“Feeling” pain in an alien hand. Evidence from skin conductance response in pathological embodiment of other people’s body part

We studied one right-brain-damaged (RBD) patient with an unusual form of contralesional asomatognosia, who claimed that the examiner’s left hand was their own left hand whenever it was positioned in egocentric perspective, congruent with the patient body schema. We aimed to verify whether, in this pathological condition, a non-owned body part can be fully incorporated into the own body schema, so deeply that the embodied alien hand elicit psychophysiological reactions specific to the patient’s own body. We recorded skin conductance response (SCR), a physiological measure of psychological and autonomic arousal, while painful stimuli was overtly applied on both the own and the alien hand (right and left). In healthy controls (ten aged-matched subjects) we expected that SCR was stronger in the “own” than in the “alien” condition for both the right and the left hand. On the contrary, in the patient who misidentified the alien left hand as the own, we expected to measure a SCR peak when the alien left hand received painful stimuli, similarly to what happens when painful stimuli were delivered to the own hand. No SCR was expected for the alien right hand (for the right hand, the pathological embodiment did not occur and the patient correctly recognized that the hand belonged to another person). Our results confirmed these predictions, providing objective evidence of how an alien hand becomes really and affectively ‘coupled’ with the own body image.