In grapheme-color synesthesia, seeing a particular letter or number evokes the experience of a highly specific color. Here we investigate the brain’s real-time processing of words in this population, by recording event-related brain potentials (ERPs) from 15 grapheme-color synesthetes and 15 controls as they judged the validity of word pairs (“yellow banana” vs. “blue banana”) presented under high and low visual contrast. Relative to high contrast words, low contrast words elicited ~30ms delayed P1/N170 visual ERP components in both groups. When color concepts were conveyed to synesthetes by a string of achromatic graphemes (“55555 banana”) individually chosen for each synesthete, visual contrast effects were like those in color words: P1/N170 components were delayed 30ms, but unchanged in amplitude. When controls saw equivalent strings of colored graphemes, visual contrast modulated the amplitude of P1/N170, leaving their latency unchanged. Color induction in synesthetes thus differs from color perception in controls. Independent from the experimental effects, all orthographic stimuli elicited larger N170 and P2 in synesthetes than controls. While P2 (150–250ms) enhancement was similar in all synesthetes, N170 (130–210ms) amplitude varied with the subjective experience of synesthetes and their self-reported imagery ability. Results suggest the extent of immediate cross-activation in visual areas processing color and shape is most pronounced in so-called projector synesthetes whose concurrent colors are experienced as originating in external space.
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