Frequency tuning of basilar membrane and auditory nerve fibers in the same cochleae

Abstract

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.

Fig. 1

Frequency tuning of responses to tones of BM sites and auditory nerve fibers with similar CF. (A and B) Comparison of the frequency-threshold tuning curve for one fiber (filled symbols connected by thin solid line) with iso-displacement and isovelocity mechanical tuning curves (open circles connected by dashed line and thick solid line, respectively). In (A) another curve (open squares connected by solid lines) indicates the result of high-pass filtering the displacement curve at a rate of 3.8 dB per octave. The tip of the BM tuning-curve in (B) appears spuriously narrow because of the low-frequency resolution of data sampling [1000 Hz, versus 250 Hz in (A)]. The fibers had spontaneous activity of 11.2 (A) and 76.3 (B) spikes per second ().