Out on a limb with extra arms and legs

THE brain can be tricked into experiencing a prosthetic as a normal part of the body, even if it is an extra limb such as a third arm, scientists have demonstrated.

While the accepted belief is that the innate “body plan” of one head, two arms and two legs governs the brain’s perception, Swedish neuroscientists showed people can experience an illusion of having an extra limb.

In an experiment with 154 healthy volunteers, researchers stroked a person’s right arm and an adjacent realistic prosthetic arm at the same time.

This created a multisensory illusion, confusing the brain about which limb belongs to their body.

They found the brain typically resolved the conflict by accepting both right arms as part of the body with the subject experiencing ownership of a third arm.

They proved this by ‘threatening’ the rubber arm with a kitchen knife, eliciting the same stress response from the person as