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

TLDR

Laser velocimetry measured basilar‑membrane responses at 3.5 mm from the cochlear base, revealing that low‑level CF tones (9–10 kHz) produce linear growth with 66–76 dB gain, while higher‑level tones exhibit compressive growth (slopes as low as 0.2 dB/dB), phase shifts, and a plateau above CF, with the largest responses at 0.4–0.5 octave below CF; death eliminates these nonlinearities and reduces CF sensitivity by 60–81 dB.

Abstract

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.