Mechanics and nonlinearity of hair cell stimulation

Many nonlinear auditory phenomena are described with a BPNL model, which consists of a nonlinear element preceded and followed by a linear filter. We aim at identifying these elements with cochlear processes. The directional sensitivity of the hair cell is assumed to provide a basis for the second filter. The nonlinearity is hypothesized at the level of mechanical impedance of the hair cell. On the basis of an approximate solution for the cochlear traveling wave, we determine radial and longitudinal components of the driving force on the hair cell. It is shown that appropriate two-tone suppression and sharpening can be obtained only if radial tuning is sharper than longitudinal tuning. This imposes severe constraints on the model. It requires specific assumptions about physical properties of, for example, the tectorial membrane. For instance, if this has significant stiffness in a direction at a small angle with the radial direction, and if stretch in this direction is nonuniform, then sharpening and two-tone suppression are explainable in terms of cochlear mechanics.