Mirror-touch synaesthesia changes representations of self-identity.

Individuals with mirror-touch synaesthesia (MTS) experience touch on their own bodies when observing another person being touched. Whilst somatosensory processing in MTS has been extensively investigated, the extent to which the remapping of observed touch on the synaesthete's body can also lead to changes in the mental representation of the self remains unknown. We adapted the experimental paradigm of the 'enfacement illusion' to quantify the changes in self-face recognition as a result of synaesthetic touch. MTS and control participants observed the face of an unfamiliar person being touched or not, without delivering touch on the participant's face. Changes in self-representation were quantified with a self-face recognition task, using 'morphed' images containing varying proportions of the participant's face and the face of the unfamiliar other. This task was administered before and after the exposure to the other face. While self-recognition performance for both groups was similar during pre-test, MTS individuals showed a significant change in self-recognition performance following the observation of touch delivered to the other face. Specifically, the images that participants had initially perceived as containing equal quantities of self and other became more likely to be recognised as the self after viewing the other being touched. These results suggest that observing touch on others not only elicits a conscious experience of touch in MTS, but also elicits a change in the mental representation of the self, blurring self-other boundaries. This is consistent with a multisensory account of the self, whereby integrated multisensory experiences maintain or update self-representations.