Is it possible that, in the not-so-distant future, we will be able to reshape the human body so as to have extra limbs? A third arm helping us out with chores or assisting a paralyzed person?

Neuroscientists at Karolinska Institutet in Stockholm report a perceptual illusion in which a rubber right hand, placed beside the real hand in full view of the participant, is perceived as a supernumerary limb belonging to the participant’s own body. The effect is verified by physiological evidence obtained from skin conductance responses when physically threatening either the rubber hand or the real one.

The illusion reported here is qualitatively different from the traditional rubber hand illusion. The subjects feel less disownership of the real hand and a stronger feeling of having two right hands. These results suggest that the artificial hand “borrows” some of the multisensory processes that represent the real hand, leading to duplication of touch and ownership of two right arms.

The scientists say this work represents a major advance because it challenges the traditional view of the gross morphology of the human body as a fundamental constraint on what we can come to experience as our physical self, by showing that the body representation can easily be updated to incorporate an additional limb.

Ref.: Guterstam A, Petkova VI, Ehrsson HH (2011) The Illusion of Owning a Third Arm. PLoS ONE 6(2): e17208. doi:10.1371/journal.pone.0017208 (open access)