BASILAR-MEMBRANE RESPONSES TO CLICKS AT THE BASE OF THE CHINCHILLA COCHLEA

Basilar-membrane responses to clicks were measured, using laser veloci metry, at a site of the chinchilla cochlea located about 3.5 mm from t he oval window (characteristic frequency or CF: typically 8-10 kHz). T hey consisted of relatively undamped oscillations with instantaneous f requency that increased rapidly (time constant: 200 mu s) from a few k Hz to CF. Such frequency modulation was evident regardless of stimulus level and was also present post-mortem. Responses grew linearly at lo w stimulus levels, but exhibited a compressive nonlinearity at higher levels. Velocity-intensity functions were almost Linear near response onset but became nonlinear within 100 mu s. Slopes could be as low as 0.1-0.2 dB/dB at later times. Hence, the response envelopes became inc reasingly skewed at higher stimulus levels, with their center of gravi ty shifting to earlier times. The phases of near-CF response component s changed by nearly 180 degrees as a function of time. At high stimulu s levels, this generated cancellation notches and phase jumps in the f requency spectra. With increases in click level, sharpness of tuning d eteriorated and the spectral maximum shifted to lower frequencies. Res ponse phases also changed as a function of increasing stimulus intensi ty, exhibiting relative lags and leads at frequencies somewhat lower a nd higher than CF, respectively. In most respects, the magnitude and p hase frequency spectra of responses to clicks closely resembled those of responses to tones. Post-mortem responses were similar to in vivo r esponses to very intense clicks. (C) 1998 Acoustical Society of Americ a.