Cochlear models: two-tone suppression and the second filter.

With advances in our understanding of the auditory periphery, models of cochlear mechanics have evolved from descriptions of existing data to the point that they can suggest meaningful observations that should be made and predict expected outcomes. As an example, we discuss relationships among mechanical nonlinearities in the inner ear, two-tone suppression, and the auditory "second filter." Electrophysiological two-tone suppression data indicate that excitatory tuning curves (stimulus intensity required to elicit a specified amount of activity in a primary fiber as a function of frequency) are more sharply tuned than suppressive tuning curves (stimulus intensity required to suppress the response to a continuous tone at the fiber's characteristic frequency by a specified amount as a function of frequency). Model simulations which attempt to account for the above difference suggest that there must be a stage of sharpening between the physical variable governing two-tone suppression (tentatively identified as velocity of the basilar membrane) and the physical variable governing neural excitation. An experiment that would unequivocally demonstrate the simultaneous existence of two representations of the stimulus with significantly different tuning properties in the cochlea is described. The future evolution of cochlear modeling, incorporating nonlinear two-dimensional models, is discussed.