Seeing Letters in Color

For a small fraction of people, roughly one in a hundred, seeing a written letter or number automatically triggers a specific color, even though nothing on the page is actually colored. The letter A might always appear tinged red, or a 7 might seem faintly green, regardless of what font or ink is used. This involuntary blending of senses is called grapheme-color synesthesia, and it is consistent for each individual: the same letter reliably produces the same color for that person over years, even though different synesthetes rarely agree with one another about which color belongs to which letter. The leading explanation, known as the cross-activation theory, points to brain geography rather than imagination. The region that processes the shapes of letters sits immediately next to a region called V4, which handles color perception, and brain-imaging studies suggest that synesthetes have unusually dense connections between these two neighboring areas. One hypothesis is that these extra connections persist from early childhood, when the brain normally prunes away many of the connections between neighboring regions as it matures; in synesthetes, some of this pruning may simply not happen. As a result, activating the letter-recognition area also activates the color area next door, and a color sensation arrives automatically, without any deliberate association being made.