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 broad-band sound of moderately high intensity, depending on exposure- and stimulus intensities. Since Hensen's cell responses have been shown to be identical in phase and directly proportional in magnitude to outer hair cell (OHC) responses (Oesterle, E.C., Dallos, P., 1989, J. Acoust. Soc. Am. 86 (3), 1013-1032.; Zwislocki, J.J., Slepecky, N.B., Cefaratti, L., Smith, R.L., 1992, Hear. Res. 57, 175-194), it was assumed that these changes were reflections of changes in OHC receptor potentials, which were of main interest. The indirect method of intracellularly recording the Hensen's cell potentials rather than OHC potentials was used to minimize damage to the organ of Corti and reduce technical difficulties associated with repeated recordings from OHCs. Continuous magnitude and phase transfer functions (TFs) were obtained before and after the exposure over a range of sound pressure levels (SPLs) extending from 40-90 dB by using frequency sweeps ranging from 0.125-18 kHz. Cochlear microphonic (CM) TFs were also acquired over the same frequency and intensity ranges for monitoring purposes. The exposure stimuli were set at 80, 86, 90 or 100 dB SPL for periods ranging from 10-40 min. When response enhancement occurred, it was most clearly seen in the peak of the transfer function determined at 90 dB SPL. Enhancement ranged from approximately 12-230% of the original peak. In contrast, control Hensen's cell recordings obtained over periods of up to 130 min revealed great response stability. In all reliable recordings, response enhancement was associated with a phase lead or no phase change. The strongest exposure stimuli tended to produce sensitivity loss accompanied by phase lag at the lower SPLs, in agreement with previous work in this laboratory (Zhang and Zwislocki, 1995). In some preparations, both sensitivity loss at lower SPLs and enhancement at higher SPLs occurred simultaneously, suggesting involvement of two different mechanisms.