Grapheme-color associations can transfer to novel graphemes when synesthetic colors function as grapheme "discriminating markers".

Synesthesia is a condition in which the perception of a stimulus in one modality automatically triggers a secondary sensation in another modality or processing stream. Our study focused on grapheme-color synesthesia, in which the visual perception of letters or numbers (graphemes) induces a specific color sensation (the synesthetic color). Grapheme-color synesthetes do not typically experience colors for novel graphemes. However, synesthetic colors associated with familiar graphemes can be transferred to graphemes learned later, even in adulthood. A previous study has shown that such a transfer can take place after only a 10-min writing exercise. In this study, we found that this immediate transfer occurs only when the synesthetic colors for familiar graphemes contribute to the discrimination of the graphemes to be learned. Synesthetes learned six novel graphemes, each of which was arbitrarily associated with one of six familiar graphemes. Half of the synesthetes were assigned to the heterogeneous condition, in which the synesthetic colors of one group of familiar graphemes were different from one another. The other half of the synesthetes were assigned to the homogeneous condition, in which the various colors of a whole group of familiar graphemes were categorically the same. After this learning session, less transfer of synesthetic colors to novel graphemes from the corresponding familiar graphemes occurred in the homogeneous condition than in the heterogeneous condition. These results support the view that synesthetic colors for graphemes may function as a grapheme acquisition aid.