Carol Crane loves most kinds of music, but concerts affect her in a peculiar way. "The sound of guitars always feels like someone is blowing on my ankles. The piano presses on me right here," she says, tapping her chest just over her heart. "And New Orleans-type jazz hits me all over like heavy, sharp raindrops." Crane's sensory reaction to letters and numbers is just as odd. At the sight of the letter a, a color flashes through her mind—invariably gray-blue. The letter b, she says, is navy blue, and c tawny crimson. Numbers trigger similar sensations. The numeral 4 causes Crane to see tomato red and, like all of her perceptual links, it has done so since childhood. Four has always been red, she says. It can't be anything else. Crane, a 47-year-old psychologist, is not delusional. Her perceptions, bizarre as they might seem, stem from an unusual intermingling of the senses known as synesthesia, a condition that can take a multitude of forms. Some synesthetes see sounds, while others feel colors or taste shapes. Simon Baron-Cohen, an experimental psychologist at the University of Cambridge, estimates that perhaps one in 2,000 people is synesthetic and goes through life with at least one sense jostling another. Little is known about what causes the condition, but one thing is certain: The sensations a synesthete experiences are quite real. As Baron-Cohen puts it, "We've ruled out that these people are simply telling tall tales." Synesthesia—from the Greek syn, for together, and aisthesis, to perceive—generated a wave of scientific and popular interest around the turn of the twentieth century. Russian composer Alexander Scriabin, himself a synesthete, featured an organ that produced multihued light beams in his symphony Prometheus, the Poem of Fire. To many fin de siècle Romantics, synesthetes appeared to be humanity's spiritual vanguard, closer to God than the sense-segregated masses. "Such highly sensitive people," wrote Wassily Kandinsky, the Russian abstract artist, "are like good, much-played violins, which vibrate in all their parts and fibers at every touch of the bow." The fascination soon peaked, however, stymied by synesthesia's sheer impenetrability. The problem: No one could crawl into synesthetes' heads to understand or share their unique perceptions. That may soon change. Propelled by recent advances in brain imaging and electrophysiological recording, as well as DNA analysis and other techniques, a small but determined cadre of researchers in the United States, Scotland, England, Canada, Australia, France, Germany, Israel, and Finland is beginning to sort out just what, exactly, distinguishes synesthetes from the rest of us. The answers promise to do more than just shed light on a quirky condition—they may illuminate a central existential conundrum. While the notion that synesthetes are semidivine no longer holds sway, cognitive scientists contend that these unusual people are precious windows into the ultimate mystery of human consciousness. As they quiz, stimulate, scan, and evaluate, researchers find themselves awed. "We tend to assume that reality is the same for everybody," says Peter Grossenbacher, a senior fellow at the National Institutes of Health (NIH) who is widely regarded as the leading American synesthesia researcher. "Synesthesia shows us that it isn't. People all around us may have a very different experience of the world." Not just different, many synesthetes say. Better. "To me, it's like you guys see the world in black and white," says Carol Steen, a New York City artist for whom letters, numbers, sounds, and pains evoke a variety of hues. "I've got it in color." Patricia Duffy, a language instructor at the United Nations, who senses color when she looks at letters or numbers or when she thinks of time units such as days or hours, emphatically agrees. "Synesthesia is wonderful," she says. "Losing it would be very upsetting, just like losing one of your senses." Seated in a windowless, soundproof room roughly five feet square, Crane waits as a disk drive grinds and a wide-screen computer monitor flickers to life. Crane is both a synesthete and a synesthesia researcher. Here, in a converted Tudor-style mansion on the dappled grounds of the nih in Bethesda, Maryland, she assists Grossenbacher in his ongoing study of synesthetes. Two dozen have been interviewed and evaluated so far; Grossenbacher, Crane, and graduate student Chris Lovelace plan to eventually assess 100. Today, Crane herself will undergo the testing. Crane's story is typical. As a child she was met with blank stares whenever she revealed that letters and numbers caused her to see different colors or that music triggered her sense of touch. So she kept quiet about the sensations and had no name to attach to them until two years ago, when an offhand comment by a psychology professor radically altered her world. "He said a researcher at NIH, a Dr. Grossenbacher, was