Acoustic reflex decay in chinchillas during a long-term exposure to noise.

Reflex decay to 0.5 kHz octave-band noise at 95 dB SPL was measured in chinchillas during four 2-hr exposure periods separated by 11-min quiet intervals. Round window electrodes were implanted in six animals. Measures of acoustic reflex decay were inferred from amplitude changes in the cochlear microphonic generated by the octave-band noise. Reflex decay followed essentially the same time course during each exposure period. Vigorous muscular contraction to the signal onset was followed by gradual decay that asymptoted between 30 and 50% of its initial value. The process of decay seemed to be complete sometime between 8 and 30 min. The results suggest that the middle ear muscles in chinchillas provide some protective function during exposures of fairly long duration.