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The difference phase between one ear and the other, the interaural time difference (ITD), is one cue used in biological sound-source localization.

In mammals, different neurons in the medial superior olive (MSO) are tuned to different ITDs.

The model of biological computation of ITDs proposed by Jeffress extracts ITDs by means of delay lines and coincidence detecting neurons:

The peaks of the sound pressure at each ear lead, via a semi-mechanical process, to peaks in the activity of certain auditory nerve fibers. Those fibers connect to coincidence-detecting neurons. Different delays in connections from the two ears lead to coincidence for different ITDs, thus making these coincidence-detecting neurons selective for different angles to the sound source.

ITD and ILD are most useful for auditory localization in different frequency ranges:

  • In the low frequency ranges, ITD is most informative for auditory localization.
  • In the high frequency ranges, ILD is most informative for auditory localization.

The granularity of representations of ITDs and ILDs in MSO and LSO reflects the fact that ITD and ILD are most useful for auditory localization in different frequency ranges: ITDs for high frequencies are less densely represented in MSO and ITDS are less densely represented in LSO.

Liu et al. model the LSO and MSO as well as the integrating inferior colliculus.

Their system can localize sounds with a spatial resolution of 30 degrees.

Liu et al.'s model of the IC includes a Jeffress-type model of the MSO.

It's easier to separate a target sound from a blanket of background noise if target sound and background noise have different ITDs.

Interaural time and level difference do not help (much) in localizing sounds in the vertical plane. Spectral cues—cues in the change of the frequencies in the sound due to differential reflection from various body parts—help us do that.

Jeffress' model has been extremely successful, although neurophysiological evidence is scarce (because the MSO apparently is hard to study).

Jeffress' model predicts a spatial map of ITDs in the MSO.

Jeffress' model predicts a spatial map of ITDs in the MSO. Recent evidence seems to suggest that this map indeed exists.

A head-related transfer function summarizes ITD, ILD, and spectral cues for sound-source localization.

Sound source localization based only on binaural cues (like ITD or ILD) suffer from the ambiguity due to the approximate point symmetry of the head: ITD and ILD identify only a `cone of confusion', ie. a virtual cone whose tip is at the center of the head and whose axis is the interaural axis, not strictly a single angle of incidence.

Spectral cues provide disambiguation: due to the asymmetry of the head, the sound is shaped differently depending on where on a cone of confusion a sound source is.

Sound-source localization using head-related impulse response functions is precise, but computationally expensive.

Yan et al. perform sound source localization using both ITD and ILD. Some of their auditory processing is bio-inspired.

Voges et al. use ITDs (computed via generalized cross-correlation) for sound-source localization.

Aarabi use ITD (computed using cross-correlation) and ILD in an array of 3 microphones for auditory localization.

Recent neurophysiological evidence seems to contradict the details of Jeffress' model.