Priming letters by colors: evidence for the bidirectionality of grapheme-color synesthesia.

In synesthesia, stimulation of one sensory modality leads to a percept in another nonstimulated modality, for example, graphemes trigger an additional color percept in grapheme-color synesthesia, which encompasses the variants letter-color and digit-color synesthesia. Until recently, it was assumed that synesthesia occurs strictly unidirectional: Although the perception of a letter induces a color percept in letter-color synesthetes, they typically do not report that colors trigger the percept of a letter. Recent data on number processing in synesthesia suggest, however, that colors can implicitly elicit numerical representations in digit-color synesthetes, thereby questioning unidirectional models of synesthesia. Using a word fragment completion paradigm in 10 letter-color synesthetes, we show here for the first time that colors can implicitly influence lexical search. Our data provide strong support for a bidirectional nature of grapheme-color synesthesia and, in general, may allude to the mechanisms of cross-modality interactions in the human brain.