The role of middle ear muscle contractions in contralateral remote masking

Abstract

Middle ear muscle contractions are shown to produce a small but significant increase in the contralateral remote masking of a low-frequency tone burst when its threshold shift is determined as a function of its time delay with respect to a high-frequency masking stimulus (either octave band noise or tone). For low levels of masking stimulus (below 85 dB re 000021 μ bar) there is a smooth fall in masking with increasing delay. However, when the masking noise exceeds 85 dB or the masking tone exceeds 95 dB, there is an increase in masking amounting to an average (over seven subjects) of 3·2 and 1·4 dB, respectively, for masking stimuli of up to 115 dB. These increases occur at delay times of between 200 and 100 msec depending upon subject and upon masking stimulus level. The behaviour of these masking increases, with stimulus level, agrees with the behaviour of the impedance changes produced in the acoustic reflex bridge when the middle ear muscle reflexes are elicited. The assumption is therefore made that the attenuation of a low-frequency tone in contralateral remote masking is not only due to central masking but a small yet significant contribution is provided by middle ear muscle contractions.