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...

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