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Arch. Otorhinolaryngol. 221, 47--60 (1978) Archives of Oto-Rhino-Laryngology @ Springer-Verlag 1978 On the Influence of the Middle Ear Muscles Upon Changes in Sound Transmission G. Sesterhenn and H. Breuninger ENT-Clinic of the University of Tiibingen (Head: Prof. Dr. D. Plester), Silcherstral3e 5, D-7400 Tfibingen, Federal Republic of Germany Zusammenfassung. An drei gefibten Versuchspersonen wird der Einflul3 erzwun- gener Impedanz/inderungen auf den Frequenzgang des Mittelohres untersucht. Der Tensorreflex wurde durch kfinstlich erzeugten fJber- und Unterdruck irn /iul3eren Geh6rgang simuliert. Die Schwellenverschiebung (A L) bei Luftdruck ist am gr6/3ten in der N/ihe der ersten Resonanz des Mittelohres (AL = 8 dB bei 500 Hz). Ober Luftleitung ist der Effekt um etwa 2 dB st/irker als fiber Knochen- leitung. Die ,S, nderung der Schalliibertragung ist f/Jr alle Pegel konstant und vor allem auf die Irnpedanz/inderung des Mittelohres zurfickzufiihren. Der Einflul3 des Stapediusreflexes ist stark pegelabh/ingig. Von der H6r- schwelle bis 70 dB (HL) betr/igt die D/impfung nur etwa 2 dB im Bereich unter 1 kHz. Bei h6chsten Pegeln kann die D/imfung bis zu 30 dB betragen, so dal3 die Erregung der Cochlea fast konstant bleibt. Es wird vermutet, dab die starke Pegelabh/ingigkeit der Ubertragungs/inderung beirn Stapediusreflex auf eine Ver/inderung der Schwingungsform des Stapes bei hohen Schalldr/icken zurfick- zuffihren ist. Der Regelmechanismus arbeitet ohne nennenswerte Verzerrungen und hat eine Einschwingzeit yon etwa 100 ms, Eine Funktion des Stapediusre- flexes k6nnte darin bestehen, bei Eigenphonation das Ohr vor hohen Amplituden zu schfitzen. Das Ph/inomen des Rekruitments bei Otosklerose ist vermutlich auf die fehlende Regelwirkung des fixierten Stapes zurfickzuffihren. Da auch Fre- quenzen fiber 1 kHz bei hohen Pegeln ged/impft werden, ist eine Schutzfunktion des Stapediusreflexes bei Belastung dutch Industriel/irm durchaus anzuneh- men. Sehliisselwiirter: Mittelohr - Akustischer Reflex -- Tensorreflex - Schallfiber- tragung - Otosklerose - Rekruitment - Verdeckung - Facialisparese. Summary. The influence of artificially induced impedance claanges on the fre- quency response of the middle ear has been investigated in three experienced listeners. The tensor-reflex has been simulated by application of positive and negative air-pressure to the outer ear canal. In this test-situation, the threshold- shift (AL) obtained is greatest in the surrounding of the first resonance of the 0302-9530/78/0221/0047/$ 02.80

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Page 1: On the influence of the middle ear muscles upon changes in sound transmission

Arch. Otorhinolaryngol. 221, 47--60 (1978) Archives of Oto-Rhino-Laryngology @ Springer-Verlag 1978

On the Influence of the Middle Ear Muscles Upon Changes in Sound Transmission

G. Sesterhenn and H. Breuninger

ENT-Clinic of the University of Tiibingen (Head: Prof. Dr. D. Plester), Silcherstral3e 5, D-7400 Tfibingen, Federal Republic of Germany

Zusammenfassung. An drei gefibten Versuchspersonen wird der Einflul3 erzwun- gener Impedanz/inderungen auf den Frequenzgang des Mittelohres untersucht. Der Tensorreflex wurde durch kfinstlich erzeugten fJber- und Unterdruck irn /iul3eren Geh6rgang simuliert. Die Schwellenverschiebung (A L) bei Luftdruck ist am gr6/3ten in der N/ihe der ersten Resonanz des Mittelohres (AL = 8 dB bei 500 Hz). Ober Luftleitung ist der Effekt um etwa 2 dB st/irker als fiber Knochen- leitung. Die ,S, nderung der Schalliibertragung ist f/Jr alle Pegel konstant und vor allem auf die Irnpedanz/inderung des Mittelohres zurfickzufiihren.

