Non-acoustic Factors influencing Activity of Middle Ear Muscles in Waking Cats

Abstract

THE intensity of sound transmitted from the ear drum to the inner ear can be attenuated by contraction of the middle ear muscles, the tensor tympani and stapedius. These muscles contract at the onset of sound, thereby reducing the size of cochlear microphonics as recorded at the round window. Previous explanations of middle ear muscle activity have only considered possible acoustic functions^1–4. However, in waking, unrestrained cats, we have found middle ear muscle activity related to non-acoustic factors, for example, bodily movement and vocalization. In animals that we have experimentally deafened by crushing both eight nerves, non-acoustic contractions persist in association with bodily movement and vocalization. In these animals all acoustic middle ear muscle activity is absent, the neural component is absent from the cochlear response recorded at the round window, and they show no behavioural reaction to loud sounds.