Lateralization of tonal signals which have neither onsets nor offsets

Abstract

In order to ascertain the special importance of binaural cues conveyed in the transient portions of dichotic signals, thresholds for interaural differences of time (delta t) and intensity (delta I) were studied using stimuli whose onsets and offsets were masked. Intense noise was used to mask all portions of each experimental trial except for the two intervals of a two-interval, forced-choice detection task. During the intervals, the noise was turned off with decay-rise times of 10 ms. What remained were tones whose interaural phase or intensity was different for intervals one and two. Performance was compared to control conditions which used unmasked gated sinusoids. For longer durations, detection without onsets and offsets was about as good as that with no masker. For the shorter signals, detection without transients was poorer than with standard lateralization, but this is attributed to forward and backward masking which reduced the effective durations of those stimuli. The ability to detect interaural differences of time with the onsets and offsets masked was extended to conditions in which the decay times of the noise were 100 ms. Performance here was slightly worse, but not by so much as to change the basic result. This is interpreted as showing that performance with the faster decay-rise times was not a product of momentary undershoots in neural following, but depended, rather, upon a true encoding of the interaural information in the stimulus fine-structure.