Der Einflul3 des Stapediusreflexes ist stark pegelabh/ingig. Von der H6r- schwelle bis 70 dB (HL) betr/igt die D/impfung nur etwa 2 dB im Bereich unter 1 kHz. Bei h6chsten Pegeln kann die D/imfung bis zu 30 dB betragen, so dal3 die Erregung der Cochlea fast konstant bleibt. Es wird vermutet, dab die starke Pegelabh/ingigkeit der Ubertragungs/inderung beirn Stapediusreflex auf eine Ver/inderung der Schwingungsform des Stapes bei hohen Schalldr/icken zurfick- zuffihren ist. Der Regelmechanismus arbeitet ohne nennenswerte Verzerrungen und hat eine Einschwingzeit yon etwa 100 ms, Eine Funktion des Stapediusre- flexes k6nnte darin bestehen, bei Eigenphonation das Ohr vor hohen Amplituden zu schfitzen. Das Ph/inomen des Rekruitments bei Otosklerose ist vermutlich auf die fehlende Regelwirkung des fixierten Stapes zurfickzuffihren. Da auch Fre- quenzen fiber 1 kHz bei hohen Pegeln ged/impft werden, ist eine Schutzfunktion des Stapediusreflexes bei Belastung dutch Industriel/irm durchaus anzuneh- men.

Sehliisselwiirter: Mittelohr - Akustischer Reflex -- Tensorreflex - Schallfiber- tragung - Otosklerose - Rekruitment - Verdeckung - Facialisparese.

Summary. The influence of artificially induced impedance claanges on the fre- quency response of the middle ear has been investigated in three experienced listeners. The tensor-reflex has been simulated by application of positive and negative air-pressure to the outer ear canal. In this test-situation, the threshold- shift (AL) obtained is greatest in the surrounding of the first resonance of the

0302-9530/78/0221/0047/$ 02.80

Page 2: On the influence of the middle ear muscles upon changes in sound transmission

48 G. Sesterhenn and H. Breuninger

middle ear (z~L = 8 dB at 500 Hz). For bone-conduction, the effect is weaker by about 2 dB than for air-conduction. The change in sound transmission is con- stant for all SPL's and is mainly due to the impedance change of the middle ear.

In contrast the influence of the stapedial reflex is strongly dependent on SPL. In the range between hearing-threshold and 70 dB (HL) the attenuation is only 2 dB below 1 kHz. At higher levels the attenuation may amount to 30 dB. Thus, excitation of the cochlea remains nearly constant. We suppose that the intensity- dependent influence of the stapedius reflex on sound transmission is due to a change of the stapes motion. The regulating device works without considerable distortion but with an attack-time of about 100 ms. The phenomenon of conduc- tive recruitment in otosclerosis is probably due to the lack of this regulating effect by the fixed stapes. One function of the stapedial reflex could be the protection of the ear from high amplitudes during phonation and shouting. Fur- thermore, because frequencies above 1 kHz are also attenuated, a protective function of the stapedial reflex in industrial noise exposure can be assumed.

Key words: Middle Ear - Acoustic reflex - Tensor reflex - Sound transmission - Otosclerosis - Recruitment - Masking - Facial palsy.

Several theories have been developed for a rational explanation of the function of the middle ear muscles (MEM). Whereas formerly some authors believed in a listening function (Ostmann, 1898) or an accomodation effect of the MEM (Kessel, 1874), a more exact knowledge of the middle ear mechanism has rejected these theories so that to date only three explanations are discussed: 1. a reduction of harmonic distor- tion of the middle ear at high levels (v. B+k+sy, 1960), 2. an improvement of time dissolution by a damping of the middle ear (Wigand, 1967) and 3. protection against high sound intensities. We think that an explanation of the MEM-function may be only derived from the physical and physiological effects of the MEM. Nevertheless, a comprehensive'study of the literature dealing with these questions reveals a rather non-uniform picture.

Physical and physiological aspects of the MEM-reflexes: below 500 Hz the input impedance of the middle ear is determined by elastic forces with a resistive compo- nent. This component is nearly independent of frequency till 1 kHz and is mainly caused by the input impedance of the cochlea. For frequencies above 800 Hz, the real part of the impedance is dominating and additional mass elements play a role, causing several resonances.

