Why Are Our Brains Susceptible to Body Illusions?

Are you bored with your body? Is bilateral symmetry feeling a little stale?

You’re in luck. You don’t need cyborg technology to give yourself an upgrade. Just grab a buddy, a towel, and a rubber hand. Line up your real hands and the fake hand on a table. Drape the towel over one arm and the rubber wrist so that it looks like you have two arms growing out of one shoulder. Then ask your friend to poke the hand on your mutant side with synchronized, unpredictable movements. As you watch, you will suddenly feel as if the rubber hand is your own.

Intrigued? You’re not alone. When a variant of the Third Hand Trick was discovered in the nineties, other psychologists jumped into the illusion game.

For inspiration, they turned to the classic Pinocchio Effect. By standing behind a friend and applying random, synched touches to the tip of your and your partner’s noses, you make it seem that you have sensation in your friend’s nose. When confronted with this bizarre scenario, your brain concludes that your nose must have stretched three feet. From there, things got Frankensteinian.

Using only a mirror, Dr. V. S. Ramachandran started resurrecting the lost limbs of amputees, thus relieving their excruciating phantom pain.

And Henrik Ehrsson began erasing the boundaries of the self. Using cameras, video goggles, and the synchronized poking technique of the Third Hand Illusion, you can now inhabit a mannequin, doll, or 30-foot dummy.

Not impressed? He can even strip you of your body entirely. Strap on some video goggles and position a camera behind you. While you stare at the back of your own head, have your some friends poke your chest and the camera’s stand in time. Soon, you’ll zip backwards out of your meat sack.

But how? How can a few seconds of trickery rewrite your body schema? It’s because you don’t have a fixed schema. Your brain is making it up as you go along. You perceive the world with two tools—bottom-up and top-down processing. Bottom-up processing starts with your senses and builds understanding from there. (Look, something white and round. Oh, it’s your mug.) Top-down processing applies assumptions to the environment. (Your mug is always on your desk, so that thing by your elbow must be your mug.)

You usually experience your body with top-down processing. There’s no reason to continually check that you haven’t sprouted a new hand. But body illusions challenge your assumptions. When the synchronicity of the poking makes it seem that you have nerves somewhere they don’t belong (a rubber hand, a friend’s nose, a reflection, a mannequin, a camera stand, etc), your bottom-up processing kicks into gear. It thinks, “If it has your nerves, it must be part of you!” Your top-down and bottom-up processes battle it out, and in most cases, bottom-up processing wins. But, really, you win.

Because if you ever do acquire a third hand, your brain will be ready.