When Sir Francis Galton first described the "peculiar habit of mind" we now call synaesthesia, he noted that it often runs in families. Modern techniques have confirmed that the condition does indeed have a strong genetic component - more than 40% of synaesthetes have a first-degree relative - a parent, sibling or offspring - who also has synaesthesia, and families often contain multiple synaesthetes.
Synaesthesia is known to affect females more than males, and although the female predominance of the condition is now known to have been exaggerated, the condition is presumed to be linked to the X chromosome. A number of genetic studies also support the theory that a single gene is responsible for synaesthesia, and that it is inherited in a dominant manner (in other words, just one copy of the gene, inherited from either parent, is sufficient to cause it).
Researchers from the University of Oxford have now conducted the first genome-wide search for genes linked to the condition. In the American Journal of Human Genetics, they report the identification of a number of genes that are likely to be involved in auditory-visual synaesthesia, in which sounds are perceived as colours. The study reveals also that synaesthesia is not X-linked, and that the genetics of this form of synaesthesia - and probably that of other forms - is far more complex than previously thought.
A group led by Julian Asher of the Wellcome Trust Centre for Human Genetics, in collaboration with Simon Baron-Cohen's group at Cambridge, studied 43 large families, all of which include multiple members with auditory-visual synaesthesia. They recruited a total of 196 individuals, of which 121 were synaesthetes as confirmed by a questionnaire designed to test for the intensity and genuinesness of the synaesthetic experience.
The researchers obtained DNA samples from each participant, and analysed more than 400 microsatellites dispersed across all the chromosomes. Microsatellites consist of very short sequences which are repeated multiple times; each allele, or variant, of a given gene contains a unique number of repeats, and this number often varies between individuals. These sequences are therefore often used to identify genetic variation in humans, as different alleles of the same gene can be distinguished from one another. In this case, however, the researchers searched for evidence of genetic linkage.
By comparing the DNA samples from different generations of synaesthetes from the same family, they identified the microsatellites which are inherited together. Rather than identifying specific genes, this analysis identified four distinct chromosomal regions located on three different chromosomes, all containing genes of interest. These regions are known to contain genes associated with a variety of disorders, including autism, dyslexia and epilepsy.
One of the candidate genes encodes the transcription factor TBR1, which regulates the activity of a number of other genes, including reelin, a signalling protein that is critical for the proper development of the cerebral cortex; another plays a role in several different developmental processes, including axon guidance, the process by which extending neuronal processes find their correct destination; and a third candidate, a sodium channel protein called SCN2A, is involved in regulating the electrical actvity of nerve cells and has been implicated in epilepsy. The region with the strongest linkage, which located on chromosome 2, is known to contain a gene associated with autism. Like synaesthesia, autism involves sensory and perceptual abnormalities, and autistics often report synaesthesia-like symptoms.
Neuroimaging shows that the connections between the brain's sensory pathways are both denser and more active in synaesthetes than in non-synaesthetes. The condition is now viewed as being developmental in origin, and it is thought that newly-established connections, which would otherwise be "pruned" during development, remain in place, and perhaps become overactive. The results of the new study therefore fit nicely with current thinking about the neural bases of synaesthesia. Specific combinations of alleles of the identified candidate genes could feasibly lead to subtle changes in developmental processes which ultimately result in alterations in neural architecture and activity thought to be involved in the condition.
As well as revealing the complexity of the genetics of synaesthesia, this study also shows that it can be inherited in a number of different ways (that is, by inheriting different combinations of alleles). The eventual identification and proper classification of all the candidate genes will inevitably lead to a better understanding not only of synaesthesia and the other conditions with which they are associated, but also of their roles in cognition more generally. The authors also suggest that advances in our knowledge of synaesthetic perceptions "may even shed light on the neural basis of consciousness".
Related:
- The neuropsychology of synaesthesia
- Tactile-emotion synaesthesia
- The sound of dots moving: A new form of synaesthesia
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.
Brain & Behavior
Comments (6)
I'm probably not qualified to make observations on this topic, but I persist in reading about it because I think I might be synaesthetic myself (I'm not sure how one would go about being definitively 'diagnosed').
Reading through your post, I get the distinct impression that we may all be synaesthetic at birth, but that the connections 'atrophy' over time as only those correlations that fit with our cultural mores are encouraged and reinforced in much the same way as we teach our children manners by reward reinforcement of acceptable behaviours and active discouragement of the unacceptable.
Before a child is able to verbalise their correlations between sounds and colours, the parents may inadvertently have begun the process of diminishing those connections just in the way they teach their children the vocabulary associated with shapes, numbers, sounds and colours. But let's say that the connections persist in sufficient strength until the child is able to move dreamily to a piece of music and say "Blue!" or clamp his hands over his ears and say "Thorny!" Unless synaesthesia is on the parents' radar, they are likely to 'correct' the child's pronouncement, because that's what good parents do. "No dear. This is music, it doesn't have colours." "Not thorny, dear. Loud."
When I was a child, I didn't eat any cabbage-related vegetables because they all tasted greyish. I imagine when I said, "I don't like this, it's grey," I caused offence to whoever had cooked the vegetables! As I grew up, the nuanced shades of flavours became more defined and I realised that Brussels sprouts actually taste olive green, which I now enjoy. White cabbage still tastes grey, but it is a light grey, with a hint of light green about it and not unpleasant at all. Green cabbage tastes a deeper green with a hint of softening grey about it.
However, most foods taste of colours for which I have no name because the texture and reflectiveness of the 'colour' are as much a component of its definition as the colour itself.
Posted by: Karyn Romeis | February 6, 2009 8:30 AM