Quantifying the passive substrate for active cochlear tuning

Abstract

Two aspects of passive mechanics that are relevant to the integration of active mechanics were examined: (i) Role of organ of Corti complex (OCC) mass in frequency tuning: Some cochlear models employ significant OCC mass to produce tuning while other models get by with zero OCC mass. We used closely spaced measurements of traveling wave phase, coupled to a cochlear model, to measure OCC mass and found it was significant, at least in the cochlear base. (ii) OCC stiffness and resistance: Active cochlear mechanics is thought to work as a negative resistance that is large enough to overcome passive damping, and the size and frequency dependence of the OCC acoustic stiffness are fundamental to traveling wave mechanics. By simultaneously measuring cochlear pressure and basilar membrane (BM) velocity in the cochlear base (BF ~ 40 kHz), the frequency-dependent OCC impedance was measured from 4 to 20 kHz. The imaginary part of the impedance was negative (stiffness dominated) with a size that decreased with frequency. The real part (resistance) often appeared negative at frequencies below ~ 8 kHz, an unexpected finding. At ~10 kHz, the real part flattened out to a positive value that remained smaller than the imaginary part.