Researchers create “third arm” illusion
Scientists at the Swedish medical university Karolinska Institute say they have succeeded in giving test subjects the illusion of having an extra arm. While still in its infancy, the research may someday lead to better prosthetics for amputees and improved treatment for brain damage caused by stroke.
Psychologists and neuroscientists have long wrestled with questions about how we experience our own bodies, and the conventional wisdom is that this image is limited by our innate body plan - in other words that we cannot experience having more than one head, two arms and two legs. But now the Karolinska scientists have shown that it’s possible to make healthy volunteers experience having three arms at the same time.
In a paper published today in the online scientific journal PLoS ONE, the researchers describe an experiment in which the participant sits at a table with a realistic prosthetic arm in plain view next to his or her right arm. The scientist touches the subject's right hand and the rubber hand simultaneously with two small brushes at corresponding locations.
"What happens then is that a conflict arises in the brain concerning which of the right hands belongs to the participant's body", says Arvid Guterstam, one of the scientists behind the study. "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."
A total of 154 of healthy volunteers were tested. To prove that the prosthetic arm was truly
experienced as a third arm, the scientist 'threatened' either the prosthetic hand or the real hand with a kitchen knife and measured the degree of sweating of the palm as a physiological response.
Surprisingly, the stress response proved to be the same whether the real or the fake hand was threatened, but only during the periods when subjects were reporting the third arm illusion.
"It may be possible in the future to offer a stroke patient, who has become paralysed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralysed arm remains within the patient's body image," says Henrik Ehrsson, who led the study at the Department of Neuroscience. "It is also conceivable that people with demanding work situations could benefit of an extra arm, such as firemen during rescue operations, or paramedics in the field."