During MEM-activity the negative reactance is increased below 500 Hz. Be- tween 0.8-1.9 kHz the resistive component is decreased and the phase is changed in the direction of a capacitive reactance. The absolute impedance values show large individual scatter.

A contraction of M, tensor tympani causes an inward movement of the ear- drum, similar to that movement seen following an increasing air-pressure in the outer ear canal. These impedance changes are very similar to those occurring upon stape- dius contractions.

The stapes probably rotates around the lower ligament. A contraction of M. stapedius causes a gliding motion in the incudo-stapedial joint and reduces the too-

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Middle Ear Muscles and Sound Transmission 49

bility of the stapes by pulling the upper part of the footplate outwards. At high amplitudes during MEM-activity, the stapes changes its axis of rotation by about 90 ~ so that it causes a volume displacement of the cochlear fluid which is more pronounced in the environment of the windows. Therefore the stapedius reflex (SR) results in a combined effect: the impedance change of the ossicular chain and a change of stapes motion at higher amplitudes. This change of movement occurs probably without latency (v. B~k~sy, 1960; Moller, 1961; Zwislocki, 1962; Tietze, 1968).

Influence of the MEM-reflexes upon sound transmission (ST): in order to deter- mine the change in ST during MEM-activity, many experiments have already been carried out, but much of the published data reveal a rather non-uniform picture. The change in ST has been measured by determination of

a) hearing threshold shift (Pichler and Bornschein, 1957; Loeb and Riopelle, 1960; Fletcher and Loeb, 1962; Ward, 1961, 1967);

b) loudness changes (Gunn, 1973; Morgan and Dirks, 1975); c) impedance changes and TTS in patients suffering from unilateral Bell's palsy

(Borg, 1968; Borg and Zakrisson, 1973, 1974; Zakrisson and Borg, 1974; Zakris- son, 1975; Zakrisson et al., 1975);

d) changes of cochlear microphonics (Wever and Vernon, 1955; Irvine and Wester, 1973, 1974; Irvine, 1976);

e) impedance changes in animals (Borg, 1971, 1972); f) transmission properties of human temporal bones (Lehnhardt, 1960; Neergard

et al., 1963; Andersen et al., 1963; Cancura, 1970). In summary, the tensor reflex (TR) seems to attenuate frequencies below 1 kHz

by about 10 dB, whereas the SR changes ST by about 15-30 dB. Nevertheless, these findings are in profound contradiction with shifts of hearing threshold (only 1-3 dB), reported by Loeb and Riopelle (1960) and Ward (1961). This contradic- tion prompted us to investigate the changes in ST during MEM-activity in man under real physiological conditions by direct psycho-acoustical measurements.

Procedure

A sustained contraction of the M. stapedius may be achieved by acoustical and electrical stimulation. In the case of an acoustical stimulation we have to take into account the phenomenons of direct and remote masking (Ward, 1961) and of reflex decay (Tietze, 1969; Anderson et al., 1970). An interrupted tone or a narrow-band noise may be able to evoke a more sustained contraction of the M. stapedius (Tietze, 1969). However due to the masking effects resulting from the broadening of the spectrum of interrupted tones, especially in the lower frequencies, and from inter- modulation products, being produced when using narrow-band noise, only a pure tone seemed to us to be appropriate for reflex elicitation. In order to avoid pro- nounced reflex decay, we first examined all subjects used in this study with regard of this effect. Direct and remote masking may be avoided by stimulation with a tone of 2-4 kHz. This frequency will not cause any masking effect on the range below I kHz, even in ipsilateral application. Thus, we used pure tones of 2-4 kHz with a maximal duration of 10 s as reflex-eliciting stimuli. The presentation levels were

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50 G. Sesterhenn and H. Breuninger

be tween 1 1 0 - 1 0 0 dB (HL). These stimuli resul ted in at least 90% o f the m a x i m a l

s tapedius con t rac t ion .

There is apparen t ly no m e t h o d which enables a sus ta ined el ici tat ion o f a T R . But

a re t rac t ion o f the ear d r u m m a y cause an effect u p o n ST in a similar w a y as a real

TR. Thus , th resho ld shifts have been de te rmined dur ing appl ica t ion o f a posi t ive and

nega t ive a i r -pressure to the outer ear canal .