looking for people to help with a study of synesthetes," she recalls. "I asked, 'What are synesthetes?' and he said, 'People who think fives are yellow.' I told him, 'No, fives are green.' " She got the job. Grossenbacher's team and others around the globe have their work cut out for them. Synesthesia is a confounding condition with few fixed facts. Studies suggest that the condition occurs more commonly among women than men—the estimated ratio varies anywhere between two and eight to one. But no one knows why. Despite their bizarre cognition, synesthetes have not been found to exhibit any more vulnerability to mental disturbance or illness than the general population. Their scores on the Minnesota Multiphasic Personality Inventory, a widely used mental-health benchmark, are within normal bounds. Nor is there evidence to back up a common view that synesthetes incline toward the arts; the likely truth is that artists revel in and talk about their experiences more than other synesthetes, who tend to be embarrassed by the condition. One intriguing recent finding is that synesthesia appears to run in families, suggesting that there is a genetic component. Richard Cytowic, a Washington, D.C., neurologist whose popular 1993 book, The Man Who Tasted Shapes, helped spark the synesthesia renaissance, discovered a family with one synesthete in each of four generations, and another in which four of five siblings are synesthetic. In fact, Carol Crane's awakening led to another startling revelation: Her son, sister, niece, and nephew all share her condition. In all instances of synesthesia, the perceptions are spontaneous and enduring. In a 1993 experiment, Baron-Cohen and his colleagues gave synesthetes and a group of nonsynesthetes a list of letters, words, and phrases and asked them to describe the color or shape each evoked. Follow-up testing found that while only 37 percent of nonsynesthetes' responses were consistent one week later, 92 percent of synesthetes' associations were identical after one year. Psychologists call this "invariance"; for synesthetes, it's a way of life. "I've been getting acupuncture for 15 years," says Steen. "The points always bring up the same colors." For Steen, at least. Another synesthete may find a needle prick in the left earlobe indelibly linked to a different hue, or to a taste or a shape. Many synesthetic experiences are triggered by external stimuli, but some—such as Pat Duffy's vivid, invariable conception of a year as an "oblong string of colored rectangles"—are purely internal. The responses are idiosyncratic and bewildering in their variety. Sean Day, a synesthete who independently studies the phenomenon, has sifted through 175 case reports, attempting to identify types of synesthesia. So far, Day, an associate professor of English at Taiwan's National Central University, has tallied 19 of them . One of the rarest, he found, is feeling phantom shapes and textures pressing the skin while viewing an object; the most common is seeing colors in response to viewing written numbers or letters. Day has also tallied the colors of synesthetes' alphabets. "The letter o tends overwhelmingly to be white," says Day; it was associated with that color by 56 percent of the synesthetes who ascribe colors to letters. But u, like many other letters, "goes all over the place, with no real color trend." According to Baron-Cohen, even synesthetes in the same family seldom agree on the colors of more than a few letters. Why? No one knows. Equally mysterious is the way secondary perceptions manifest themselves to synesthetes. For Day, the perceptions are part of the external world. "A piano is a blue mist, electric guitars are orange or reddish streaks about five feet away, hovering in the air," he says. Likewise, when Carol Steen tore a ligament in her left leg on a Canadian beach, "the whole world turned orange," she recalls, "like there was a gel over the scene—orange rocks, orange water, orange husband running to help me." But when Crane sees a black 3 printed on a white background, "I see the numeral in my head," she notes, "and in my head, it's always yellow." But which yellow? Enter Grossenbacher and software created for his project by Lovelace. When Crane's computer screen finally clicks on, it presents her with a two-and-a-half-inch-tall numeral 3 and a color wheel, the kind graphic artists use to select precisely the shade they want for their creations. Crane painstakingly maneuvers the mouse to place the cursor over the specific spot that matches her internal 3. She clicks. Her 3—the color of American hot-dog mustard—has a hue angle of 57, a 90 percent color saturation, and a 95 percent brightness value. "That's it," she says. "We see this in all of our subjects," says Lovelace, watching Crane squint, frown, and hunt for the color of the next numeral. "Their 5 isn't just green, it's that green— very, very specific." Crane