Digit synaesthesia: A case study using a Stroop-type test

Two experiments tested the effect that synaesthesia has on the processing of digits for a single participant, a 22-year-old female college student, who experiences colour mental images (photisms) for digits, music, sounds, etc. The experiments used Stroop-type materials that were digits in the colours of her photisms for two tasks: colour naming and digit naming. For colour naming, the hypothesis was that when the colour of the actual print of the digit mismatched the colour of the participant's digit photism, colour naming times would be slower than when the print and digit photism matched, or when the digit was in black print. For digit naming, it was predicted that naming the digit corresponding to a coloured circle (that corresponded to one of her photisms for digits) would take longer than naming digits printed in any colour. ANOVAs and Tukey tests supported these hypotheses (P < .01). Synaesthesia seems to occur automatically, involuntarily, and unidirectionally for this participant. Details of her synaesthesia and its implications are discussed.