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Acta physiol. scand. 1973. 88. 382-391 From the Institute of Neurophysiology, University of Oslo, and the Ear, Nose and Throat Department, Rikshospitalet, Oslo, Norway Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear BY ERIK TEIG Received 14 October 1972 Abstract TEIG, E. Differential effect of graded contraction of middle ear muscles on the sound transmission of the ear. Acta physiol. scand. 1973. 88. 382-391. The middle ear muscles in cats have been brought to different degrees of contraction by electric stimulation through electrodes placed in the proper nuclei in the brain stem. The resulting change in the amplitude of the cochlear microphonic potentials produced by pure tones from 250-7000 Hz has been used to determine the change in the sound transmission of the ear. Later the muscle tension produced by the same stimulus strengths have been determined myographically. The main effect of weak contractions involving only a relatively small number of motor units was more selective than the effect of stronger muscle contractions. Contractions of the stapedius muscle involving only up to 10-15 per cent of the maximal contraction force reduced the microphonics below 2000 Hz, with most marked effect on the lowest frequencies tested. Stronger tensions produced an additional modest reduction, equally pronounced for all sound frequencies. The most marked effect of contractions of the tensor tympani, amounting to 10-15 per cent of the maximal contraction force, was a reduction of microphonics of frequencies below 750 Hz, while higher frequencies could be either slightly enhanced or reduced. Stronger contractions had a less selective effect with respect to various frequencies and gave less reduction of the sound transmission per gram tension. In most studies where different degrees of middle ear muscle tension have been cor- related with the corresponding effect upon the sound transmission of the middle ear, relatively strong tensions have been used, and a certain degree of reduction of the sound transmission has been the common finding (Wever and Bray 1937, 1942, Neer- gaard et al. 1963, Cancura 1970), giving support to the notion that the muscles have mainly a protective function against excessive sound (Wever and Lawrence 1954). However, in experiments in which the muscles have been brought to light contrac- tion either through reflex activation (Wever and Vernon 1956, Price 1963 a, b) or by other means (Starr 19691, enhancement of certain frequencies has been the com- mon finding, suggesting that weak contraction of the middle ear muscles has a dif- ferent and more selective influence on the sound transmission than strong contrac- tion of the muscles.

Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear

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Page 1: Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear

Acta physiol. scand. 1973. 88. 382-391 From the Institute of Neurophysiology, University of Oslo, and the Ear, Nose and Throat

Department, Rikshospitalet, Oslo, Norway

Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear

BY

ERIK TEIG

Received 14 October 1972

Abstract

TEIG, E. Differential effect of graded contraction of middle ear muscles on the sound transmission of the ear. Acta physiol. scand. 1973. 88. 382-391.

The middle ear muscles in cats have been brought to different degrees of contraction by electric stimulation through electrodes placed in the proper nuclei in the brain stem. The resulting change in the amplitude of the cochlear microphonic potentials produced by pure tones from 250-7000 Hz has been used to determine the change in the sound transmission of the ear. Later the muscle tension produced by the same stimulus strengths have been determined myographically. The main effect of weak contractions involving only a relatively small number of motor units was more selective than the effect of stronger muscle contractions. Contractions of the stapedius muscle involving only up to 10-15 per cent of the maximal contraction force reduced the microphonics below 2000 Hz, with most marked effect on the lowest frequencies tested. Stronger tensions produced an additional modest reduction, equally pronounced for all sound frequencies. The most marked effect of contractions of the tensor tympani, amounting to 10-15 per cent of the maximal contraction force, was a reduction of microphonics of frequencies below 750 Hz, while higher frequencies could be either slightly enhanced or reduced. Stronger contractions had a less selective effect with respect to various frequencies and gave less reduction of the sound transmission per gram tension.

In most studies where different degrees of middle ear muscle tension have been cor- related with the corresponding effect upon the sound transmission of the middle ear, relatively strong tensions have been used, and a certain degree of reduction of the sound transmission has been the common finding (Wever and Bray 1937, 1942, Neer- gaard et al. 1963, Cancura 1970), giving support to the notion that the muscles have mainly a protective function against excessive sound (Wever and Lawrence 1954). However, in experiments in which the muscles have been brought to light contrac- tion either through reflex activation (Wever and Vernon 1956, Price 1963 a, b) or by other means (Starr 19691, enhancement of certain frequencies has been the com- mon finding, suggesting that weak contraction of the middle ear muscles has a dif- ferent and more selective influence on the sound transmission than strong contrac- tion of the muscles.

