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EXPERIMENTAL STUDIES ON THE FUNCTION OF THE INMAN

AKADEMISK AVHANDLING

som med vederbörligt tillstånd av Medicinska fakulteten vid Umeå Universitet för vinnande av medicine doktorsgrad offentligen försvaras i Samhällsvetarhuset, sal D, lördagen den 25 maj 1974 kl. 9.15 f.m.

av JOHN-ERIK ZAKRISSON med.lic.

UMEÅ 1974

UMEÀ UNIVERSITY MEDICAL DISSERTATIONS No. 18 1974 From the Department of , University of Umeå, Umeå, Sweden and the Division of Physiological Acoustics, Department of Physiology II, Karolinska Institutet, Stockholm, Sweden

EXPERIMENTAL STUDIES ON THE FUNCTION OF THE STAPEDIUS MUSCLE IN MAN

BY

JOHN-ERIK ZAKRISSON

UMEÂ 1974

To Karin Eva and Gunilla

The present thesis is based on the following papers which will be referred to in the text by the Roman numerals:

I. Zakrisson, J.-E., Borg, E. & Blom, S. The acoustic impedance change as a measure of stapedius muscle activity in man. A methodological study with electromyography. Acta Otolaryng, preprint.

II. Borg, E. & Zakrisson, J.-E. Stapedius reflex and monaural masking. Acta Otolaryng, preprint.

III. Zakrisson, J.-E. The role of the stapedius reflex in poststimulatory audi ­ tory fatigue. Acta Otolaryng, preprint.

IV. Borg, E. & Zakrisson, J.-E. The activity of the stapedius muscle in man during vocalization. Acta Otolaryng, accepted for publication.

CONTENTS

ABBREVIATIONS ...... 8

INTRODUCTION...... 9

MATERIAL...... 11

METHODS ...... 11

RESULTS AND CONCLUSIONS...... I3

The acoustic impedance change as a measure of stapedius muscle activity (I) ...... 13

Masking produced by low frequency noise (II) ...... 13

Poststimulatory (III) ...... 14

Stapedius muscle activity during vocalization (IV) .... 15

SUMMARY...... 16

ACKNOWLEDGEMENTS ...... 17

REFERENCES 18 8

ABBREVIATIONS used in the thesis and the separate papers

CF Center frequency

HL level

ISO International standard organization

PTS Permanent threshold shift

RMS Root mean square

SD Standard deviation

SEM Standard error of the mean

SPL Sound pressure level (re 2 x Ì0 Newton/m )

TTS Temporary threshold shift 9

INTRODUCTION

The stapedius muscle, the smallest striated muscle of the body, was first de ­ scribed by Varolius 1591 (cit. by Wever & Lawrence, 1954 p. 5). It is inner­ vated by the facial (Politzer l86l) and can be activated as an and by several nonacoustic factors. The course of the acoustic stapedius reflex arc in man is not known but in the rabbit it has 3-4 neurons in addition to parallel multisynaptic connections (Borg, 1973)« In man, the stapedius mus ­ cle alone participates in the acoustic middle reflex, while in many species the other muscle, the , is also regu­ larly activated by sound.

Various methods have been used for studying the activity of the stapedius mus ­ cle, such as visual inspection (LUscher, 1929; Jepsen, 1955; Fisch & v Schult- hess, 1963), recording of pressure change in the (Terkildsen, 1957; Holst et al., I963; Neergaard & Rasmussen, I960; Lidén et al., 1970) and elec­ tromyography (EMG) (Perlman & Case, 1939; Fisch & v Schulthess, 1963; Salomon & Starr, 1963; Djupesland, 1965). Measurement of the change in the ear's acoustic impedance in response to sound, first used by Geffcken (1934), has become of great importance in auditory research and clinical diagnosis (Metz, 1946; for review see also Jepsen, I963 and Anderson, I969). Though much utilized, the change in the ear's acoustic impedance has previously not been adequately re­ lated to more direct measures of stapedius muscle activity, e.g. EMG.

