Cochlear frequency sharpening—a new model

Abstract

The basilar membrane is not sufficiently sharply tuned to account for the frequency selectivity of sensitive eighth-nerve fibers [E. F. Evans in Facts and models in hearing, edited by E. Zwicher and E. Terhardt (Springer, NY, 1974), pp. 118–129]. Anatomical evidence indicates no obvious pathways which could mediate classical lateral inhibition and physiological data indicate no time delays in the establishment of the selectivity of eighth-nerve fibers. Any proposed “second filter” must be physiologically vulnerable and be interposed between the basilar membrane response and nerve fiber output. The model proposed here assumes that the uniformity of threshold and response properties of spiral ganglion units [D. Robertson and G. A. Manley, J. Comp. Physiol. 91, 363–375 (1974)] indicates all or almost all recordings originate from inner hair cell afferents. A model is proposed of lateral interaction between hair cells via current pathways in scale media. At frequencies other than the characteristic frequency, current is shunted away from the velocity sensitive inner hair cells of the characteristic frequency location by all active outer hair cells. The model predicts sharp tuning curves with “tails,” two-tone inhibition, the effects on units of electrical polarization of the cochlea, the pattern of inhibition produced by stimulation of the crossed olivocochlear bundle, and is compatible with evidence for longitudinal interactions [A. Ryan and P. Dallos, J. Acoust. Soc. Am. 57, S41 (1975)].