Detectability of varying interaural temporal differences.

Abstract

Detectability and salience of time-varying interaural temporal differences (IATD's) were measured in three experiments by determining observers' ability to follow the temporal fluctuations of a "moving stimulus"--a 3000-Hz low-pass computer-generated noise presented binaurally with a sinusoidally varying IATD. In the first two experiments the peak IATD (deltat the "extent of movement") was manipulated to determine, for different rates of interaural variation (fm), threshold discriminability of the "moving" stimulus from a reference (two-interval forced-choice paradigm). The nonmoving reference was either a dichotic noise stimulus (experiment 1) or a dichotic noise stimulus whose "image width" matched that of the excursions traced by the "moving stimulus" (experiment 2). Threshold deltat's in the two experiments were similar, increasing from 30 microns at fm = 0 Hz to 90 microns at fm = 20 Hz, indicating a "low-pass characteristic" for the binaural system. Thresholds decreased again for fm = 50 Hz, apparently because at these high rates of "movement" observers used other cues than the varying IATD's to perform the task. The third experiment measured the threshold of a binaural click in the presence of a "moving noise" masker as a function of fm and of the instantaneous IATD of the masker when the click was presented. As fm increased, click threshold gradually became independent of the masker's instantaneous IATD, again suggesting a "low-pass" characteristic for the binaural system; additionally, there was some evidence for a lag in the system's response for fm greater than 5 Hz. The data from the three experiments are discussed in terms of results from other studies which have investigated temporal aspects of the binaural system. The possible existence of movement detectors in the auditory system is discussed.