Response phase: A view from the inner hair cell

Inner hair cell (IHC) responses are recorded from the apical three turns of the guinea pig cochlea in order to define the relationship between hair ce ll depolarization and position of the basilar membrane. At low frequencies, inner hair cell depolarization is generally observed near basilar membrane velocity to scala vestibuli, reflecting the putative freestanding nature o f the IHC's stereocilia. While this is consistent with previous IHC results , independent of location, and with neural responses for fibers with low be st frequencies, it is inconsistent with single-unit results from the base o f the cochlea, where response phase is associated with basilar membrane vel ocity to scala tympani. Results suggest that the temporal disparity between IHC and neural data from the base of the cochlea may relate to several fac tors that influence transmembrane voltage in IHCs. First, extracellular vol tages (Ingvarsson, 1981; Sellick et al., 1982; Russell and Sellick, 1983) c an potentially affect low- and high-frequency regions differently because e lectrical interactions are more likely in the base of the cochlea than in t he apex (Dallos, 1983, 1985). Second, waveform distortion and kinetic prope rties associated with voltage-dependent ion channels in the IHC's basolater al membrane can both influence response phase by adding harmonic components and lagging the receptor potential by as much as 90 deg. Third, the veloci ty dependence of IHCs in the apex appears to extend to higher frequencies t han the velocity dependence demonstrated for IHCs in the base of the cochle a. These features. which influence the timing of discharges in the auditory nerve, are compared and evaluated. (C) 1999 Acoustical Society; of America . [S0001-4966(99)01702-6].