An active process in cochlear mechanics

A model for cochlear mechanics is proposed to take account of its two systems, one passive and one active. The classical passive system stimulates the inner hair cells directly at levels above about 40 dB SL. At intensities below about 60 dB an active process, the 'cochlear amplifier' (CA), somehow provides additional energy that enhances the vibration of a narrow segment of the basilar membrane near the apical foot of the familiar, traveling wave envelope. The outer hair cells are essential for CA. The active system acts like a high-Q acoustic resonator, and it accounts for the great sensitivity and sharp tuning expressed by the 'tips' of neural tuning curves. The tips are selectively vulnerable to anoxia, noise exposure and other trauma. The CA model explains the detection of small differences in time as well as in frequency, the dual character of the electrocochleogram, recruitment of loudness in cochlear hearing impairment, the long latency of normal neural responses near threshold, acoustic emissions (both stimulated and spontaneous) and the locus of TTS in the frequency range above the exposure tone. Both the classical high-intensity system and the active low-level CA system are highly nonlinear and they combine to compress the great dynamic range of hearing into a much narrower range of mechanical movement of the cilia of the inner hair cells. The mechanism of CA is unknown, and the problem remains of how its action can be triggered by submolecular movements near threshold.