Each time she bites into an apple, she hears the sound of bees buzzing in her ears. Whenever the Rolling Stones come on the radio, visions of silver triangles appear in his head. These experiences aren't from people reliving the '70s. They are normal for the 4 percent of the population with synesthesia – a perceptual condition that's marked by a mixture of the senses.
"Synesthesia is a harmless condition," says David Eagleman, Ph.D., assistant professor of neurobiology and anatomy. "There doesn't seem to be any disadvantage to it. In fact, it might only be advantageous because synesthetes often have better memories than non-synesthetes. That's because they're tagging information with an extra dimension."
Because synesthesia can involve any of the senses, it's estimated that there are more than 100 different forms. Synesthetes might see colors when they listen to music; feel shapes while tasting food; or experience tastes when hearing words.
"With synesthesia, you have more cross activation between neighboring neural areas than you do in normal brains," Dr. Eagleman says. "In a synesthete's brain, neighboring areas are talking to each other more than normal."
The most common form of synesthesia is associating colors with letters of the alphabet or with numbers. For example, a synesthete might see the letter "H," and that induces the color pale yellow, or see the number "2" as lime green.
Dr. Eagleman stresses that the blending of senses synesthetes experience is not a hallucination. "It's not the same as a hallucination," he says. "Hallucination is something that you see, and you believe it exists in the outside world. Synesthesia is not like that.
"It's automatic, involuntary, and unconscious," Dr. Eagleman continues. "You know that it's in the mind's eye. There's no confusion about what they're looking at."
Another common form is experiencing a series of numbers or units of time in a particular sequence. To research this type, Dr. Eagleman developed a virtual reality program.
"It allows people to place all of their weekdays, months, or numbers into a threedimensional space and arrange them in relationship to their body space," he says.
Synesthesia appears to be heritable, and this intrigues Dr. Eagleman, who is interested in its genetic basis. Through the study, "The Genetics of Synesthesia: Linking Genes to Perception," he wants to find the gene responsible for synesthesia.
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