Page 2: Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear

MIDDLE EAR MUSCLES AND SOUND TRANSMISSION 383 The pattern of this possible selective action is not clear. During spontaneous con-

tractions of the middle ear muscles in the guinea pig Wiggers (1937) found a reduc- tion in the transmision of frequencies below 1000 Hz and a slight enhancement of frequencies from 1000 Hz up to 2500 Hz, while Mprller (1965) in both cats and rabbits found that contraction of either or both middle ear muscles reduced fre- quencies below 2000 Hz leaving higher frequencies practically unaffected.

In apparent contrast to these findings are the results of Wever and Vernon (1965) and Price (1963 b) who used 450 Hz as a test tone in cats and rabbits and found enhancement of the transmission of this frequency during weak reflex activa- tion of the middle ear muscles.

The main difficulty in correlating the reported data is that none of the investiga- tions included measurements of the muscle tensions being exerted. Since the tensions of individual motor units of the middle ear muscle are now known (Teig 1972 b) and since ordinarily the weakest motor units in a muscle are activated first and used most frequently (Henneman and Olson 1965) we can estimate the tensions most often produced by the muscles under physiological conditions.

The purpose of the present investigation was to test if the effect of different degrees of weak contractions involving only a relatively small number of motor units was more selective and different from the effect of stronger muscle tensions. It was found that the main effect of weak to moderate contraction of the muscles was a reduction of the transmission of tones below certain frequencies (2000 Hz and 750 Hz for the stapedius and the tensor tympani, respectively), while the transmission of higher frequencies could be moderately enhanced or reduced. Additional contraction force resulted in a further moderate reduction of the transmission of all frequencies.

Methods Operative procedure. Adult cats ( 1.9-3.9 kg) were anesthetized with sodium pentobarbital (3.0 mg/kg) , tracheotomized and fixed in a headholder. A retroauricular incision through the skin and the platysma exposed the posterolateral parts of the left mastoid bulla, and after cutting through the cartilagenous part of the ear canal, good access to the mastoid bulla was obtained. This was drilled open exposing the round window, and after removal of a part of the septum between the middle ear and the left mastoid bulla, access to the belly of the tensor tympani muscle was obtained. The canal of the facial nerve was drilled open from the posterolateral part of the bulla to the level of the stapedius muscle. A small part of the stapedius muscle was exposed to allow electromyographic recording. Bipolar 0.5 mm insulated silver electrodes were then placed on the surface of the tensor tympani and the stapedius muscles. A 1 mm ball point silver electrode was put on the rim of the round window to record the cochlear microphonic potentials while a similar indifferent electrode was placed on the surface of the parotid gland which was exposed during the operation. The septum between the middle ear cavity and the mastoid bulla, and later the mastoid bulla itself, were then reconstructed by dental acrylic, leaving the electrodes secured in place, The posterior part of the skull was opened and the posterior part of the cerebellum removed by- suction, exposing the floor of the fourth ventricle. Using the obex as a reference, stimulating electrodes held in a stereotactic manipulator could be placed in the motor nucleus of the trigeminal nerve and in the facial nucleus ('Teig 1972 b ) .

Stimulation and recording technique. Since it has proved difficult to reproduce acoustically elicited middle ear muscle contractions in anesthetized cats (M0ller 1965), electric stimulation of the proper motor nuclei in the brainstem was used to activate the muscles. Bipolar insulated tungsten electrodes (tip diameter about 10 pm, 1.5-2.0 mm apart) were used as stimulation electrodes and inserted in the facial nucleus or in the motor nucleus of the trigeminal nerve,

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384 ERIK TEIG

depending on which muscle being tested. When a preliminary localization of the appropriate nucleus has been obtained stereotactically, negative square pulses of 0.2 ms duration and an amplitude sufficient to elicite muscle action potentials in one of the middle ear muscles were applied. Checks were made that only one muscle was excited.

Usually the electrodes had to be moved several times to find a location close enough to the motor nucleus of m e muscle to avoid simultaneous stimulation of the other muscle when the stimulus strength was increased. Pure tones from 250 to 7000 Hz were applied to the operated ear through an earphone type Telephonics T D H 39 attached to a rubber tube (30 mm long- diameter 12 mm). To ensure a closed cavity, the rubber tube was glued to the temporal bone around the ear canal with dental cement. When calibrated in a 2 cm3 coupler with a Briiel and Kjaer condenser microphone type 4134, pure tones of 60-90 dB would give a cochlear micro- phonic potential of about 400 pV. Input loutput curves of applied sound /microphonic potentials showed that this was within the linear response when plotted out in dB. The phenomenon of overloading (Stevens and Davis 1938) did thus not interfere with the interpretation of the results.

