VOLUNTARY CONTRACTION OF MIDDLE-EAR MUSCLES - EFFECTS ON INPUT IMPEDANCE, ENERGY REFLECTANCE AND SPONTANEOUS OTOACOUSTIC EMISSIONS

Two types of measurements were performed on a subject able to voluntar ily contract her middle ear muscles (MEM). First, wideband measurement s (0-11 kHz) of middle ear input impedance and energy reflectance were obtained when the subject was relaxed and when she contracted her MEM . The changes in impedance observed with voluntary MEM contraction wer e similar to those reported in the literature for acoustically-elicite d MEM contractions. The energy reflectance increased for frequencies b elow about 4 kHz. Second, the effects of voluntary MEM contraction on the frequencies and levels of spontaneous otoacoustic emissions (SOAEs ) were measured and compared to effects evoked by contralateral acoust ic stimulation. Effects on SOAEs appear to be a more sensitive indicat or of MEM activity than changes in impedance, and the effects due to v oluntary MEM contraction were qualitatively similar to those evoked by contralateral acoustic stimulation. These results suggest that in sub jects with normally-functioning middle ears, only some effects on otoa coustic emissions caused by contralateral stimuli whose levels are bel ow the contralateral acoustic reflex threshold can be unequivocally at tributed to the action of cochlear efferents. The temporal aspects of SOAE frequency shifts caused by voluntary contraction of MEM show that voluntary contraction fatigues rapidly over a time period of tens of seconds.