The acoustic stapedius reflex in man has been extensively studied with respect to basic physiological properties such as excitability, temporal characteris­ tics and attenuation provided on sound transmission through the middle ear (for review see Miller, 1972). Nonacoustic stimuli such as tightly shutting the eyes, swallowing and cutaneous stimulation of the external ear and auditory ca­ nal have also been shown to change the ear's acoustic impedance or to give EMG activity from the stapedius muscle (Metz, 1951; Klockhoff & Anderson, 1959; Klockhoff, 1961; Djupesland, I965). Activation of the human stapedius muscle during vocalization has been shown (Klockhoff, 1961; Salomon & Starr, I963; Djupesland, 1*965; Shearer & Simmons, 1965). Interpretation of the results is, however, difficult since the speech sound can directly interfere with the im­ pedance measurements and since activity from face- and jaw-muscles might be picked up by the electrode* in the stapedius muscle.

Several theories about the function of the stapedius muscle have been presented during the last century (for review see Jepsen, I963 and Miller, 1972). Many 10 authors have regarded the function of the muscle to be mainly protective (Hall- pike, 1935; Potter, 1936; Shapley, 1954; Fletcher & Riopelle, I960; Fletcher, 1962). Others have suggested that one important function of the stapedius reflex may be to reduce masking of high frequency components by low frequency components in complex sounds (Stevens & Davis, 1938; Perlman, I960; Lidén et al., 1964). Wever & Lawrence (1954) and Simmons & Beatty (1962) proposed that the stapedius muscle could serve several parallel functions such as protection, fixation of the ossicular chain and aid in auditory analysis.

One possible reason for the diversity of ideas is incomplete information re­ garding the role of the stapedius muscle in relation to important auditory functions (e.g. masking and auditory fatigue) and speech production. Results of auditory fatigue measurements in patients stapedectomized in one ear are not conclusive (Steffen et al., 1963; Ferris, 1965, 1967; Mills & Lilly, 1971). Recently it has been shown that speech discrimination at high levels is poor in without a functioning stapedius muscle (Borg & Zakrisson, 1973). The reason for this impairment is not known in detail but might be an increase in masking of high frequency speech components in the absence of normal stapedius reflex.

The aims of the work described in the present thesis were:

1) to evaluate the acoustic impedance change as a measure of stapedius muscle activity by comparing it with the integrated EMG from the muscle.

2) to analyse the effect of the stapedius muscle on masking and auditory fa­ tigue .

3) to study the mechanism for and to determine the degree of stapedius muscle activity during speech production. 11

MATERIAL

A total of 74 subjects were used for the experiments. Thirty-two of these sub ­ jects constituted a control group with normal hearing and normal in both ears (I, II). Thirty-three subjects had unilateral peripheral facial para­ lysis (Bell's palsy) (II, III). They had normal hearing up to at least 3.O kHz and normal eardrums. In 29 of these subjects the Bell's palsy included a uni ­ lateral stapedius muscle paralysis. Nine subjects had an perforation or eardrum scar in one ear with visible and stapedius tendon (I, IV). In the other ear the eardrum was either normal or had a perforation or scar.

METHODS

The impedance change of the ear was measured at 8OO Hz in response to acoustic stimulation with the method developed by Miller (i960, 1961, 1962) (I, II, III). Simultaneous recordings of ipsilateral and contralateral impedance change upon pure tone stimulation were made in the subjects of the control group and in the subjects with Bell's palsy. The impedance change, expressed in per cent of the maximal value obtained in each ear, was plotted as a function of stimulus in­ tensity to yield stimulus-response curves. On the basis of the ipsilateral and contralateral reflex recordings subjects from a large group of Bell's palsy patients were selected for experimental studies on the stapedius reflex func ­ tion.

The attenuation provided by the stapedius reflex at 0.5 kHz was determined with the method described by Borg (1968) (II). It was measured as the shift of cor­ responding contralateral stimulus-response curves obtained during paralysis and after recovery of the stapedius muscle function. Recovery was assessed on the basis of determinations of a symmetry index. The normal symmetry index was obtained from the stimulus-response curves in the control group.

