Extra-sense perception

01/06/2003

By SUSAN GAIDOS / Special Contributor to The Dallas Morning News

When Carol Steen injured her knee while hiking, she didn't just feel the pain she saw a bright orange hue. And the prickling needles of her acupuncture treatment evoked colorful geometric shapes.

Patricia Duffy sees letters and numbers in brilliant color, and words take on the cast of their first letter. When envisioning the new year before her, she sees the months written in their individual shades, linked together like a multicolored ribbon.

Ms. Steen and Ms. Duffy share an extraordinary condition called synesthesia a "blending of the senses." In synesthesia, a sight, sound, taste, touch or smell prompts a response from that sense plus another one. Synesthetes may taste shapes, see numbers or letters in color, or perceive colors when hearing particular sounds.

"For as long as I could remember, each letter of the alphabet had a different and distinct color," says Ms. Duffy, author of Blue Cats and Chartreuse Kittens, a book on synesthesia published in November. "Until I was 16, I took it for granted that everyone shared those perceptions with me."

Although doctors first described synesthesia in 1710, scientists have only recently begun to understand the biology behind it. The latest studies of synesthesia, using brain scans and genetic screening, have exploited synesthesia's commingling of the senses to learn how the brain perceives and categorizes sensory information. Research is even providing new clues to how language evolved.

Studies suggest that at least one in 2,000 people experience a blending of the senses while smelling, seeing, tasting or hearing. For some, more than two senses are involved. Synesthesia runs in families and occurs up to six times more frequently among women than men. The Russian writer Vladimir Nabokov was a synesthete, as was physicist Richard Feynman.

Synesthesia comes from the Greek words syn (together) and aisthesi, (sensation). In its most common form, people see letters or numbers in color. Most synesthetes see such colors internally, in "the mind's eye." Some, such as Ms. Duffy, see their visions projected outside the body, as if on a movie screen.

"I would compare it somewhat to watching a black-and-white movie," she says. "Even though in the movie everything is in shades of gray, as you see the young actress your mind says, 'Oh, she has blond hair and blue eyes.' Your mind is not saying she has gray hair, because somehow it is making that correction for what you know the colors ought to be."

Unlearned, enduring

Ms. Duffy's experience illustrates several features that neurologists use to diagnose synesthesia it's involuntary, unlearned and enduring.

"Once these associations are made in childhood, they remain fixed for life," says neurologist Richard Cytowic, whose investigations with a synesthetic friend and 1993 book The Man Who Tasted Shapes helped renew scientific interest in the topic.

Confirmation of such long-term associations was first established in the late 1980s by Simon Cohen-Baron and colleagues at the University of London. The researchers read a list of 100 words to a synesthete and asked her to describe a color associated with each word. Retested without warning a year later, the synesthete was more than 90 percent consistent in her initial responses. Comparison subjects given the same list and told they would be retested in two weeks were consistent only 20 percent of the time.

In the mid-1990s, the same research group found that in synesthetes who perceive colors when hearing words, a part of the brain used in vision is activated in response to sound. No such activity occurs in nonsynesthetes.

"These studies showed that the synesthetic experiences were real, that synesthetes are doing something different from the rest of us," says Edward Hubbard, a researcher at the University of California, San Diego.

Some researchers have argued that synesthesia is a "conceptual" event, based on learned associations between colors and letters or numbers. Such associations might be made in early childhood from playing with objects such as colored blocks or refrigerator magnets.

Convincing evidence

"We now have several convincing lines of evidence that synesthetic colors are not just over-learned associations," says psychologist Randolph Blake of Vanderbilt University. "The synesthetic colors behave just like real colors do for those of us who do not have synesthesia."

For example, the colors seen by synesthetes can help in distinguishing objects from a cluttered background.

Dr. Blake's group studied a man who saw letters and numbers in vivid hues. A visual search task involving motion and stereo-vision required considerable effort for nonsynesthetic subjects. The synesthete, however, completed the task with relative ease because the target of the search appeared to be a different color from the distractors.

