Animals constantly detect and encode the location of sound sources in the environment. In order to determine the location of sounds in the horizontal plane, or azimuth, the auditory system employs the interaural time differences (ITDs) that arise when sound reaches one ear before the other. Given the tiny time differences involved, animals are remarkably accurate at localizing sounds. The range of time differences that are useful to the animal depends on the head size—for humans, it's about 600 µs, for gerbils about 150 µs. Humans and barn owls are both localization champions, with an ability to resolve sounds about 2° apart . The task is easier for humans than for barn owls, because our heads are bigger (we have more microseconds per degree of azimuth), but all localizing animals detect time differences on the order of tens of microseconds. This temporal accuracy is remarkable, especially considering that individual neurons fire action potentials that can last a millisecond or more in duration.
ITDs are detected by specialized neurons that act as coincidence detectors in an area of the brainstem called the medial superior olive. The name reflects how these neurons work—they respond most reliably when they receive precisely synchronized, essentially simultaneous inputs from each ear. For coincidence detection to be useful in detecting very small time differences, the incoming sound information must first be encoded very precisely in the periphery, such that neurons fire action potentials in phase with the sound on a cycle-by-cycle basis. The phase-locked activity of the auditory nerve fibers in the periphery are relayed by the cochlear nucleus specialized for timing. The nervous system throws everything it's got at both making and keeping a temporally precise signal, from fast synapses to short sharp responses . Precise, phase-locked inputs from the left and right cochlear nuclei converge on the coincidence detector neurons. These neurons respond maximally when inputs from the two sides coincide (or almost coincide), and minimally at an unfavorable ITD. When pure tones are used, the tuning curves have multiple peaks 2pi apart, revealing their dependence on interaural phase differences.