Brain Scientists Create Illusion of Third Human Arm

The human brain can be tricked into believing it has three arms, Swedish scientists said Thursday, giving hope to the advancement of prosthetic limbs for paralyzed stroke victims.

Scientists from the Karolinska Institute were able to convince 154 healthy volunteers that they owned multiple limbs -- to such an extent that participants broke out in a sweat when their fake limbs were threatened with a knife.

During the experiment, participants had a realistic prosthetic arm placed next to their right arm, and researchers stroked the real hand and fake hand with two small brushes in the same place. By synchronizing the strokes, the volunteers’ brains were tricked into feeling that the false arm was part of the body.

"A conflict arises in the brain concerning which of the right hands belongs to the participant's body," said researcher Dr. Arvid Guterstam. "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 threatened either the prosthetic hand or the real hand with a kitchen knife while measuring the amount of sweating from the palm. They found results the participants had the same stress response when the prosthetic hand was threatened as when the real hand was in danger.

"It may be possible in the future to offer a stroke patient, who has become paralyzed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralyzed arm remains within the patient's body image," said Dr. Henrik Ehrsson, who led the study.

The research was published in the journal Public Library of Science ONE.