A neural window into the cochlea

In the initial stage of auditory processing, the snail-shaped cochlea in the inner ear performs an exquisite frequency analysis via a mechanical coupling that has intrigued physicists for more than a century yet is still not entirely understood (see, for example, Physics Today, April 2008, page 26 ). Each point along a membrane that spirals up the cochlea responds maximally to a small band of sound frequencies; the center frequencies of those bands decrease from the broad cochlear base to the narrow cochlear apex. Neurons of the auditory nerve are arrayed along the length of the cochlea, and their sequences of electrical spikes provide an avenue for examining cochlear processing.

Philip Joris and colleagues at the University of Leuven in Belgium have recorded and studied the impulses that white noise produces along individual auditory nerve fibers in a cat’s ear. Each of the curves here, arranged from bottom to top in increasing frequency, plots the autocorrelations in a single fiber’s spike sequence. The curves reveal a transition in the neuronal encoding, from instantaneous sound-waveform information at low frequencies, as evidenced by the oscillations, to envelope information at high frequencies, where there is only a single peak. And when the researchers compared an individual fiber’s responses to differing sound intensities, they found surprisingly little shift in the correlation patterns; that, they suggest, contributes to the stability of sound localization based on timing differences in the signals from the left and right ears (see Bill Hartmann’s article in Physics Today, November 1999, page 24 ). (P. Michelet, D. Kovačić, P. X. Joris, J. Neurosci. 32, 9517, 2012; image submitted by Pascal Michelet and Philip Joris.)

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