Scientists have uncovered evidence that particular genes related to neural connectivity may be linked to synaesthesia, a type of perception in which an experience of one sense automatically and involuntarily triggers the experience of another sense.
In a new study conducted by a team of researchers from the Max Planck Institute for Psycholinguistics in the Netherlands and the University of Cambridge in the UK, researchers found genetic evidence that could help scientists better understand synaesthesia and its causes. The team published their findings on Monday in the journal Proceedings of the National Academy of Sciences.
Around one in 25 people, or 4% of the world’s population, have synaesthesia. The trait develops during childhood and is experienced as crossing of the senses, such as seeing colours when listening to music or associating letters and numbers with particular colours or personalities.
Scientists have known for more than 100 years that synaesthesia runs in families, suggesting that inherited traits are connected to the condition. Previous neuroimaging-based research has shown atypical neural connectivity in people who experience synaesthesia, also known as synaesthetes. Until now, however, little was known about which genes, if any, could predispose people to synaesthesia, and the molecular mechanisms which underlie the trait.
“Brain imaging of adults with synaesthesia suggests that their circuits are wired a little differently compared to people who don’t make these extra sensory associations,” said Dr Amanda Tilot, co-author of the study and geneticist at the Max Planck Institute for Psycholinguistics in the Netherlands. “What we don’t know yet is how these differences develop. We suspect some of the answers lie in people’s genetic makeup.”
To find out more about the molecular structures that regulate synaesthesia, researchers studied three families with multiple synaesthetes spanning several generations. All of the individuals studied experience colour when hearing sounds, a type of synaesthesia known as sound-colour or auditory-visual.
Using a type of gene sequencing called “whole-exome sequencing,” scientists analysed the participants’ DNA. According to the study, they focused “on rare DNA changes that altered the way genes code for proteins, and that perfectly matched the inheritance of synaesthesia in each of the three families.”
The researchers found 37 “genes of interest,” with six genes associated with axonogenesis, a process where connections between neurons, known as axons, are developed. These six genes are expressed during early childhood, the same period in which formation of synaesthetic associations occurs. The study notes that although none of the genetic variants identified were shared between families, the enrichment for genes tied to neural development is suggestive of a link between synaesthesia and hyperconnectivity of neurons.
“This research is revealing how genetic variation can modify our sensory experiences, potentially via altered connectivity in the brain,” said Professor Simon Baron-Cohen, who was involved in the research.
Baron-Cohen, who works as director of the Autism Research Centre at Cambridge University, added, “Synaesthesia is a clear example of neurodiversity which we should respect and celebrate.”
To further explore their findings and answer some of the remaining questions, the team intends to conduct additional research into the phenomenon of synaesthesia.
The findings may also open up possibilities for research into other conditions involving altered connectivity in the brain. Since synaesthesia seems to be more common among people with autism spectrum disorder, Science Magazine reports that the findings could have relevant applications for autism research.