Fig. 4.

Motions associated with OHC electromotility are low-pass filtered. (A) OHC-DC displacement waveform for an 8-kHz, 70-dB SPL stimulus in a WT mouse. Low-pass filtering is indicated by the fact that the tonic displacement (gray curve) was larger than the cycle-by-cycle displacements (note that the waveform’s downward excursions do not reach the OHC-DC’s initial baseline position; dotted line). (B) Average cycle-by-cycle OHC-DC displacements for 70-dB SPL tones swept from 1 to 15 kHz (in ∼200-Hz steps) and tonic displacements for stimuli swept from 3 to 11 kHz (in ∼1-kHz steps). Tonic displacements were sharply tuned to the CF, where they were typically as large as displacements at the stimulus frequency and much larger than the harmonic responses. (C) Average interpolated OHC-DC displacements for stimulus levels producing 8 nm of BM displacement across frequency. Tonic displacements (plotted versus f) were roughly flat with stimulus frequency, while the cycle-by-cycle displacements (plotted versus their own frequency) declined by ∼7 dB/octave. Averages are shown when data from at least three mice were above the noise floor. (D) Average interpolated OHC-DC displacement (at f) for stimulus levels producing 0.5 nm of BM displacement across frequency. Magnitudes and phases were referenced to the those of the BM. The response of a first-order low-pass filter with corner frequency of 1.75 kHz is shown for comparison, with the filter’s phase response shifted by −0.02 cycles. Data analyzed in D were obtained over a wider range of stimulus levels but using coarser frequency steps (0.5 kHz) compared to the data presented in B and C.