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June 2, 2007

BehindTheMedspeak: The brain of a synaesthete


An article in the May 24, 2007 Economist on new research into the strange world of synaesthesia featured an image of a synaesthete's brain (above) obtained using a technique called diffusion-tensor imaging.

The story follows.

    Synaesthesia: Too well connected

    Why certain people see words in colour

    Some people have a more complicated sensory life than others. Most taste tuna only when they eat it. A few, though, taste it when they hear a particular word, such as “castanet”. Others link the colour red with the letter “S” or make some other inappropriate connection between stimulus and response. Such people are known as synaesthetes, and the phenomenon of synaesthesia has puzzled brain scientists since it was recognised over a century ago.

    Most researchers in the field suspect synaesthesia is caused by crossed wires in synaesthetes' brains, but until recently they have had no way to check this hypothesis. However, the development of a technique called diffusion-tensor imaging has changed that. And Romke Rouw and Steven Scholte at the University of Amsterdam have just applied it to the brains of 18 women (the sex more likely to experience synaesthesia) who have the most common form of the condition. This is called grapheme-colour synaesthesia. It is a tendency to see letters and numbers in colour.

    Diffusion-tensor imaging measures the direction of movement of water molecules. Since the filaments that connect distant nerve cells are surrounded by fatty sheaths which restrict the movement of water, such molecules tend to move along a filament rather than out of it. The upshot is that the technique can detect bundles of such filaments running from one part of the brain to another.

    Dr Rouw and Dr Scholte chose grapheme-colour synaesthesia to study for two reasons. One was that it is common. The other was because there is a specific hypothesis as to its cause. Earlier brain-scanning studies have shown that the part of the brain which identifies word shape is in an area called the fusiform gyrus. This is next to an area known as V4, which identifies colour. Both light up simultaneously in traditional scanners when someone is experiencing grapheme-colour synaesthesia, so an inappropriate link between them is an obvious thing to look for.

    As the two researchers report in the latest edition of Nature Neuroscience, diffusion-tensor imaging did, indeed, show strong connections between these two areas in the brains of the synaesthetes when they slid into the scanner and viewed colour-evoking letters and numbers. Other unusual connections showed up too, suggesting the phenomenon is more complex than had been appreciated. By contrast, the brains of 18 non-synaesthetes matched with the volunteers by age and sex showed no such strong connections.

    And the revelations went further. Certain types of grapheme-colour synaesthetes have more connectivity than others. As part of the study, Dr Rouw asked her volunteers to fill out questionnaires about how they experience their colour sensations. Some reported seeing colour projected on to whatever word or number they were shown. Members of this group are known as “projectors”. Others, known as “associators”, reported colour only in their mind's eye. Although both groups of synaesthetes had much more connectivity than non-synaesthetes, projectors had noticeably more again than associators.

    Dr Rouw and Dr Scholte are not yet sure what form the stronger connections take. They could be the result of more filaments than normal connecting the areas in question. Or the filaments might be broader than normal. Or the fatty coatings of the filaments might be thicker, which would amplify the signal passing along them as well as keeping water molecules on the straight-and-narrow.

    How synaesthesia starts (in other words, how the connections become so strong) is also unclear. There is evidence of a genetic component but, as Dr Rouw points out, learning must be involved as well. People are not born with the concept of the letter “A”. And they certainly are not born with a bright-red, cherry-coloured “A”—however much it feels, as synaesthetes insist, as though they were.


Here's the abstract of the Nature Neuroscience paper.

    Increased structural connectivity in grapheme-color synesthesia

    Diffusion tensor imaging allowed us to validate for the first time the hypothesis that hyperconnectivity causes the added sensations in synesthesia. Grapheme-color synesthetes (n = 18), who experience specific colors with particular letters or numbers (for example, 'R is sky blue'), showed greater anisotropic diffusion compared with matched controls. Greater anisotropic diffusion indicates more coherent white matter. Anisotropy furthermore differentiated subtypes of grapheme-color synesthesia. Greater connectivity in the inferior temporal cortex was particularly strong for synesthetes who see synesthetic color in the outside world ('projectors') as compared with synesthetes who see the color in their 'mind's eye' only ('associators'). In contrast, greater connectivity (as compared with non-synesthetes) in the superior parietal or frontal cortex did not differentiate between subtypes of synesthesia. In conclusion, we found evidence that increased structural connectivity is associated with the presence of grapheme-color synesthesia, and has a role in the subjective nature of synesthetic color experience.


The best book ever written on synaesthesia remains A. R. Luria's 1968 classic,


"The Mind of a Mnemonist."

June 2, 2007 at 02:01 PM | Permalink


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What does one record on such a device.."if you took my bag, please do my laundry before you return it?"
" A large reward to call me?" Perhaps you could use the old hotel key " if found please drop in the nearest mailbox"..no wait, that won't work. Mail's bad enough already without a Louis Vuitton in every collection box.

Posted by: Carlitos | Jun 3, 2007 2:16:24 AM

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