Finding the Gene that Makes People Hear Shapes and Taste Words (continued)

By Camille Webb

"There's never been a study of this kind before – trying to link genes to how you perceive the world," Dr. Eagleman says. "Synesthesia struck me as a great example of how people can, with a slight genetic difference, actually see the world differently."

A series of tests, posted online and created by Dr. Eagleman (www.synesthete.org), helps him clearly distinguish synesthetes from nonsynesthetes for the study. Individuals taking the test will see a letter or number appear on screen and pick the color that best represents their synthetic perception. During the test, the letter or number will appear randomly three times over the course of 108 times.

"Synesthetes will pick the same color," Dr. Eagleman says. "If you're faking it, it's difficult to remember what color you assigned to "J" 57 trials ago. You have to nail the "J" with the same color three times. For a real synesthete, it's no problem."

He then evaluates the distance in color space between their answers. "A real synesthete will have small distances between their colors, and a non-synesthete will have larger distances," Dr. Eagleman says. "From this, I'm able to calculate a score for each person, and synesthetes are clearly discriminable from non-synesthetes this way."

Over the past one and a half years, Dr. Eagleman and his research assistants have collected DNA from more than 100 family members with synesthesia – mostly from the United States and Australia – for the study, which is funded with seed money from the University Clinical Research Center (UCRC). In collaboration with Dianna Milewicz, M.D., Ph.D., professor and director of the Division of Medical Genetics, Dr. Eagleman and his team have begun to sequence the DNA in the hunt for the responsible gene or genes.

"What's clear is that synesthesia results from increased cross talk in the brain," he says. "From looking at the pattern of inheritance, it appears to be an X-linked dominant trait. It looks like it could be a single gene."

If it proves to be a single gene, Dr. Eagleman plans to explore new ground. "It turns out that there may be some other, nonsynesthetic fraction of the population who expresses this gene," he says. "The gene may cause cross talk between other areas of their brain, for example in the frontal lobes, which are involved in reasoning, planning, and decision making. We don't know what it would mean to have increased cross talk between other parts of the brain. Once I find the gene, then I can look at the normal population, try to find who else is expressing this gene and where, and see what's different or special about them."

Are you synesthetic? Go to Dr. Eagleman's website at www.synesthete.org to take the battery of online tests.