a 1000 Hz. tone was used; vocal e f f o r t was not controlled for; etc.) may explain why t h e i r f i n d i n g was not repeated. McBay's (1971) study also d i f f e r e d i n that three phonation conditions were used vs. one phonation condition i n t h i s study. She found the tendency f o r females to e x h i b i t greater TTS differences to occur only during the "humming comfortably" conditions while the "humming loudly" condition, which most c l o s e l y approximated t h i s study's - 5 7 --58-H T condition, resulted i n s i m i l a r TTS differences f o r males and females. The r e s u l t s of t h i s study, therefore, do not appear to c o n f l i c t with previous findings concerning sex re l a t e d TTS diffe r e n c e s . Examination of TTS recovery revealed that humming during the fatigue exposure r e s u l t s i n a s i g n i f i c a n t l y shorter i n i t i a l TTS recovery, indicated by the l v a l u e s , than i f no humming i s performed. This shorter i n i t i a l recovery rate f o r the humming condition would appear to support Karlovich and Luterman's (1970) p r e d i c t i o n "that return to pre-exposure threshold l e v e l s would take longer f o r the non-voiced than f o r the voiced conditions." (Ibid., 1970, p.514). The data, however, seems to show a plateau of TTS recovery f o r the humming condition between 2' and 4* post-exposure (See F i g . 5.1), while the corresponding TTS curve f o r the non-humming condition i s s t i l l decaying noticeably. Thus, i t would be necessary to continue the post-exposure tracking u n t i l the pre-exposure threshold l e v e l i s reached before the above p r e d i c t i o n could be applied to a complete TTS paradigm. The otoadmittance procedures revealed changes i n conductance and susceptance at the beginning and at the end of the exposure period to be s i g n i f i c a n t l y larger when humming accompanied the exposure. I t i s known that 1) large changes i n impedance (thus large changes i n admittance) can r e s u l t from a large degree of MEM a c t i v i t y (Metz, 1951? M i l l e r , 196la, 1965? Simmons, 1964; Shearer and Simmons, 1965; Karlovich, et a l . , - 5 9 -1972) and 2) MEM contraction attenuates transmission of low frequency tones by up to 20 d3 (Simmons, 1959? Reger, I960} Jepsen, 1963; Borg, 1968; Brasher, et a l . , 1969? Kevanishvili and Gvacharia, 1972) . From these f a c t s , one might assume that the reduction of transmission of the exposure tone during the humming condition could be due to increased MEM a c t i v i t y . Such an assumption appears v a l i d but should be regarded with caution because, although humming produced larger changes i n G and B than no humming, these changes with humming appeared to become negative thus making i n t e r p r e t a t i o n d i f f i c u l t . A r e p e t i t i o n of these otoadmittance procedures, possibly with technical improvements, would be desirable to resolve the above mentioned problem. Ka r l o v i c h and Luterman (1970, p.513) "suspect involve-ment of a mechanism which increases the e l a s t i c reactance component of the impedance" to account f o r the reduction of TTS that occurs i f phonation accompanies exposure. The "elas-t i c reactance" i s the negative part of the reactance or the c a p a c i t i v e component of the impedance. As the absolute value of t h i s reactance increases, the absolute value of the suscept-ance component of admittance decreases. I f Karlovich and Luterman's hypothesis i s true one should f i n d the negative reactance to become larger or, i n t h i s study, the p o s i t i v e susceptance to become smaller when phonation accompanies exposure. The l a t t e r r e s u l t was found i n the current study. In f a c t , a t - t e s t f o r related measures showed susceptance to be reduced by the humming condition s i g n i f i c a n t l y more (0.01 -60-l e v e l ) than the conductance was. Such a f i n d i n g suggests that the susceptance component of admittance i s affected more than the conductance by whatever mechanism reduces TTS a f t e r phonation. When no phonation accompanied exposure i t was found that the change i n conductance at the beginning of exposure was l a r g e r than the corresponding change i n susceptance. This d i f f e r e n c e between changes i n G and B was also present, though not s i g n i f i c a n t l y so, at the end of exposure ( i e . the change i n G decreased during exposure). I t i s well known that MEM a c t i v i t y due to the acoustic r e f l e x decays during continuous acoustic stimulation (Metz, 1951? Karlovich, et a l . , 1972) . The f a c t that the change i n G (but not the change i n B) decreased during exposure seems to be an i n d i c a t i o n of such a r e f l e x decay. I t may be that G i s the admittance component most affected by acoustic stimulation of