keeps clicking until her number palette is complete: 1 is black; 2, grayish-blue; 3, mustard yellow; 4, tomato red; 5, lime green; 6, navy blue; 7, dark purple-blue; 8, a blushing flesh tone; 9, tan; and zero is her only two-color number—white with a black border. To outside observers, such associations, if real, may seem inconsequential, but synesthetes bristle at the notion. "The colors are as intrinsic as the shapes," maintains Pat Duffy. To state, as she does, that p is yellow seems no more an invention than does asserting that circles are round. In fact, synesthetes often remember the secondary perception more vividly than the primary one. A typical complaint: "I can't remember his name, but I know it's purple." The ultimate challenge facing synesthesia researchers is to discover the brain mechanisms that underlie these strange sensory links. In a seminal experiment two decades ago performed by neurologist Cytowic, a synesthete named Michael Watson inhaled radioactive xenon gas, which soon permeated his blood. Watson then sniffed various fragrances and—just as he had all his life—felt invisible cones, spheres, and columns pressing against his skin. A tracking device disclosed that the blood flow to his cortex—the outer layer of the brain that handles most sensory processing and abstract thought—had almost shut down. "We were amazed," says Cytowic. Though Watson had an iq of 130 and seemed healthy, "he had the kind of cortical blood flow that you see in people who have had severe strokes." The experiment helped lead Cytowic to theorize that synesthesia is based in the limbic system—the evolutionarily primitive, emotion-mediating part of the brain that sits just atop the spinal column. Newer brain imaging efforts have painted a more complex picture. In 1995, British and Italian researchers used Positron Emission Tomography (pet), a sophisticated cerebral blood-flow monitoring technique, to scan the brains of six nonsynesthetes and six synesthetes who link colors to words, as they listened to a series of words being recited. The team's findings, replicated in subsequent studies by other researchers, are in startling contrast to Cytowic's: Cortical blood flow did not decrease; indeed, it soared. One possible explanation for the discrepancy may be that different forms of synesthesia spring from different brain anomalies. But there was yet another surprise: Not only did language-processing areas of the cortex activate in both subjects and controls, the synesthetes' brains, relatively speaking, lit up like bulb-laden Christmas trees, and several areas normally used to process vision and color also blinked "on." Baron-Cohen, who participated in the experiment, has suggested that the key to synesthesia is "unusual anatomic connectivity" between different sensory modules in the cortex. One provocative notion is that all human beings are born synesthetes. "At six months of age, you get a very similar cortical response in infants regardless of whether the stimulus is a bright light or a loud noise," Baron-Cohen says. After that, responses become segregated. "Sounds show up in the temporal lobe, and a visual stimulus gives you an occipital response," he explains. Baron-Cohen points out that several studies have confirmed that "selective cell death" is part of infant brain development. In most children, according to theory, dying neurons create discrete sensory islands in the brain; in synesthetes, the synaptic bridges remain—for some unknown reason—more or less intact. This spring at a meeting of the Cognitive Neuroscience Society, Grossenbacher advanced an alternative theory: that synesthetes have what he calls abnormally strong "feed-backward" activity in their cortical sensory pathways. In all human brains, sensory input—say, vision and touch—goes from single-sense cortical modules into multisensory areas such as the superior temporal sulcus, a quarter-sized region behind the tip of the right ear. Such sense-associating areas allow, for example, a person to see a ball, then, with eyes closed, pick out that ball from a pile of cubes. There are also pathways leading back from multi- to single-sense areas. "In most people, activity in these feed-backward routes is somewhat inhibited," says Grossenbacher. "In synesthetes, I propose that it is less so." The theory, he says, jibes with brain-imaging results and solves a logical contradiction in theories such as Baron-Cohen's. "If you give people enough lsd or mescaline, they will often experience synesthesia," Grossenbacher notes. "These people obviously aren't growing new connections in their brains. They're using connections we all have, but in a novel way." Even if solid conclusions remain elusive, the modern revival in synesthesia studies encourages synesthetes. They hope news of such efforts will trickle into the mainstream and inform their many fellow synesthetes who have never been able to put a name