Can we fully incorporate into our body schema the body parts of others, altering our sense of ownership []? And, to what extent, given the tight link between body and motor representations, does an altered sense of body-ownership affect motor awareness [] and the sense of agency []? The new study we report here demonstrates that a body part of one individual can become so deeply embedded in another’s sensory-motor circuits as to have objective effects on the latter’s motor execution. Indeed, we found, in right-brain-damaged hemiplegic patients who identified another person’s hand as belonging to themselves, significant interference effects of the alien hand movements on the actual movements of their own intact hand.

Main Text

5 Zeller D.

Gross C.

Bartsch A.

Johansen-Berg H.

Classen J. Ventral premotor cortex may be required for dynamic changes in the feeling of limb ownership: a lesion study. 6 Gandola M.

Invernizzi P.

Sedda A.

Ferrè E.R.

Sterzi R.

Sberna M.

Paulesu E.

Bottini G. An anatomical account of somatoparaphrenia. 7 Fotopoulou A.

Jenkinson P.M.

Tsakiris M.

Haggard P.

Rudd A.

Kopelman M.D. Mirror-view reverses somatoparaphrenia: dissociation between first- and third-person perspectives on body ownership. 1 Ehrsson H.H.

Spence C.

Passingham R.E. That’s my hand! Activity in premotor cortex reflects feeling of ownership of a limb. Our work involved right-brain-damaged hemiplegic patients affected by an atypical form of hemisomatoagnosia []. While not explicitly denying that their contralesional (left) limbs belonged to themselves (as in the well-known delusion of disownership []), these patients claimed that the examiner’s left hand was their own whenever it was positioned, in egocentric coordinates, on the table next to their real left hand. This delusion of ownership, although resembling the ‘rubber-hand-illusion’ [], was spontaneous and not induced by any experimental procedure. Interestingly, when the examiner moved his/her left hand, patients claimed that they were moving their own (paralysed) hand. (For clinical and anatomical details see the Supplemental Information and Supplemental Movie 1 ). We reasoned that, if this phenomenon is not the result of a mere verbal confabulation, but rather reflects a physically implemented embodiment mechanism, then it should alter patients’ motor representation and motor behavior.

+) and five without (E−) alien-limb-embodiment, as well as ten age-matched healthy controls, were tested with a circles-lines paradigm [ 8 Franz E.A.

Zelaznik H.N.

McCabe G. Spatial topological constraints in a bimanual task. 8 Franz E.A.

Zelaznik H.N.

McCabe G. Spatial topological constraints in a bimanual task. 9 Wenderoth N.

Debaere F.

Sunaert S.

Van Hecke P.

Swinnen S.P. Parieto-premotor areas mediate directional interference during bimanual movements. 2 Garbarini F.

Rabuffetti M.

Piedimonte A.

Pia L.

Ferrarin M.

Frassinetti F.

Gindri P.

Cantagallo A.

Driver J.

Berti A. ‘Moving’ a paralysed hand: bimanual coupling effect in patient with anosognosia for hemiplegia. 10 Franz E.A.

Ramachandran V.S. Bimanual coupling in amputees with phantom limbs. To investigate this embodiment, eight hemiplegic patients, three with (E) and five without (E) alien-limb-embodiment, as well as ten age-matched healthy controls, were tested with a circles-lines paradigm []. Here, when normal subjects are asked to simultaneously draw lines with one hand and circles with the other, both trajectories assume oval shapes, indicating that the motor programs of one hand affects the motor programs of the other (a ‘bimanual coupling’ effect []). Previous studies [] have shown that this coupling effect strictly depends on the normal functioning of the intention-programming system. Indeed, having an effective intention to move is essential for triggering a cascade of operations that, in bimanual conditions, affect motor execution of both hands.

2 Garbarini F.

Rabuffetti M.

Piedimonte A.

Pia L.

Ferrarin M.

Frassinetti F.

Gindri P.

Cantagallo A.

Driver J.

Berti A. ‘Moving’ a paralysed hand: bimanual coupling effect in patient with anosognosia for hemiplegia. Figure 1 Ovalization results for the patients and controls. Show full caption Bar plots show the Ovalization Index mean values (with standard error) for each group — controls (C), E− and E+ patients — in each condition. Different lines show the individual mean values for each E+ patient (as single case) in each condition. ∗ P < 0.01; ∗∗ P < 0.001. The two boxes at the top of the figure show examples of right hand ovalization in one control subject performing BCL-c and in one E+ patient performing AEC-c. In the present experiment, hemiplegic patients were asked to draw lines with their right (healthy) hand and to ‘try’ to draw circles with the left (paralysed) hand in three conditions: in the absence of any alien hand (Bimanual-Circles-Lines-condition; BCL-c); simultaneously as the alien left hand drew circles in an egocentric perspective (Alien-Egocentric-Circles-condition, AECc); or in an allocentric perspective (Alien-Allocentric-Circles-condition, AAC-c). As a baseline, we used a Unimanual-Lines condition (UL-c), in which only the right hand drew lines (see Supplemental Information and Figure 1 ). Control subjects were asked to actually move both hands in BCL-c, whereas in the alien hand conditions (AEC-c and AAC-c) they were requested to draw lines with the right hand while the alien hand drew circles. By means of a tablet personal computer, an Ovalization Index was calculated as the standard deviation of the right hand trajectory from an absolute vertical line []. The coupling effect can be described as a significant increase of the Ovalization Index value with respect to the baseline (UL-c).

