Well - Tara Parker-Pope on Health
February 24, 2011, 1:59 pm
Need an Extra Hand?
By TARA PARKER-POPE
Doctoral student Arvid Guterstam conducts experiments aimed at tricking the brain into believing the body has three arms.

If you’ve ever felt like you needed an extra hand around the house, a bizarre new study may have found a way to make it possible.

Researchers at the Karolinska Institute in Stockholm conducted an unusual experiment aimed at creating the illusion — and physical sensation — of having a third arm. In five separate laboratory experiments, 154 volunteers were seated with their hands on a table and a rubber arm was placed next to their right arm. A sheet covered their shoulders and elbows, creating the illusion that the person had three arms.

The scientists then gently brushed the real and fake hands. Sometimes the brushing occurred simultaneously, while other times the hands were touched separately. When the brushing occurred at the same time, the respondents were far more likely to report feeling the sensation in the rubber hand as well and even feeling as if they had two right hands.

In one part of the experiment, both the real hands were “threatened” with a knife after the brushing exercise. The presence of the knife near the rubber arm caused the participant to flinch, and monitors on the left hand indicated a similar stress response as when the real hand was targeted.

The research, led by doctoral student Arvid Guterstam and Henrik Ehrsson, an associate professor in the department of neurology, is described in a paper published in the online scientific journal PLoS ONE.

Mr. Guterstam said about 7 out of 10 participants experienced the sensation of having a third hand. The reason may be that the visual illusion creates a conflict for the brain. But instead of settling on just one arm as the real thing, the brain accepts both right hands as part of the body, causing the subjects to experience the sensation of having a third arm.

Mr. Guterstam said he and his colleagues were surprised at the power of the visual illusion to trick the brain, suggesting that the brain seems to continuously construct a “dynamic model of our own body.”

“The fact that it does work tells us something very deep about how our perceptual systems operate,’’ he wrote in an e-mail. “Rather than choosing one solution, the brain seems to accept that it is quite likely that each of the two hands are part of one’s body, and therefore an internal body model is constructed that includes both hands. Consciously people experience this as both hands being their own.”

The findings may one day be used to improve the experience of wearing a prosthetic limb, or it could be used to help people with paralysis learn to operate robotic arms.

“This might have important bearing on the development of advanced prostheses, where the patient can experience and control an extra robotic arm,’’ said Mr. Guterstam. “Such prosthesis might be five or 10 years in the future, but our research is at least demonstrating that this is not an impossible idea.”

You can watch a portion of the experiment in the video below. And for more on a related topic, read about how doctors are using mirror illusions to stop phantom limb pain.

NYT_VideoPlayerStart({playerType:"blog",videoId:"100000000658567",adxPagename:"well.blogs.nytimes.com/video"});