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.

Basilar-membrane responses to clicks were measured, using laser velocimetry, at a site of the chinchilla cochlea located about 3.5 mm from the oval window (characteristic frequency or CF: typically 8-10 kHz). They consisted of relatively undamped oscillations with instantaneous frequency that increased rapidly (time constant: 200 µs) from a few kHz to CF. Such frequency modulation was evident regardless of stimulus level and was also present post-mortem. Responses grew linearly at low stimulus levels, but exhibited a compressive nonlinearity at higher levels. Velocity-intensity functions were almost linear near response onset but became nonlinear within 100 µs. Slopes could be as low as 0.1-0.2 dB/dB at later times. Hence, the response envelopes became increasingly skewed at higher stimulus levels, with their center of gravity shifting to earlier times. The phases of near-CF response components changed by nearly 180 degrees as a function of time. At high stimulus levels, this generated cancellation notches and phase jumps in the frequency spectra. With increases in click level, sharpness of tuning deteriorated and the spectral maximum shifted to lower frequencies. Response phases also changed as a function of increasing stimulus intensity, exhibiting relative lags and leads at frequencies somewhat lower and higher than CF, respectively. In most respects, the magnitude and phase frequency spectra of responses to clicks closely resembled those of responses to tones. Post-mortem responses were similar to in vivo responses to very intense clicks.

Responses to tones of a basilar membrane site and of auditory nerve fibers innervating neighboring inner hair cells were recorded in the same cochleae in chinchillas. At near-threshold stimulus levels, the frequency tuning of auditory nerve fibers closely paralleled that of basilar membrane displacement modified by high-pass filtering, indicating that only relatively minor signal transformations intervene between mechanical vibration and auditory nerve excitation. This finding establishes that cochlear frequency selectivity in chinchillas (and probably in mammals in general) is fully expressed in the vibrations of the basilar membrane and renders unnecessary additional ("second") filters, such as those present in the hair cells of the cochleae of reptiles.

We review the mechanical origin of auditory-nerve excitation, focusing on comparisons of the magnitudes and phases of basilarmembrane (BM) vibrations and auditory-nerve fiber responses to tones at a basal site of the chinchilla cochlea with characteristic frequency Ϸ 9 kHz located 3.5 mm from the oval window. At this location, characteristic frequency thresholds of fibers with high spontaneous activity correspond to magnitudes of BM displacement or velocity in the order of 1 nm or 50 m͞s. Over a wide range of stimulus frequencies, neural thresholds are not determined solely by BM displacement but rather by a function of both displacement and velocity. Near-threshold, auditory-nerve responses to low-frequency tones are synchronous with peak BM velocity toward scala tympani but at 80 -90 dB sound pressure level (in decibels relative to 20 microPascals) and at 100 -110 dB sound pressure level responses undergo two large phase shifts approaching 180°. These drastic phase changes have no counterparts in BM vibrations. Thus, although at threshold levels the encoding of BM vibrations into spike trains appears to involve only relatively minor signal transformations, the polarity of auditory-nerve responses does not conform with traditional views of how BM vibrations are transmitted to the inner hair cells. The response polarity at threshold levels, as well as the intensity-dependent phase changes, apparently reflect micromechanical interactions between the organ of Corti, the tectorial membrane and the subtectorial fluid, and͞or electrical and synaptic processes at the inner hair cells. In the mammalian cochlea, the bulk of auditory information is transmitted to the brain via the inner hair cells, which provide the sole synaptic inputs to 90-95% of the afferent fibers of the auditory nerve (1). Auditory-nerve excitation is triggered by the depolarization of inner hair cells upon deflection of their ''hair'' bundles toward the taller ''hairs'' or stereocilia (2, 3). Presumably, the forces that deflect the stereocilia are derived from the vibrations of the basilar membrane (BM) but it is not known how these vibrations are transmitted to the inner hair cells (4). Although it is clear that the BM and auditory-nerve fibers are similarly tuned at frequencies close to the characteristic frequency (CF) (5, 6), there is no consensus on whether neural thresholds correspond to a constant magnitude of BM displacement, velocity, or some function of these variables. Neither is it known with certainty what phases of BM vibrations trigger auditory-nerve excitation.Our ignorance on the relationship between the magnitudes and phases of cochlear vibrations and auditory-nerve responses stems in part from the dearth of measurements of BM vibration in relatively healthy cochleae, which until recently were available from a single site in the cochlea of each species. Even when adequate mechanical data are available, comparisons with the responses of auditory-nerve fibers have been perfunctory, typically involving a frequency-threshold tuning c...