Trading of interaural differences in high-rate Gabor click trains

In this study, combinations of interaural time differences (ITD) and interaural level differences (ILD) were applied to trains of 4000 Hz Gabor clicks (Gaussian-filtered impulses) and presented to listeners over headphones. ITD/ILD equivalence functions, or “trading ratios” (TR) were estimated using two different procedures: a “closed-loop” procedure in which subjects adjusted (via head-turn) the ILD of a target click train to counteract the effects of an imposed ITD, and an “open-loop” procedure in which subjects indicated (also via head-turn) the lateral position of click trains containing independent combinations of ITD and ILD. For both tasks, TR values increasingly favored ILD over ITD as inter-click interval (ICI) decreased from 10 to 2 ms. Subsequent analysis confirmed that this change reflected a loss of sensitivity to envelope ITD at short ICI rather than a gain in sensitivity to ILD, consistent with prior studies demonstrating rate-limited processing of ongoing envelope ITD. Significant intersubject differences in the data included two subjects whose TR values obtained under both procedures were consistently lower (greater influence of ITD) than other subjects', and did not vary with ICI. Such differences suggest that multiple mechanisms of ITD/ILD combination may be utilized to varying degrees by individual listeners. By at least one of those mechanisms, ITD sensitivity (but not ILD sensitivity) is limited to low modulation rates.