The regions of our DNA that wire some people to “see” sounds have been discovered. So far, only the general regions within chromosomes have been identified, rather than specific genes, but the work could eventually lead to a genetic test to diagnose the condition before it interferes with a child’s education.

It has long been known that synaesthesia – which can take many other forms, but generally involves a cross-wiring between the senses – seems to run in families, although it also appears to be affected by environmental factors.

To investigate the nature of the genetic component of the condition, Julian Asher, now at Imperial College London, and colleagues from the Wellcome Trust Centre for Human Genetics at the University of Oxford took genetic samples from 196 individuals of 43 families. Of these, 121 individuals exhibited the synaesthetic trait of seeing a colour in response to a sound.

“When I hear a violin, I see something like a rich red wine,” says Asher, who is a synaesthete himself. “A cello is more like honey.”


Autism link

The team performed a genetic analysis that tracked common “markers” – specific sets of base pairs that are repeated throughout the genome and which vary from person to person. By comparing these markers within the family, the researchers were able to infer which regions had been inherited from each parent. Together with knowledge about who exhibited synaesthesia within the families, the team identified the regions of DNA linked to the condition.

A region on chromosome 2, which has been associated with autism, exhibited the strongest link. This is particularly intriguing: the autistic savant Daniel Tammet, for example, possesses extraordinary abilities and also has a combination of the two conditions, as do some other autistic savants.

The region is also thought to be involved in epilepsy – which Tammet also suffered from as a child – and this could indicate that the three conditions may share some underlying genetic or neurological mechanisms.

Quiet please

Three other areas on chromosomes 5, 6 and 12 also returned promising results. All these regions are known to be involved in regulating neuronal activity and the way the brain structures itself, and future work that pinpoints the exact genes and their functions could help to explain exactly how synaesthesia occurs.

The work could also lead to a simple diagnosis of the condition. Although synaesthesia is thought to help improve memory, it can also be very distracting – particularly in a teaching environment. “For some it is benign, but for others it can impact on their learning,” says Asher. “Early diagnosis is then key for their future academic success.”

For example, synaesthetes who “see” sounds might find it hard to work in a noisy classroom. “It’s a more intense experience,” says Asher.

People who attach colours to letters might also find reading confusing. Some synaesthetic children get confused between the colour of numbers and their numerical properties, thinking that a “blue” number plus a “yellow” number must make a “green” number, for example.

Julia Simner at the University of Edinburgh believes a genetic test would have many advantages over behavioural tests, which can be difficult to perform on children who don’t understand the procedures. “A genetic test is very appealing,” she says.

Journal reference: American Journal of Human Genetics, in press

Read our interview with Daniel Tammet