The electromyograms were amplified, displayed on separate beams on the oscilloscope, and served as control to show which muscle was tested. With the depth of anesthesia employed no reflex contraction of the middle ear muscles took place, judged from the electromyographic response. Trains of 0.2 ms negative square pulses with a frequency of 100 Hz and duration of 150 ms, which was known to give tetanic fusion of the middle ear muscles (Teig 1972 a ) were then applied. This relatively short stimulation period was employed to avoid muscular fatigue even if the period was not long enough for the muscles to reach the tension plateau (Teig 1972 b) . The stimulus intensity was varied in small steps, and the amplitude of the electro- myogram and the effect on the transmission of a 500 Hz tone were used as a control of the stability of the preparation. Series and experiments, during which these control records changed were discarded. I n experiments where the motor nuclei of the muscles were found only after prolonged manipulation, they had a tendency of being reduced, probably due to local damage of the brainstem.

T o rule out the possibility that accidental stimulation of the olivocochlear bundle should in- fluence the results (Fex 1962), the tendons of the middle ear muscles were cut in two separate experiments. Stimulation of the motor nuclei then did not give any change in the microphonic potentials.

At the end of each experiment, the bulla and the middle ear were widely exposed, and the tensor tympani and the stapedius muscles were attached to an electromechanical trans- ducer. This part of the procedure has been described previously (Teig 1972 b ) . The optimal length of the muscles was determined by applying strong single pulses (0.2 ms duration) and observing the size of the resulting twitch (Teig 1972 a ) . The tetanic contraction of the muscles at optimal length was then recorded a t the stimulus intensities used during the microphonic experiments. The tension was maximal approximately 10 ms after the last stimulus pulse, and the tetanic tension as we]! as the effect on the microphonic potentials were, therefore, both measured at this point.

Results

The effect upon the sound transmission of small tensions of either of the two muscles was dependent both on the sound frequency tested and upon the particular tension being used, In preliminary experiments single muscle twitches were used to modulate the microphonic potentials. Fig. 1 A shows the transient change of the microphonic potentials of a 500 H z tone caused by a tensor tympani muscle twitch, and Fig. 1 B shows the effect of the same twitch upon the 2000 Hz microphonic potentials. The mechanical record of the twitch is shown in the lower row for comparison. Muscle contraction caused a reduction of the microphonic potentials of the test tones. This reduction often matched the time course of the contractions, but could also, particu- larly with stronger contractions have a different shape. The matching was best at lower frequencies (500 Hz) . When testing with higher sound frequencies (2000 Hz) DC components made the recordings difficult to interpret. These changes were

Page 4: Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear

MIDDLE EAR MUSCLES AND SOUND TRANSMISSION

A 500Hz B 2000Hz

385

Fig. 1. Effect of single twitches (lower row) & of the tensor tympani upon the microphonic

A I l g

potentials (upper row) produced by a 500 Hz (A) , and by a 2000 Hz tone (B).

H

20ms

similar to those described by Galambos and Rupert (1959). Because of the distorted shape of the recordings at higher test frequencies it was decided to used tetanic stimulation. The smoother tetanic contraction curve reduced these disturbances. Measurements at the end of a tetanic contraction further avoided the initial acoustic transients caused by abrupt tension changes not only in the ossicular chain but also in the ear drum and in the supporting ligaments.

Fig. 2 exemplifies some of the effects of tetanic contraction of the tensor tympani muscle upon the microphonic potentials recorded. The upper row shows the myographic recording of a submaximal muscle contraction ( A ) with peak tension of 1.2 g, and the effect of this contraction upon the 500 Hz (B), 750 Hz (C) and 1000 Hz (D) microphonic potentials. The corresponding electromyograms (EMGs) of the tensor tympani and the stapedius muscles are shown under each recording of the microphonic potentials. Only the tensor tympani muscle was active. The lower row (E, F, G and H) shows the corresponding records of a stronger contraction with peak tension of 4.5 g. A light contraction reduced the transmission of the 500 Hz tone (Fig. 2 B) while at the same time enhancing the transmission of the 750 Hz tone (Fig. 2 C) .

