Cognitive neuroscientists at the Brain, Body & Self Laboratory, Department of Neuroscience, Karolinska Institutet (KI), Stockholm, Sweden, have demonstrated that people can experience the perceptual illusion of having a third arm. According to the scientists, the results have bearing on the research field of neuroprosthetics because while effort has been made to design brain-machine interface prosthetic systems, the problem of how to achieve somatic perception of prostheses has not yet been tackled. Thus, say scientists Arvid Guterstam, medical and predoctoral research student, Valerie Petkova, MSc, doctoral student, and Associate Professor H. Henrik Ehrsson, MD, PhD, body-ownership illusions may contribute to the incorporation of future advanced limb-prostheses into the body-representation.
A total of 154 healthy volunteers were tested in five experiments using variants of the “rubber arm illusion.” Participants observed a rubber hand being touched in synchrony with touches applied to their real hand, hidden out of view, which created an illusory experience that the applied touches were felt on the rubber hand and that the rubber hand was their own. Results of the study demonstrate that the perceptual illusion of having a third arm can only be induced by synchronous tactile stimulation of a person’s real hand and an artificial limb that matches the former in respect of limb type (i.e., the illusion does not work with a rubber foot), laterality (i.e., left and right), and anatomical alignment. The perceptual illusion also involves stronger duplication of touch and ownership of two right hands, accompanied by a weaker feeling of disowning the real hand. These results are important because they demonstrate that the central nervous system, under certain conditions, when faced with two equally probable locations of a seen limb, can “split” the limb representation in two, making people experience a supernumerary limb as being part of their own body.
“What happens…is that a conflict arises in the brain concerning which of the right hands belongs to the participant’s body,” Guterstam said in a KI press release. “What one could expect is that only one of the hands is experienced as one’s own, presumably the real arm. But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of the body image, and the subjects experience having an extra third arm.”
The scientists conclude that it could be possible to develop supernumerary limb prostheses and that their findings provide a “proof of concept” that the central nervous system has the capacity to represent more than two upper limbs at the same time. Thus, paralyzed patients could experience a supernumerary prosthesis as part of their own bodies while maintaining ownership of their real limbs. Or a person with a functionally impaired arm could perform everyday tasks using a supernumerary arm prosthesis.
The paper was published February 23, 2011, in the online scientific journal PLoS ONE.
Editor’s note: This story has been adapted from materials provided by PLoS ONE.