From Science:
Ever flinch at the sight of an actor being punched in the face? The reason is that neurons in the brain light up when we watch others suffering. Now a team of psychologists has added evidence to the theory that such mirror systems in our brains are what lie behind our ability to empathize with others. The conclusions are based on a rare group of individuals who feel a touch upon their own bodies when they see someone else being touched. Only one such case of mirror-touch synesthesia had been reported previously in the literature; University College London’s Michael Banissy and Jamie Ward investigated the phenomenon in 10 other individuals.
In the new study, the researchers first established that the subjects had mirror-touch synesthesia. They had the individuals and members of a control group report where they felt a touch on their bodies while observing another person being touched. During the task, an actual touch was applied to their bodies as well–either at the same location as the person being observed or at a different location. The researchers found that mirror-touch synesthetes were quicker at detecting actual touch when it was applied to the same location as that of the person they were watching. They were also more likely than control subjects to report a synesthetic touch as a real touch.
More here.