This enhancement of the transmission of certain frequencies was regularly seen to follow light contraction of both the stapedius and the tensor tympani. The effect of a relatively strong muscle contract;on (Fig. 2 F, H) usually reached its maximum be- fore the muscle contraction itself reached its maximum, as judged by the myographic recording (Fig. 2 E) . This apparent saturation of the effect of the muscle contrac- tion was seen with both stapedius and tensor tympani contractions.

The effect upon the sound transmission of stapedius contractions giving tensions between 400 mg and 6.5 g was studied in seven experiments. Selected frequencies from 250 Hz to 6000 or 7000 Hz were used as test tones. The reduction of the micro- phonic potentials was most pronounced for the lower frequencies, particularly at small tensions. The experiment shown in Fig. 3 A and 4 A exemplifies the general conclusion on the stapedius muscle. In Fig. 3 A the change in microphonic potential amplitude produced by different muscle tensions is given in dB with the initial 400 PV microphonic potentials as reference level. Increasing muscle tension up to

7-733003. Acta phyriol. scond. 88: 3

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386

68

- 5

.-;0

-15

-20

ERIK TEIG

0 -

-

-

-

-

A B 500Hz c 750Hz 0 lOOOHz

F I;. H

100 ms

Fig. 2. Upper row shows a tetanic contraction (.4) of the tensor tympani with a peak tension of 1.2 g, and the effect of this contraction upon the microphonic potentials produced by a 500 Hz (B) , a 750 Hz (C) and a 1000 Hz (D) tone. The two tracings below the microphonic potential trace are from above the electromyograms of the tensor tympani and the stapedius muscles, respectively. Lower row (E, F, G, and H ) gives similar data, but with stronger tetanic tension (peak tension of 4.5 g ) .

2.10 g led to a decrease in the microphonic potential amplitude at frequencies below 3000 Hz. The effect was most pronounced for the lowest frequency tested, 250 Hz. Small tensions up to 1.27 g gave improvement of the transmission as seen by in- creased microphonic potentials, up to 0.5 dB in the 3000-4000 Hz range. Addi- tional increase in the muscle tension gave a reduction of the microphonic potentials at all frequencies.

A M STAPEDIUS 8

I I , , 1 , 1 1 1 1 I I I , # ' / I 250 530 1000 2000 4000 800CHr

0 - 0 9 0 g 0 - 3 2 8 9 0-073; A - 3 6 6 9

I , 8 ' , , I I , 1 8 , '

250 5CI 1300 2000 1000 @0c:?z

e-1 0 4 9 m - 4 3 5 9

a-1 27 g A - 2 1 0 9 0-517 g

. - l ? E ; ~ - 5 8 4 g

A-1 9 4 3 m - 1 1 8 g

Fig. 3. Change in the microphonic potential amplitude at different test frequencies with the initial 400 yV responses as the reference level, produced by the muscle tensions indicated by the symbols under the graph. A. Stapedius muscle. B. Tensor tympani muscle.

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MIDDLE EAR MUSCLES AND SOUND TRANSMISSION 387

4 M SIAPEOIUS

- 5

Fig. 4. Incremental effect upon the cochlear microphonic potentials at different fre- quencies produced by stepwise changes in muscle tension as indicated above each diagram. A. Stapedius muscle. B. Ten- sor tympani muscle.

Fig. 4 A, which is from the same experiment as Fig. 3 A, shows the incremental effect upon the transmission of different sound frequencies of stepwise increased tensions. The upper diagram shows the effect of an increase in muscle tension from zero to 0.9 g, the next graph the effect of increasing the muscle tension from 0.9 g to 1.04 g. The subsequent graphs similarly show the effect of increases in the muscle tension given by the numbers above each diagram. In a previous inactive muscle an increase in muscle tension to about 1.0-1.5 g mainly affected the frequencies below 2000 Hz. At the lowest frequencies the sound reduction caused by such a tension increase could be up to 12-15 dB. When a similar degree of tension was added to a previous tension of 2-3 g, this gave an additional reduction in the sound trans- mission of 2-3 dB only, and had an almost equal effect over the whole frequency range.

