Mechanical bases of frequency tuning and neural excitation at the base of the cochlea: Comparison of basilar-membrane vibrations and auditory-nerve-fiber responses in chinchilla

We review: the mechanical origin of auditory-nerve excitation, focusing on comparisons of the magnitudes and phases of basilar-membrane (BM) vibration s and auditory-nerve fiber responses to tones at a basal site of the chinch illa cochlea with characteristic frequency approximate to 9 kHz located 3.5 mm from the oval window. At this location, characteristic frequency thresh olds of fibers with high spontaneous activity correspond to magnitudes of B M displacement or velocity in the order of 1 nm or 50 mum/s. Over a wide ra nge 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 synch ronous with peak BM velocity toward scala tympani but at 80-90 dB sound pre ssure level (in decibels relative to 20 microPascals) and at 100-110 dB sou nd pressure level responses undergo two large phase shifts approaching 180 degrees. These drastic phase changes have no counterparts in BM vibrations. Thus, although at threshold levels the encoding of BM vibrations into spik e trains appears to involve only relatively minor signal transformations, t he polarity of auditory-nerve responses does not conform with traditional v iews of how BM vibrations are transmitted to the inner hair cells. The resp onse 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, and/or electr ical and synaptic processes at the inner hair cells.