ENHANCED COCHLEAR RESPONSES AFTER SOUND EXPOSURE

Alternating potentials produced in Hensen's cells of Mongolian gerbils by sinusoidal stimuli were enhanced or depressed after exposure to br oad-band sound of moderately high intensity, depending on exposure-and stimulus intensities. Since Hensen's cell responses have been shown t o be identical in phase and directly proportional in magnitude to oute r hair cell (OHC) responses (Oesterle, E.C., Dallos, P., 1989, J. Acou st. Soc. Am. 86 (3), 1013-1032.; Zwislocki, J.J., Slepecky, N.B., Cefa ratti, L., Smith, R.L., 1992, Hear. Res. 57, 175-194), it was assumed that these changes were reflections of changes in OHC receptor potenti als, which were of main interest. The indirect method of intracellular ly recording the Hensen's cell potentials rather than OI-IC potentials was used to minimize damage to the organ of Corti and reduce technica l difficulties associated with repeated recordings from OHCs. Continuo us magnitude and phase transfer functions (TFs) were obtained before a nd after the exposure over a range of sound pressure levels (SPLs) ext ending from 40-90 dB by using frequency sweeps ranging from 0.125-18 k Hz. Cochlear microphonic (CM) TFs were also acquired over the same fre quency and intensity ranges for monitoring purposes. The exposure stim uli were set at 80, 86, 90 or 100 dB SPL for periods ranging from 10-4 0 min. When response enhancement occurred, it was most clearly seen in the peak of the transfer function determined at 90 dB SPL. Enhancemen t ranged from approximately 12-230% of the original peak. In contrast, control Hensen's cell recordings obtained over periods of up to 130 m in revealed great response stability. In all reliable recordings, resp onse enhancement was associated with a phase lead or no phase change. The strongest exposure stimuli tended to produce sensitivity loss acco mpanied by phase lag at the lower SPLs, in agreement with previous wor k in this laboratory (Zhang and Zwislocki, 1995). In some preparations , both sensitivity loss at lower SPLs and enhancement at higher SPLs o ccurred simultaneously, suggesting involvement of two different mechan isms.