While exploring tide pools with her husband, Carol Steen jumped from a rock and tore a ligament in her knee. “At the point of impact, everything I saw suddenly became orange,” she recalls. “My husband was orange. The waves were dark orange, and the sand was light orange.”
For Steen, orange is the color of pain. She has synesthesia, a condition in which the senses are cross-wired. She perceives colors and shapes in her mind’s eye when she processes certain sounds, smells, or tactile sensations. She also associates specific colors with numbers and letters. To Steen, the letter “K” is green. Ammonia smells blue. The sound of her apartment’s elevator bell is a flashing magenta triangle.
Synesthesia comes in many forms. Some synesthetes experience vivid tastes or scents when they hear certain sounds. Others strongly associate days or months with specific points in space; July might be elevated and to the left, while October is located to the far right, near the floor. Colored letters and digits, or color-grapheme synesthesia, is one of the more common (and best studied) forms of the condition.
In the past decade, synesthesia research has really come of age, says Michael Dixon, a psychologist at the University of Waterloo in Ontario. “When I started doing this research in late 1990s, the big push was just to prove that it was real,” he says. Studies using functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) handily settled that debate. When individuals with color-grapheme synesthesia listened to numbers being spoken aloud, for instance, their brain activity increased in regions involved in processing colors.
Now that synesthesia is a proven phenomenon, researchers have moved on to asking more penetrating questions. For starters: How common is it? Estimating prevalence is tricky since it depends on what you define as synesthesia, Dixon says. Dozens of forms have been recognized, and new types are frequently added to the list. But the best current estimates suggest 1.1 to 4.4 percent of the population — as many as 1 in 23 people — are synesthetic.
Synesthesia is known to run in families, though the gene for the condition has yet to be unearthed. And despite successful imaging studies, researchers are still sorting out exactly what goes on in the brains of synesthetes like Steen. Some neuroscientists suggest they might have extra wiring between adjacent brain regions. Others propose they simply have extra activity in the existing wiring.
Some recent research even suggests we might all be hard-wired for synesthesia. As a graduate student at McMaster University, Ferrine Spector studied sensory associations in people with normal perception. She asked volunteers to sniff various odors and describe the colors and textures they called to mind. Many of the associations were clearly a product of experience; lemon was typically described as yellow, for example. But other associations weren’t so easy to peg. The scent of mushrooms was often described as blue. The odor of lavender was frequently described as green and sticky.
Spector, now an associate lecturer at the University of Wisconsin, Madison, presented her findings at the annual meeting of the Cognitive Neuroscience Society in April. She suggests that all our brains may be wired for sensory crosstalk. In most of us, those connections are inhibited. But in a few lucky individuals, the cross-wired brain regions are free to chatter away. “Connections that lie dormant in most of us are selectively active in synesthetes,” Spector theorizes.
Researchers also point out that those of us with “normal” perception comfortably commingle our senses in speech all the time. Orange is a warm color. Cheese tastes sharp. The winter air is bitter. Even beyond metaphor, Dixon says, the average person tends to make certain associations repeatedly. For example, most people automatically attribute low sounds to big objects and high sounds to small objects. “We have these implicit associations,” says Dixon. “That suggests the connections may be there.”
For most of us, those connections are present only in the subconscious. For synesthetes, they exist in living color. Steen is a successful artist, and she draws on her vivid sensory perceptions when creating new pieces. “If you have the ability to experience the world in mixed media, it’s very rich,” she says.
Synesthesia has been linked to other creative personalities as well, from the artist David Hockney to the writer Vladimir Nabokov (who described the letter “M” as a fold of pink flannel, and “L” as a pale, limp noodle.) Some studies suggest synesthesia benefits memory, too, Dixon says. “I think the cognitive advantages are slowly beginning to emerge.”
But synesthesia may offer even more than creative inspiration and memory aids. It may also be a window into the machinery of the mind. “We’re like researchers in a cognitive candy store when it comes to studying synesthetes,” says Dixon. “They show us the limits of what’s possible in terms of how the brain works.”
SOME FORMS OF SYNESTHESIA:
- GRAPHEME, OR COLOR, SYNESTHESIA: Letters or numbers are perceived as inherently colored.
- SOUND-COLOR SYNESTHESIA: Voice, music, and assorted environmental sounds make the individual see flashes of color and simple shapes.
- SPATIAL-SEQUENCE, OR NUMBER-FORM, SYNESTHESIA: Numbers, months of the year and/or days of the week elicit precise locations in space. (For example, October may be located to the far right, near the floor.)
- ORDINAL LINGUISTIC PERSONIFICATION: Numbers, days of the week and months of the year evoke personalities.
- SOUND SYNESTHESIA: People hear sounds in response to visual motion and flicker.
- GUSTATORY SYNESTHESIA: Individual words evoke taste sensations.