The group's findings, published last year in the Proceedings of the National Academy of Sciences, provide strong evidence that synesthetic experiences originate from a binding of color and form in the central stages of visual processing.

Another synesthesia study published last year identified activity in the central processing region.

The London research group used MRI scanning to determine precisely what regions of the brain were activated in synesthetes with colored-hearing. For them, the researchers found, a word evoked neural activity in the brain region V4, the visual area associated with color perception and color processing. Comparison subjects showed no such activity, despite being trained to associate colors with certain words, the researchers reported in Nature Neuroscience .

An MRI study under way at the UC, San Diego, is testing what areas of the brain are activated in four synesthetes who see letters and numbers in color. Mr. Hubbard and colleagues Vilayanur Ramachandran and Geoffrey Boynton at the Salk Institute plan to present their results at a synesthesia conference in May.

Various explanations

Although scientists agree that additional imaging studies are needed to determine how the senses merge in synesthesia, various explanations have been offered.

Perhaps synesthesia reflects "cross-wiring" between areas of the brain that process different sensations. Advocates of this theory point out that babies are born with many neural connections between adjacent brain regions.

"We propose that some type of genetic factor prevents these connections from being eliminated or pruned the way they normally would be, leaving the adjacent areas of the brain connected," Mr. Hubbard says.

He notes that a primary color area lies next to an area that processes numbers and letters. Another color region in the brain borders a primary auditory area. Mr. Hubbard says that such natal neural connections, if not pruned, might explain why some people see colors when they read or hear sounds.

He argues that the same sort of cross-wiring that might give rise to synesthesia may also be present in a weaker form in all of us. Thus, brain wiring lies along a continuum and synesthetes just happen to be at one end of the spectrum.

Another theory suggests that sensory information from separate brain areas is somehow merged during processing. Phil Merikle, of the University of Waterloo in Ontario, notes that perception does not occur at once, but forms as low-level sensory areas send and receive information from higher-level sensory areas. A link between these processing regions may somehow allow the concepts associated with letters and numbers to become associated with colors, he says.

Dr. Merikle currently is collaborating with neurogeneticist Maria Karayiorgou of Rockefeller University to pursue studies on the genetics of synesthesia. Last year, his research group published a study on identical twin girls. One twin has synesthesia and the other doesn't.

"Just because they have the same gene structure doesn't necessarily mean all of the genes will function the same especially if it's an X-linked gene," Dr. Merikle says.

Because synesthesia is often seen in families, yet never passed from father to son, it is widely believed to be a genetic, X-chromosome linked trait, he says. Although each of the twin girls possesses two X chromosomes one from each parent only one X chromosome is activated in each. Activation is randomly selected and can be influenced by differences in the uterine environment during development.

Language theories

In addition to providing information on neural pathways, studies on synesthesia are prompting linguists to take another look at theories of how language evolved, Mr. Hubbard says.

For example, a concept referred to as the "arbitrariness of the sign" argues that there's no connection between the sound of a word and the object or idea to which it refers. People in linguistics are now viewing this bit of wisdom with fresh eyes, he says, and arguing that the correspondence between sounds and specific types of meaning is not arbitrary.

"For example, if you look at words like fudge, trudge and sludge, all these 'dge' words seem to involve some sort of arrest of motion. And there seems to be similar stickiness in the meaning," Mr. Hubbard says.

The similarity in these words might be explained by common features of the brain. "We might choose certain words like fudge and trudge and sludge because of a similar structure in the brain for the physical sense of this action or the motion that you would make with your mouth," he says.

Studies with synesthetes may help scientists understand how such neural connections are made.

"In synesthesia we see a very clear example of how the senses are being combined," says Mr. Hubbard, "how information from vision and audition, for example, combine to create colored-hearing. By exploring these multisensory interactions, we can see what sort of biases are present in the brain."

Susan Gaidos is a free-lance writer in Maine.