Auditory Attention is Not a Spotlight: Evidence for the Representation of Unattended Sound Sources in Working Memory and Their Impact on Target
Symonds, Renee M.
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The ability to attend and discriminate sounds of interest is essential to communication. To do so, our auditory system must organize and process the sounds from the environment that arrive at our ears as a mixture of multiple sound sources that vary over time and space. Known as Auditory Scene Analysis (ASA), listeners often do not experience this process consciously. The goal of this thesis was to investigate the dichotomy between listeners' limited awareness of the auditory scene and the underlying processing that supports perception. Namely, does the brain process sounds that we do not perceive, and if so, what are the consequences of this processing for task performance? Selective attention is known to play a key role in ASA by enhancing the processing of selected stimuli for perception. Current theories therefore posit that attention acts as a perceptual spotlight, enhancing the processing of selected sounds and suppressing unattended sounds. While listeners are typically unaware of unattended sounds, listeners' ability to perceive their own names in the background during selective listening suggest that the processing of unattended sounds is not as limited as current theories propose. We posit that attention serves to bias processing towards attended sounds without suppressing unattended sound processing. Specifically, we hypothesize that normal-hearing listeners maintain representations of unattended sounds in parallel with attended sounds during selective listening. We predicted that this concurrent processing of unattended sound sources would compete with the processing of attended sound sources for limited cognitive resources. While the implications of resource competition remain an open area of inquiry, we also expected that the competition between attended and unattended sound streams would hinder task performance in noisy listening environments. Four studies were conducted to examine how the acoustic features defining each sound source, the age of participants, and participants' sensitivity to statistical regularities in the listening environment, affected processing of unattended sound streams. In all of the studies, we used a three-frequency-stream paradigm to assess unattended sound processing, while participants attended to one of the three streams to perform a task. Human event-related potentials (ERPs) were used to index ongoing cognitive processing of unattended sounds, because ERP responses do not require participants to actively report on their perception. We assessed ERP components of change detection and target processing in attended and unattended sound streams to measure processing of within-stream pattern violations and the effects of attention. Additionally, we analyzed participants' behavioral responses during a selective listening task to evaluate the impact of unattended sound processing on task performance in different listening contexts. Overall, our results demonstrate that for all ages of participants, the degree of processing of the unattended sounds depends on the listeners' attentional strategy and the processing load of the listening task (Chapters 2 and 3). In adults, processing of unattended sound sources is sufficient to allow implicit cues to facilitate task performance (Chapter 4). Further, listeners were able to establish target-specific expectations based on implicit cues even when the temporal association between cues and target was unreliable (Chapter 5). Taken together, our results are consistent with the hypothesis that unattended sound sources are processed in parallel with attended sounds. These data align with a growing body of literature that suggests that rather than acting as a spotlight, attention serves to bias the competition for cognitive resources towards attended stimuli, enhancing perception of the attended stimuli without suppressing the ongoing processing of additional, unattended signals. Accordingly, we find that the concurrent processing of unattended sound sources depends on processing load and that this alters performance by competing for cognitive resources with listeners' primary task. Our findings thus offer novel insights into the neural processes influencing selective listening in normal-hearing individuals and provide a necessary baseline for future investigations into communication and auditory processing disorders in clinical populations.
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