The effect of distractor frequency on judgments of target laterality based on interaural delays

A two-dimensional stimulus-classification paradigm was used to examine the ability of listeners to judge the laterality of an interaurally delayed low- frequency target component presented concurrently with a distractor component. Of primary interest was the effect on performance of the frequency difference (Δf) between the target and distractor. In one set of conditions, the target was fixed at 753 Hz and the distractor was 353, 553,653, 703, 803, 853, 953, or 1153 Hz (fixed within a block of trials). In a second set of conditions, the distractor was fixed at 753 Hz and the target frequency was 353, 553, 653, 703, 803, 853, 953, or 1153 Hz. The listeners were presented with a target component with an interaural delay that varied from trial to trial, taking on one of ten values, five leading to the left ear and five leading to the right. A distractor component was simultaneously presented with an interaural delay that also took on one of the same ten values. Delays ranged from -90 to +90 μs in 20-μs steps. During a block of 100 trials, each of the possible combinations of target and distractor delay was presented once and only once in a random order. Listeners were instructed to make left-right judgments based on the target delay. Each condition was repeated ten times, and the slopes of the best linear boundaries between left and right responses were used to derive the relative weights given to the target and distractor. The duration of the signals was 200 ms. Two of the eight listeners weighted the target heavily when the target and distractor were spectrally remote but gave the two components equal weight when the difference in frequency was small. These two listeners yielded similar target weights regardless of which component was designated as the target. One listener gave nearly equal weight to the target and the distractor regardless of Δf. Five of the listeners gave greater weight to the higher of the two frequencies regardless of which was assigned as the target. This high frequency dominance is explained in terms of cross-correlation functions based on the composite two-tone waveforms.