Mirror-touch synaesthesia: the role of sharedrepresentations in social cognition

Synaesthesia is a condition in which one property of a stimulus results in conscious experiences of an additional attribute. In mirror-touch synaesthesia, the synaesthete experiences a tactile sensation on their own body simply when observing touch to another person. This thesis investigates the prevalence, neurocognitive mechanisms, and consequences of mirror-touch synaesthesia. Firstly, the prevalence and neurocognitive mechanisms of synaesthesia were assessed. This revealed that mirrortouch synaesthesia has a prevalence rate of 1.6%, a finding which places mirror-touch synaesthesia as one of the most common variants of synaesthesia. It also indicated a number of characteristics of the condition, which led to the generation of a neurocognitive model of mirror-touch synaesthesia. An investigation into the perceptual consequences of synaesthesia revealed that the presence of synaesthesia is linked with heightened sensory perception - mirror-touch synaesthetes showed heightened tactile perception and grapheme-colour synaesthetes showed heightened colour perception. Given that mirror-touch synaesthesia has been shown to be linked to heightened sensorimotor simulation mechanisms, the impact of facilitated sensorimotor activity on social cognition was then examined. This revealed that mirror-touch synaesthetes show heightened emotional sensitivity compared with control participants. To compliment this, two transcranial magnetic stimulation (TMS) studies were then conducted to assess the impact of suppressing sensorimotor activity on the expression recognition abilities of healthy adults. Consistent with the findings of superior emotion sensitivity in mirror-touch synaesthesia (where there is facilitated sensorimotor activity), suppressing sensorimotor resources resulted in impaired expression recognition across modalities. The findings of the thesis are discussed in relation to neurocognitive models of synaesthesia and of social cognition.