The effect of tensions in the tensor tympani between 650 mg and 12.6 g (6 expts.) upon the sound transmission of different frequencies is exemplified by the experi- ment shown in Fig. 3 B and 4 B. In Fig. 3 B the change in transmission produced by various tensions in the tensor tympani is given in dB with the initial 400 pV micro- phonic potentials as reference level. Fig. 4B, which is from the same experiment, shows the net effects for the six step increases of muscle tension as indicated above each graph. Tensions up to 5.84 g gave a reduction in the sound transmission of most frequencies below 4000 Hz. The reduction was most marked for frequencies below 750 Hz. This effect was accompanied by peaks of absolutely or relatively in-

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388 ERIK TEIG

creased transmission in certain frequency ranges. In the 6 ears tested 3 peaks of absolutely or relatively increased transmission were found, one in the 750-1200 Hz range, a second in the 1500-2500 Hz range, and a third in the 4000-6000 Hz range. In each ear these peaks of absolutely or relatively increased transmission had a surprisingly narrow range such as shown in Fig. 3 B.

As in the stapedius muscle, the effect of increasing the tension of the tensor tym- pani muscle was dependent upon the previous tension in the muscle. In an inactive muscle an increase in muscle tension to 5-6 g had its strongest effect upon fre- quencies below 750 Hz giving maximal sound reduction of more than 15 dB. A similar tension added to a tension of 5.84 g gave nearly equal effect upon all fre- quencies up to 4000 Hz, amounting to 3-5 dB only (Fig. 4 B) .

The effect of simultaneous contraction of both muscles was studied in four experi- ments. Since the relative activity of the two muscles under physiological conditions is not known, the experiments were limited to test if the overall effect of the two muscles was qualitatively the same as that of each muscle alone. The effect of simul- taneous contraction of the two muscles was similar to that produced by each muscle alone, and there was no simple summation of the effect of the two muscles.

Discussion

E f f e c t of contraction of the tensor tympani muscle Except for the very weakest tensions, light to moderate contraction of the tensor tympani had a rather selective depressive effect upon the transmission of frequencies below 750 Hz, while the additional effect of stronger contraction was a reduction of the transmission of all frequencies. The effect of a muscle tension of 2-3 g closely resembled that found by Wever and Vernon (1955) during tensor tympani contrac- tion elicited by acoustic stimulation of the contralateral ear, 2 dB above reflex threshold (stapedius tendon cut) . The difference between the present findings and the relative insignificance of the tensor tympani in the reduction of cochlear micro- phonic potentials in awake versus anesthetized cats (acoustic reflex inactive) reported by Simmons (1959) is hard to account for. Since frequencies below 500 Hz were not tested by Simmons, and probably weak tensor tympani contractions were elicited, a possible depression of transmission of the lower frequencies might have escaped notice. In the present study stronger tensions always produced reduction of the transmission of low frequency sound. This, naturally, also applied to the effect of tetanic versus twitch contractions. This is in contrast to Starr’s (1969) observation that single shocks produced stronger reductions of low frequency sound than tetanic stimulation did. Since the present experimental arrangement resembled that used by Starr (1969) there is no obvious explanation for this apparent discrepancy.

The tetanic tensions or sizes of limb muscle motor units are inversely related to the reflex excitability of their motoneurones (Henneman and Olson 1965). Further- more, in the tensor tympani low voltage motor units with low firing frequency are activated before high voltage motor units with high firing frequency (Okamoto,

Page 8: Differential Effect of Graded Contraction of Middle Ear Muscles on the Sound Transmission of the Ear

MIDDLE EAR MUSCLES AND SOUND TRANSMISSION 389

Sat0 and Kirikae 1954). I t therefore seems probable that the slow twitch motor units (Teig 1972 b) are activated first during physiological activation of the tensor tympani muscle. These motor units had an average twitch tension of 26.4 mg. Each tensor tympani muscle was estimated to consist of an average of 40 such slow twitch motor units. Assuming a tetanus/twitch ratio of 4, these motor units together, when maximally activated, should be capable of producing a tension of approximately 4 g. This is the tension range within which the relatively selective reduction of low fre- quency tones (below 750 Hz) was found to take place.

