How to really feel like you've got three arms

We can imagine having two heads or three arms, but we can't really comprehend what that would be like...at least, that's what scientists always assumed. But now a new experiment has shown we really can experience three arms at once.

As humans, we have a basic, innate body plan, one that includes one head, two arms, two legs, and so on. Outside of some fairly severe aberrations, nobody has more features than what is included in our body plan, and psychologists and neuroscientists assumed this fundamentally limited our body image, meaning nobody could ever really experience the sensation of having, say, three arms.

But researchers at Sweden's Karolinska Institutet have offered an audacious response to this assumption. They created an experiment where subjects had a prosthetic but realistic rubber arm placed right next to their right arm. The experimenters then started touching both right arms with a brush in the corresponding locations, trying to make the two brushes identical in time and location.

That's when something weird happened, as researcher Arvid Guterstam explains:

"What happens then is that a conflict arises in the brain concerning which of the right hands belongs to the participant's body. 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."

You can see the illusion in action in this video, courtesy of New Scientist. But how do we know the brain really accepted this third arm as part of the body? Once the illusion was established, the experimenters "threatened" either the real or prosthetic hand with a kitchen knife, and then they measured the subjects' physiological response. Real or fake, it didn't matter - the subjects had exactly the same stress reaction, and this only worked when the illusion was in effect.

For all intents and purposes, the subjects' body images had expanded to include a third arm. Researcher Henrik Ehrsson says there could be important medical applications to all this...and then he gets a little crazy with it:

"It may be possible in the future to offer a stroke patient, who has become paralysed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralysed arm remains within the patient's body image. It is also conceivable that people with demanding work situations could benefit of an extra arm, such as firemen during rescue operations, or paramedics in the field."

Via PLoS ONE.