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THE BENEFITS OF SPATIAL SEPARATION ON THE CORTICAL REPRESENTATIONS OF SPEECH SOUNDS

Abstract
Spatial separation between competing speech streams reduces their confusion (informational masking) and improves speech processing under challenging listening conditions. The precise stages of auditory processing and the bottom-up and top-down mechanisms involved in this spatial release from informational masking are not fully understood. Event-related potentials (ERPs) were used to measure the cortical processing of relevant speech under conditions of informational masking and its spatial release, and to examine the preattentive and attentive mechanisms that benefit listeners. Participants were asked to detect noise-vocoded target speech presented with noise-vocoded two-talker masking speech. In separate conditions, the same set of targets were spatially co-located with maskers to produce a high degree of informational masking and spatially separated from maskers using a perceptual manipulation to release the informational masking. Cortical auditory evoked potentials (N1/P2 ERP waveforms) elicited by targets were only apparent under conditions in which informational masking was released. Furthermore, when targets were presented at an intensity above masking threshold in both spatial conditions, N1 and P2 latencies were shorter when targets were perceptually separated compared to co-located with maskers. These effects of spatial separation were observed regardless of whether participants attended to the auditory task or attended away from the sounds to engage in a challenging two-back visual task. Benefits of attending to the sounds were apparent in later time windows (P2 and P3), while there was tentative evidence of attentional benefits earlier in processing under some conditions. These results show that spatial separation between competing speech streams can facilitate preattentive, bottom-up processes that reduce confusion and improve the early perceptual representations of relevant speech. Top-down selective attention is necessary for supporting higher-level task-relevant cognitive benefits of spatial separation that occur at later stages of processing, and may play a more crucial role in the early perceptual processing of speech under especially challenging listening conditions. These studies shed light on the underlying processes that contribute to the spatial release from informational masking, and establish methods and measures that may be applied in future research aiming to benefit listeners who experience difficulties processing speech within noisy, complex environments.
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dissertation
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http://creativecommons.org/licenses/by-nc-sa/4.0/
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