Interaural time differences: Implications regarding the neurophysiology of sound localization

Abstract

Interaural time differences (ITD) were measured from 400-7000 Hz in cats for a quantitative analysis of the behavior of sound localization. ITD derived from clicks and the initial portion of tone bursts showed a pronounced roughness and frequency dependence. This frequency dependence was most evident at higher angles of incidence. A single ITD will not always represent a single position on the azimuth. Most of the roughness in these functions was due to reflections off the surface supporting the control animals and the measured ITD corresponded to predictions made by steady-state theory. Measurements were made with and without the pinnae in position. They had relatively little effect on these ITD functions, particularly for frequencies below 2500 Hz and for small angles of incidence. In spite of acoustic limitations exemplified by the roughness and frequency dependence of these functions, ITD generated by sound sources situated close to the midline provided reliable localization cues that were much better than from sources well out on the azimuth. Another ITD, the group ITD, was ascribed to an acoustic signal. Calculations besed on the measured steady-state ITD showed differences between the group and steady-state ITD over a given range of frequencies. Differences between the group and steady-state ITD were significant. The group ITD apparently can provide a localization cue to the auditory system that is distinct from the steady-state ITD; and it is possible that these group ITD are used by the nervous system to localize sound sources in realistic situations.