Development of color-grapheme synesthesia and its effect on mathematical operations.

SE, a 19-year-old female college student and a color-grapheme synesthete, reported consistent synesthetic experiences (photisms) for digits, and most letters in the Roman and Hebrew alphabets over a 6-month period. Photisms for later-learned Hebrew letters were influenced by those of the Roman alphabet and photisms for auditorily presented words were influenced by the spelling of the words. As hypothesized, in two studies, SE's solution times for addition problems were significantly slower when the colors of the digits in the problems mismatched her synesthetic photisms than when they matched or were in black (p < 0.001), whereas color had no effect on solution times for six female non-synesthetes (aged 19 to 22 years). Implications for learning and cognitive processing are discussed.