Basilar membrane vibrations near the round window of the gerbil cochlea.

Using a laser velocimeter, responses to tones were measured at a basilar membrane site located about 1.2 mm from the extreme basal end of the gerbil cochlea. In two exceptional cochleae in which function was only moderately disrupted by surgical preparations, basilar membrane responses had characteristic frequencies (CFs) of 34-37 kHz and exhibited a CF-specific compressive nonlinearity: Sensitivity near the CF decreased systematically and the response peaks shifted toward lower frequencies with increasing stimulus level. Response phases also changed with increases in stimulus level, exhibiting small relative lags and leads at frequencies just lower and higher than CF, respectively. Basilar membrane responses to low-level CF tones exceeded the magnitude of stapes vibrations by 54-56 dB. Response phases led stapes vibrations by about 90 degrees at low stimulus frequencies; at higher frequencies, basilar membrane responses increasingly lagged stapes vibration, accumulating 1.5 periods of phase lag at CF. Postmortem, nonlinearities were abolished and responses peaked at approximately 0.5 octave below CF, with phases which lagged and led in vivo responses at frequencies lower and higher than CF, respectively. In conclusion, basilar membrane responses near the round window of the gerbil cochlea closely resemble those for other basal cochlear sites in gerbil and other species.