To demonstrate the perceptual reality of synesthetic colors, researchers have introduced synesthetic color differences into a variety of traditional visual-perception tasks. Searching for a among ¿s is a difficult task because the digits are so visually similar, differing by only a mirror reflection. If the was colored orange and the ¿s were colored green, the search task would be trivially easy because the orange digit visually pops out from the background of green digits. When shown a display consisting of monochromatic digits, we found that a synesthete could quickly find the target because for him was orange but was green (see image).
Vilayanur Ramachandran and Edward M. Hubbard of the University of California at San Diego, have reported complementary findings supporting the perceptual reality of synesthetic colors. In one task, they presented synesthetes with an array of equally-spaced letters and digits. Synesthetes reported that these arrays organized themselves into distinct rows or columns depending on whether the rows or columns of characters were the same synesthetic color. This perceptual grouping based on synesthetic color is analogous to the kind of perceptual grouping non-synesthetes experience with real colors.
Claims for the perceptual reality of synesthetic colors have been bolstered by recent functional brain imaging studies by researchers in the U. K. showing that synesthetic color activates central visual areas of the brain thought to be involved in perceiving real colors.
The neural mechanism by which synesthetic colors are automatically bound to alphanumeric characters remains a mystery. One possibility is that synesthesia might arise from some kind of anomalous cross-wiring between brain areas that are normally segregated in nonsynesthetic individuals. For grapheme-color synesthesia, there may be cross-wiring between digit and letter processing areas and color processing areas in the visual cortex, which occupy neighboring regions of the human brain.
The causes of synesthesia also remain unknown. Some scientists have suggested that everyone is born synesthetic but that the typical developmental trajectory results in these highly interconnected brain areas becoming far more segregated. We do not know why synesthetes retain some of these anomalous connections. A biological determinant may be partially at work in certain cases of synesthesia, because the condition tends to run in families; moreover, nearly six times as many women as men report synesthesia. Whatever its etiology, synesthesia provides cognitive neuroscientists with a unique opportunity to learn more about how the brain creates our perceptual reality.
Answer originally posted on June 17, 2002.