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Romke Rouw, Steven Scholte; Increased structural connectivity in Grapheme-Color Synesthesia. Journal of Vision 2007;7(9):108. doi: https://doi.org/10.1167/7.9.108.
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© ARVO (1962-2015); The Authors (2016-present)
We studied grapheme-color synesthetes, who experience particular colors while viewing particular graphemes (e.g. the A is red). While it is now clear that synesthesia is a genuine sensory phenomenon, no consensus exists on the cause of synesthetic experiences. Models diverge mainly on two issues. The first issue is whether anatomical brain differences exist between synesthetes as compared with non-synesthetes. For example, grapheme-color synesthesia may be caused by additional cross-connections between the visual word form area in inferior temporal cortex and the adjacently located color processing region. The second issue concerns the cognitive and neurological mechanisms involved in synesthesia. We studied 18 grapheme-color synesthetes by means of fMRI, questionnaires, and diffusion tensor imaging (DTI). DTI measurements showed increased diffusion anisotropy in synesthetes as compared with non-synesthetes. Increased anisotropic diffusion indicates more coherent white matter structures. This finding validates for the first time the hypothesis that differences in brain structure differentiate synesthetes from non-synesthetes. Of particular interest is a cluster of increased anisotropy in inferior temporal cortex. Synesthetes, as compared with non-synesthetes, also show increased brain activation in inferior temporal cortex while viewing graphemes that elicit synesthetic color. Next, individual differences in grapheme-color synesthesia were studied. In particular, a different cognitive and neurological architecture might underlie the degree to which the synesthetic color is experienced in the outside world (‘projectors’) as compared with seeing the synesthetic color in the mind's eye only (‘associators’). Indeed, we found differences in brain activation and brain structure between these subtypes of grapheme-color synesthesia. Importantly, increased connectivity in inferior temporal cortex is particularly strong for grapheme-color synesthetes who see synesthetic color in the outside world. In conclusion, we found evidence that increased structural connectivity is associated with the presence of grapheme-color synesthesia, and furthermore plays a role in the subjective nature of synesthetic color experience.
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