Synesthesia Gene Discovered
People who connect letters and numbers with speci c colors now have a genetic explanation
some families argue over
politics, others over who controls the remote. But Carol Steen and her father fight over the color of numerical digits. She once insisted that 5 was yellow. “And my father said, ‘No, it’s yellow ocher,’” she says.
Steen and her father share a trait called synesthesia, in which senses are crossed. An experience in one sense stimulates an experience in another, so you might hear colors or taste sounds or see letters in vivid hues.
Scientists have long suspected that this peculiar ability is written somewhere in our DNA. The Steens support that notion: Several of their distant relatives are also synesthetes. And new research, published in Proceedings of the National Academy of Sciences in March, not only supports that idea but pinpoints the genes responsible.
Amanda Tilot, a geneticist at the Max Planck Institute for Psycholinguistics, recruited three families to help her probe the genetic underpinnings of the trait. In two of the families, three generations of women experienced sounds as colors. In the third, a man and his mother, daughter, sister and grandson experienced that same phenomenon.
Tilot and her colleagues wanted to know where synesthesia comes from. Prior research seeking genes had identified wide swaths of the genome with potential connections; Tilot wanted a more precise look, and she turned to a method called whole exome sequencing. “That gave us the accuracy we needed to point to specific genes,” she says.
Among the approximately 20,000 genes in the human genome, the researchers found six that differed slightly in people with synesthesia. In these genes, just one or a handful of nucleotides—the compounds that make up our genes—were swapped for ones that aren’t usually there. Each of the synesthesia genes has a direct connection to how the brain works. Known as COL4A1, ITGA2, MYO10, ROBO3, SLC9A6 and SLIT2, they are related to a process called axonogenesis, which is how neurons connect to each other as the brain develops in the womb and into childhood. The altered genes didn’t travel as a group; they appeared in just one or two of the three families. But changes in just a few were enough to cause a radical difference in perception.
Finding altered versions of genes encoding essential information about neuron connections matches previous findings. One study found that the brain cells of synesthetes were more interconnected than those of people without this trait. That tighter network may be the result of thicker nerve endings or extra myelin, a mix of protein and fat that makes impulses move faster.
Even though just 4 percent of the global population has synesthesia, the work, says Tilot, is broadly relevant. She cites the famous internet frenzy triggered by the image of a dress that some people saw as blue and black and others as white and gold. “Sensory perception is something that has natural variation,” says Tilot.
It’s a variation that frequently goes undetected. “We usually have no idea that there are differences in how our neighbors perceive aspects of the world,” says Tilot. The debate surrounding the colors of the dress brought this phenomenon to the surface. We may not all be synesthetes, says Tilot, but we all experience the world differently.
Synesthesia hasn’t always been easy for Steen. When she was 7, she told her best friend that the letter A was, “the prettiest pink I’ve ever seen,” she recalls. The friend told her she was weird and never talked to her again.
Steen, an artist and digital multimedia design professor at Touro College in New York, co-founded the American Synesthesia Association in 1995. Over the years, she’s come to see her condition as a strength and advantage. When she shops for paint, for example, she listens to music and searches for the color matching the one in her mind (a rather poetic idea, come to think of it). Synesthesia has also kept her healthy: Steen once told her dentist that her tooth was “glowing orange.” It turned out she needed a root canal.
Steen and her father, meanwhile, have learned to avoid tension by never discussing their number-color disagreements—proving synesthesia families are just like every other.
“We usually have no idea that there are differences in how our neighbors perceive different aspects of the world.”