EFFECT OF THE MIDDLE-EAR REFLEX ON SOUND-TRANSMISSION TO THE INNER-EAR OF RAT

The effect of the acoustic middle ear reflex (MER) was quantified usin g electrodes chronically implanted in the middle ears of rats. Cochlea r microphonics (CM) and middle ear muscle EMG were measured under ligh t Ketamin anesthesia after stimulation with tone pulses of 5-20 kHz ra nging between 75 and 120 dB SPL. With increasing intensity, the CM mea sured before the onset of the MER increased to a maximum amplitude and then decreased with higher SPLs. At 10 kHz this maximum was reached a t 95 dB SPL, for other stimulus frequencies at higher SPLs. After a la tency of 10-20 ms, CM to 10 kHz stimuli of 80-95 dB SPL were decreased by the attenuating action of the MER. The lowest threshold of the MER was also measured at 10 kHz (77 dB SPL in the mean). To stimuli great er than 100 dB SPL after a latency of 6-10 ms, the CM amplitude was in creased. That this CM increase to intense stimuli is caused by the act ion of the MER was confirmed by control experiments such as cutting th e tendons of the middle ear muscles. The CM decrease to stimuli below 100 dB SPL, as well as the increase to very intense stimuli, can be ex plained by sound attenuation caused by the MER, together with the nonl inear dependence of CM amplitude on stimulus level. The observed shift of the maxima of the CM input-output function by the MER to higher st imulus levels probably indicates an increase of the dynamic range of t he ear.