The brain usually has a pretty good idea of what is part of the body and what is not—although the classic rubber hand illusion can convince people to adopt a fake hand as their own when one of their real hands is hidden from view. Researchers at the Karolinska Institute in Stockholm have added a strange new twist to this experiment, persuading volunteers to believe that they have three hands rather than two.

The psychologists accomplished this sensory legerdemain by placing a false rubber right hand next to the subject’s real right hand and covering both with a cloth from the wrist up (to obscure which one was connected to the body). With the left hand also in view, an experimenter stroked each right hand in parallel with a small brush—a technique that tricks the brain into “feeling” the touch on the fake hand. The experimenter then swiftly picked up a kitchen knife and swiped it toward one of the right hands.

Participants reacted with a flash of fear regardless of whether the knife was plunging toward the real or rubber right hand, indicating that the brain had started to think of the false hand as part of the body, too.

The findings, which were published online February 23 in PLoS ONE, sug­gest that the nervous system—and a lifetime of experience—may not in fact hardwire our somatosensory cortex to expect and accommodate just two arms. The brain might be far more flexible in what it can perceive as part of the body. This discovery could one day help create operational pros­thetics for paralyzed stroke patients or people who could just use an extra hand on the job.

The mind is not entirely dupable, though. Exchange the false right hand for a left hand—or a prosthetic foot—and the brain does not buy it. No amount of brushstroking or knife waving could trick subjects into sweating that a chest-level foot was about to lose a toe.