Basilar-membrane responses to tones at the base of the chinchilla cochlea

Basilar-membrane responses to single tones were measured, using laser velocimetry, at a site of the chinchilla cochlea located 3.5 mm from its basal end. Responses to low-level (< 10-20 dB SPL) characteristic-frequency (CF) tones (9-10 kHz) grow linearly with stimulus intensity and exhibit gains of 66-76 dB relative to stapes motion. At higher levels, CF responses grow monotonically at compressive rates, with input-output slopes as low as 0.2 dB/dB in the intensity range 40-80 dB. Compressive growth, which is significantly correlated with response sensitivity, is evident even at stimulus levels higher than 100 dB. Responses become rapidly linear as stimulus frequency departs from CF. As a result, at stimulus levels > 80 dB the largest responses are elicited by tones with frequency about 0.4-0.5 octave below CF. For stimulus frequencies well above CF, responses stop decreasing with increasing frequency: A plateau is reached. The compressive growth of responses to tones with frequency near CF is accompanied by intensity-dependent phase shifts. Death abolishes all nonlinearities, reduces sensitivity at CF by as much as 60-81 dB, and causes a relative phase lead at CF.