The precedence effect and its possible role in the avoidance of interaural ambiguities.

The precedence effect, the observation that sound-source localization is determined largely by the interaural cues associated with the earlier-arriving direct sound to the neglect of later-arriving reflections, was investigated in several psychophysical experiments. The first experiment employed a stimulus composed of a continuous noise and its delayed repetition to simulate a direct sound and a single reflection. Comparison of the interaural amplitude and phase differences in this stimulus with its judged lateral position showed that the interaural amplitude and phase differences in this stimulus with its judged lateral position showed that the interaural differences do not predict lateralization judgments as simply as does a knowledge of the temporal pattern of stimulation, given the precedence effect. Most of the experiments were attempts to outline the time course of the precedence effect by measuring just-noticeable differences in interaural time and intensity of brief (less than 5 ms) wide-band noise bursts. The principal finding of these experiments was that interaural sensitivity to changes in both time and intensity follows a nonmonotonic course after the abrupt onset of as ound. Sensitivity is degraded for a period from approximately 0.5 to 10 ms after onset, with the largest jnd's at delays of 2-3 ms. The precedence effect can be understood as resulting from this temporary lapse of interaural sensitivity. It was speculated that this temporal variation in interaural acuity serves to inhibit: (1) the representation of multiple and ambiguous interaural time delays between corresponding points on the waves of periodic stimuli; and (2) the extraction of interaural phase at frequencies greater than approximately 1400 Hz.