Nonuniform temporal weighting of interaural time differences in 500 Hz tones.

The discrimination and lateralization of interaural time differences (ITD) in rapidly modulated high-frequency sounds is dominated by cues present in the initial portion of the sound (i.e., at sound onset). The importance of initial ITD at low frequencies is, however, less clear. Here, ITD discrimination thresholds were measured in 500 Hz pure tones with diotic envelopes and static or dynamic fine-structure ITD. Static-ITD thresholds improved as tone duration increased from 40 to 640 ms but by an amount less than expected from uniform temporal weighting of binaural information. Dynamic conditions eliminated ITD from either the beginning or end of the sound by presenting slightly different frequencies to the two ears. While overall thresholds were lower when ITD was available at sound onset than when it was not, listeners differed appreciably in that regard. The results demonstrate that weighting of ITD is not temporally uniform. Instead, for many listeners, ITD discrimination at 500 Hz appears dominated by ITD cues present in the initial part of the sound. To a variable degree, other listeners rely more equally on ITD cues occurring near sound onsets and offsets, although no listeners appear to utilize such cues uniformly throughout the sound's duration.