Low-passed outer hair cell response and apical-basal transition in a nonlinear transmission-line cochlear model

The low-pass characteristic of the outer hair cell (OHC) voltage response to mechanical stimulation could be considered a serious problem for cochlear models aiming at explaining high-frequency active amplification by introducing instantaneous nonlinear terms because active gain would dramatically decrease at high frequency. Evidence from experimental studies by Nam and Fettiplace [(2012). PloS One 7, e50572] suggests that the local cutoff frequency significantly increases approaching the cochlear base, somehow mitigating this problem. In this study, low-pass filtering of an internal force term, derived from a physiologically plausible OHC schematization by Lu, Zhak, Dallos, and Sarpeshkar [(2006). Hear. Res. 214, 45-67] is included in a simple one-dimensional (1-D) two-degrees-of-freedom transmission-line model by Sisto, Shera, Altoè, and Moleti [(2019). J. Acoust. Soc. Am. 146, 1685-1695] The frequency dependence of the low-pass filter phase-shift naturally yields a transition from sharp tuning and wide dynamical gain range in the basal cochlea to low tuning and poor dynamical range in the apical region. On the other hand, the frequency-dependent attenuation of low-pass filtering makes it more difficult to obtain the high gain (40-50 dB) of the basal basilar membrane response that is experimentally measured in mammals at low stimulus levels. Pressure focusing in the short-wave resonant region, which is not accounted for in this 1-D model, may help in acquiring the additional gain necessary to match the experimental data.