Around four percent of the world’s population has some form of synesthesia, a neurological phenomenon that blurs some of the lines around the senses. In two of the more common variants, synesthetes may involuntarily associate letters with colors, or see colors for musical notes—but there are many other forms of synesthesia, all involving the crossover of one form of perception to another.

Writing this week in the Proceedings of the National Academy of Sciences, researchers report that they’ve identified several regions of the genome that may be involved in synesthesia. The team, based at the Max Planck Institute in the Netherlands, examined the genomes of several people in three different families with histories of synesthesia. Within each family, they found gene differences in regions dealing with the formation of axons in the brain during early childhood—though the specific gene variants involved were different from family to family. The work supports the idea that synesthesia may be the result of additional or crossed wiring in certain brain regions.

Amanda Tilot, one of the authors of the study, and Ed Hubbard, an educational psychologist, join Ira to discuss the work, what it implies, and how it fits into what’s already known about this perceptual ability.

Find more information on Tilot’s synesthesia study and to learn how to participate here.

Interview Highlights

On why synesthetes see different shades of colors when listening to musical notes.
Ed Hubbard: Our thinking about this is that some of the same brain processes that are involved in say imagining the music as being dark might be present both in people who experience synesthesia and in all the rest of us that don’t experience synesthesia. But in people who have synesthesia, these processes are heightened or exaggerated in such a way that they do this involuntarily. They often describe it as something that happens to them as opposed to something that they do, and they report that it’s been that way for as long as they can remember, [or] ever since childhood. So it’s quite different at the level of sort of how it feels to you, that subjective experience. But we think at the brain-level, there might be a sort of continuum between what you and I might do voluntarily and what synesthetes report experiencing.

On the potential causes of synesthesia.
Ed Hubbard: One of the main theories about what’s causing synesthesia is a story of crossed wires in the brain that, for example, brain areas that are involved in recognizing letters and words lie directly adjacent to brain areas that are involved in perceiving colors. About 15 years ago, we put forth this idea that there was some sort of what we called “cross-activation.”

So every time somebody who has synesthesia sees a letter or number, in addition to activating those neurons in the brain that are involved in recognizing letters and numbers, they also activate some of those color cells in the brain. And that’s why they then get this automatic, involuntary additional experience of colors.

On whether someone who doesn’t have synesthesia can induce the effects.
Ed Hubbard: Since the late 1800s really, people have asked that same question. And the answer seems to be kind of. With lots and lots of intensive training, people can learn to have synesthesia-like associations. A recent study has even shown that you get these really nice systematic changes in brain activity along with these changes in people’s report. But it doesn’t seem to be quite as automatic. It doesn’t seem to be quite as stable. And the changes may go away fairly quickly after you stop the training. So there does seem to be something still different between synesthetes and non-synesthetes in that respect, but those of us that are curious about it might find a way to at least get some insight into what it’s like.

On why synesthesia is often difficult to detect.
Amanda Tilot: A lot of people don’t realize that their perceptions are anything unusual until maybe they hear about it in a book or in a neuroscience class in college. So that’s actually a tricky thing. It’s very common, but people don’t talk about their perceptions with their friends and realize that there’s something a little different.

On determining if it’s genetically passed down.
Amanda Tilot: Yes, that’s what we think. And we’ve had some indication that that was the case for about 130 years. So that idea that it runs in families is not new. But it’s been taking a long time to figure out exactly what was happening…

Our next task was to figure out if there were any functions of [the genes that contained changes specific to synesthesia] that would tie them all together. So maybe they’re all different, but they act in similar biological pathways. Maybe they have similar roles, maybe even in the brain. And when we look for that, what activities were overrepresented in this big list that we had from the three families [of people with synesthesia from a continued 10-year study], we found that genes related to how neurons connect with each other during development—how they know where to go to send their connections to link the right circuits together—that was a function that was overrepresented in our list of genes that were showing differences in the synesthetes. And that was surprising and exciting for us.

On how synesthesia might affect learning.
Ed Hubbard: Synesthetes will say that sometimes their colors help them to remember things like phone numbers or math facts. They will also say that sometimes they get in the way when, for example, the colors for three and four don’t mix to create the color that seven should be. Or two plus five, which are different colors, are supposed to give them the same color for the seven.

So synesthetes say it helps them and it hurts them in all sorts of interesting ways. And we’re still trying to understand this. We’re looking at adults, college students here that have synesthesia, and also looking at children trying to understand better how these synesthetic associations help and hurt their learning.

This interview was edited for length and clarity.

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