Figure 5. Nonlinear, cycle-by-cycle OHC electromotility is present in some impaired mice but is not strongly correlated with tonic displacements.

(a-b) Average displacement gains (a) and phases (b) for the OHC-DC junction and TM as a function of frequency and stimulus level in WT mice (n = 17). Average phase differences between the TM and OHC-DC junction are shown in the bottom panel of b. Gains and phase differences are shown for stimulus levels of 30 to 90 dB SPL in 10 dB steps, while phases are shown only for 80 dB SPL stimuli, for clarity. Responses to higher levels are shown with darker/thicker lines. Dashed-dotted lines indicate ± 1 SE. Nonlinear gain was observed down to at least 2 kHz, and roughly out-of-phase motion was observed at low frequencies, even at high stimulus levels. (c-d) As in a-b, but for an individual salsa mouse. Gains and phase differences are shown for stimulus levels of 70 to 100 dB SPL in 10 dB steps, and phases are shown only for 80 dB SPL stimuli. Gain curves largely overlap, indicating response linearity, and phase differences are close to 0. (e-f) As in c-d, but for an individual Triobp mouse whose responses exhibited a small amount of nonlinearity and modest phase differences. Such nonlinearity and phase differences disappeared postmortem, as shown in Supplementary Fig. S5. (g) Absolute tonic OHC-DC displacement magnitudes plotted vs. the nonlinear gain observed in OHC-DC displacements for all salsa and Triobp mice. Nonlinear gain was quantified by taking the average difference between displacement gains at 70 and 100 dB SPL for stimulus frequencies of 1 to 4 kHz. (h) Absolute tonic OHC-DC displacement magnitudes plotted vs. the phase difference between TM and OHC-DC displacements for all salsa and Triobp mice. Phase differences were calculated for 80 dB SPL stimuli and averaged for stimulus frequencies of 1 to 4 kHz. In g-h, Spearman’s rank correlations (ρ) and R² values are provided, with asterisks indicating significant correlations (* p < 0.05, ** p < 0.005, *** p < 0.0005, ns = not significant). Dashed lines are linear fits. Similarly weak-to-modest correlations were found when comparing nonlinearity observed in TM motion to tonic RL displacements, as shown in Supplementary Fig. S6.