Is “Σ” purple or green? Bistable grapheme-color synesthesia induced by ambiguous characters

•The temporal relation of inducer identification and color emergence was explored.•W/M and 6/9 were presented at varying angles to color-graphemic synesthetes.•Synesthetes were slower naming the experienced color than identifying the character.•The degree of ambiguity tended to interact with slowed response in “color” naming. People with grapheme-color synesthesia perceive specific colors when viewing different letters or numbers. Previous studies have suggested that synesthetic color experience can be bistable when induced by an ambiguous character. However, the exact relationship between processes underlying the identity of an alphanumeric character and the experience of the induced synesthetic color has not been examined. In the present study, we explored this by focusing on the temporal relation of inducer identification and color emergence using inducers whose identity could be rendered ambiguous upon rotation of the characters. Specifically, achromatic alphabetic letters (W/M) and digits (6/9) were presented at varying angles to 9 grapheme-color synesthetes. Results showed that grapheme identification and synesthetically perceived grapheme color covary with the orientation of the test stimulus and that synesthetes were slower naming the experienced color than identifying the character, particularly at intermediate angles where ambiguity was greatest.