Synesthesia is a condition in which one sense (for example, hearing) is simultaneously perceived as if by one or more additional senses such as sight. Another form of synesthesia joins objects such as letters, shapes, numbers or people’s names with a sensory perception such as smell, color or flavor.
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Synesthesia’s mysterious ‘mingling of the senses’ may result from hyperconnected neurons
When you hear a B-flat music note, do you see the color blue? Do the words in this sentence look red or green? If so, you may have synesthesia, a mysterious condition in which one sense consistently mingles with another. Now, for the first time, scientists have identified a handful of genes that might predispose people to synesthesia, offering a window to better understand disorders such as autism, which is also thought to involve abnormal brain connections.
“It’s very exciting,” says Romke Rouw, a cognitive psychologist who studies synesthesia at the University of Amsterdam but who wasn’t involved in the study. “It provides a fascinating suggestion of a link between particular genetic variations and hyperconnectivity in the synesthetic brain.”
For decades, many psychologists and neuroscientists were reluctant to research synesthesia. Some refused to acknowledge its existence, whereas others believed the phenomenon’s individual, subjective nature made it virtually impossible to study. But increasingly sophisticated survey methods have allowed scientists to confirm that some people—it’s unclear how many—do consistently and involuntarily experience this unusual condition.
Synesthesia is thought to be at least somewhat heritable, as it frequently clusters within families. But genomic investigations so far have failed to turn up individual genes that might be responsible for it.
In the new work, a team led by neuroscientist Simon Fisher at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands, decided to take a slightly different tack. Using a gene-sequencing technique known as whole-exome sequencing that targets only the DNA that encodes proteins, the researchers cataloged virtually every significant DNA variant in three families in which the condition is common.
Fisher obtained DNA sequences from four or five synesthetes and at least one nonsynesthete from each family, covering three generations. The synesthetes had one of the most common forms of the condition, which blends the perception of sound and color. For example, certain sounds trigger the perception of certain colors, and vice versa.
The team found 37 genes that predicted whether family members inherited synesthesia. No particular genetic variant was shared across all synesthetes in all three families, suggesting there is no single “synesthesia gene” or set of genes. But when the researchers looked at what these genes did, they noticed a pattern: Six of the variants were in genes related in some way to the development of connections between neurons known as axons . What’s more, these genes are expressed in both the auditory and visual cortices of the brain during childhood development, the researchers report today in the Proceedings of the National Academy of Sciences.
That’s interesting because previous brain imaging studies of synesthetes have suggested that they might have an abnormally high number of neuronal connections. Together, the two lines of research suggest that an unusually high degree of connectivity in certain brain regions might predispose people to have synesthesia.
Types of Synesthesia
There are many different types of synesthesia, but they may be categorized as falling into one of two groups: associative synesthesia and projective synesthesia . An associate feels a connection between a stimulus and a sense, while a projector actually sees, hears, feels, smells, or tastes a stimulation. For example, an associator might hear a violin and strongly associate it with the color blue, while a projector might hear a violin and see the color blue projected in space as if it were a physical object.