Across-frequency influences in lateralization for interaural time and level differences

The localization of simultaneously presented sounds was investigated in two experiments by introducing interaural disparities over headphones (lateralization). In the first experiment, interaural time difference (ITDs) and interaural level differences (ILDs) were informative in one band of noise (the target), but not in a spectrally remote second band of noise (the distractor). The interference produced by the presence of a distractor was measured by the increase in target ITD and ILD that was necessary for threshold performance. The stimuli were bands of noise centered at either 500 Hz or 4000 Hz with bandwidths that were 10% of center frequency. Each band of noise served as both a target and as a distractor. For ITD discrimination, the interference was greater with the 4000 Hz target than with the 500 Hz target. For ILD discrimination, the interference with the 500 Hz target was greater than or equal to that obtained with the 4000 Hz target. The distractor was either diotic (containing no ITD or ILD), or had randomly varied ITDs across intervals, or had randomly varied ILDs. Randomization of the distractor ITD or ILD increased the interference relative to the diotic distractor. In a second experiment, observers centered the perceived lateral position of a target by manipulating the target ITD or ILD. The influence of a simultaneously presented distractor was measured for a range of center frequencies and interaural disparities. Both discrimination performance and position judgments indicate that the distractor position influenced the perceived position of the target. The nature of this influence is explored, leading to the hypothesis that the perceived location of the target is a function of the interaural information contained across separate frequency regions. The relative weights of different frequency regions vary across ITD and ILD domains in a manner qualitatively consistent with relative sensitivity in those regions