Mind-bending body illusions

The “rubber hand illusion” – is one of the weirdest and most-studied illusions in neuroscience. As Lawrence Rosenblum at the University of California, Riverside shows in the above video, it’s surprisingly easy to fool people that a rubber hand is somehow their own. Place a rubber hand strategically in front of someone, hit it with a hammer, and people react as if their real hand were in real danger. At the same time, their heart rate leaps, and they break into a sweat – physiological reactions that you don’t see when people take a sham version of the experiment.

Why do we react this way? The eerie feeling that you have swapped hands is thought to be caused by the way our brains combine information from the different senses, and compare it to our internal map of how the body should look.

Most of the time, the process works perfectly, but in cases like the rubber hand illusion, it begins to confuse two different inputs: the touch on the real hand seems to correspond to the movements on the rubber one, so the brain wrongly updates its internal map with the rubber fake and assumes it as its own.

‘Marble-hand illusion’
Perhaps unsurprisingly, this creepy effect was discovered at a Halloween party by student Matthew Botvinick. Taking a fake hand – one of the props from the party – he placed it in front of his body while hiding his real hand from view, and he asked a friend to stroke both of the hands simultaneously. The growing sensation that the plastic hand was his own was reportedly so spooky that he threw the hand across the room.

Since then we’ve discovered other weird ways to warp our minds in this manner. Some sound like they are straight out of Alice in Wonderland. A team in Sweden, for instance, have shown that a similar set-up can fool your brain into inhabiting a Barbie doll or a giant mannequin, so they felt like their body had shrunk or grown.

Others have studied the “enfacement” illusion; in this case, two people sit in front of each other while a third strokes their cheeks in synchrony – slowly leading them to feel that they are really looking at a reflection of themselves. When asked to look at photos, for instance, they are more likely to mistake their partner’s features for their own.

Perhaps the strangest experiment so far is the “marble hand illusion”. Rather than using a rubber hand, scientists tapped a real hand with a small hammer, while out of sight they played the sound of a hammer hitting marble. Slowly, the participants began to feel that their hand was stiffer, heavier, and harder. Crucially, that also corresponded to reduced sensitivity in the hand – they were less likely to break into a sweat when the researcher moved a needle towards their finger, for instance.

One giant leap for mannequin - can your mind make a dummy out of you? (Thinkstock)

There is a serious point to all of this research, since it helps show that our sense of self is far more fluid than you might imagine. We might experience something similar, naturally, when someone mimics our body language – leading us to blur identities a little so we like them more. Along these lines, some scientists are looking at whether theseembodiment illusions could alter things like implicit racism; you might be less prejudiced if you felt that you had lived in another person’s body for a little while.

For the average person, however, you might find more inspiration from Botvinick’s first experiment. Why not try them for yourself and see if you can spook your friends into believing that they have swapped bodies or turned to stone?