Wiring the brain for synaesthesia

Neurophilosopher has a great article on a brain scanning study showing that people with synaesthesia have different patterns of brain connections compared to non-synaesthetes.

You read a lot of articles on the brain that use phrases like “wired differently”, suggesting that the connections in the brain are altered.

As the connections in our brain are changing all the time at the dendrite level, often this is just a meaningless way of saying “there’s a difference”.

Perhaps these sort of phrases are best applied to white matter which is the nearest you’ll find to genuine wires in the brain.

White matter fibres run in bundles, they carry electrical signals, and they are insulated by a fatty covering called myelin.

The connections of white matter have been quite hard to study in living people until the development of diffusion tensor imaging (DTI), a brain scanning technology that can specifically pick out the white matter fibres and create maps like the one in the picture.

Rarely when articles talk about “different brain wiring” do they actually mean detectable differences in white matter though.

In the DTI study covered by Neurophilosopher this is exactly what was studied, and it does indeed seem to be different in people who experience synaesthesia, a condition where some of the senses are crossed so, for example, numbers might be also experienced as colours.

DTI is a type of magnetic resonance imaging (fMRI) that measures the diffusion of water molecules. In the brain, water diffuses randomly, but tends to diffuse easier along the axons that are wrapped in myelin, the fatty protein that insulates nerve fibres. Diffusion tensor imaging can therefore be used to infer the size and direction of the bundles (or “fascicles”) of white matter tracts that connect different regions of the brain (above).

The Dutch researchers show that synaesthetes have more connections between the two adjacent areas in the fusiform gyrus than non-synaesthetes. They report their findings in the June issue of Nature Neuroscience.

As well as showing these differences between synaesthetes and non-synaesthetes, the authors also show that there are also differences in connectivity between synaesthetes who differ in the intensity of their sense-mixing experiences.

In other words, the researchers found people with synaesthesia had white matter ‘wiring’ between sensory areas that others don’t have, and that this wiring differed depending on how much synaesthesia the participants experience.

Just from the fantastically straight-forward explanation of DTI imaging given above, you can see that it’s a wonderfully written article.

Have a look at the full piece for more on this fascinating study.

Link to Neurophilospher on ‘Imaging of connectivity in the synaesthetic brain’.
Link to abstract of scientific study.