The Illusion of Owning a Third Arm

TLDR

Future advances may allow humans to acquire extra limbs, such as a third arm for tasks like grocery shopping or aiding paralysis. The study reports a perceptual illusion in which a rubber right hand placed beside the real hand is perceived as a supernumerary limb of the participant. Four well‑controlled experiments demonstrate the minimal required conditions for eliciting the supernumerary hand illusion. Questionnaire and skin‑conductance data confirm the illusion, and a fifth experiment shows it differs from the classic rubber hand illusion by preserving real‑hand ownership while inducing a stronger sense of two right hands, suggesting the artificial hand shares multisensory processes and demonstrating that body representation can be updated to incorporate an additional limb.

Abstract

Could it be 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 the weekly shopping in the local grocery store, or an extra artificial limb assisting a paralysed person? Here we 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. This effect was supported by questionnaire data in conjunction with physiological evidence obtained from skin conductance responses when physically threatening either the rubber hand or the real one. In four well-controlled experiments, we demonstrate the minimal required conditions for the elicitation of this "supernumerary hand illusion". In the fifth, and final experiment, we show that the illusion reported here is qualitatively different from the traditional rubber hand illusion as it is characterised by 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. 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.