ASA 127th Meeting M.I.T. 1994 June 6-10

1pPP4. The olivocochlear bundle and protection from acoustic overexposure.

M. Charles Liberman

Dept. of Cellular and Molecular Physiol. and Dept. of Otol. and Laryngol., Harvard Med. School, Harvard---MIT Div. of Health Sci. Technol., and Eaton--Peabody Lab., Mass. Eye & Ear Infirmary, 243 Charles St., Boston, MA 02114

Activity in the olivocochlear bundle (OCB), either sound- or shock-evoked, has been reported to decrease the temporary threshold shift (TTS) produced by acoustic overexposure. For example, in anesthetized guinea pigs, a TTS of 40 dB measured after a 1-min, monaural exposure to a high-intensity 10-kHz tone could be reduced by 20 dB when simultaneous electric stimulation was applied to the OCB [R. Rajan, J. Neurophysiol. 60, 549--568 (1988)]. Interpretation of such data was challenged by experiments in which unilaterally de-efferented cats, binaurally exposed to high-intensity 6-kHz tones for 10 min while shocking the OCB, shows no interaural differences in TTS [M. C. Liberman, J. Neurophysiol. 65, 123--132 (1991)]. More recent experiments suggest this discrepancy arose because OC-mediated effects on TTS are only present for short-duration (~1 min) exposure to high-frequency (10 kHz and above) tones. This OC mediated ``protection'' may be related to ``slow'' suppressive effects of OCB stimulation on cochlear responses, which are also most robust for high-frequency stimulation and for OC shock trains from 1 to 2 min in duration [T. Sridhar et al., Abstracts---XVIIth ARO Midwinter Meeting (1994)]. [Work supported by NIH Grant No. DC 00188.]