Post-stimulatory auditory fatigue, measured as temporary threshold shift (TTS), was induced by steady-state narrow band noise (bandwidth 0.3 kHz) with CF at 0.5 or 2.0 kHz, presented monaurally at intensity levels between 90 and 120 dB SPL RMS level and with varied durations (ill). The hearing thresholds before and after the noise exposure were traced with a Békésy audiometer (Grason- Stadler type E-8OO) using a pulsating tone with frequency fixed at half an oc­ tave above the CF of the exposure noise. TTS, measured as the difference in hearing thresholds before and 20 sec or 2 min after the noise exposure, was determined in the subjects with Bell's palsy. 12

Monaural masking was produced by the same type of noise (with CF 0.5 kHz) as was used for induction of TTS while the RMS levels were varied between 85 and 120 dB SPL (II). The masked thresholds were traced at fixed frequencies be ­ tween 1.0 and 8.0 kHz with a Békésy audiometer using a pulsating pure tone.

The masking- and TTS-measûrements were repeated in those subjects who recov­ ered completely from their stapedius muscle paralysis.

EMG recordings were made in six of the subjects with eardrum perforation (I, IV). A unipolar tungsten microelectrode (Vallbo & Hagbarth, 1968) was inserted into the stapedius tendon as near the muscle as possible. The reference elec­ trode was a similar tungsten electrode inserted in the ipsilateral ear lobe. Neither local anesthesia nor sedation of the subjects were utilized. EMG from the stapedius muscle was recorded during acoustic stimulation (I, IV) and dur ­ ing the subjects" own vocalization (IV). It was shown that activity from jaw- and face-muscles could be picked up by the electrodes, making the interpreta ­ tion of the stapedius muscle EMG difficult. Special precautions were taken thereafter in order to minimize such activity during the subjects'own vocali­ zation.

The subjects vocalized an [a:] in a standardized manner with a minimum of movements of jaw- and face-muscles and with a distinct onset. The intensity of the vocalized [a:], measured 20 cm in front of the mouth, was approximately the same as at the entrance of the auditory canal. During vocalization the EMG was recorded from the stapedius muscle and the threshold for movement of the stapes or stapedius tendon was determined by observation of the stapes through an operating microscope (IV). Direct comparison could be made of the stapedius muscle activity during vocalization and acoustic stimulation by obtaining these two measures in the same experimental session (IV). 13

RESULTS AND CONCLUSIONS

The acoustic impedance change as a measure of stapedius muscle activity (I)

The results of the EMG recordings revealed that the stapedius reflex had proper ­ ties like those of a polysynaptic reflex.

The impedance change and the integrated EMG averaged both approximately linear functions of stimulus sound level up to at least 20 dB above the stapedius re­ flex threshold. Direct comparisons between the stimulus-response curves for in­ tegrated EMG and acoustic impedance change at 800 Hz were made after correction for the difference in sensitivity between the ipsilateral and contralateral re­ flex responses in the control group. This difference ranged from 5 dB (at thres­ hold) to 8 dB (at the stimulus intensity level giving 70 % of the maximum ob ­ tained impedance change). The direct comparison showed that the two methods of measuring stapedius muscle activity differed less than 6 dB at all stimulus intensity levels, the EMG generally being more sensitive.

It was concluded that recording of acoustic impedance change at 800 Hz is a valid method for determination of the activity of the stapedius muscle in a wide range of sound intensities. It was furthermore suggested that such measure ­ ments at suprathreshold levels can give clinically valuable information on dis ­ turbances of the lower .

Masking produced by low frequency noise (II)

The measurements on masking produced by low frequency noise were performed in three of the subjects with unilateral stapedius muscle paralysis. Narrow band noise with CF at 0.5 kHz was found to have a great masking effect on high fre­ quency pure tones, especially at high noise levels. Masking was considerably greater in the ear with stapedius muscle paralysis (affected ear) than in the ear with normal stapedius muscle function (nonaffected ear).

