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 colors – this 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, the letter 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 the word blue, printed in pink, and "blue" to the
word pink, printed in blue). 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
6 among 9s is a difficult task because the digits are so visually similar, differing by only a mirror
reflection. If the 6 was colored orange and the 9s 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 6 was orange
but 9 was green.
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.