Lateralization of very-short-duration tone pulses of low and high frequencies

The position and image-width of the simultaneous images produced by very short tone pulses were measured as a function of interaural time difference (ITD) at both low- (250 and 800 Hz) and high- (2500 and 8000 Hz) frequencies using a direct-estimation technique. Primary images are lateralized towards the ear receiving the leading stimulus. At low frequencies image position is proportional to interaural phase-difference (IPD) below 90 degrees and remains at the lead-ear for larger values. At high frequencies images are reported is proportional to ITD up to 500-1000 microseconds. Secondary images are reported on the opposite side of the head for IPDs greater than 180 degrees at low frequencies, and at ITDs greater than 500 microseconds at high frequencies. Image width is approximately constant for all ITDs and both images at a given frequency, but becomes more compact as frequency increases. The data are discussed in terms of onset cues and stimulus fine-structure cues. The best explanation is in terms of an onset mechanism, but one that is calibrated in terms of IPD at low frequencies. The existence of double images is explained in terms of a breakdown in the mechanism determining fusion.