Unidirectional amplification as a mechanism for low-frequency hearing in mammals

Abstract

Sound detection is achieved in the inner ear, or cochlea, which spatially separates frequencies. The highest frequencies are detected at the organ's base and successively lower frequencies at progressively more apical positions. Although a mechanism termed critical-layer absorption accounts for the spatial separation of frequencies above 1 kHz, hearing at lower frequencies appears to operate by a distinct mechanism that is poorly understood. We discuss a recently proposed ratchet mechanism for spatial separation of these low frequencies. This mechanism involves the synergistic interplay of two known active processes in the mechanoreceptive hair cells - active hair-bundle motility and membrane-based electromotility - to implement unidirectional amplification. It thus represents a mechanical analogue of the operational amplifier in electrical engineering. The mechanism provides a general design principle for unidirectional mechanical amplification that we have also employed to construct an active microphone. © 2011 American Institute of Physics.