A comparison of models for sharpening of the frequency selectivity in the coachlea

The mechanical tuning of the basilar membrane has not been shown to be sharp enough to explain the very narrow tuning curves observed in primary auditory fibers. A mechanism for sharpening of the frequency selectivity or an unknown second filter is therefore assumed to exist in the cochlea. In the present paper, several theories for frequency selectivity sharpening in the cochlea are studied with the aid of a basilar membrane model. It is shown that the model proposed by Zwislocki (1974) for interaction between inner and outer hair cells results in the greatest increase in selectivity. It is also shown that nonlinear basilar membrane motion may very well explain the discrepancy between mechanical and neural tuning. Besides an increased selectivity at low sound intensities due to an increased low-frequency cut-off slope in the frequency transfer function, nonlinear basilar membrane motion also results in an increased high-frequency cut-off slope not present in other models for frequency selectivity sharpening.