The less selective influence upon the sound transmission when the muscle tension exceeded 5-6 g and the relatively modest additional damping effect of such con- tractions suggest that reduction of low frequency transmission is only one aspect of the function of this muscle. I t should be kept in mind that the 5-6 g tension which gives the most selective high-pass filter action represents only about 10 per cent of the maximal contraction force of this muscle (Teig 1972 a ) . Effect of contraction of the stapedius muscle Weak to moderate contraction of the stapedius muscle produced a change in the transmission of the middle ear similar to that found during stapedius muscle con- traction in cat (tensor tympani anesthetized) elicited by acoustic stimulation of the contralateral ear ( M ~ l l e r 1965), suggesting that in the latter experiments the con- traction force of the muscle was about 1-1.5 g. The effect of the weakest contrac- tions used during the present investigation (400 mg) was similar to, but more pronouned than that obtained by Wever and Vernon (1955) using acoustic stimula- tion of the contralateral ear 2 dB above reflex threshold and having the tensor tympani cut. The effect on the transmission pattern was also similar to that found in man during electric stimulation of the external ear canal (Pichler and Born- schein 1957).

The twitch tension of the stapedius motor units ranges between 11 and 168 mg (Teig 1972 b) . Assuming a tetanus/twitch ratio of 4, the tetanic tension of indivi- dual motor units upon maximal stimulation should range between 40 and 750 mg. The supposedly slow muscle fibres of the stapedius, which had a smaller diameter than the fast ones (Teig and Dahl 1972), may constitute even weaker motor units than the fast twitch type. Since the smallest motor units in a muscle are most numerous (Henneman and Olson 1965, Teig 1972 b) , the stapedius muscle is probably capable of an extremely fine gradation of the effect upon the sound trans- mission of frequencies below 1500 Hz. Stronger contractions of the stapedius muscle produced a change in the sound transmission similar to that found in artificial ten- sion studies on temporal bone preparations in cat (Wever and Bray 1942) and in man (Neergaard et al. 1964). The effect upon the sound transmission of tension in excess of 2 g was almost simliar for all frequencies. Since 2 g represent only about 15 per cent of the total contraction force of the stapedius muscle in the cat (Teig 1972 a ) ~ these data suggest an additional function for this muscle to that of reducing the transmission of low frequency sound. The reductions of transmission for a given amount of tension were much greater for the stapedius muscle than for the tensor

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390 ERIK TEIG

tympani, an observation which was also made by Wever and Bray (1937, 1942). From the description of the middle ear as a mechanical transformer (Wever and Lawrence 1954) it appears that the distance from the rotational axis of the ossicular chain to the insertion of the stapedius muscle is considerably longer than the distance to the insertion of the tensor tympani. The lever arm of the stapedius muscle is thus longer than the lever arm of the tensor tympani. This offers one explanation for the difference in effectiveness per gram force of the two muscles. Possible physiological significance of middle ear muscle contraction Since low frequency tones mask high frequency tones far more effectively than vice versa, the differential suppression of low frequency tones, as pointed out by Stevens and Davis (1938) improves hearing for faint high frequency tones in the presence of loud low frequency tones. The present finding that the tensor tympani muscle upon mild to moderate contraction had the strongest damping effect upon sound below about 750 Hz might give this muscle a role for selective removal of masking when the animal is directing its awareness against weak sounds of higher frequencies. The importance of the stapedius muscle as a high-pass filter was emphasized by Lid& Nordlund and Hawkins (1964) who found stapedectomized patients with the stape- dius tendon cut to have a poorer discrimination in noise than stapedectomized pa- tients with comparable hearing and an intact stapedius muscle function.

Even if peaks of absolutely or relatively increased transmission were a frequent finding in the present study, these peaks had almost the same location during dif- ferent degrees of muscle tension. The continual shift in resonant loci believed to take place during small changes in muscle tension (Simmons 1964) could not be con- firmed.

Enhancement of low frequency sound such as the 450 Hz frequency found in a certain proportion of cats (Wever and Vernon 1956) and rabbits (Price 1963 b) during acoustically elicited contraction of both middle ear muscles was not observed with the muscle tensions used during the present study. I t is possible that this occurs in some animals during very weak contractions of the muscles as a by-product of the change in tension as suggested by Stevens and Davis (1938). The possibility that the two muscles should have a quite different effect when operating together than when operating alone seems unlikely both from the present experiments and from previous work (Mdler 1965, Wever and Vernon 1955, Neergaard et al. 1964).

The relatively moderate reduction in the transmission of all frequencies which developed when the tension of the middle ear muscles exceeded about 10-15 per cent of the total contraction force is surprising. However, this sound reduction could be a less important effect of the muscle tension necessary to give sufficient contact pressure between the auditory ossicles at higher sound levels (BCkCsy 1960).

References

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