the MEM. On the other hand, when, phonation accompanied expo-sure, the changes i n susceptance during exposure were larger than the corresponding changes i n conductance. Since we do not know which anatomical parts of the middle ear contribute which components of admittance, i t i s d i f f i c u l t to explain t h i s f i n d i n g . I t i s possible that the l a r g e r change i n B r e s u l t s from an increased MEM contraction with phonation, from the change i n stapes v i b r a t i o n thought to occur with phonation (Beklsy, i 9 6 0 ) , or from a combination of these and other, as yet undefined, mechanisms. Further research, such as that by -61-M ^ l l e r ( 1961a) , Onchi (1961), and Zwislocki (1965) on e l e c t r i c a l or mechanical analogues of the middle ear, i s needed, Following otoadmittance data analysis, the apparently negative G and B values, during exposure i n the humming condi-t i o n , remained a problem. I t was thought that acoustic feedback during the humming might have resulted i n the negative values. Since the p l o t t i n g of G and B had involved a r e l a t i v e l y slow sampling rate (approximately 1 Hz), mingograms of the humming condition f o r each subject were made to determine i f pertinent information had been missed i n the between-sampling periods. The mingograms confirmed that, f o r a l l subjects, G and B, during exposure accompanied by humming, f e l l below the zero mmho baseline. G and B did not become p o s i t i v e , however, dur-ing the 2-3 sec pauses between two i n t e r v a l s of humming thus, the hypothesis of acoustic feedback r e s u l t i n g i n negative G and B values was rule d out. A study i n v e s t i g a t i n g changes i n G and B when no exposure i s presented during a humming a c t i v i t y might help resolve t h i s problem. Since G and B did not become p o s i t i v e during the between-humming pauses i t appeared that G and B do not change r a p i d l y . I t i s known (Salomon and Starr, 1963; Djupesland, 1967) that MEM contractions continue up to 300 msec af t e r phonation ends. Results of the current study suggest that mechanisms that a l t e r G and B, when phonation accompanies an intense exposure, remain active much longer than 300 msec a f t e r phonation ends. The possible mechanisms involved were discussed -62-e a r l i e r ( c f . Sec. 2.2 and 2.3) but i t i s not known which com-bination of mechanisms reduces admittance (and therefore reduces TTS) when phonation accompanies exposure or which mechanisms are responsible f o r separate changes i n the two admittance components. In the comparison of TTS and otoadmittance data only-one s i g n i f i c a n t trend was found. Figures 5.2 and 5.3 show that the l a r g e r the change i n conductance, at the beginning and end of exposure with no humming, the smaller the TTS d i f -ference between humming and non-humming conditions (at early post-exposure times). In other words, the greater the amount of.MEM contraction, r e f l e c t e d by the change i n conductance with exposure (condition N Q), the smaller the protection given the ear by the humming, r e f l e c t e d .by TTS difference values. This may mean that i f the MEM are contracted strongly i n response to an acoustic stimulus they may not contract much more during phonation thus humming provides l i t t l e extra pro-t e c t i o n from the exposure. Figure 5«2 and 5«3 appear to sub-s t a n t i a t e t h i s hypothesis i e . i n d i v i d u a l s having the l a r g e s t amounts of MEM contraction, as indicated by percentage changes i n conductance, have the smallest amounts of extra protection from humming, as indicated by TTS difference values. The re-verse also holds i n that i n d i v i d u a l s with a weak MEM contraction i n response to acoustic stimulation show considerable extra p r o t e c t i o n from the humming condition. These r e s u l t s must be considered as preliminary. A repeat of these procedures with a l a r g e r population, of both -63-normals and subjects with c e r t a i n middle ear abnormalities, might v e r i f y the above trend or even show more s i g n i f i c a n t c o r r e l a t i o n s . Such a study would help indicate the normal range of r e s u l t s from such procedures and might determine whether those not c l o s e l y following the trend e i t h e r f a l l w i t h i n normal l i m i t s or poss i b l y have some undiagnosed abnor-mality that was not apparent from accepted screening techniques. Changes i n conductance and admittance (Fig. 5»5) were c o r r e l a t -ed with the TTS difference values but changes i n susceptance did not show any such r e l a t i o n s h i p ( F i g . 