A Quantitative Study of the Effect of the Acoustic Stapedius Reflex on Sound transmission Through the Middle Ear of Man

Abstract

The influence on sound transmission through the middle ear as exerted by the acoustic stapedius muscle reflex, has been investigated in subjects with unilateral peripheral facial paralysis of short duration. Reflex activity in both ears was recorded simultaneously as a change in the acoustic impedance at 800 cps. Bursts of pure tones between 300 and 3000 cps were used to elicit this reflex. The influence of the stapedius reflex on the transmission of the sound stimulus through the middle ear was obtained from measurements made during paralysis and after complete recovery. The amplitude of the crossed reflex (recorded as an impedance change) was used as a relative measure of the excitation in the cochlea on the affected side. The magnitude of the influence on the transmission of the stimulus tone was obtained from the shift of the stimulus response curve of the crossed reflex which was caused by the stapedius reflex after recovery. Attenuation was found to be greater at frequencies below the resonant frequency of the middle ear (about 1000 cps) than above. A pure tone of 500 cps (20 dB above reflex threshold) was attenuated 12-15 dB whereas a tone of 1450 cps (16 dB above threshold), was attenuated 0-6 dB. After the recovery of stapedius function, the excitation in the cochlea reached a maximal level (at 500 cps, 115 dB SPL) which was lower than during palsy. This decrease in maximal excitation was interpreted as showing that a “perfect regulatory” effect is exerted by the stapedius reflex in governing sound transmission through the middle ear when the reflex is near its maximal degree of activity. A conceivable explanation for the increased amplification in the feed-back circuit is that a strong stapedius contraction might change the mode of vibration of the stapes and thus diminish the energy transmitted to the cochlea. The oscillations in the amplitude of the impedance change as elicited by low frequency stimulation, were shown to be due to the activity of the stapedius of the stimulated side.

Indirect methods were discussed for evaluation of the protective capacity of the stapedius reflex in normal subjects. The advantages offered by recordings of the ipsilateral reflex were stressed.