This proposal is in response to Notice Number NOT-OD-09-058: NIH Announces the Availability of Recovery Act Funds for Competitive Revision Applications. The proposed research will examine how the human auditory cortex (AC) responds to combinations of different types of binaural information (specifically, information from interaural time differences, or ITD, and interaural level differences, or ILD) over time. 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. Echoes and reverberation, however, distort ITD and ILD values over time, degrading localization and speech understanding. These effects are especially severe in aging, hearing impairment, and cochlear-implant use, possibly reflecting dysfunction in normal compensatory mechanisms that emphasize aspects of sound (such as ITD at onset) that are more robust to effects of echoes. Previous research in this area has investigated how spatial sensitivity varies over the time-course of a sound, and some studies have suggested that the AC maintains multiple representations of auditory spatial cues, combining information across these cues in a manner that is sensitive to characteristics of the reverberant environment. The proposed revision uses functional magnetic resonance imaging (fMRI) to measure the sensitivity of AC responses to changes in the binaural configuration of a stimulus.
The specific aims of address whether activity in the AC exhibits sensitivity to the temporal structure of binaural information (for example, does the AC respond to cues contained in the onset of a sound more than later parts of a sound?) and in turn whether AC activity reflects the perception or the physical configuration of a stimulus (when the two differ).
The aims further address whether various candidate spatial cues (ITD and ILD at onset and thereafter) contribute together or independently in shaping AC responses, and whether those contributions vary across functionally distinct sub-regions of human AC.
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. Furthermore, mapping the sensitivity of auditory cortical regions to variation in binaural parameters will improve our current understanding of how the brain processes sound, and how auditory cortex is organized, in normal and disordered populations.
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