An interesting report from Banissy and Ward (PDF here)... People who have 'mirror-touch' synesthesia, when watching another person being touched, have the same experience as being touched themselves...an extreme form of empathy! They developed a protocol to provide evidence for the authenticity of this form of synesthesia, in which participants have difficulty in discriminating between actual and synesthetic touch. Their abstract:
Watching another person being touched activates a similar neural circuit to actual touch and, for some people with 'mirror-touch' synesthesia, can produce a felt tactile sensation on their own body. In this study, we provide evidence for the existence of this type of synesthesia and show that it correlates with heightened empathic ability. This is consistent with the notion that we empathize with others through a process of simulation.