Interaural Time Delay and Apparent Direction of Clicks

Abstract

The relationship between interaural time delay and the apparent direction of the image of a click was investigated at 30 and 60 dB SL (sensation level). A variation of the method of constant stimuli was used to collect the data. The constant stimulus, a steady stream of air, was in the tactile modality; the variable stimulus, a train of dichotic clicks, was in the auditory modality. The stream of air was set 15°, 60°, and 75° from the median plane in the right or left frontal quadrants. The train of dichotic clicks, each 0.2 msec in duration, and the stream of air were presented simultaneously for 3 sec. The subject had to decide whether the sound image was to the right or to the left of the stream of air. Psychometric functions were fitted to the data of three inexperienced subjects. The data show that, up to ±0.5 msec, the same interaural delay produces the same angular displacement at 60 as at 30 dB SL. The relationship between the interaural time delay and the angular displacement of the sound image is linear up to ±0.5 msec. Time delays longer than ±0.5 msec produce different displacements at different intensities; longer time delays are needed to shift the sound image to the same angular displacement at 60 than at 30 dB SL. The relationship between the interaural time delay and the angular displacement of the sound image also departs from the linearity that is found for time delays up to ±0.5 msec. A model based upon a coincidence detector is proposed to explain these results in a manner consistent with the results of experiments on interaural time-intensity trading.