The Jeffress model (Jeffress, 1948) describes a neural mechanism for the brain to detect very small differences in the time of arrival of sound at one ear compared to the other, and thus determine the horizontal (azimuth) origin of the sound. It operates through a combination of coincidence-detecting neurons and axonal delay lines.
Follow the tabbed panels below from left to right. The first two describe the basic mechanism, the last two two deal with the more advanced topic of phase ambiguity and its resolution.
[Note that the cartoons are qualitative illustrations of the phenomena, not accurate quantitative simulations.]
The spatial separation of ears means that sound from an off-centre source will not arrive simultaneously at both ears, leading to an interaural time difference (ITD).
Delay lines and coincidence detectors convert a time code (inter-aural time difference [disparity]: ITD) into a line-labelled space code
Pure tone (single frequency) sounds result in phase ambiguity - phantom sound sources indistinguishable from the real one
Broad-spectrum (noise) sound resolves phase ambiguity because only the correct locality sums at all frequencies at the space-map integrator.
The Jeffress mechanism was originally completely hypothetical, but there is now strong evidence that something like this operates in the nucleus laminaris in birds (Carr and Konishi, 1988). The mechanism may also operate in the medial nucleus of the superior olivary complex in mammals, although this is more debatable (Grothe et al., 2010).
Carr, C.E. & Konishi, M., 1988. Axonal delay lines for time measurement in the owl’s brainstem. Proceedings of the National Academy of Sciences of the United States of America, 85(November), pp.8311–8315.
Grothe, B., Pecka, M. & Mcalpine, D., 2010. Mechanisms of Sound Localization in Mammals. Physiological Reviews, 90, pp.983–1012.
Jeffress, L.A., 1948. A place theory of sound localization. Journal of Comparative and Physiological Psychology, 41(1), pp.35–39.
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