Defensive responses in brain-damage patients with pathological embodiment of someone else’s body part

Our research focused on a monothematic delusion of body ownership, in which brain-damaged patients systematically claim that the examiner’s hand is their own hand (E+ patients). Previous studies have shown that this delusion of ownership is not merely a verbal confabulation, but reflects an embodiment mechanism able to alter the patients’ motor and somato-sensory behaviors. In the present study, we explored, in 4 E+ patients and in healthy controls, how defensive responses are modulated by the sense of body ownership. To this aim, we took advantage of the hand-blink reflex (HBR), a subcortical defensive response enhanced when the threated hand is located close to the face, inside the defensive peripersonal space (DPPS). Here, we investigated weather in E+ patients the magnitude of the HBR is enhanced irrespective of weather the threat is brought by the own or the alien (embedded) hand. This paradigm, applied to E+ patients, can provide evidence of how top-down processes (i.e., the delusional belief that an alien hand is a part of the own body) can modulate physiological reactions. Electromyographic activity (EMG) was recorded from the orbicularis oculi bilaterally and the HBR was elicited by delivering electrical stimuli to the median nerve at the wrist, in two different conditions. In the “own hand” condition the participant’s hand was located either outside (far) or inside (near) the DPPS of the face; in the “alien hand” condition the examiner’s hand was located outside or inside the participant’s DPPS. Two separate sessions of the experiment investigated, either the “affected side” where the pathological embodiment occurred or the “intact side” where the pathological embodiment did not occur. In our results no difference between the E+ patients’ intact side and the healthy controls was found: in both groups the far-near modulation (i.e. the HBR increase in the near with respect to the far position) was significantly higher in the own than in the alien condition. On the contrary, a significant difference between the E+ patients’ affected side and controls was found, suggesting that, when the pathological embodiment occurs, the far-near modulation is the same for the own and the alien hand. The crucial aspect of this experiment is that, in the affected side, during the alien hand condition, E+ patients were convinced that the examiner’s hand was their own hand moving close to their face. This altered sense of body ownership modulates the patients’ defensive response: they showed the same HBR enhancement as that induced by the own hand. The key finding of the present study is that an alien hand that becomes a part of the own body triggers the same defensive physiological response as the real hand.