Department of Speech and Hearing Sciences, University of Washington, 1417 NE 42nd St, Seattle, WA 98105, USA.
Hear Res. 2010 Sep 1;268(1-2):202-12. doi: 10.1016/j.heares.2010.06.002. Epub 2010 Jun 12.
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.
在这项研究中，通过耳机向听众呈现了 4000 Hz 加伯点击（高斯滤波脉冲）的耳间时间差（ITD）和耳间强度差（ILD）组合。使用两种不同的程序估计了 ITD/ILD 等效函数，或“交易比率”（TR）：一种是“闭环”程序，其中受试者通过转头调整目标点击序列的 ILD，以抵消施加的 ITD 的影响；另一种是“开环”程序，其中受试者通过转头指示包含 ITD 和 ILD 独立组合的点击序列的横向位置。对于这两个任务，随着 ICC 从 10 毫秒降至 2 毫秒，TR 值越来越倾向于 ILD 而不是 ITD。随后的分析证实，这种变化反映了对短 ICC 下包络 ITD 的敏感性丧失，而不是对 ILD 的敏感性增加，这与先前研究一致，表明对进行中的包络 ITD 的处理受到限制。数据中的受试者间差异显著，包括两名受试者，他们在两种程序下获得的 TR 值始终较低（对 ITD 的影响较大），而与 ICC 无关。这些差异表明，个体听众可能会以不同程度利用多种 ITD/ILD 组合机制。至少有一种机制限制了 ITD 敏感性（而非 ILD 敏感性）至低调制速率。
