The three-channel model of sound localization mechanisms: interaural level differences.

The current understanding of mammalian sound localization is that azimuthal (horizontal) position assignments are dependent upon the relative activation of two populations of broadly-tuned hemifield neurons with overlapping medial borders. Recent psychophysical work has provided evidence for a third channel of low-frequency interaural time difference (ITD)-sensitive neurons tuned to the azimuthal midline. However, the neurophysiological data on free-field azimuth receptive fields, especially of cortical neurons, has primarily studied high-frequency cells whose receptive fields are more likely to have been shaped by interaural level differences (ILDs) than ITDs. In four experiments, a selective adaptation paradigm was used to probe for the existence of a midline channel in the domain of ILDs. If no midline channel exists, symmetrical adaptation of the lateral channels should not result in a shift in the perceived intracranial location of subsequent test tones away from the adaptors because the relative activation of the two channels will remain unchanged. Instead, results indicate a shift in perceived test tone location away from the adaptors, which supports the existence of a midline channel in the domain of ILDs. Interestingly, this shift occurs not only at high frequencies, traditionally associated with ILDs in natural settings, but at low frequencies as well.