The exper iments have been car r ied ou t in three exper ienced l isteners (six ears).

P recond i t ions were n o r m a l hear ing f rom 0 . 1 2 5 - 8 k H z [ 0 - 1 5 dB (HL)] , n o r m a l

t y m p a n o g r a m s and in tac t s tapedial reflexes with large i m p e d a n c e changes and no

re f lex-decay within 10 s for f requencies up to 4 kHz .

Methods and Results

Experiment 1

A positive and negative air pressure (_+ 300 m m H20 ) was applied to the outer ear canal and the shift of hearing threshold was determined for air and bone conduction. The bone vibrator was held in place by a steel ring, the contraiateral ear was masked with WN presented at 40 dB (HL) during bone conduc- tion measurements. The results obtained are presented in Table 1 and 2 and Figure 1. At higher frequencies a slight improvement in hearing sometimes occurred. In one subject threshold determina- tions were difficult at higher frequencies because the air pressure produced tinnitus. In a control experiment the change in ST was determined for low frequency tones presented at 70 dB (HL) using the method described in experiment 3. The results at 70 dB (HL) were identical with those shifts obtained at threshold levels.

Experiment 2

The shift in hearing threshold was determined during contralateral and ipsilateral elicitation of the SR for air and bone conduction. While using contralateral stimulation the SR was elicited by a tone of 2-3 kHz and presented at 110 dB (HL). This stimulus was presented for 10 s, followed by a silent period of 20 s to avoid reflex decay. Table 3 presents the results obtained for air and bone conduction.

When using ipsilateral stimulation the SR was elicited by a tone of 3 -4 kHz presented at 105-110 dB (HL). Table 4 and Figure 2 present the results obtained for air conduction only. For comparison, the threshold shifts reported by Pichler and Bornschein (1957) while using ipsilateral electrocutaneous stimulation are presented. In a control experiment using two ears, the threshold shifts during electrical ipsilateral stimulation was determined. The test situation was very uncomfortable and the evoked impedance change was only about 50% of the maximal impedance change in acoustical elicitation. The measured hearing threshold shifts were only 2 -3 dB in the frequency range below 1 kHz and thus were identical with those obtained while using acoustical stimulation.

It is supposed in this experiment that the electrical stimulation indeed evokes a stapedius contrac- tion. However during maximal acoustical stimulation, the additional electro-cutaneous elicitation re- suited in a further impedance change. Thus, a tensor tympani contraction may be contributing to this impedance change.

Experiment 3

In order to determine the change in ST at higher levels, we carried out the following masking experi- ment: a masking tone of 125,250, and 500 Hz was presented continuously at 70 dB (HL). The masked

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Middle Ear Muscles and Sound Transmission

Table 1. Air conduction

51

kHz 0.125 0.25 0.5 1 2 4

zSL (dB) 5.3 6.5 8.6 6.6 3.5 1.6 +300 mm H20 Extreme values (dB) 2--8 5--13 4--12 +2--13 0--7 0--6

AL (dB) 5.7 7.8 8.5 5.8 2.8 1 --300 mm H20 Extreme values (dB) 4 -8 4-12 6--11 +2-- 11 0-5 0--4

Table 2. Bone conduction

kHz 0.125 0.25 0.5 1 2 4

AL (dB) 3.6 5.2 5.6 2.8 +0.2 0 +300 mm H20 Extreme values (dB) 2--4 4--6 3--9 +4--8 +2--1 +2--2

AL (dB) 4 4.6 4.2 2.2 1 +0.5 --300 mm H20 Extreme values (dB) 3--5 2--7 2--6 + 1--6 +4--6 +2--0

Hearing threshold shift (AL) for air- and bone-conduction during positive and negative air pressure in the outer ear canal (+ sign means improvement of hearing)

z~L

(riB)

lO

O. 125 O. 2 5 0.5 1 2 g,

k H z

Fig. 1. Change in ST (AL) for air and bone conduction during air pressure application of + 300 mm H20

threshold for a 1-2 kHz-tone was then determined. Sometimes the frequency of the maskee was modified by a few Hz in order to avoid beats. Initially the threshold of the maskee was determined during ipsilateral and contralateral elicitation of the SR by a tone of 2 -4 kHz presented at 105-110 dB (HL). After determination of the masked threshold the level of the maskee was held constant. Using this method the acoustic SR reduced the transmission of the low-frequency masker through the middle ear to a certain degree. When the reflex eliciting stimulus was swiched off, no SR was active and the masker reached the cochlea at a somewhat higher level so that the maskee vanished completely. When

