A knife-wielding researcher is bearing down on your right hand—or is it your hand? You see three arms in front of you, and you can feel your palms dampen with fear-induced perspiration. But is it your right hand the kitchen knife is plunging toward, or a false, rubber right hand?

The dilemma might sound ridiculous, but tell that to the 154 healthy adult volunteers who found themselves persuaded into feeling like they really did have three arms. Their uncanny ordeals are detailed in a study published online February 23 in PLoS ONE.

The rubber-hand illusion has been a popular perception experiment for more than a decade. In it, a research subject's real hand is hidden from view while a fake rubber hand is substituted in plain sight. Both hands are simultaneously stroked with a brush until the person's mind has come to perceive the fake hand as part of their body. In some people—especially those prone to a poorly developed body schema—the real hand then starts to get ignored by the brain, marked by a discernable temperature drop. The concept has also helped some amputees alleviate pain in phantom limbs.

But the false-hand illusion has been based on the notion that the brain is maintaining a normal, symmetrical body plan: two arms, two hands. A strange new study throws that model out the window—or at least adds on a new twist. Researchers at the Karolinska Institute in Stockholm, have shown these healthy adults could easily be tricked into feeling as if they had three arms.

This sensory legerdemain was accomplished by placing a false rubber right hand (matched to the subject only in gender) on a tabletop next to the subject's real right hand (with the two index fingers approximately 12.5 centimeters apart). A cloth covered the lower arm up to the shoulder to obscure which hand might actually be connected to the body. With the left hand also on view, an experimenter stroked each right hand in parallel with a small brush.

"What one would expect is that only one of the hands is experienced as one's own, presumably the real arm," Arvid Guterstam, of the Institute's Brain, Body and Self Laboratory and co-author of the paper, said in a prepared statement. "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."

Guterstam and his colleagues then tested the extent to which subjects had adopted this supernumerary limb. After convincing the subjects that the third hand was indeed part of their body with the brush strokes, the experimenter then swiftly picked up a kitchen knife and swiped it toward one of the right hands. A skin conductance response sensor on the left hand was used to detect any flashes of fear-based sweat in response to body threats. Sure enough, sweat glands became active when the knife swept toward the rubber hand as well as when it was approaching subjects' real right hands (even after controlling for any general fear-response upon seeing the knife), showing that the participants had not disowned their real hand as happened in traditional rubber-hand experiments.

The new findings are more than just an odd appendage to the rubber-hand illusion, though. Presumed to be relatively hardwired—by the nervous system and a lifetime of experience—to expect and accommodate two arms, the human body (and our somatosensory cortex) might be far more flexible in what it perceives as part of itself. As the researchers noted in their paper, the results show that, "the body representation can easily be updated to incorporate an additional limb." (Previous research has also shown that the brain starts to incorporate a hand-held tool, such as a hammer, as part of its body schema.)

The mind isn't entirely dupable, though. Exchange the false second right hand for a left hand—or a prosthetic foot—and the brain isn't buying it. No amount of brush stroking and knife waving could trick the participants into sweating that a chest-level foot was about to lose a toe.

And the researchers contend that there might be applications of this phenomenon beyond laboratory trickery. "It may be possible in the future to offer a stroke patient, who has become paralyzed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralyzed arm remains within the patient's body image," Henrik Ehrsson, also of the Karolinska Institute and co-author of the study, said in a prepared statement.

And what about those of us who could just occasionally use an extra hand? "It is also conceivable that people with demanding work situations could benefit [from] an extra arm," Ehrsson said. But as even the researchers pointed out, it's unclear whether the real right hand has to know what the fake right hand does—or whether the brain would be able to tell them to reach for different condiments.


Image courtesy of Henrik Ehrsson; video courtesy of Kjell Erlandsson/Karolinska Institute