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Is synesthesia more than unusual associations? : examining cue combination and various forms of learning in synesthetes.
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Abstract:
Thesis (Ph. D.)--University of Rochester. Department of Brain and Cognitive Sciences, 2016. ; Synesthesia is a phenomenon defined by the experience of unusual associations. For instance, a lexical-gustatory synesthete may taste "cold, hard bacon" when hearing the word jail. Unlike associations common in the general population (e.g., associating relatively high pitches with relatively bright colors), synesthetic associations are conscious, incredibly precise, involuntary, and consistent. Although these associations are diagnostically the hallmark of synesthesia, a growing body of research suggests that synesthetes differ from nonsynesthetes in other ways as well. Across behavioral, neuroimaging, and genetic studies, evidence suggests that synesthetic associations are the obvious manifestation of more general, widespread cognitive differences between synesthetes and nonsynesthetes. Behaviorally, synesthetes demonstrate heightened perceptual and memory abilities in their synesthetic domain. In neuroimaging studies, group differences arise in brain areas other than those responsible for processing the synesthetic association itself. Lastly, genetic investigations of synesthesia nominate genes that are widely expressed throughout the brain and involved in multiple cognitive processes. Taken together, the current literature suggests that synesthesia is a phenomenon encompassing more than its diagnostic associations. In this dissertation, I investigated two particular cognitive functions that may operate differently in synesthetes: learning and cue combination. There is abundant evidence that synesthetic associations are neither innate (e.g., they occur in response to letters that must be learned) nor entirely random (e.g., common patterns of associations emerge when looking at populations of synesthetes), but are instead shaped in part by some form of learning. By using fine-grained measures of learning and introducing shifts in the available statistics, I investigated synesthetes' ability to learn environmental statistics and adjust to unannounced changes. Across multiple tasks and domains, I found that synesthetes and nonsynesthetes differed in the way that they learn about their environment. For explicit associations learned in a self-directed manner, I provided evidence that synesthetes learned associations more quickly than controls. In contrast, I found that synesthetes were slower to benefit from implicitly learned probabilities. Across all of these tasks, synesthetes demonstrated superior memory for the learned statistics and, potentially as a consequence, greater interference from unexpected changes in these statistics. These findings suggest that general aspects of learning and memory differ between synesthetes and nonsynesthetes, due--at least in part--to mechanisms of learning and not solely to genetic differences. Thus, my investigation of synesthetes' learning supports the hypothesis that synesthesia is a broad phenomenon, The motivation for investigating cue combination stems from mixed results regarding synesthetes' susceptibility to multisensory illusions compared to controls' susceptibility as well as neuroimaging findings that synesthetes' parietal cortex structure and activation differs from that of controls. In this dissertation, quantitatively examining synesthetes' integration of audio and visual cues revealed optimal use of both sensory and categorical information. As controls also demonstrate optimal use of sensory and categorical information during cue combination, these findings suggest that synesthetes and controls rely on the same cognitive mechanism for integration. Thus, this portion of the dissertation demonstrates a particular cognitive mechanism that is not affected by synesthesia. Combined, my evaluation of synesthetes' learning and cue combination mechanisms assists in creating a more complete picture of synesthesia.
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URL: http://hdl.handle.net/1802/31473
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