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52 G. Sesterhenn and H. Breuninger

Table 3. Threshold shift (AL) during contralateral elicitation of the SR for air and bone-conduction

Air conduction 125 Hz 250 Hz 500 Hz 1000 Hz

AL (dB) 1.65 1.65 3.15 1.3

Extreme values (dB) 1-3 1--2 2--4 + 1-3

Bone conduction zSL (dB) 0 0 0 0

Table 4. Threshold shift (AL) during ipsilateral elicitation of the SR for air-conduction

125 Hz 250 Hz 500 Hz 1000 Hz

AL (dB) 1.5 2.2 2 0.3

Extreme values (dB) 1-2 1-3 1-3 + 1--1

AL

( d B )

15

10

P ich le r and Bornsche in | lll l .

p r e s e

7"- -> ",, 0.125 0.25 0,5 I 2 kHz

Fig. 2. Shift of hearing threshold during ipsilateral acoustical elicitation of the SR. For comparison, the shifts reported by Pichler and Bornschein (1957) during electrical stimulation

this occurred the level of the masker was reduced by A L, until the maskee became audible again. Thus the change in ST AL can be determined directly even at levels up to 70 dB (HL). A further advantage of this experiment is that masked thresholds may be determined more accurately than normal hearing thresholds. Of course, the masker never should elicit the SR. This was certified for every subject. Table 5 presents the results obtained for ipsi- and contralateral reflex elicitation.

It is assumed in this experiment that the SR affects ST at threshold levels in the frequency range below 1 kHz only, whereas the range above this frequency is uninfluenced. This has been ascertained

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Middle Ear Muscles and Sound Transmission 53

Table 5. Change in sound transmission (AL) at 70 dB (HL) during ipsi- and contralateral elicitation of the SR for air conduction

125 Hz 250Hz 500Hz

Contralateral

,5,L (dB) 2 2.3 2.1 Extreme values (dB) 1-3 1-5 1-5

Ipsilateral

~L (dB) 2.3 2.8 2.6 Extreme values (dB) 1-5 2--6 1-7

Table 6. Change in sound transmission at 70 and 90 dB (HL) of subject U.D. during contralateral elieitation of the SR for air conduction

125 Hz 250 Hz 500 Hz

Contralateral

70 dB (HL) 1 2 2 AL (dB) 90 dB (HL) 6 4 7

by control experiments using a masker of 500 Hz and a maskee of 2--4 kHz. These controI experiments showed the same results of changes in ST.

In one of our subjects we found an elevated reflex threshold in the left ear (90 dB). In this person the masking experiment (described above) has been carried out using masker levels of 70 and 90 dB (HL). The SR was elicited from the contralateral ear. Table 6 shows the measured values of the change in ST. It is obvious that in spite of a constant contraction of the stapedius muscle, the change in ST increases at higher levels.

Experiment 4

Alternate binaural loudness balance tests in patients, suffering from unilateral Bell's palsy: when the limiting effect of the SR is cancelled on the affected side, considerable differencies in loudness sensation between both ears should occur at levels above 80 dB (HL). Therefore, an automatic Fowler test was performed in six patients with normal bearing and unilateral Bell's palsy. Figure 3 shows the results obtained at 250-4000 Hz. The highest levels used were 120 dB (HL). At 125 Hz no significant side- difference could be obtained because the maximal available sound intensity was only 85 dB (HL). The side-differences, and, thus, the amounts of sound attenuation determined are:

Hz At dB (HL) Mean AL (dB) Extreme values AL (dB)

125 85 5 0-10 250 105 12 10-15 500 115 23 15--25 750 120 22 15-25

1000 120 21 10-25 2000 120 15 10-20 4000 115 5 0 - 5

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54

250 5 0 0 750 tO00

G. Sesterhenn and H. Breuninger

2 0 0 0 4 0 0 0 H z

7 0 , m , , , l ~ - - ,d ~ . .