The difference in masking covered the frequency range 2.0 - 8.0 kHz, most pro­ minent betweën 4.0 - 8.0 kHz where it could amount to about 50 dB. Noticeable difference in masking appeared only for masking noise intensities above the in­ dividual stapedius reflex threshold, i.e., at 95 dB SPL of the masking noise but not at 85 dB SPL. The increase in masking in the affected ear was shown to agree well with what could be-expected from lack of attenuation normally pro­ vided by the stapedius reflex at 0.5 kHz. 14

The attenuation provided by the stapedius reflex on 0.5 kHz pure tones was de ­ termined in subjects with unilateral stapedius muscle paralysis, in whom the stapedius reflex recovered completely. The attenuation amounted to a maximum of about 20 dB with a growth rate of 7 dB per 10 dB increase of the stimulus in­ tensity level in the auditory canal.

It was thus shown that masking was considerably greater in the affected ear and that it covered important parts of the speech spectrum. The reduced speech dis ­ crimination in subjects with unilateral stapedius muscle paralysis shown by Borg & Zakrisson (1973) might therefore be explained by the increase in masking on the high frequency speech sounds due to lack of attenuation of the strong low frequency components of the speech.

Poststimulatory auditory fatigue (III)

In subjects with unilateral stapedius muscle paralysis, TTS after exposure with narrow band low frequency noise was considerably greater in the affected ear than in the nonaffected ear. The average TTS difference between affected and nonaffected ear was significant at and above 110 dB SPL of the narrow band noise. TTS difference appeared only in the range of noise levels where the sta­ pedius reflex was active. Great individual variations in the sensitivity for TTS were obvious, the noise intensity required to give 10 dB TTS in the affec­ ted ear varying throughout a range of nearly 15 dB for the different subjects. In the nonaffected ear only a slight TTS was obtained in all measurements even at the highest noise exposure levels. A tendency to negative rank order corre­ lation between the TTS values in the nonaffected ear and the attenuation pro ­ vided by the stapedius reflex at the exposure intensities was found.

No difference in TTS was found between the affected and nonaffected ear after low frequency noise exposure in Bell's palsy subjects whose stapedius muscle function was estimated to be normal according to the symmetry index.

When the measurements were repeated after recovery of stapedius muscle function the TTS in the affected ear was significantly reduced after low frequency noise exposure and did not differ from the TTS in the nonaffected ear.

After exposure with narrow band noise, CF 2.0 kHz, there was no significant difference in TTS at 3*0 kHz between the affected and nonaffected ear.

In short, the stapedius reflex significantly decreased poststimulatory auditory fatigue after exposure to low frequency noise at levels above the stapedius reflex threshold. The lack of attenuation in the ears with stapedius muscle paralysis would sufficiently explain their great sensitivity to auditory 15 fatigue in the low frequency range. The results are compatible with the assump ­ tion that the stapedius reflex provides protection from hearing damage by noise within the low frequency range. Low frequency sounds may not be attenuated by the stapedius reflex if most of their energy lies within the latency of the stapedius reflex and may induce fatigue if they are repetitive. Similarly low frequency sound may be fatiguing if the stapedius reflex responses are abnor ­ mally weak, for instance when the central nervous system is influenced by alco­ hol or barbiturates (Borg & Miller, 1967).

Stapedius muscle activity during vocalization (IV)

EMG activity of the stapedius muscle and movements of the stapes during the subject's own vocalization of [a:] were studied in subjects with eardrum per­ foration. With EMG, stapedius muscle activity was recorded in the entire range of vocalization from the weakest to the strongest the subjects could produce. The integrated EMG activity increased as the vocal intensity was increased in a wide range of vocal intensities.

The threshold for stapedius muscle activity was near the weakest vocal intensi ­ ty which the subjects could produce. On the average this corresponded to 68 and 73 dB SPL for EMG and visible movement of the stapes respectively. The thres­ hold values during vocalization were on the average 36 dB below the stapedius reflex threshold in response to contralateral pure tone stimulation.

At normal vocal effort the stapedius muscle activity was 50-60 per cent of the maximum obtained.

It was concluded that the stapedius muscle activity significantly attenuated the transmission of low frequency components of a person's own speech. It was suggested that the consequence could be an improvement of intelligibility of simultaneous speech from other persons.

