A neural code adapted to the statistical structure of sensory cues may optimize perception. We investigated whether interaural time difference ITD statistics inherent in natural acoustic scenes are parameters determining spatial discriminability. To test this hypothesis, sounds with invariant statistics were presented to measure human spatial discriminability and spatial novelty detection.
Human auditory spatial perception showed correlation with natural ITD statistics, supporting our hypothesis. Further analysis showed that these results are consistent with classic models of ITD coding and can explain the ITD tuning distribution observed in the mammalian brainstem. When a person hears a sound, how do they work out where it is coming from? A sound coming from your right will reach your right ear a few fractions of a millisecond earlier than your left. The brain uses this difference, known as the interaural time difference or ITD, to locate the sound.
But humans are also much better at localizing sounds that come from sources in front of them than from sources by their sides. This may be due in part to differences in the number of neurons available to detect sounds from these different locations. It may also reflect differences in the rates at which those neurons fire in response to sounds. But these factors alone cannot explain why humans are so much better at localizing sounds in front of them.
Firstly, the change in ITD for sounds coming from different sources in front of a person is greater than for sounds coming from their sides. And secondly, the ITD for sounds that originate in front of a person varies more over time than the ITD for sounds coming from the periphery. The results also provide clues to how other senses, including vision, may have evolved to respond optimally to the environment. Humans and other species localize sound sources in the horizontal plane using sub-millisecond interaural time difference ITD between signals arriving at the two ears Middlebrooks and Green, Classical psychophysical studies demonstrated that humans detect sound location better in the front than in the periphery Mills, ; Yost, ; Makous and Middlebrooks, Enhanced performance at frontal locations could be efficient for hunting and foraging, as proposed for vision Collins and Opthalmological Society of the United Kingdom, ; Cartmill, ; Changizi and Shimojo, Physiological evidence indicates that coding of binaural spatial cues could support finer discriminability in the front van Bergeijk, ; Feddersen et al.
Better sound discrimination and localization in frontal locations can also be predicted from the geometry of the head and the placing of the ears, causing higher ITD rate of change as a function of azimuth in the front Woodworth, ; Feddersen et al. For azimuth detection based on ITD, sound diffraction by structures surrounding the ear can affect the ITD-azimuth relationship Aaronson and Hartmann, ; Roth et al. In addition, because the brain computes ITD in narrow frequency bands, the interaction of nearby frequencies within a given cochlear filter may also be a source of ITD variability over time ITDv.