90 ~ - ~ ~ ""

1oo .qp ~ ~ Ik

, , o \ \ \. 120

ct i e i el i q i o

)" , ~ ,

o i

70

80

90

100

1 1 0

120 d B ( H L )

Fig. 3. Alternating binaural loudness balance test (automatic Fowler-test) in patients suffering from complete unilateral Bell's palsy, a = affected, i = intact side

dB 720 �9

170-

100

9O

80 .

? 0 �9

60

5O

40

3 0 ,

2O

10

out put

/ /

/

4 L

: d B )

3 0

2 0

10

�9 w

tO 2 0 3 0 4 0 50 6 0 70 8 0 9 0 100 170 d B

i n p u t (HL )

Fig. 4. Input-output-function of the middle ear at 500 Hz and change in ST caused by the SR

Page 9: On the influence of the middle ear muscles upon changes in sound transmission

Middle Ear Muscles and Sound Transmission 55

The highest limiting effect occurred at 500 Hz (= 23 dB). This amount may be still larger at higher levels. Thus, the attenuating mechanism is most effective in the range where the human voice is loudest. It is therefore interesting that two patients reported pronounced phonophobia during shouting. How- ever, it is obvious that a pronounced limiting effect of the SR also exists for the frequencies above 1 kHz.

If we assume a maximal attenuation of the SR to be 20 dB at 500 Hz and 120 dB (HL), we may thus derive the limiting characteristics of the middle ear during SR activity. Figure 4 shows these relations. In the range from 0--70 dB (HL) the attenuation is only 2 dB and reaches 20 dB at 120 dB (HL).

Experiment 5

A non-linear transmission characteristic as presented in Figure 4 should produce considerable har- monic distortion when the regulation works without attack-time. This has been suggested by Moiler (1961). However, distortions are apparently not audible. A regulation without attack-time should also influence short tone-pulses in the same manner as continuous tones; i.e., the attenuation should increase at higher levels. In order to clarify the existence of an attack-time, we performed the following experi- ment in two ears: the same technique as in experiment 3 was used, but instead of continuous tones, short pulses of 30 ms were presented. These short pulses will not elicit the SR because the latency of the reflex is about 30--40 ms. The presentation levels of the masking pulses (125, 250, and 500 Hz) were 110 dB (SPL). The SR was elicited by stimulating the contralateral ear as described in experiment 3. The measured changes in ST for these short pulses were 2-5 dB in the frequency range below 1 kHz. Therefore the regulation during reflex activity must occur with attack-time. The stapes needs some time to change its position and mode of vibration, and only then is the entire limiting device active.

Discussion

The two middle ear reflexes affect ST in a very different way and are based upon very different physical and physiological mechanisms. Whereas the impedance changes in principal are very similar, the TR alters the impedance of the eardrum more than does the SR.

T h e air pressure affects air conduction more than bone conduction (Huizing, 1960; Thullen, 1965; Irvine and Wester, 19 73, 1974). The low frequencies are atten- uated at low and high SPL's alike, while there appears to be little change in ST above 2 kHz (Opitz et al., 1973). Sometimes in fact, an improvement in hearing may be obtained.

It was assumed in this experiment that air pressure and a real TR have similar effects on ST. This conclusion may be drawn from a comparison of our results with those reported from investigations of human temporal bones (Andersen et al., 1963).

In contrast to the SR the change in ST of the TR is constant for all SPL's. Nevertheless, because bone conduction is also affected, this change may not be due to changes of the middle ear's impedance alone, but also to changes in the vibration mode of the stapes. This is not the place to discuss the problems of bone conduction. However as it will be outlined later, bone conduction is certainly influenced by the relations of the middle ear (L6gouix and Tarab, 1959; Huizing, 1960). Furthermore it is probable that the occlusion effect in our test situation changed the relations of bone conduction.

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56 G. Sesterhenn and H. Breuninger

For the SR, the change in ST is only about 2 dB at threshold. This finding is in good agreement with results reported by Loeb and Riopelle (1960) and Ward (1961, 1967). We think that this change is due to the impedance change of the middle ear only and affects only air conduction. Bone conduction is totally uninfluenced at threshold. These findings are in agreement with those of Irvine and Wester (1973, 1974) who reported that the SR affects air conduction by about 3 dB more than bone conduction.