It was furthermore shown that EMG activity during the subjects' own vocaliza­ tion often could be recorded before the start of the speech sound, up to 150 msec.

The mechanisms for stapedius muscle activation during vocalization were dis ­ cussed. It was pointed out that air-conducted sound could play a role for activation of the stapedius muscle only at high voice levels. The role of the -conducted sound from the person's own voice as activator of the stapedius muscle was difficult to estimate. On the basis of the temporal relation between stapedius muscle EMG and the speech sound it seems justified to conclude that 16

stapedius muscle activation from the central nervous system as a part of the vocalization process is one important mechanism during speech production at least at low and medium intensity levels.

SUMMARY

The following are the main results and conclusions reached:

1) Thé acoustic impedance change of the ear agreed well with the integrated EMG of the stapedius muscle in a wide intensity range.

2) Results of EMG recordings showed that the stapedius reflex has properties to be expected from a polysynaptic reflex.

3) The ipsilateral stapedius reflex was on the average 5-8 dB more sensitive than the contralateral.

4) The stapedius reflex considerably reduced masking on 2.0 - 8.0 kHz pure tones produced by low frequency narrow band noise. The decrease was greatest between 4.0 and 8.0 kHz and could amount to about 50 dB.

5) The stapedius reflex significantly decreased TTS produced by exposure with low frequency narrow band noise.

6) The attenuation provided by the stapedius reflex on 0.5 kHz pure tones was 7 dB per 10 dB increase of the stimulus intensity level in the auditory canal. The maximum attenuation was about 20 dB.

7) The findings mentioned in 4 and 5 could be explained by the attenuation provided by the stapedius reflex.

8) The stapedius reflex had no effect on TTS at 3.0 kHz after exposure with 2.0 kHz narrow band noise.

9) Stapedius muscle activity was recorded during largely all intensity levels of the subject's own vocalization.

10) During normal vocal effort the stapedius activity was 50-60 % of its maxi ­ mum value.

11) The stapedius muscle can be activated from the central nervous system as a part of the vocalization act.

12) The functional significance of stapedius muscle activity during speech pro­ duction was suggested to be reduction of masking of the strong low frequency components of the subject's own voice. 17

ACKNOWLEDGEMENTS

To the Head of the Department of Otorhinolaryngology, University of Umeå, Professor Herman Diamant, I wish to express my sincere gratitude. He has pla­ ced the facilities of the department at my disposal and encouraged and sup ­ ported me throughout the whole course of the study.

I am deeply indebted to my supervisor, Assistant Professor Aage Miller, who made me a member of his team, introduced me into the field of physiological acoustics and initiated this study. Throughout its execution he has guided and helped me with diverse theoretical and practical problems.

I want to thank Assistant Professor Sigfrid Blom who allowed me to use his la­ boratory facilities. He has also supported me with advice, constructive criti­ cism and many valuable discussions.

I am very grateful to Assistant Professor Erik Borg for his enthusiastic en­ gagement in all parts of the investigations.

To Assistant Professor Åke Vallbo I want to express my appreciation for assist­ ing in the analysis of the EMG recordings.

I would like to express my heartfui gratitude to the following persons:

Mr. Olov Stensson and Mr. Stig Wiklund, for their skilful technical assistance,

Ms. Inger Nordberg for excellent secretarial work,

Ms. Margareta Danielsson for making calculations on which the curves, diagrams and tables are based,

Ms. Margareta Jonsson for capable contribution to the preparation of the manu ­ scripts, especially the figures and tables,

Mr. Per-Olov Carlsson for expert assistance during preparing reproductions of the EMG recordings,

Ms. Pamela Boston for giving me editorial assistance with the manuscripts

and Dipl. Ing. Günter Rosier for translating the summaries into German.

All members of the staffs of the Department of Otorhinolaryngology, University of Umeå, and the Department of Physiology II, Karolinska Institutet, Stockholm, have kindly cooperated at every stage and made my task stimulating.

The studies were supported by grants from the Swedish Medical Research Council (Project No 61P-4274), the Medical Faculty, University of Umeå, P. Frenckner's fund, B. Fromm's fund and the funds of Karolinska Institutet. 18

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