Enhanced self-reported affect and prosocial behaviour without differential physiological responses in mirror-sensory synesthesia

Mirror-sensory synesthetes mirror the pain or touch that they observe in other people on their own bodies. This type of synesthesia has been associated with enhanced empathy. We investigated whether the enhanced empathy of people with mirror-sensory synesthesia influences experience of situations involving touch or pain, and whether it affects their prosocial decision making. Mirror-sensory synesthetes (N=18, all female), verified with a touch-interference paradigm, were compared to a similar number of age-matched control individuals (all female). Participants viewed arousing images depicting pain or touch; we recorded subjective valence and arousal ratings, and physiological responses, hypothesizing more extreme reactions in synesthetes. The subjective impact of positive and negative images was stronger in synesthetes than in control participants; the stronger the reported synesthesia, the more extreme the picture ratings. However, there was no evidence for differential physiological or hormonal responses to arousing pictures. Prosocial decision making was assessed with an economic game assessing altruism, in which participants had to divide money between themselves and a second player. Mirror-sensory synesthetes donated more money than non-synesthetes, showing enhanced prosocial behaviour, and also scored higher on the Interpersonal Reactivity Index as a measure of empathy. Our study demonstrates the subjective impact of mirror-sensory synesthesia and its stimulating influence on prosocial behaviour.