5*4 and Table 3 ) . Further research, hopefully, w i l l indicate why t h i s occurs. I f EMG studies were possible with normal humans we might determine the l e v e l s of MEM contraction that r e s u l t from acoustic stimu-l a t i o n and from acoustic .stimulation accompanied by phonation. This could indicate whether changes i n TTS and admittance during the humming condition are due to MEM contraction alone or whether other mechanisms, such as a change i n stapes v i b r a -t i o n (Bekesy, I960), are involved. The r e s u l t s of t h i s study indicate two basic types of MEM a c t i v i t y i n response to acoustic and to acoustic plus phonatory stimulation. Some in d i v i d u a l s show strong acoustic a c t i v a t i o n of t h e i r MEM with l i t t l e further protection provided by phonation during the fatigue exposure. Most i n d i v i d u a l s , however, show some acoustic a c t i v a t i o n of t h e i r MEM while phonation during exposure provides considerable extra protec-t i o n of the ear as evidenced by a s i g n i f i c a n t l y reduced TTS. The r e s u l t s do not, however, reveal how phonation protects -64-these ears from intense low frequency stimulation. One could hypothesize that the MEM contract more strongly during phonation plus exposure than during acoustic exposure alone, however, t h i s claim can he substantiated only through further research. Even i f i t i s shown that the degree of contraction i s the same for phonatory and auditory stimula-t i o n , MEM may s t i l l provide extra protection during phonation. Since we know that introduction of a new frequency during acoustic stimulation i s s u f f i c i e n t to stimulate renewed MEM contraction (Metz, 1951; Brasher, et a l . , 1969) i t would seem that, as shown by the current r e s u l t s , humming should keep the MEM maximally contracted throughout the exposure. We also know that continuous single frequency stimulation, as i n the non-humming condition, r e s u l t s i n a decrease of REM a c t i v i t y (Metz, 1951s Karlovich, 1972) . One might expect that the more rapi d the decrease i n MEM a c t i v i t y during the non-humming condition, the greater the TTS produced by exposure. Using a Madsen (Model ZO 70) electroacoustic impedance bridge with 6 subjects, McBay (1971) found t h i s r e s u l t for the non-humming condition. For the current group of subjects, however, no consistent pattern of decrease of MEM a c t i v i t y was found (as defined b y ^ G ^ - ^ G^) thus i t would appear that either MEM a c t i v i t y i s d i f f e r e n t i n the two conditions (N^ and H T) or that other mechanisms are involved during phonation to account f o r the l a r g e r change i n admittance and thus reduced TTS with phonation. -65-Summary This study was set up to investigate one of the f a c t o r s that may be responsible f o r TTS reduction that occurs when phonation accompanies exposure to a 500 Hz 117.5 dB pure tone. Through the use of admittance measurements, the r o l e of the MEM i n TTS reduction was investigated. The r e s u l t s of the study indicated thats 1) TTS i s a function of the post-exposure time at which the threshold i s measured. The early post-exposure times ( 0 " , 7.5"• 15". 30") resulted i n the largest TTS values. 2) TTS from the exposure tone accompanied by phonation (humming) was consistently and s i g n i f i c a n t l y smaller than TTS from the exposure tone with no supplementary a c t i v i t y . 3) Differences between TTS from phonation (humming) during exposure and TTS from exposure with no - supplementary a c t i v i t y were most s i g n i f i c a n t at the early post-exposure times ( 0 " , 7.5". 15". 30"). k) The rate of i n i t i a l TTS recovery when phonation accompanied exposure was s i g n i f i c a n t l y shorter than when no supplementary a c t i v i t y accompanied exposure. 5) There were no s i g n i f i c a n t TTS differences between sexes. 6) Changes i n the conductance and susceptance components of admittance at the beginning and end of exposure were s i g n i f i c a n t l y larger when phonation accompanied exposure than when no supplementary a c t i v i t y accompanied exposure. 7) When phonation accompanied exposure the changes i n susceptance at the beginning and end of exposure were s i g n i f i c a n t l y larger than the corresponding changes i n conductance. When no supplementary a c t i v i t y accompanied exposure, however, the changes i n conduc-tance at the beginning of exposure were s i g n i f i c a n t l y l arger than the corresponding changes i n susceptance. 8) Changes i n conductance and susceptance at the beginning and end of exposure did not correlate s i g n i f i c a n t l y with TTS values measured at any of the post-exposure times. -66-9) The rate of MEM relaxation during exposure accompanied by no supplementary a c t i v i t y did not correlate s i g n i -f i c a n t l y with the rate of i n i t i a l TTS recovery or with TTS values measured at several of the post-expo-sure times (0", 30", 1', 2 \ 2«-4'). 