YOU may know the crossed-hands illusion. Hold your arms out in front of you and cross them over, rotate your hands so your palms face each other, then mesh your fingers together. Now slowly rotate your hands up between your arms so you're staring at your knuckles. Ask someone to point to one of your index fingers, then attempt to move it. Did you move the wrong one?
If so, you've just experienced a minor failure of your body schema - your mental representation of the location, position and boundaries of your body. Your brain builds this up by drawing on data from vision, touch and a body-wide network of proprioceptive sensors that monitor position. Your body schema is a critical part of self-awareness, which is why it feels so odd when it goes wrong.
In the crossed-hands illusion, the schema fails because of a confusing visual input. You don't normally see your hands in this convoluted position; the finger you move is the one that is pointing in the direction that the correct one would be pointing if you had simply clasped your hands as if in prayer.
An even odder way of disturbing your body schema is an illusion that taps straight into your sense of body ownership. Known as the rubber-hand illusion, it fools you into thinking a rubber hand - or even a piece of wood, or a table - is part of your body.
To experience the illusion, get hold of a model hand (it doesn't have to be very realistic) and put it on the table in front of you. If it is a left hand, put your actual left hand somewhere you can't see it, in the same pose as the rubber hand. Now get someone to touch and stroke your unseen hand and the rubber hand with identical movements. If you concentrate on the rubber hand, you will probably get the uncanny feeling that it is your own. (See a video of the rubber hand illusion here)
What this illusion shows is that your sense of body ownership is less anchored in reality than you think. Your brain will happily override information from proprioception to conjure up an incorrect yet coherent body schema based on vision and touch.
In fact, your mental body map is an absolute sucker for visual information. This year Frank Durgin of Swarthmore College in Pennsylvania set up the illusion as described above but instead of touching the rubber hand he merely "stroked" it with light from a laser pointer, leaving the unseen hand alone. Two-thirds of 220 subjects reported a sense of ownership of the rubber hand and said they had the sensation of heat and even touch from the laser pointer (Psychological Science, vol 18, p 152). "It's obvious the hand is rubber - no one is fooled at all," says Durgin. "But if your brain decides it's your hand, all the conscious awareness in the world won't change it."
If you can't get hold of a fake hand, there are other (though less reliable) ways to experience the illusion. Some people can be fooled into believing a piece of wood has replaced their hand. Around half of people can even be made to feel a table top is part of their body. Sit at a table and put your hand out of sight underneath. Get someone to tap and stroke this hand while doing exactly the same to the table top directly above. If you watch the table top, you may experience the illusion that the table has become part of your body.
Proprioception may be the junior partner to vision and touch in creating your body schema, but it still plays a key role. You can demonstrate this with an illusion that taps into proprioception alone. This Pinocchio illusion is hard to do without a specialist piece of equipment called a physiotherapy vibrator, but if you can get hold of one, try this. Close your eyes, touch the tip of your nose and then get somebody to apply the vibrator at about 100 hertz to skin at the very top of your bicep. This creates the strong sensation that you are straightening your elbow, and that your nose is simultaneously growing longer and longer, like Pinocchio's.
Vibrating the skin above a tendon excites stretch receptors in the muscle, creating a powerful sensation that the muscle is stretching and the joint is extending. This confuses your proprioceptors, which revise your body schema accordingly. The result is rather like having a phantom limb: the sensed position of your arm in space doesn't correspond to its actual position.
If you're touching your nose at the same time, this leads to a weird sensation that it is growing. Your brain integrates the touch sensation from your fingers with the "movement" of your arm and comes to the erroneous conclusion that your nose must be growing to fill the gap.
The Pinocchio illusion is an important tool for understanding how the brain calculates the size and shape of our bodies. This isn't just an academic question. When it goes wrong, such as in body dysmorphic disorder, anorexia and phantom limb, the results can be devastating (PLoS Biology, vol 3, p e412).