ASA 126th Meeting Denver 1993 October 4-8

3aPP1. Direct evidence for automatic gain control in the cochlea.

J. J. Zwislocki M. Chatterjee

Inst. for Sensory Res. and Dept. of Bioeng. and Neurosci., Syracuse Univ., Syracuse, NY 13244-5290

Compression of the input/output functions of auditory nerve fibers was ascribed to automatic gain control by Rose and his associates many years ago. It was ascribed to synaptic processes [C. D. Geisler and S. Greenberg, J. Acoust. Soc. Am. 80, 1352--1363 (1986)]. But compression is known to take place in the cochlea, as has been documented at the basilar membrane and hair cell levels. By comparing measured waveform distortion and the size of the second harmonic to those predicted from the input/output functions of Hensen's cells, it is demonstrated that the cochlear compression is due to automatic gain control at least up to a sound pressure level of 70 dB in Mongolian gerbils. Strong compression has been found at levels as low as 20 dB SPL. Since the response magnitude of Hensen's cells has been shown by the present authors and others to be directly proportional to that of the outer hair cells, this demonstration should be applicable to the hair cells themselves. The almost complete lack of waveform distortion was present independent of frequency distance from CF and could not have been caused by spectral filtering. [Work supported by NIDCD Grant No. DC00074.]