Changing roles in the cochlea: Bandpass filtering by the organ of Corti and additive amplification on the basilar membrane

The classical role of the basilar membrane in cochlear sound analysis is being reassessed on the basis of quantitative models of cochlear nonlinear responses and biophysical evidence for tonotopically organized tuning by outer hair cells. It is hypothesized that the primary function of cochlear bandpass filtering is performed by the organ of Corti, whose nonlinear responses to stapes signals are collected by the basilar membrane. Bilateral nonlinear signal processing characteristics for the organ-of-Corti filters are quantified with a new feedback reconfiguration of the multiple-bandpass nonlinearity (MBPNL) model [J. Goldstein, Hear. Res. 49, 39–60 (1990)]. The basilar membrane is modeled classically, but its role is hypothesized to be analogous to the transmission-line collector of a distributed amplifier. Traveling waves on the basilar membrane provide a means for coherent addition of responses to a given tone from different organ-of-Corti filters. Computations with the model using biophysical knowledge of the density of outer hair cells and the tonotopic map yield a maximum distributed gain of 43 dB, which agrees with known data. Bilateral nonlinear transduction in the cochlear model is responsible for an I/O response versus level for tones that compresses at much lower sound levels than the isolated organ-of-Corti filter, also consistent with data. These results encourage further modeling research with our working hypothesis that redefines the roles of basilar membrane and organ of Corti in cochlear function. [Work supported by NIDCD Grant No. DC00737.]