10) The degree of MEM a c t i v i t y at the beginning and end of exposure accompanied by no supplementary a c t i v i t y , measured by the percentage change i n conductance at these times, correlated s i g n i f i c a n t l y with the amount of protection provided the ear by phonation (humming), measured by the differences between TTS values at early post-exposure times f o r the humming and non-humming conditions. From the r e s u l t s , i t was hypothesized that some i n d i v i d u a l s , who have MEM that contract strongly with intense acoustic stimulation, gain l i t t l e extra protection from phona-t i o n (humming),during exposure, while the majority of i n d i v i -duals, who have MEM that contract weakly with intense acoustic stimulation, gain ..a .significant amount of protection from phona-t i o n (humming) during exposure. The r e s u l t s suggest that the MEM play a major r o l e i n the attenuation of sound transmission that occurs when phonation accompanies exposure. Other mechan-isms, however, including i n s u f f i c i e n t stapes v i b r a t i o n and a t t e n t i o n a l f a c t o r s , also may be involved but more research i s necessary before we can determine the exact r o l e each mechanism plays i n the reduction of TTS with phonation. References vo n Bekesy, G. (I960). 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(1964). " E f f e c t of Repeated Exposure to High Intensity Sound," J . Acoust. Soc. Amer. __6, 1195-H98. Rodda, M. (1964) . "Role of Test Tone i n Producing Temporary Threshold S h i f t , " Arch. Otolaryngol. 80, 160-166. Salomon, G., and Starr, A. (1963). "Electromyography of Middle Ear Muscles in Man During Motor A c t i v i t i e s , " Acta Neurol. Scand. 32, 161-168. „ Shearer, W.M., and Simmons, F.B. (1965). "Middle Ear A c t i v i t y During Speech in Normal Speakers and Stutterers," J . Speech Hearing Res. 8, 203-207. Simmons, F.B. (1959). "Middle Ear Muscle A c t i v i t y at Moderate Sound Levels," Ann. Otol. Rhinol. Laryngol. 68, 1126-1143. (1964) . "Perceptual Theories of Middle Ear Muscle Function," Ann. Otol. Rhinol. Laryngol. 21* 724-739. Ward, W.D. (1962)". Damage Risk C r i t e r i a f o r Line Spectra," J . Acoust. Soc. Amer. J_4, 1610-1619. (.1963)"Auditory Fatigue and Masking," i n Modern Developments in Audiology, J . Jerger, Ed, (Academic Press IncTi New York). 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You w i l l f i r s t hear a pulsed tone of low p i t c h i n your r i g h t ( l e f t ) ear that w i l l gradually become higher i n p i t c h . Press the button as soon as you hear the tone, then l e t go as soon as the tone d i s -appears . 2. You w i l l now hear a pulsed tone i n your r i g h t ( l e f t ) ear that w i l l remain at the same p i t c h . Press the button as soon as you hear the tone, l e t go as soon as i t disappears. 3. Now i n your l e f t (right) ear Step 1 w i l l be repeated then Step 2. Press the button when you hear the tone, l e t go when you don't. 4. You w i l l now hear a loud continuous tone i n your l e f t (right) ear f o r 5 minutes. You w i l l be given i n s t r u c t i o n s about what to do during t h i s time and w i l l get s u f f i c i e n t practice of the a c t i v i t i e s i n -volved. During the 5 minutes r e f r a i n from excessive body movements or unnecessary c l e a r i n g of the throat coughing, yawning, or swallowing. About 10 seconds before the tone i s turned o f f ? I w i l l jump on the f l o o r to a l e r t you to be ready f o r the next step. 5. As soon as the loud tone i s turned o f f ? l i s t e n f or the pulsed tone that does not change i n p i t c h . As - 7 2 -soon as you hear the pulsed tone^press the button u n t i l i t disappears, then l e t go, etc. as before. This w i l l continue f o r 4 minutes. Do you have any questions? B. Otoadmittance Procedures When the probe i s placed i n one ear^you w i l l hear a moderately loud tone that w i l l remain on fo r the duration of the session. I w i l l obtain an a i r - t i g h t seal i n that ear, then vary the pressure within the sealed cavity and make a number of measurements. A f t e r t h i s time }you must remain as s t i l l as possible. Do not move your feet, arms, hands, etc. and t r y not to swallow, cough or clear your throat u n t i l I t e l l you the session i s over. 1. You w i l l hear the probe tone only, f o r 2 minutes. 2. For the next 5 minutes you w i l l also hear the loud exposure tone (same as i n the TTS conditions) i n the ear opposite to the probe. S i t very s t i l l or hum as directed, 3. When t h i s loud tone i s turned o f f remain very s t i l l f o r another 4 minutes. Probe tone w i l l remain on. Wait u n t i l advised to move before doing so. Do you have any questions? 

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