Imagine if every time you heard the word "Wednesday" or "supermarket" and you tasted orange and felt yellow. That is synesthesia, a condition where the senses appear to be cross-wired. And the root cause is genetics.
The Synesthetic Experience
Synesthesia is neither an illness nor a disability. It is a neurological condition that is believed to affect up to about 1% of the population and more women than men. The synesthetic experience differs amongst people who have the condition. Some may "see shapes" or "taste sounds" and others can experience vivid colours when they hear a piece of music.
If you're looking for a definition of synesthesia it can be described as a mixing of the senses, where a stimulation of one sense creates perception in another sense (Encyclopaedia Britannica). The composer Franz Liszt had the disorder, as did the author Vladimir Nabokov who wrote Lolita.
Synesthesia Research
Functional magnetic resonance imaging scans of the brains of people with synesthesia (of the type where individuals see colours when they hear words) have revealed that areas of the brain associated with vision are activated when synesthetes hear sounds. The work was conducted by researchers at Goldsmiths College, London.
It has also been known for some time that synesthesia runs in families, though the specific regions of DNA and the genes involved have not been located. So scientists have been turning their gaze to parts our chromosomes that could be linked to the condition.
Scientists at Oxford University studied the DNA from 196 people representing 43 families in which there were several members with the neurological condition. They focused their attention on the synesthetic condition where sounds trigger colours, and they compared 410 microsatellite DNA regions. These are sections of the genetic code where short sequences of nucleotides are repeated many times over.
Every allele, or version of a gene, contains a unique number of microsatellites, which varies between individuals. So the Oxford scientists compared the DNA of different generations of people with the condition from the same family; and they found common regions of DNA that were inherited together. A total of four chromosomal regions were pinpointed - 2q24, 5q33, 6p12, and 12p12. Prior to this research it had been thought that the disorder was created by mutations of a single gene on the X chromosome. However, the Oxford study provided no evidence of a link to the X chromosome.
Are We All Synesthetes?
There is a school of thought that believes that we all have synesthesia to some degree. Psychologist Jamie Ward conducted an experiment in 2006 where 200 random visitors to the Science Museum in London were asked to watch two musical animations and asked which animation better suited the music. Now one of the animations was designed by synesthetes, and the other by non-synesthetes. The synesthete-designed animation was the clear favourite and Ward saw this as evidence that our senses are very closely attuned to each other, as is the case with people who have synesthesia. Further research into the molecular basis of the condition will not only help us to develop a deeper understanding of synesthesia, but it may also be used to better understand how ordinary brains function.
Reference
Asher, J. E. et al (2009). A Whole-Genome Scan and Fine-Mapping Linkage Study of Auditory-Visual Synesthesia Reveals Evidence of Linkage to Chromosomes 2q24, 5q33, 6p12, and 12p12. Am. J. Hum. Genet. DOI: 10.1016/j.ajhg.2009.01.012