Cari Nierenberg writes: If two hands are better than one, then imagine what you could do with three. In a new study, Swedish researchers were able to trick participants' brains into believing their body had an extra arm.
Brain scientists at the Karolinska Institutet in Stockholm conducted five different experiments on 154 healthy men and women, in the paper published in the February issue of the online journal PLoS ONE. To create the impression of having a third arm, participants sat with their right arm resting on a table and their left arm hidden from view, behind a screen.
A rubber prosthetic right hand was placed beside the person's real hand and a cloth was placed over the participant's right shoulder to hide everything but the forearms on both the real and fake right arms. If the volunteer glanced at their right hand, they appeared to have two of them.
Known to neuroscientists as "the supernumerary hand illusion," researchers used two different paintbrushes to stroke the skin on the index and middle fingers of the participant's real right hand and false one at the exact same time for a minute or two. During this process, participant's were told to look at the artificial limb.
After each brushing session, volunteers completed a questionnaire. Participants reported that they felt the touch of the brush on both the real and fake hand. They also described feeling as if they had two right hands and both limbs felt like a part of their body.
"Our study shows that the human brain has the ability to experience an extra third arm," says Arvid Guterstam, the study's lead author and a neuroscience doctoral student. You might think that being born with two arms and two legs would limit your body image to this idea, he says. "But within less than a minute, you can fool the brain into believing it has an extra arm, which is quite fantastic."
In another experiment, the researchers held a knife close to the volunteer's hand and the rubber hand as if it might pierce the skin. They wanted to see if the body and mind perceived the knife as a threat. The same stress response was seen in both the real hand and the prosthetic one. (The researchers measured the amount of sweat coming from the skin as a measure of emotional arousal of the nervous system; the same response to the threat of the knife was seen whether it was close to touching the real hand or the fake one.)
As for why the mind is so easily duped by the third-hand illusion, Guterstam explains that when a study participant sees a fake right hand next to his own, the brain wonders "Which right hand is mine?"
"Instead of choosing to experience only one hand as your own, we surprisingly found that the brain accepts both right hands as part of the body image, and the participant experiences ownership of an extra third arm," say Guterstam.
While some of these experiments might seem like fun parlor tricks, the results have practical applications.
They could be important in developing advanced prosthetics, suggests Guterstam. The findings, for example, may benefit stroke patients who need an artificial arm because one side of their body is paralyzed. Scientists would better understand how patients can control this extra arm and experience it as their own.