Inner hair cell response patterns: implications for low-frequency hearing.

Inner hair cell (IHC) responses to tone-burst stimuli were measured from three locations in the apical half of the guinea pig cochlea. In addition to the measurement of ac receptor potentials, average intracellular voltages, reflecting both ac and dc components of the receptor potential, were computed and compared to determine how bandwidth changes with level. Companion phase measures were also obtained and evaluated. Data collected from turn 2, where best frequency (BF) is approximately 4000 Hz, indicate that frequency response functions are asymmetrical with steeper slopes above the best frequency of the cell. However, in turn 4, where BF is around 250 Hz, the opposite behavior is observed and the steepest slopes are measured below BF. The data imply that cochlear filters are generally asymmetrical with steeper slopes above BF. High-pass filtering by the middle ear serves to reduce this asymmetry in turn 3 and to reverse it in turn 4. Apical response patterns are used to assess the degree to which the middle ear transfer function, the IHC's velocity dependence and the shunting effect of the helicotrema influence low-frequency hearing in guinea pigs. Implications for low-frequency hearing in man are also discussed.