8 Franz E.A.

Zelaznik H.N.

McCabe G. Spatial topological constraints in a bimanual task. + and E− patients who, being aware of their motor impairment (see Supplemental 2 Garbarini F.

Rabuffetti M.

Piedimonte A.

Pia L.

Ferrarin M.

Frassinetti F.

Gindri P.

Cantagallo A.

Driver J.

Berti A. ‘Moving’ a paralysed hand: bimanual coupling effect in patient with anosognosia for hemiplegia. + patients arises from an abnormal embodiment of an alien hand that automatically triggers the intention-programming processes for their own hand, then, when the alien hand draws circles in the egocentric position (AEC-c, where E+ patients show their delusion of ownership), the lines should be ovalized as in healthy controls actually performing the bimanual task. On the other hand, we should not find any coupling effect in the alien-hand-conditions of E− patients (who, given the absence of any embodiment phenomenon, should not activate any effective motor program for the paralyzed limb) and of control subjects (who, not requested to move their left hand, only program right-hand movements). It is worth noting that these latter conditions were introduced because they are crucial for distinguishing between coupling effects induced by motor activation or coupling effect induced by visual feedback. Indeed, if by simply viewing a hand drawing circles can affect the hand drawing lines, then we should find an interference effect in both AEC-c and AAC-c of control subjects. According to the literature, we would expect to find a coupling effect in BCL-c with normal subjects [], but not with the Eand Epatients who, being aware of their motor impairment (see Supplemental Table S1 ), do not program any movement with their left, paralysed hand []. Moreover, if the delusion of ownership shown by Epatients arises from an abnormal embodiment of an alien hand that automatically triggers the intention-programming processes for their own hand, then, when the alien hand draws circles in the egocentric position (AEC-c, where Epatients show their delusion of ownership), the lines should be ovalized as in healthy controls actually performing the bimanual task. On the other hand, we should not find any coupling effect in the alien-hand-conditions of Epatients (who, given the absence of any embodiment phenomenon, should not activate any effective motor program for the paralyzed limb) and of control subjects (who, not requested to move their left hand, only program right-hand movements). It is worth noting that these latter conditions were introduced because they are crucial for distinguishing between coupling effects induced by motor activation or coupling effect induced by visual feedback. Indeed, if by simply viewing a hand drawing circles can affect the hand drawing lines, then we should find an interference effect in both AEC-c and AAC-c of control subjects.

+ nor E− patients showed a significant coupling effect in BCL-c, confirming the fact that, although requested ‘to try’ to move their left hand, hemiplegic patients, fully aware of their paralysis, do not produce any effective motor programming [ 2 Garbarini F.

Rabuffetti M.

Piedimonte A.

Pia L.

Ferrarin M.

Frassinetti F.

Gindri P.

Cantagallo A.

Driver J.

Berti A. ‘Moving’ a paralysed hand: bimanual coupling effect in patient with anosognosia for hemiplegia. + patients, and only in AEC-c, that is, in the condition where they showed the embodiment phenomenon (P < 0.01 for each comparison; see + patient (P < 0.001 for each comparison). The results ( Figure 1 ) show a significant coupling effect in healthy controls actually performing the bimanual task (BCL-c; P < 0.01 for each comparison; for statistical details, see the Supplemental Information ). On the contrary, neither Enor Epatients showed a significant coupling effect in BCL-c, confirming the fact that, although requested ‘to try’ to move their left hand, hemiplegic patients, fully aware of their paralysis, do not produce any effective motor programming []. Crucially, we found a significant coupling effect in Epatients, and only in AEC-c, that is, in the condition where they showed the embodiment phenomenon (P < 0.01 for each comparison; see Supplemental Movie 2 ). Notably, the interference of the alien left hand movements, performing circles, on the actual movements of the patients’ right hand, performing lines, was not different from the interference found in BCL-c of the healthy controls (P = 0.15). Moreover, in the alien-hand-conditions of healthy controls we did not find any coupling effect. This demonstrates that simply looking at an alien hand drawing circles is not sufficient to induce line ovalization (for a further discussion of this point see the Supplemental Information ). Finally, single subjects analysis confirms the presence of a significant coupling effect in the crucial AEC-c in each Epatient (P < 0.001 for each comparison).