At presentation levels of 70 dB (HL) the change for air conduction is about 2 dB for the contralateral ear and about 3 dB for the ipsilateral ear. In the ipsilateral test mode the change in ST may be a little larger because the reflex eliciting stimulus causes a larger stapes displacement. At highest levels (i.e. 120 dB) the attenuation may amount to more than 20 dB. Thus it is obvious that in spite of a constant contraction of the M. stapedius, the change in ST is largely dependent on sound pressure. Therefore the constant impedance change cannot be responsible for an intensity-dependent change in ST. Furthermore we were able to confirm the assump- tion of Loeb and Riopelle (1960) that the SR works like a limiter and not like a linear resistor.

Borg (1968) suggested that at intensity levels up to 20 dB above reflex threshold, the increased stiffness of the ossicular chain may be sufficient to explain the attenua- tion of sound, whereas, at higher levels, an additional effect (change of stapes dis- placement) must occur. In later studies Borg (1971, 1972) suggested a decoupling of the middle ear from the cochlea at highest levels. A change of stapes motion at highest levels has already been suggested by v. B~k~sy (1938, 1960) and by Wan- derer (I 953). Nevertheless, other authors failed to confirm this assumption (Cancura and Stark, 1977). However, it must be emphasized that none of these studies have been performed under real physiological conditions in vivo.

Another possibility is that sound energy is abolished in the elastic incudo-stape- dial joint. However, Irvine (1976) reported that even after interruption of the incudo- stapedial joint, the SR affects bone conduction at higher presentation levels. In view of this result and the fact that the change in ST is dependent on sound pressure level, it must be concluded that the cause for the change in ST during SR activity is dependent on the kind of stapes motion. The impedance change of the ossicles is therefore of minor importance.

It is well known that the amplitude of the stapes is greatest at frequencies below 3 kHz (Tonndorf and Khanna, 1966; Rubinstein et al., 1966; Guinan and Peake, 1967). The SR likewise affects ST mainly in the lower frequency range. Thus, it seems probable that the SR protects the cochlea from high amplitudes by way of the reflex-induced change of stapes motion. Furthermore this may be one reason for the enormous dynamic range of the human ear. When the stapes changes its axis of rotation by 90 ~ as has been suggested by v. Btktsy (1960), and rotates around the longitudinal axis of the footplate, it may cause a displacement of the cochlear fluid more in the surrounding area of the windows. Thus the lower frequency range is less excitated. However, because the frequencies up to at least 2 kHz are also affected, a pronounced protective function of the SR in industrial noise exposure must exist. Only against impulsive noise such as gun fire, the SR will not be effective on account of the latency of the muscle contraction.

When the SR is cancelled, as in otosclerosis, the change of stapes motion is also

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Middle Ear Muscles and Sound Transmission 57

cancelled, and, thus the attenuating effect is abolished. In 1966, Anderson and Barr reported on the phenomenon of conductive recruitment. They found that in cases of unilateral otosclerosis hearing- and SR-thresholds approach one another in a similar manner as it often occurs in cochlear pathology. They confirmed the presence of recruitment in such cases when using the Fowler-test. Nevertheless, conductive re- cruitment was never as complete as that obtained in cases with inner ear disease and amounted to 20 -30 dB, which is about the attenuating effect of the SR. The authors observed, that the degree of recruitment was proportional to the degree of stapes fixation. They suggested a change in the hydromechanical excitation pattern as a consequence of the fixed stapes. We think that this conclusion is correct. A further observation, pointing in this direction, is the pronounced phonophobia of patients after stapedectomy, probably occurring as a consequence of the vibration mode of the artificial "stapes". Also the increased loudness sensation on the affected side may be one reason for the lateralisation effect in the Weber tuning-fork test.

The mechanism of the middle and the inner ear consist of one complete system, i.e. the mechanical status of the cochlear windows influence largely the sensitivity of the inner ear (Cancura, 1969) and middle ear disease can affect both air and bone conduction (Ranke et al., 1952). When the SR regulates the excitation of the cochlea by way of stapes motion, it would be interesting to know, whether intermodulation products of the low frequencies are also influenced by the SR. This question may be answered from experiments in animals.

