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Synesthesia is a condition in which individuals experience atypical responses to certain types of stimuli, in addition to the typical responses elicited by those stimuli. For example, a synesthete may perceive tastes when seeing certain shapes or might perceive colors when seeing achromatic letters. Synesthesia comes in many forms, covering a wide range of sensory interactions both cross-modally and within a single modality.

It will be useful to highlight some characteristics of synesthesia that serve to distinguish it from other perceptual phenomena, such as visual imagery and certain forms of imagistic memory. There are three such characteristic features: (1) automaticity, (2) reliability, and (3) consistency. First, there is ample evidence that synesthetic associations are automatic in nature. They are typically produced outside the intentional control of the individual and cannot be directly inhibited. Second, it is typically the case that synesthetes reliably experience synesthetic responses when presented with triggering stimuli. Finally, although there is variability across synesthetes, synesthetic associations within an individual appear to remain relatively consistent over time in that the same types of stimuli (e.g., specific auditory tones) tend to elicit the same types of synesthetic responses (e.g., specific colors).

Grapheme-color synesthesia is a condition characterized by enduring and consistent associations between letter/digits and colors. For people with grapheme-color synesthesia, letters or digits have fixed, enduring, conscious color associations. For example, the letter A might be red, B might be blue, C might be yellow, and so on. In the terminology of the literature, letters and digits are the “inducers” of grapheme-color synesthesia, and the color itself (i.e., the synesthetic experience) is the “concurrent.”