The Synesthesia Census

Bibliologue / by Veronique Greenwood /

Author and synesthesia expert David Eagleman on subjective realities, the genes behind mixed sensory experiences, and taking stock of the condition that everyone wants.

A color-tagged alphabet from the perspective of synesthete Marti Pike. Courtesy: Richard Cytowic

Although The Cure’s Robert Smith was likely being metaphorical when he sang, “I don’t care if Monday’s blue, Tuesday’s grey and Wednesday too,” for a significant portion of the population such bizarre sensory experiences are quite literal. In search of how crossed signals in the brain can transform sight into sound, smells into colors, or tastes into shapes, neuroscientist David Eagleman has performed the first comprehensive, large-scale analysis of the synesthete population, who number about one in 20. Eagleman explores these findings in his provocative book, Wednesday is Indigo Blue — coauthored with Richard Cytowic, the clinician who introduced synesthesia to the psychology community. Seed magazine’s Veronique Greenwood spoke with Eagleman about his hunt for the synesthesia gene, how the rest of us may be expressing the gene, and what those gifted with a different sensory experience can tell us about the nature of consciousness.

Seed: In your book you write that you’ve identified a spot on a chromosome that’s associated with synesthesia?
DE: Yeah! We’ve actually been at this for years now. We’ve found this hotspot on chromosome 16, but what we need is more statistical power, which means testing more and bigger families. Once we have that, we’ll be able to narrow the region until we know what the actual gene is.

Seed: How big is the hotspot?
DE: Right now it’s 23 million base pairs long, with about 400 genes. We have no way of knowing which of those genes is involved.

Seed: What do you mean when you write that the genes causing increased communication between brain regions in synesthetes may also be present in non-synesthetes, but with effects unrelated to perception?
DE: It’s possible that increased “crosstalk,” as we call it, occurs between regions of the brain that aren’t focused on perception. Once I’ve been able to identify the gene, I’m going to try to determine who’s expressing it in the non-synesthestic population and find out if these people are the geniuses or the madmen. Is there something special about them?  They wouldn’t come to me — I wouldn’t find them in my lab — because they wouldn’t have the perceptual consequences of the synesthesia gene. So I’d have to find them.

Seed: How has this field been impacted by your online synesthesia test?
DE:   The Synesthesia Battery has been a really big deal. Essentially, all the literature on synesthesia has been case studies of one person. The field was pretty fuzzy when I first got into it — about seven years ago — and I thought, “we can do a lot of clean-up here.” So I set up this online battery of tests, and it spread by word of mouth very quickly. We’ve now had 17,000 people come out of the woodwork and take these tests.

Seed: How has increased interest in consciousness and subjective realities played into synesthesia research?
DE:   When I first got into neuroscience, you couldn’t even talk about consciousness. It was a word you couldn’t even say. But then people like Francis Crick — who was a friend and mentor of mine during my postdoc years — said, “wait, consciousness is a real scientific problem!” It’s because of people with the gravitas such as Crick that people started asking questions about how we can we study consciousness. And consciousness is a fascinating scientific problem because brains are made of mere physical matter, and yet somehow we have this private, subjective experience.

What quickly struck me is that synesthesia is a fantastic inroad into understanding consciousness because it’s a private, subjective experience that’s a little bit different. You have some change in the physical structure — we think it’s a genetic change — and all of a sudden you’re seeing the world very differently. That gives us a really neat path into correlating physical/genetic differences with differences in how we perceive the world. That’s the goal of consciousness research.

Seed: What else can large-scale analyses tell us about synesthesia?
DE:   One of the things we’ve found is that there may be at least five different types of synesthesia. When you look at the types of synesthesia, they cluster. If you have colored numbers, you’re much more likely to have colored months and weekdays, but you’re no more likely to have synesthesia of taste or smell than anyone else. And vice versa. It could be there are different genetic underpinnings for each of these, or maybe we’re lucky enough that there’s only a single gene, and it just depends on where it’s being expressed. The gene I’m pulling for is a potential promoter for color-sequence synesthesia — colored weekdays, months, and letters. These are all learned sequences that we practice at some point in our lives. It looks like the gene has to do with these sequences.

Originally published April 16, 2009

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