In the 1880s, Francis Galton described a condition in which “persons…almost invariably think of numerals in visual imagery.” This “peculiar habit of mind” that Galton was describing is today called synaesthesia, a condition in which stimuli of one sensory modality elicit sensations in another of the senses.
There are several different kinds of synaesthesia, which is now known to be far more common than was previously thought. Galton was describing a specific type of the condition, called grapheme-colour synaesthesia, in which printed numbers or letters elicit the sensation of specific colours. The Nobel Prize-winning physicist Richard Feynman was a grapheme-colour synaesthete; he reported seeing equations in colour. But there are other forms of the condition muscial tones elicit the experience of colours; the expressionist artist Wassily Kandinsky, for example, was a tone-colour synaesthete, in whom musical tones elicited specific colours. Kandinsky used his synaesthesia to inform the artisitc process – he tried to capture on canvass the visual equivalent of a symphony.
There are a number of theories which seek to explain synaesthesia in terms of neurobiological mechanisms. One of them holds that synaesthetes have unusual connections between different sensory regions of the cerebral cortex, perhaps because a failure to prune improper, under-used or redundant synaptic connections during development of the nervous system. Thus, stimuli entering one sensory system will generate activity in another sensory system, and activity in the latter system will also evoke sensations in the synaesthete, despite an absence of the appropriate stimuli for that modality.
According to the other, the differences between the brains of synaesthetes and non-synaesthetes are functional and not anatomical. This theory holds that synaesthesia occurs because of impaired inhibition between the cortical circuits involved, such that there is abnormal feedback to a region of the brain involved in processing colour information (area v4 in the inferior temporal gyrus) from a region at the junction of the temporal, parietal and occipital lobes that processes information from multiple sensory modalities. Thus, disinhibition of the feedback to area v4 leads to inappropriate perceptions of colour.
Previous studies have provided indirect support for the first theory, and suggest a mechanism underlying grapheme-colour synaesthesia. Neuroimaging has shown that two regions in the fusiform gyrus of the temporal lobe become active when grapheme-colour synaesthetes perceive letters that evoke sensations of colour: area v4 in the inferior temporal cortex and the region immediately anterior (in front) to it, which is known to be involved in the perception of word shape. This co-activation of area v4 and the adjacent region suggest that an inappropriate interaction between these two parts of the brain may underly the extraordinary sensory experiences that occur in grapheme-colour synaesthesia.
Now, Romle Rouw and Steven Scholte of the University of Amsterdam provide direct evidence for the first hypothesis. They used a technique called diffusion tensor imaging (DTI) to compare the connectivity of the brain in grapheme-colour synaesthetes and in non-synaesthetes during viewing letters and numbers that evoked sensations of colour. 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. Some grapheme synaesthetes (called “projectors”) actually see the colours associated with the letters or numbers, while others (called “associators”), only experience mental representations of the colours. Rouw and Scholte show that projectors have more connections between area v4 and the fusiform gyrus than associators. However, they are unsure whether this increased connectivity between the two regions of the fusiform gyrus are the result of wider axons, increased myelination, or a greater number of axons.
Rouw, R. & Scholte, H. S. (2007). Increased structural connectivity in grapheme-color synesthesia. Nat. Neurosci. 10: 792-797. [Abstract]
Galton, F. (1881). Visualising numerals. J. Anthrop. Inst. 10: 85-102. [Full text]