Non-linear processing of sound by the cochlea

Before the brain can process sounds, their different frequencies must be separated; intensities, compressed to a small response range; and mechanical energy converted into electrical signals. These tasks are performed by the cochlea. Low-frequency sounds are processed in the apex of the cochlea. Using auditory-nerve data we analyzed the amplitude and phase of cochlear excitation. We descibed the gradual change of amplitude and phase curves with characteristic frequency (CF) and sound intensity. Generally, apical cochlear mechanics is considerably nonlinear and of a different character than basal cochlear mechanics. We recorded basilar membrane motion at the base. Frequency selectivity and intensity effects were very similar for single-tone and multi-tone responses and seemingly offset by a constant intensity difference. Analysis of distortions showed that the linear response generally dominates, except for stimulus frequencies well above CF. By varying the intensity of one component we studied suppressive effects on amplitude and phase. Suppressors below CF had effects similar to increased sound intensity of all components. Above-CF suppressors affected probe frequencies slightly below the suppressor frequency and higher. A gain-control scheme accurately predicted the effects of low-side suppressors, but not of high-side suppressors. These results are best interpreted in a spatial framework, where a traveling wave passes many local gains and propagation speeds that are displacement and frequency dependent. This spatial buildup of local effects explains how seemingly contradictory responses are in fact different manifestations of the same displacement-dependent effects. It is widely assumed that the cochlea amplifies sound. We tested this hypothesis.

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