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Vilayanur S. Ramachandran, Edward M. Hubbard, Peter A. Butcher; “Higher” and “lower” forms of synesthesia may arise from cross-wiring at different cortical stages. Journal of Vision 2002;2(7):265. doi: 10.1167/2.7.265.
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Although synesthesia (“seeing sounds” or seeing letters and numbers tinged with specific colors) has been known for over 100 years, only recently has progress been made towards understanding its underlying mechanisms. We have previously demonstrated that synesthesia is a perceptual phenomenon, and suggest that, in some synesthetes, it may be caused by cross-wiring between V4 and the number grapheme area, both of which lie in the fusiform gyrus (Ramachandran & Hubbard, SFN 2000; 2001, VSS 2001). Roman numbers and subitizable clusters of dots did not evoke colors, suggesting that the visual grapheme is critical for the experience of colors. To explore this further, we presented graphemes in unusual fonts, in lower case, or made from unusual forms (i.e. a drawing of three cats to make up the form of an “A”). Synesthete JC reported that he still experienced the appropriate synesthetic colors, but lighter and “shinier.” These results further show that the visual grapheme is critical for the experience of colors. However, we have also identified a second group of “higher synesthetes” in whom the colors are driven by the numerical concept (e.g., by Roman numerals). For these synesthetes, the experience of colors does not change depending on visual properties such as font. We propose that in this group of synesthetes, cross-wiring may occur near the angular gyrus, where higher color areas and more abstract number areas lie. Intriguingly, in some of these synesthetes, even days of the week and months of the year are colored. We therefore suggest that the angular gyrus may be involved in representing abstract sequences or ordinality, and this would explain why numbers, days of the week and month of the year are colored in these synesthetes. This heterogeneity has often led researchers to avoid studying synesthesia. Our results suggest that there may be an underlying logic to the phenomena, and may help to guide further research into the neural basis of synesthesia.
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