Chinchilla auditory-nerve responses to low-frequency tones_a)

Single unit activity was recorded in the auditory nerves of chinchillas. Period histograms were constructed for responses to tones with frequencies 30–1000 Hz. For low-frequency tones at near-threshold levels, peak period histogram phases for low- and medium-best-frequency (BF) neurons (<3 kHz) ranged from synchronous with condensation at the eardrum to 90° leading it. At near-threshold (but high absolute) levels, high-BF (>8 kHz) neurons responded in phase with rarefaction. At even higher levels, period histograms for responses of high-BF neurons tended to become bimodal, with one of the modes lagging rarefaction by 90°. Using cochlear microphonics as an indicator of basilar membrane (BM) displacement, at threshold levels, response phase of low- and medium-BF neurons fall within a range between displacement and velocity of the BM toward scala vestibuli. High-BF neurons respond, at threshold (but high) intensities, in phase with BM displacement toward scala tympani. The rates of growth of frequency sensitivity in responses of low-BF (+ 18 dB/oct) and high-BF (+ 12 dB/oct) neurons are consistent with preferred response phases corresponding to BM SV velocity and ST displacement, respectively. At suprathreshold levels high-BF neurons may fire preferentially to both scala tympani displacement and scala vestibuli velocity. These results support the notion that, for high-intensity, low-frequency stimuli, OHC hyperpolarization can induce excitation of the dendrites innervating IHCs.