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The blended senses of synesthesia

If you ask Emma Anders about the number five, she'll tell you that it's red. She'll also tell you that five is a mischievous, self-centered brat — like a kid throwing a temper tantrum at a party.

"Two is yellow, three is purple, four is an intense sky blue," says the 21-year old student at UC San Diego. "An eight is very noble and kind of held together, almost like a parent figure to five. Nine is a brown-haired guy, and he's pretty calm — but he's really into seven."

For most people, a number is simply an arithmetical value that represents a quantity. But for Anders, it is also a thing that has a particular color and an entire suite of personality traits. And it's not just numbers — she also ascribes colors to flavors and smells. (Vaseline, for instance, smells burgundy, and a green apple tastes yellowish-orange.)

This is the world of synesthesia, a perceptual phenomenon in which one sense kindles sensation in another. The condition, which is harmless, is caused by increased connectivity between areas of the brain that are normally separated. As a result, when Anders sees a five, the region of her brain that perceives colors is stimulated along with the region that processes numbers.

Other synesthetes see colors when they hear music, taste words before they say them or feel textures on their fingertips when they discern the flavors of particular foods. Virtually any combination between the senses is possible in the 1% to 4% of people who have inherited the condition.

No one is trying to cure synesthesia — in fact, most synesthetes will tell you they love their synesthetic experiences and would never want to lose them. But scientists have begun studying people like Anders in hopes that what they discover about the way their brains are wired will provide clues for understanding other neurological disorders, like autism and schizophrenia.

"We're using the synesthetic brain as a model for neural hyper-connectivity," says Steffie Tomson, a neuroscientist at Baylor College of Medicine in Houston. "What we're learning is that there are very specific delicate relationships between different regions of the brain that can cause it to function normally — or to tweak."

Scientists have been aware of synesthesia for more than 100 years, but only in the last decade or so has it been considered more than a strange quirk. Recent advances in neuroimaging have allowed researchers to visualize what's going on inside a synesthete's brain when it makes its unconventional connections. The Internet has inspired the creation of online tests that have gathered data from tens of thousands of synesthetes throughout the world. And genetic sequencing has enabled scientists to come closer to pinpointing the genes that cause this condition.

David Brang, a UC San Diego neuroscientist, says nature provides a strong hint that the brains of synesthetes may have some kind of cognitive advantage. The genes for synesthesia appear to be dominant, and family trees depict the trait marching through the bloodline. This high degree of heritability suggests the genetic mutation that causes synesthesia provides some significant evolutionary benefit.

Brang's hypothesis is that the benefit is related to creativity, enhanced perception and overall smarts. So far, studies have found that so-called colored sequence synesthetes (who experience color when they see numbers or letters) have a heightened ability to discriminate between similar colors, while mirror-touch synesthetes (who experience touch sensations when watching another person touch themselves) are more sensitive to touch in general.

The search for the genes that trigger synesthesia is underway in David Eagleman's lab at Baylor College of Medicine, where Tomson works. Eagleman calls this nascent field "perceptual genomics," or the study of how specific genes influence how people experience the world.

"I see in synesthesia a really good inroad into understanding the brain in general and consciousness in particular," says Eagleman, who has identified a region on chromosome 16 that is linked to colored sequence synesthesia. "Here we have a condition where some small change, presumably a very tight genetic change, causes the internal experience to be completely different from someone else's."

The study of synesthesia has helped shift the way scientists think about the brain. In the past, they have focused on matching different areas with specific functions; now, the entire organ is viewed as a tapestry of interwoven connections.

"The whole system is a giant network," Eagleman says. "It's no longer sufficient to think about single areas in isolation."

Like synesthesia, many neurological disorders — such as schizophrenia, autism,Alzheimer's disease, depression and epilepsy — have been linked to abnormal communication between brain regions. The hope is that as neuroscientists learn about how the connections in the synesthetic brain differ from those in normal brains, they will also gain insight into how these differences develop — and how they sometimes manifest as harmful disorders.

"We're trying to understand how a different activity pattern in your brain can change the way you perceive reality," says Tomson, pointing out that studying disorders such as depression or schizophrenia in people who already have the disorder can be tricky. Not only are the network properties of these illnesses more complex than the relatively simple circuitry involved with synesthesia, but patients are often on medication, which makes it impossible to tell how their brains would function on their own.

"Synesthesia is a perfect model because we have a healthy brain that has some kind of perceptual tweak that changes the relationship between various regions of the brain," she says

Researchers in Eagleman's lab are also studying sensory processing dysfunction (SPD), which is a hallmark characteristic of autism. People with this disorder have temper tantrums and other extreme reactions when exposed to particular tastes, sounds, textures or other stimuli.

The prevailing idea is that people with SPD experience certain stimuli as louder or more intense than normal. But Eagleman's studies of synesthesia have caused him to look at individuals with SPD in a different way.

"I think that what they're experiencing is a form of synesthesia where instead of some sense connecting to their color area, it's connecting to an area involving pain or aversion or nausea," Eagleman says. "If that's true, what we're doing in synesthesia will give us an actual molecular target for helping that."

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