The most common form of synesthesia, grapheme to color synesthetes associate letters and numbers to very specific colors. These experiences are involuntary and often unique to the individual, though large-scale studies have found some commonalities. For instance, the letter "A" is most commonly paired with the color red.

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While the exact causes of this condition are yet to be known, there are several hypotheses that attempt to explain the phenomena. Recent research has found that grapheme synesthetes have increased grey matter in the fusiform gyrus of the brain, and area responsible for color, letter, and word processing.

Grapheme-Color Synesthesia