Cochlear mechanics without the traveling wave: the cochlea as a surface acoustic wave resonator

Abstract

Current models of the cochlea are almost invariably based on a traveling wave set up on the partition by a pressure difference across it, a von Békésy-style picture in which the pressure common to both sides – the common-mode pressure – is ignored. But here the speculation is made that the sensing elements – the outer hair cells – respond to common-mode pressure in a way that creates reverberating wave activity between the cells and produces an active amplifying system analogous to a surface acoustic wave (SAW) resonator. In particular, the three rows of outer hair cells appear to act like the interdigital electrodes of a SAW resonator so that, through their combined motile responses to intracochlear fluid pressure, they impart slow ripples of very short wavelength – tens of micrometres – to the undersurface of the gelatinous tectorial membrane immediately above. A repeated cycle of sensing ripples and executing motile responses to them creates reverberation between the rows, establishing a virtual resonant cavity with inherent positive feedback, amplification, and high Q – characteristics that underlie the cochlear amplifier. Also emerging naturally from such a local interaction on the partition are spontaneous, evoked, and distortion-product emissions, the profile of the cochlear tuning curve, and, perhaps, even musical properties. Differential pressure is seen to have an effect, but again it is localised. Oscillating differential pressure, at moderate intensities, moves the tectorial membrane up and down, a motion that creates a second ripple source at the point where the jelly-like membrane contacts the hard edge of the vestibular lip. The slow ripples emanating from this interaction produce cycles of delay at the inner hair cell which emulate delays attributed to the conventional traveling wave; moreover, interaction between the two sources provides a straightforward account of peak-splitting and interference notches. Thus, using some novel but not physically unreasonable assumptions, a comprehensive, consistent, and place-specific account of cochlear mechanics – in many ways akin to von Helmholtz’s resonance theory of hearing – can be given without recourse to the usual traveling wave