Phase-Locked Responses to Tones of Chinchilla Auditory Nerve Fibers: Implications for Apical Cochlear Mechanics

Responses to tones with frequency ≤ 5 kHz were recorded from auditory nerve fibers (ANFs) of anesthetized chinchillas. With increasing stimulus level, discharge rate–frequency functions shift toward higher and lower frequencies, respectively, for ANFs with characteristic frequencies (CFs) lower and higher than ∼0.9 kHz. With increasing frequency separation from CF, rate–level functions are less steep and/or saturate at lower rates than at CF, indicating a CF-specific nonlinearity. The strength of phase locking has lower high-frequency cutoffs for CFs >4 kHz than for CFs 2 kHz, the inflections coincide with the tip-tail transitions of threshold tuning curves. ANF responses to CF tones exhibit cumulative phase lags of 1.5 periods for CFs 0.7–3 kHz and lesser amounts for lower CFs. With increases of stimulus level, responses increasingly lag (lead) lower-level responses at frequencies lower (higher) than CF, so that group delays are maximal at, or slightly above, CF. The CF-specific magnitude and phase nonlinearities of ANFs with CFs