Because the limiting device of the SR works without distortion, and because of the non-linear function presented in Figure 5, it must be assumed that the regulating mechanism works with attack time. Our control experiment confirms this assump- tion. This attack time must be caused by the change of stapes motion because a maximal constant contraction of M. stapedius causes only very little attenuation at threshold (2 dB). Moller( 1962) and Borg (1971, 1972) reported having measured an oscillation of the SR-activity of about 5 Hz as a consequence of this regulating effect. Thus the maximal contraction of M. stapedius occurs at T/2 of the oscillation, and the attack time of the stapes must be assumed to be about 100 ms.

Consequences of these findings for the audiologist are: 1. Threshold audiometry: in special cases it is necessary to mask the better ear by white noise. This noise may elicit the SR. The artificially induced threshold shift for the ear being examined is only 2 dB below 70 dB (HL), when the white noise is presented contralaterally. This is a neglegible shift. Thus it is not possible to significantly elevate the hearing thresh- old, as it has been mentioned by Ward (1961). 2. Fitting of hearing aids: in cases of moderate hearing loss with normal SR, it is not useful to amplify the low frequencies too much, because sound energy below 2 kHz is abolished by the SR. An ascending frequency response of 20 dB/decade seems to be optimal from this point of view.

Because the TR and SR affect ST at low sensations levels in a very different way, it may be possible to discern between these reflexes by an observation of threshold and loudness changes. But it must be emphasized that the stimuli eliciting the TR may be accompanied by a low frequency noise which may have some masking effect and thus influence threshold as well as loudness sensation.

Gunn (1973) determined changes in ST of 4.5 dB at 500 Hz, whereas Pichler and Bornschein (1957) reported on threshold changes of about 12 dB for 125 and

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58 G. Sesterhenn and H. Breuninger

250 Hz during electro-cutaneous elicitation of the SR. These latter results must be seriously questioned.

A further phenomenon that must be doubted is the "voluntary TR". Two of our subjects were able to evoke a voluntary impedance change of the ear drum. This change caused a distinct change in loudness sensation at higher levels, as well as a hearing threshold shift. But otoscopic examination revealed only a motion of the posterior quadrant of the ear drum without any movement of the hammer. Thus we conclude that the voluntary impedance change of the ear drum is not a TR but a reaction of M. tensor veli palatini which causes an air-pressure change in the middle ear and an inward movement of the ear drum. Registration of air pressure changes in the outer ear canal (Terkildsen, 1960; Mendelsson, 1966; Lid~n et al., 1970) delivered negative deflections during voluntary impedance change. However, in our subjects, neither acoustical nor tactile-mechanical stimulation caused such pressure changes.

Conclusions

1. The TR changes sound transmission in the frequency range up to 2 kHz by about 7 dB for air conduction and 5 dB for bone conduction. This change is caused main- ly by the impedance change of the middle ear and is constant for all intensity levels.

2. The SR changes ST at threshold by only 2 dB. This change is caused by the impedance change of the middle ear. At presentation levels above 70 dB (HL) the change in ST increases considerably and may amount to 30 dB at 130 dB (SPL). This change in ST is caused by a modification in the motion of the stapes and also affects bone conduction. The attenuation occurs without distortion, but with an attack-time of about 100 ms.

3. Because tones up to 2 kHz are distinctly attenuated, it is probable that the SR protects the inner ear also in situations of industrial noise exposure.

4. The phenomenon of conductive recruitment in otosclerosis is due to the fixa- tion of the stapes and as a consequence of a modified excitation pattern in the cochlea. The loss of the regulating effect of the stapes is the reason for phonophobia after stapedectomy.

5. The inner ear mechanism is largely dependent on the status of the middle ear. Both mechanisms form one complete system.

6. The voluntary impedance change of the ear drum is not a TR but a reaction of M. tensor veli palatini.

Acknowledgement. The authors are indepted to Mrs. Cathy Liebner (MA) for reading and correct- ing the English manuscript and for helpful discussions. We also want to thank Doz. Dr. G. Esser and Dipl. Ing. R. Schunieht from the Forschungslabor ftir reed. Akustik of the University of Dfisseldorf and to Dr. D. Nagel from the ENT-Clinic of the University of Ulm for their assistance in several experi- ments. And last but not least our special thanks to Dr. A. Rademacher and Dr. U. Hoppe for their otoscopic examinations.

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Middle Ear Muscles and Sound Transmission 59

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Received December 13, 1977