to their strange sensations. Despite the pleasure their colorful perceptions can bring, an undercurrent of sadness and isolation often flows beneath the life stories of synesthetes. In a poignant essay posted on a synesthesia Web site, Duffy writes, "In life, so much depends on the question 'Do you see what I see?' That most basic of queries binds human beings socially . . . Having one's perceptions go uncorroborated can make one feel peculiarly alone in the world. . . . marooned on my own private island of pale yellow p's, turquoise Thursdays and wine-colored v's." Synesthetes envision a day when schoolteachers will be on the lookout for synesthetic comments, and teach kids—and their parents—to cherish these unique perspectives as a valid way to see, hear, touch, taste, smell, and think. If such children have no fellow synesthetes physically near, they can at least be directed to the burgeoning Internet communities, where e-mail correspondents giddily compare letter colors, G-sharp shapes, and peppermint textures. Many varieties of synesthesia happen every second of every day throughout an entire life, and Carol Steen found that denying the dazzling reality of her perceptions was a heavier burden than even she had realized. In 1993, at age 49, she was molding a sculpture when, on personal-stereo headphones, she chanced to hear a National Public Radio interview with Richard Cytowic. He defined synesthesia and explained that it was a real and even enviable condition. Steen's eyes grew wider with each revelation. At the end of the program, she understood, for the first time, who she was. "Then I did something I thought I would never do," she recalls, speaking softly. "I burst into tears. I cried for 45 minutes straight." Famous Synesthetes Vladimir Nabokov In his 1966 autobiography Speak, Memory, the Russian-born novelist recalled arguing with his mother, herself a synesthete, about the precise colors evoked by letters. He wrote: "The long a of the English alphabet . . . has for me the tint of weathered wood, but a French a evokes polished ebony. This black group also includes hard g (vulcanized rubber) and r (a sooty rag being ripped). Oatmeal n, noodle-limp l, and the ivory-backed hand-mirror of o take care of the white. . . . Passing on to the blue group, there is steely x, thundercloud z and huckleberry h. Since a subtle interaction exists between sound and shape, I see q as browner than k, while s is not the light blue of c, but a curious mixture of azure and mother-of-pearl." Olivier Messiaen The avant-garde French composer attributed his success to his synesthesia, explaining to an interviewer in 1979, "Whenever I hear music, or even if I read music, I see colors . . . The piece I composed about Bryce Canyon is red and orange, the color of the cliffs." Wassily Kandinsky At least one historian maintains that the Russian-born abstract artist was an "invented" synesthete who claimed the distinction to inflate his reputation in synesthesia-struck fin de siècle Europe. Alexander Scriabin The Russian composer and pianist peppered his scores with notations such as "luminously and more and more flashing," and planned a work called Mysterium— too logistically nightmarish to actually perform—that would have combined dance, decor, an orchestra, light, fire, scents, and sculpture. He was among the first to rigorously catalog his color-note associations, declaring that C-sharp was violet and E evoked "pearly white and shimmer of moonlight." David Hockney The British painter's synesthesia inspired the phantasmagoric sets he created for New York's Metropolitan Opera in the 1980s. Hockney told neurologist Richard Cytowic, "When it came time to paint the tree for Ravel, I put on the tree music from the opera, and it had a certain weight and color. The music would dictate the shape." Strange Sensations Researcher Sean Day has cataloged 19 different types of synesthesia from 175 case histories. But the permutations are virtually limitless, especially because a synesthete may have more than just one form of the condition. Although the spontaneous secondary perceptions can't be suppressed, synesthetes can, with varying success, ignore them. Sometimes, however, the sensations can be overwhelming. In an interview with Russian psychologist A.R. Luria, synesthete Solomon Shereshevsky related one incident in a daily life of jumbled perceptions: "I walked over to the ice cream vendor and asked her what flavor she had. 'Tutti frutti,' she said, but she answered in such a tone that a whole pile of coals, of black cinders, came bursting out of her mouth, and I couldn't bring myself to buy any ice cream after she answered that way. Some people's voices are a bouquet, and I get so interested in their voices, I can't follow what's being said. Other times, smoke or fog appears, and the more people talk, the harder it gets, until I reach a point where I can't make anything out."