Abstract:
This thesis expands on the knowledge of the cognitive neuroscience of synaesthesia. Three experiments examined aspects of cognition and neural activity in synaesthetes. Experiment 1 examined how synaesthesia affects visual binding. Grapheme-colour synaesthetes were presented with sets of differently coloured letters, followed by a single coloured letter which made up a colour-letter combination that had either appeared in the preceding array, or not. Synaesthetes indicated whether or not they thought it had. In addition, the colour-letter combination was either synaesthetically congruent or not. No evidence was found that synaesthesia affected the pattern of binding errors, with congruent and incongruent letters equally subject to binding errors. Experiment 2 used EEG to examine brain activation in synaesthetes when they view graphemes presented in the synaesthetically congruent colour versus graphemes presented in an incongruent colour. Matched nonsynaesthetes were also as yoked controls. For incongruent stimuli. synaesthetes had a more negative N200 component. The N200 indexes visual mismatch, indicating that synaesthetes responded to incongruently coloured letters as perceptually different from congruently coloured letters, and underscoring that synaesthesia is a perceptual phenomenon rather than simply memory associations. Experiment 3 used EEG to explore how the brain activations of synaesthetes differ from that nonsynaesthetes when they are viewing stimuli that induce a synaesthetic experience. Grapheme-colour synaesthetes and yoked controls viewed black graphemes. The synaesthetes differed significantly from nonsynaesthetes on three negativities, peaking at 230ms, 305ms, and 375ms poststimulus onset. These results are consistent with the theory that synaesthesia involves brain activity comparable to colour processing.