Color Synesthesia

Synesthesia is a condition in which stimulation in one sensory or cognitive stream involuntarily, or automatically, leads to associated internal or external (illusory or hallucinatory) experiences in a second unstimulated sensory or cognitive system [1–9]. Although most cases of synesthesia are developmental and run in families, acquired cases have also been reported following traumatic brain injury, demyelination, ischemia, tumors, post-traumatic total ocular blindness, and neuropathology involving the optic nerve and/or chiasm [10–12]. Color synesthesia is a special kind of synesthesia that comprises cases of synesthesia in which a noncolored sensory or cognitive stimulus involuntarily leads to internal or external color experiences. The prevalence of color synesthesia is unknown. Estimates range from 1 in 200 to 1 in 250,000 [13, 14]. Some speculate that color synesthesia may be present in more than 4 % of the population [5]. One of the best-known forms of color synesthesia is grapheme-color synesthesia, in which numbers or letters are seen as colored. But lots of other forms of color synesthesia have been identified, including week-color synesthesia, sound-color synesthesia, taste-color synesthesia, fear-color synesthesia, etc. [5] For lack of space, this entry shall focus primarily on grapheme-color synesthesia. One mark of color synesthesia is that the synesthetic colors are seen either as projected out onto the world (“projector synesthesia”) or in the mind’s eye (“associator synesthesia”) [15]. Another mark is that it exhibits test-retest reliability [1, 16]: colors identified by the subject as representative of her synesthetic experiences relative to a given stimulus in the initial testing phase are nearly identical to colors identified by the subject as representative of her synesthetic experiences relative to the same stimulus in a retesting phase at a later time (see Fig. 1). Because of the automatic nature of synesthesia and its test-retest reliability, color synesthesia is not to be confused with memory associations or stereotypical colors of objects. For example, there is no evidence that color synesthetes simply remember the colors of entities or images they were exposed to earlier in their lives or associate stimuli with their stereotypical colors [16].