This research will examine how listeners combine auditory spatial information across different types of binaural cues-interaural time differences (ITD) and interaural level differences (ILD)-and over time in order to better understand the dynamic processes underlying spatial hearing in reverberant environments. ITD and ILD provide the major set of cues that allow listeners to localize sounds in space and to segregate competing sound sources. In ideal situations, the two cues agree with one another, providing redundant information about auditory space, but echoes and reverberation distort these cues over time, thereby degrading localization and speech understanding. Whereas the normal binaural system compensates by emphasizing robust aspects of the sound (such as ITD at onset), listeners who are aging, hearing-impaired, or using cochlear implants continue to experience difficulty in the presence of echoes and reverberation. Identifying the specific nature of these auditory processing deficits is difficult because so little is known about the compensatory mechanisms at work in normal-hearing listeners. Past research has investigated how spatial sensitivity changes over the time-course of a sound (thereby emphasizing sound onsets), but a key question remains unanswered: Are these changes specific to the processing of ITD and/or ILD, or do they reflect the manner in which the cues are combined over time? The specific aims of the proposed study address this question by mapping the time-course of listeners'sensitivity to ITD, ILD, and combinations of the two. Listeners will judge the spatial positions of sounds with time-varying ITD and/or ILD, and multiple regression will be used to measure the dependence of those judgments on ITD and/or ILD. Regression coefficients will be used to construct temporal weighting functions (TWFs) that illustrate the dynamics of neural mechanisms for integration of ITD and ILD over time.
The aims further address whether TWFs vary between tasks that do and do not involve overt localization (e.g., discriminating vs. pointing to sounds). Because a number of patient populations (aging, hearing impaired, cochlear implant users) are impaired when listening in noisy and reverberant environments, an improved understanding of the mechanisms that allow normal-hearing individuals to deal with echoes and reverberation will improve (a) theoretical descriptions of auditory processing deficits, and (b) algorithms for signal processing in hearing aids and cochlear implants. Specifically, mapping the time-course of sensitivity to ITD and ILD could guide the distribution of digital signal-processing resources to most effectively preserve relevant spatial information and de-emphasize potentially misleading information.
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