Your hand is my hand! Bimanual coupling effect in a somatoparaphrenic patient

When people are required to draw circles with one hand while drawing lines with the other, both the trajectories tend to become ovals (Franz, 2003). Recently, we showed (Garbarini et al. under review) that these effects occur in both actual bimanual movements and when an active limb movement is combined with an illusory limb movement (i.e., anosognosia for hemiplegia). Those results would support the view that coupling effects arises from central signals (e.g., sensory predictions) rather than on actual feedbacks (Franz, 2003). Here we examine whether coupling effects are present also when an active movement is combined with a movement performed by another person’s hand, misidentified as one’s own hand. One right-brain-damaged-patients affected by left hemiplegia and misidentifications of the experimenter’s hand as his own hand (somatoparaphrenia), five hemiplegic patients and ten healthy subjects were administered a bimanual circle-line drawing task. Participants were asked to draw lines with the right hand (baseline) and a) to draw circles with their own left hand b); to draw circles with their own left hand passively moved; c) to observe the examiner drew circle with his left hand positioned in a compatible orientation with respect to the subject’s body. Results showed that, in the crucial c) condition, only in the patient who misidentified the experimenter’s hand as his own, the lines produced with the right hand were significantly ovalized in respect to the baseline (p = 0.0001). These results give interesting new hints regarding the link between body ownership and sense of agency.