Friday, August 27, 2010
Coloured hearing in Williams syndrome
Posted by Kevin Mitchell
The idea that our genes can affect many of the traits that define us as individuals, including our personality, intelligence, talents and interests is one that some people find hard to accept. That this is the case is very clearly and dramatically demonstrated, however, by a number of genetic conditions, which have characteristic profiles of psychological traits. Genetic effects include influences on perception, sometimes quite profound, and other times remarkably selective. A recent study suggests that differences in perception in two conditions, synaesthesia and Williams syndrome, may share some unexpected similarities.
Williams syndrome is a genomic disorder caused by deletion of a specific segment of chromosome 7. Due to the presence of a number of repeated sequences, this region is prone to errors during replication that can result in deletion of the intervening stretch of the chromosome, which contains approximately 28 genes. The disorder is characterised by typical facial morphology, heart defects and a remarkably consistent profile of cognitive and personality traits. These include mild intellectual disability, with relative strength in language and extreme deficits in visuospatial abilities (including being able to perceive the relationships of objects in 3D space and to construct and mentally manipulate 3D representations). Williams patients are also highly social – often to the point of being over-friendly – empathetic and very talkative. This behaviour may belie a high level of anxiety, however.
One of the most remarkable features of Williams syndrome is the strong attraction of patients for music. Many show a strong interest in music from an early age and greater emotional responses to music. They are also more likely to play a musical instrument, some using music to reduce anxiety. A recent study from Elisabeth Dykens and colleagues adds a new twist to this story. They found in a neuroimaging experiment that in addition to activating the auditory cortex, music also stimulates visual activity and perceptions in Williams patients. In fact, this is not specific to music – non-musical sounds had the same or even stronger effects.
This is very reminiscent of what happens in a form of synaesthesia, called “coloured hearing”. In this condition, which is also heritable, sounds, sometimes specifically music or words, sometimes general sounds, are accompanied by a visual percept. These percepts are typically fairly simple – patches of colour, for example – and can be experienced out in the world or “in the mind’s eye”. (They are alternatively sometimes felt more as an “association” with a visual property, so that the sound of a trombone might be blue, while a piano might be green). Importantly, these visual percepts are both idiosyncratic and extremely consistent – middle C may evoke the image of a small purple cloud, a dog’s bark may set off yellow starbursts, etc.
Neuroimaging experiments in synaesthesia have also found activation of visual areas in response to sounds. Various models have been proposed to account for this, which I have discussed previously. They all involve cross-activation from auditory circuits to those that represent visual information. This may be mediated by extra physical connections in the brains of synaesthetes, presumably due to genetic effects on how the brain is wired during development. Alternatively, the wires could be there in everyone but just working differently in synaesthetes. It has so far been very difficult to distinguish between these possibilities.
The situation in Williams syndrome may be much more amenable to investigation. Unlike synaesthesia, we know what the genetic cause is in Williams syndrome. We know which genes are deleted and researchers are beginning to dissect which ones are associated with which symptoms. Some of these genes are known to function in nerve growth and guidance. It has also been demonstrated very clearly using diffusion tensor imaging that there are large differences in various circuits in the brains of Williams patients, including the presence of additional fibre bundles to or from the intraparietal sulcus, a region involved in visuospatial construction. It will be very interesting to determine whether similar extra connections can be detected between auditory and visual areas.
It is important to recognise, however, some crucial differences between the auditory-visual effects observed in Williams syndrome and in synaesthesia. The visual percepts reported in Williams syndrome are far more complex than those reported in synaesthesia. The former reportedly involve objects and scenes, more like a dreamscape than a simple blob of colour. They also lack the consistency which is one of the defining characteristics of synaesthesia. There may thus be more than one way to end up with coloured hearing.
Whatever the cause in these conditions, they both highlight the fact that genetic differences can have profound effects on how people perceive the world.
Thornton-Wells, T., Cannistraci, C., Anderson, A., Kim, C., Eapen, M., Gore, J., Blake, R., & Dykens, E. (2010). Auditory Attraction: Activation of Visual Cortex by Music and Sound in Williams Syndrome American Journal on Intellectual and Developmental Disabilities, 115 (2) DOI: 10.1352/1944-7588-115.172
Marenco, S., Siuta, M., Kippenhan, J., Grodofsky, S., Chang, W., Kohn, P., Mervis, C., Morris, C., Weinberger, D., Meyer-Lindenberg, A., Pierpaoli, C., & Berman, K. (2007). Genetic contributions to white matter architecture revealed by diffusion tensor imaging in Williams syndrome Proceedings of the National Academy of Sciences, 104 (38), 15117-15122 DOI: 10.1073/pnas.0704311104