Can you imagine how life would be like if you had an extra limb? It’s hard to think about isn’t it? But apparently it’s quite easy to convince our brains that we do have a third arm. In fact all it takes is a little brush. Scientists at the Brain, Body and Self Laboratory of the Department of Neuroscience, Karolinska Institutet in Stockholm, Sweden conducted a relatively simple experiment where they were able to make participants feel as if they had a third arm.
In the experiment conducted by Arvid Guterstam, Valeria I. Petkova and H. Henrik Ehrsson, participants were asked to put their right arm next to a prosthetic right arm. As you can see in the left half of the picture below, both hands were simultaneously stroked by a brush at similar points. You’d expect that a participant would only feel the brush on his real hand, but what happens is that their brain gets confused. The blanket hides the fact that only the real arm is connected to the participant, so his brain is comfortable enough with treating BOTH hands as part of the body. Most participants responded that they felt the brush on both hands. Awesome.
To further prove the illusion, the scientists then threatened the confused participants by holding a knife over the fake right arm and then doing the same to their actual arm. The stress response for both events were very similar, again proving that the participants thought that the prosthetic was also their arm. But our brains are not that dumb – when the scientists tried placing a prosthetic left arm or a rubber foot next to a participants’ real right arm, the experiment failed.
So what applications could this result have? Ehrsson says that maybe in the future, stroke patients and other people with paralyzed limbs will be able to use prosthetic arms, or even people who have functional arms but could use an extra limb to do their job more effectively. Check out the source links below for more details on the experiment.