In the 1880s, Francis Galton described a condition in which “persons…almost invariably think of numerals in visual imagery.” This “peculiar habit of mind” is today called synaesthesia, and Galton’s description clearly defines this condition as one in which stimuli of one sensory modality elicit sensations in another of the senses.
There are several different kinds of synaesthesia, and the condition is now known to be far more common than was previously thought. Galton was describing a specific type of synaesthesia, called grapheme-colour synaesthesia, in which printed numbers or letters elicit the sensation of specific colours. The Nobel Prize-winning physicist Richard Feynman, who reported seeing equations in colour, was a grapheme-colour synaesthete, while the expressionist artist Wassily Kandinsky, in whom musical tones elicited specific colours, was a tone-colour synaesthete. Kandinsky used his synaesthesia to inform the artisitic process – he tried to capture on canvass the visual equivalent of a symphony. There is also a type of synaesthesia called mirror-touch synaesthesia, which was discovered only very recently (see below).
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.
Earlier this year, 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 (see the image at the top of the post). 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.
The condition known as mirror-touch synaesthesia was described for the first time only two years ago. People with this type of synaesthesia experience tactile sensations when they observe another person being touched. Another recent study, also published in Nature Neuroscience, shows that mirror-touch synaesthetes are more empathetic than non-synaesthetes.
The study was led by Jamie Ward, of the Department of Psychology at University College London, who first described mirror-touch synaesthesia, and gave the condition its name, in 2005. Together with Michael Banissy, one of his graduate students, Ward recruited 10 synaesthetes who claimed to experience tactile sensations when they observed someone else being touched. In a series of experiments, the authenticity of the synaesthetes were verified. The participants were touched on the cheeks or the hands. At the same, they observed someone else being touched, either on the same part of the body that they were touched, or elsewhere. They were asked to report where they were touched, while ignoring their observations of someone else being touched.
It was found that the synaesthetic participants were much faster than the non-synaesthetes at reporting being touched when their observations corresponded to their own experiences. But the researchers were more interested in the situations when the observations of the synaesthetes did not correspond to their own sensations. They had reasoned that synaesthetes should find it harder than non-synaesthetes to distinguish between the sensations elicited by actually being touched and those elicited by observing someone else being touched. And this was indeed found to be the case: the synaesthetic participants made more “mirror-touch errors” than non-synaeshetic controls. That is, they often reported being touched on the same part of the body as those people they observed, even if they were touched on part of the body and person they observed was touched on a different part.
Banissy and Ward then measured the empathy quotients of their synaesthetic and non-synaesthetic participants. Both the experimental and control groups were asked to respond to a list of statements designed to measure their emotional and social skills. It was found that there was a strong correlation between mirror-touch synaesthesia and empathy – the synaesthetes responded more positively than non-synaesthetes to statements such as “I am good at predicting how someone will feel” and “I get upset when I see people suffering on news programmes.”
A region of the brain called the somatosensory cortex receives inputs from; the body is mapped onto this part of the brain, such that when one is touched, the subregion of the somatosensory cortex corresponding to that part of the body becomes active. It is activity in the somatosensory cortex that leads to the sensation of being touched, and it is now known that observing another person being touched also activates the somatosensory cortex. And several years ago, a neuroimaging study conducted by Ward and his colleagues showed that this region of the brain is hyperactivated in mirror-touch synaesthetes when they observe someone else being touched.
The somatosensory cortex and the areas surrounding it (including the primary motor cortex) are hypothesized to be a major component of the brain’s “mirror system”. The mirror system is composed of neurons which fire not just when one is performing a particular action, but also when one observes another performing that action. Thus, it is believed that these cells are involved in “mirroring” the behaviour of others so that the brain can generate simulations of their experiences. It has further been suggested that the mirror system is crucial for the acquisition of behaviors that are learnt through imitiation, such as language, and that it is impaired in conditions such as autism.
Banissy and Ward show for the first time that the sensations elicited in mirror-touch synaesthetes while observing someone else being touched are indistinguishable from those felt when they are actually touched. The researchers had no difficulty in recruiting participants for their study, and all those involved were actually unaware that they had the condition – they believed that their synaesthetic experiences were completely normal.
Mirror-touch synaesthesia may, therefore, be relatively common. Perhaps the condition has gone by another name; the somatosensory hyperactivation that occurs when observing others may cause feelings of discomfort, or even pain, when observing someone being hurt. Thus, maybe mirror-touch synaesthesia is in fact the condition formerly known as “squeamishness”.
Banissy, M. J. & Ward, J. (2007). Mirror-touch synesthesia is linked with empathy. Nature Neurosci. doi: 10.1038/nn1926.
Blakemore, S. -J., et al. (2005). Somatosensory activations during the observation of touch and a case of vision-touch synaesthesia. Brain 128: 1571-1583. [Abstract]
Galton, F. (1881). Visualising numerals. J. Anthrop. Inst. 10: 85-102. [Full text]
Rouw, R. & Scholte, H. S. (2007). Increased structural connectivity in grapheme-color synesthesia. Nat. Neurosci. 10: 792-797.