Thomas J. Palmeri, Randolph B. Blake and Ren Marois of the psychology department and the Center for Integrative and Cognitive Neuroscience at Vanderbilt University study synesthesia. They provide the following explanation:

When you eat chicken, does it feel pointy or round? Is a week shaped like a tipped-over D with the days arranged counterclockwise? Does the note B taste like horseradish? Do you get confused about appointments because Tuesday and Thursday have the same color? Do you go to the wrong train station in New York City because Grand Central has the same color as the 42nd Street address of Penn Station? When you read a newspaper or listen to someone speaking do you see a rainbow of colors? If so, you might have synesthesia.

Synesthesia is an anomalous blending of the senses in which the stimulation of one modality simultaneously produces sensation in a different modality. Synesthetes hear colors, feel sounds and taste shapes. What makes synesthesia different from drug-induced hallucinations is that synesthetic sensations are highly consistent: for particular synesthetes, the note F is always a reddish shade of rust, a 3 is always pink or truck is always blue.

The estimated occurrence of synesthesia ranges from rarer than one in 20,000 to as prevalent as one in 200. Of the various manifestations of synesthesia, the most common involves seeing monochromatic letters, digits and words in unique colorsthis is called grapheme-color synesthesia. One rather striking observation is that such synesthetes all seem to experience very different colors for the same graphemic cues. Different synesthetes may see 3 in yellow, pink or red. Such synesthetic colors are not elicited by meaning, because 2 may be orange but two is blue and 7 may be red but seven is green. Even more perplexing is that synesthetes typically report seeing both the color the character is printed in as well as their synesthetic color. For example, is both blue (real color) and light green (synesthetic color).

Synesthetes report having unusually good memory for things such as phone numbers, security codes and polysyllabic anatomical terminology because digits, letters and syllables take on such a unique panoply of colors. But synesthetes also report making computational errors because 6 and 8 have the same color and claim to prejudge couples they meet because the colors of their first names clash so hideously.

For too long, synesthetes were dismissed as having overactive imaginations, confusing memories for perceptions or taking metaphorical speech far too literally. Recent research, however, has documented the reality of synesthesia and is beginning to make headway into understanding what might cause such unusual perceptions.

Research has documented that synesthetic colors are perceived in much the same way that nonsynesthetic individuals perceive real colors. Thus, synesthetic color differences can facilitate performance on tasks in which real color differences facilitate performance for nonsynesthetes and can impair performance on tasks in which real color differences impair performance for nonsynesthetes.

In one such task, people are asked to say the color of the ink a word is printed in as quickly as possible (for example, responding "pink" to and "blue" to ). For lexical synesthetes, these words take on unique colors. When the synesthetic color matches the ink color, responses are fast. But when the synesthetic color mismatches the ink color, responses are slow, presumably because subjects need to resolve the conflict over which color name to respond with. Although such results demonstrate that synesthesia is automatic, in the sense that they cannot turn off their synesthesic experience even when it interferes with a task, these results do not reveal whether synesthetic colors are perceptions or memories.

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.