Frequency tuning of basilar membrane and auditory nerve fibers in the samecochleae

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 displace ment 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 selecti vity in chinchillas (and probably in mammals in general) is fully expressed in the vibrations of the basilar membrane and renders unnecessary addition al ("second") filters, such as those present in the hair cells of the cochl eae of reptiles.