Dr. Richard Cytowic is one of the leading researchers of synesthesia, a condition in which two normally separated sensations - such as sight and sound, or touch and taste - occur at the same time. As a result, a synesthetic person might experience the taste of a dish on her fingertips, or be convinced that the letter X is a vibrant turquoise. Mind Matters editor Jonah Lehrer chats with Cytowic about his new book, Wednesday is Indigo Blue, which he co-wrote with David Eagleman.

LEHRER: What first got you interested in synesthesia?

CYTOWIC: It was an accident. I like etymology and so knew the word, whereas my colleagues back in 1979 had never heard of synesthesia. In fact, they refused to believe it could be real, and warned that looking into such “weird” and “New Age” nonsense would ruin my career. Their denial was the typical reaction of orthodoxy to something it can’t explain.

It is said that chance favors the prepared mind, so I guess I was ready when a dinner host apologized that there weren’t “enough points on the chicken.” For Michael Watson, who I later wrote about as “The Man Who Tasted Shapes,” flavor was more than a mouthful. Taste was also a touch sensation felt on his face and in his hands. “With an intense flavor,” he explained, “a feeling sweeps down my arm and I feel weight, shape, texture, and temperature as if I’m actually grasping something.”

Fortunately, I could use university resources to quietly study Michael in depth and write papers. What interested me most was pondering an experience that “wasn’t supposed to be.”

LEHRER: How has our scientific understanding of synesthesia changed in recent years?

CYTOWIC: It has to do with possibilities of how the senses couple in the brain. My first idea that the emotional brain served as the link gave way, based on observations in neonatal synesthesia, to the possibility of faulty pruning. That is, the gene in synesthesia might fail to prune the extra synapses that are normally made in great excess in all newborns. We thought their persistence might plausibly explain why some people are synesthetes.

Today, we know that far from being rare, synesthesia is common––one in 23 individuals has some kind of synesthesia, and one in 90 has colored letters and numerals. That being so, in Wednesday is Indigo Blue David Eagleman and I favor a genetically–determined imbalance between excitation and inhibition. We’ve learned that the normal brain is already highly cross–wired. We think synesthesia occurs due to increased activity in existing wiring rather than the result of extra wiring.

LEHRER: What can synesthetes teach us about the nature of human perception?

CYTOWIC: Far from being a mere curiosity, synesthesia is a consciously elevated form of the perception that everyone already has. Minds that function differently are not so strange after all, and everyone can learn from them.

Synesthesia has opened up a window onto a broad expanse of the brain and perception. Younger researchers are now active in 15 countries. Because the trait runs strongly in families, it is easy to collect DNA from a large number of synesthetic relatives. This means that synesthesia may be the very first perceptual condition for which science can map its gene. This inherited quirk is teaching us that cross–talk among the senses is the rule rather than the exception––we are all inward synesthetes who are outwardly unaware of sensory couplings happening all the time.

For example, sight, sound, and movement normally map to one another so closely that even bad ventriloquists convince us that whatever moves is doing the talking. Likewise, cinema convinces us that dialogue comes from the actors’ mouths rather than the surrounding speakers. Dance is another example of cross–sensory mapping in which body rhythms imitate sound rhythms kinetically and visually. We so take these similarities for granted that we never question them the way we might doubt colored hearing.

LEHRER:In Wednesday Is Indigo Blue, you argue that investigations of synesthesia can help us better understand the neurological basis of metaphor and even creativity. Could you explain?

CYTOWIC: Artists are at ease using metaphors, and we have known for a long time that synesthesia is more common in creative individuals. Famous synesthetes include novelist Vladimir Nabokov, whose mother and son Dmitri also had it; composers Olivier Messiaen, Amy Beech and Billy Joel; and painters David Hockney and Wasily Kandinsky. Dmitri Nabokov, incidentally, wrote a charming afterword about his father and himself for “Indigo Blue.”

There is more to creativity than a capacity for metaphor, of course. Nonetheless, begin with the assumption that the gene for synesthesia lashes together normally unconnected brain areas, thus linking seemingly unrelated qualities such as sound and color. Having one kind of synesthesia gives a person a 50 percent chance of having a second or third kind, meaning that the gene expresses itself in two or three separate areas in that person’s brain. Suppose, however, that brain hyper–connectivity occurred not selectively here and there, but diffusely. One would have a generalized talent for cross connecting apparently unrelated concepts, which is the definition of metaphor: seeing the similar in the dissimilar.

And this is the reason several of us suspect that the synesthesia gene maintains itself at such a high frequency in the population. After all, one in 23 people are walking around with a mutation for an apparently useless trait. It must be doing something of inapparent value in order for evolution to select so strongly in its favor. When the gene expresses itself in sensory parts of the brain, people are outwardly synesthetic. But what are they like when the mutation expresses itself in non–sensory brain parts such as those concerned with memory, planning, or moral reasoning? Might it contribute to increased creativity, thereby making humans smarter as a whole?

We are beginning to find out. The strongest link so far is a region on chromosome 2 that is associated with autism and epilepsy, conditions that occur together with synesthesia more often than chance predicts. The autistic savant Daniel Tammet, whose best–selling autobiography is Born on a Blue Day, has all three conditions––indicating that they might share an underlying genetic mechanism. Tammet first shot to fame in Britain when he set a record for reciting 22,514 digits of pi from memory.

LEHRER: Has there been one case of synesthesia that you've been particularly astonished by?

CYTOWIC: What is astonishing about The Man Who Tasted Shapes is how rare Michael Watson’s type of flavor–touch synesthesia turned out to be in retrospect: less than one percent. So, the odds of him having been the first case were vanishingly small.

One feature that still fascinates me is the “screen” phenomenon that some people with colored hearing have. That is, they see their sound–triggered hues, geometric shapes, and moving configurations projected a foot or so in front of their face as if on a screen. One college professor particularly likes seeing rising and falling lines. Lines that go up are the best. “My favorite music,” she says, “makes the lines go right off the top of the screen.”

In the end, the most astonishing thing I’ve experienced over and over during 30 years of study has been the trust strangers placed in me and their willingness to allow me into their private worlds. That is a brave thing to do when no one has believed you all your life. So it is impossible not to remain fascinated with synesthesia, and even more so synesthetes themselves.

Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His latest book is How We Decide.