Auditory Neuroscience: A Time for Coincidence? Dispatch

Mammals and birds appear to encode timing differences between the ears, a major cue for auditory localization, in fundamentally different ways. It now appears that results from different species can be accommodated within a single general framework. An ability to localize sounds both accurately and rapidly — particularly if they occur outside the field of view — is of obvious survival value. Because the afferent nerves from the cochlea do not convey spatial information directly, neural computations have to be performed within the brain in order to determine the direction of a sound source. This involves comparing the intensity and time of arrival of the sound at the two ears. Together with the spectral filtering imposed by the external ear, these binaural cues are responsible for auditory localization [1]. Psychophysical studies in humans have shown that the principal cue for sound localization in the horizontal plane, at least at low frequencies, is the interaural time difference (ITD) [2]. The maximum ITD encountered depends on the distance between the ears; in adult humans it is about 600 µs. Humans can discriminate ITDs as small as 10–20 µs [3] — an astonishing achievement given that the duration of an action potential is two orders of magnitude greater than this. Over the last few years, electrophysiological studies have indicated that the neural basis for ITD coding may vary among different species. Recent modelling data [4], however, suggest that the optimal coding strategy depends primarily on head size and the sound frequency range over which ITDs can be discriminated, rather than a more intrinsic difference between species