With the help of sophisticated behavioral brain-imaging and molecular genetic methods, researchers are coming closer to understanding what drives the extraordinary sensory condition called synesthesia.
Guitar music doesn't just tickle Carol Crane's fancy--it also brushes softly against her ankles. When she hears violins, she also feels them on her face. Trumpets make themselves known on the back of her neck.
In addition to feeling the sounds of musical instruments on her body, Crane sees letters and numbers in brilliant hues. And for her, units of time each have their own shape: She sees the months of the year as the cars on a ferris wheel, with July at the top, December at the bottom.
Sean Day, PhD, tastes in technicolor.
"The taste of beef, such as a steak, produces a rich blue," says Day, a linguistics professor at National Central University in Taiwan. "Mango sherbet appears as a wall of lime green with thin wavy strips of cherry red. Steamed gingered squid produces a large glob of bright orange foam, about four feet away, directly in front of me."
Crane and Day share an extraordinary sensory condition called synesthesia.
The phenomenon--its name derives from the Greek, meaning "to perceive together"--comes in many varieties. Some synesthetes hear, smell, taste or feel pain in color. Others taste shapes, and still others perceive written digits, letters and words in color. Some, who possess what researchers call "conceptual synesthesia," see abstract concepts, such as units of time or mathematical operations, as shapes projected either internally or in the space around them. And many synesthetes experience more than one form of the condition.
The condition is not well known, in part because many synesthetes fear ridicule for their unusual ability. Often, people with synesthesia describe having been driven to silence after being derided in childhood for describing sensory connections that they had not realized were atypical.
For scientists, synesthesia presents an intriguing problem. Studies have confirmed that the phenomenon is biological, automatic and apparently unlearned, distinct from both hallucination and metaphor. The condition runs in families and is more common among women than men, researchers now know. But until recently, researchers could only speculate about the causes of synesthesia.
Now, however, modern behavioral, brain-imaging and molecular genetic tools hold exciting promise for uncovering the mechanisms that drive synesthesia--and, researchers hope, for better understanding how the brain normally organizes perception and cognition.
Research suggests that about one in 2,000 people are synesthetes, and some experts suspect that as many as one in 300 people have some variation of the condition. The writer Vladimir Nabokov was reputedly a synesthete, as were the composer Olivier Messiaen and the physicist Richard Feynman.
The most common form of synesthesia, researchers believe, is colored hearing: sounds, music or voices seen as colors. Most synesthetes report that they see such sounds internally, in "the mind's eye." Only a minority, like Day, see visions as if projected outside the body, usually within arm's reach.
Some synesthetes report experiencing sensory overload, becoming exhausted from so much stimulation. But usually the condition is not a problem--indeed, most synesthetes treasure what they consider a bonus sense.
"If you ask synesthetes if they'd wish to be rid of it, they almost always say no," says Simon Baron-Cohen, PhD, who studies synesthesia at the University of Cambridge. "For them, it feels like that's what normal experience is like. To have that taken away would make them feel like they were being deprived of one sense."
For psychologists, interest in synesthesia extends far beyond just the study of the few individuals who experience the phenomenon.
"Synesthesia taps into a lot of other domains that are more familiar to many psychologists," says Marks. "It tells us something about the nature of perception and what makes things perceptually similar to one another. Synesthesia may help us to understand how the concept of similarity is embedded within the nervous system."
In addition, Dixon suggests, the fact that synesthetic perception interferes with the perception of physical stimuli highlights an important aspect of cognition.
"We tend to think of our experiences, and especially the visual system, as being bottom-up," he remarks. "But there are many instances where meaning goes back down and influences our lower-order perception of the world. Synesthesia is just one very rare and exceptional example of that."
The possibility that synesthesia has genetic roots is equally tantalizing, says Grossenbacher--especially if it turns out that a single gene controls the condition, as some have speculated.
"If indeed something as central to mental life as [synesthesia] is controlled by a single gene, this might be rather a new kind of gene to know about," says Grossenbacher. "It would be a gene that, in either of its forms, results in a healthy human but has a profound effect on the organization of the nervous system."
At a practical level, many researchers observe, research on synesthesia will help raise the condition's visibility, reducing the risk that clinicians might mistake it as a sign of mental illness.
In addition, Grossenbacher, Lovelace and Crane--who conducts research on synesthesia while completing her doctorate in clinical psychology--are beginning to examine whether common mechanisms might underlie both synesthesia and hallucination. If so, synesthesia may be an ideal laboratory for studying those mechanisms.
"This is a group of people who would be available for research," explains Crane. "Unlike patients who experience hallucinations, synesthetes are not medicated, so you don't have that confounding factor. We're able to talk about our experiences. We offer something very valuable."