Unraveling the Genetics Behind Why Some People “See” Sound and “Hear” Color

It’s difficult to imagine for many people, but for a certain percentage of the human population, music may evoke colors, words stir up flavors or sounds may even curl into shapes. This mash-up of senses is known as synesthesia and has baffled scientists for decades. Now, reports Michael Price at Science, researchers have identified some of the genes that may be responsible for these unusual experiences.

According to a press release, synesthesia commonly runs in families and usually develops in early childhood, which means it’s likely there is a genetic basis for the ability. “Brain imaging of adults with synesthesia suggests that their circuits are wired a little differently compared to people who don’t make these extra sensory associations,” geneticist Amanda Tilot, co-author of the new paper in the Proceedings of the National Academy of Sciences says. “What we don’t know yet is how these differences develop. We suspect some of the answers lie in people’s genetic makeup.”

As Price reports, despite the suspected genetic link, researchers have not been able to use genomic analysis to identify the genes that might be responsible for the condition. So the latest study used a new technique known as whole-exome sequencing to only target genes that encode proteins. They used the process to catalog the protein-producing genes from three generations of three families with a history of sound and color synesthesia. They then compared the DNA to members of the same family without the condition, looking for differences. According to the study, the researchers found 37 “genes of interest” that could be related to the development of synesthesia.

Those clusters of genes, however, were not the same in the three families studied, which suggests that the development of the trait is complex and not carried by a single gene or even a single set of genes. This was not necessarily surprising. “We knew from earlier studies by the Cambridge team that no single gene can account for this intriguing trait; even families who experience the same form of synesthesia are likely to differ in terms of specific genetic explanations,” team leader Simon Fisher, director of the Max Planck Institute, says in the release.

Pinpointing particular genes is further complicated by a small sample size, Tereza Pultarova at LiveScience reports. So instead of searching for the specific genes responsible, the researchers examined the function of each of the genes of interest. As Fisher puts it: “Our hope was that the DNA data might point to shared biological processes as candidates for involvement in synesthesia.”

They found that six of the genes identified were related to the process that helps neurons in the brain link up with the correct partners. The genes are expressed in the auditory and visual parts of the brain during early childhood development—the time when synesthesia commonly starts to take shape. While the study does not point to a genetic “smoking gun” for synesthesia, it hints at the physical processes that causes the sensory mixing.

Price reports that previous studies of synesthetes showed they have more than the normal number of neuronal connections in their brains. Combined with the new study, that hints at the beginnings of an outline for how the trait works. “[It] suggests we are moving in the right direction with these studies,” Fisher tells Price.

The team hopes to find more synesthetes so they can expand their genetic research. While it’s interesting to know how the process works, it could also be useful in understanding other conditions as well. For instance, Price reports that many people on the autism spectrum have sensitivities to sound, touch and other stimuli that may also be related to abnormal brain connections.

It’s possible that you might be a synesthete and not even realize it. While it’s currently estimated about four percent of the global population has some sort of synesthesia, a study last year suggested that up to 20 percent of the population could have a low-level type of synesthesia. When tested, one in five people claimed they heard faint sounds associated with flashing lights, even though no sound was present.

But if you’re not one of this select population, you still can get a taste—literally—of what synesthesia is like. The British dance troupe BitterSuite uses food, perfume and touch to help its audience feel the music in an entirely new way. There are even immersive VR aimed at providing such experiences. Take a look or a listen, perhaps it’ll stir up some smells.

source: smithsonianmag.com By Jason Daley

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