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Cross your index finger over the middle finger of the same hand. Close your eyes and rest the tip of your nose between the two crossed fingertips. Many people feel two noses where there is only one.
This is Aristotle's illusion, which he described more than two thousand years ago. In the usual position, the outer side of the index finger and the outer side of the middle finger cannot touch a single small object at the same time: each surface reports an object in a different place. By crossing your fingers you bring those two surfaces into an exceptional combination, but the learned map keeps its usual interpretation and assigns two causes to two contacts. This first test does not touch the muscle sensors: it touches the map of the skin, which works from the posture proprioception hands it.
It works just as well with a pea or with the rubber on the end of a pencil, and it falls apart the moment you look: opening your eyes is enough for the number of objects to go back to one. Vision wins the argument immediately because it offers a more stable explanation: one nose, two points of contact.
What you have just fooled is called proprioception, the sense of the position and movement of your own body. Now stretch a hand out under the table and move your thumb without looking at it: you know where it is because your brain calculates its position. It has no tape measure inside your arm and receives no continuous photograph. It combines signals from muscles, tendons, joints and skin, picks the ones that are usually reliable and builds an answer before you have to think about it. If those sources tell incompatible stories, it does not stop the system to investigate: it chooses the interpretation that best fits its experience.
There are four more set-ups. You can do the next two on your own; the last two need a helper.
Open your arms wide, close your eyes and slowly bring the tips of your two index fingers together in front of your chest. Do it once, without hunting for them. Most people miss by a centimetre or two: one finger passes above the other, or they cross. Open one eye and repeat: now they meet first time. It is the same test a neurologist asks for in a consulting room, and it does not measure aim, it measures how much your brain trusts what your arms tell it when it has nothing to check them against. With your eyes open it does not need to trust: it looks.
Oskar Kohnstamm described the third one in 1915, and it is still used in the laboratory to study involuntary movement. Stand in a doorway, let your arms hang and push the backs of your hands outwards against the jambs, with steady pressure and no jerking, for a good thirty seconds. Count slowly. When you finish, step forward, out of the frame, and let your arms go dead without trying anything.
In many people the arms start to rise on their own. It is not that you feel like lifting them: they lift, slowly, and if you try to stop them you notice you have to use force to prevent it. An intense, sustained contraction can leave an involuntary after-contraction in the muscles that lift the arm away from the body, and when the jamb disappears that activation no longer finds anything to push against. It does not happen to everyone and how long it lasts varies a great deal from person to person; the arm keeps rising for tens of seconds. In the laboratory the test is done with sight blocked, because looking at the arm introduces a signal that competes with the muscle's. And while the arm goes up, your sense of standing still remains intact.
For the fourth test, find a realistic-looking rubber hand or stuff a glove until it keeps a shape something like your own. You will also need a piece of card, two identical brushes and another person. Sit at a table. Put one of your hands behind the card so that you cannot see it and place the false one in front, oriented the same way and roughly in the position your arm could occupy. Look only at the false one.
Ask the other person to run one brush over a finger of your hidden hand and the other over the same finger of the false hand, at the same time, in the same direction and at the same rhythm. In the original experiment the brushing was kept up for ten minutes; count on several, not on one. Concentrate on the brush you can see. Some of the participants ended up locating the touch on the artificial hand and feeling a degree of ownership over it.
Matthew Botvinick and Jonathan Cohen described that transfer in Nature in 1998. Synchrony is decisive: if the two brushes are out of step, the joint explanation loses its force. Sight says "they are touching that hand" and skin says "I am being touched now"; when both signals agree, the brain can accept that the visible hand is part of the body and shift the felt position of the real hand towards it.
Save the last test for when you have a massage device able to keep up a fast, steady vibration, close to a hundred cycles per second.
Sit down, close your eyes and touch the tip of your nose gently with the fingers of one hand. Keep your elbow bent without straining. Ask the other person to rest the device on your biceps tendon, the firm band near the inner fold of the elbow and never on the joint, for a few seconds, and to take it away if it hurts.
The vibration activates the muscle spindles, the sensors that report changes in the length of the muscle. To the nervous system, that discharge looks like the signal that the biceps is lengthening and the elbow extending. But your fingers are still touching your nose. The brain has to reconcile the two pieces of news: the hand seems to be moving away from the face and, at the same time, is still joined to it by touch. Some people then feel that their nose is stretching, that their fingers are getting longer, or that both are changing shape.
James Lackner published experiments in Brain in 1988 in which muscle vibration altered perceived body shape and orientation within seconds. The so-called Pinocchio illusion is especially clear because it sets a manipulated muscle signal against a facial contact that stays put. Faced with the conflict, the calculation believes the muscle before it believes the face.
The frequency matters, and that is why not any device will do: the sensors that respond like this work mainly between seventy and a hundred and twenty cycles per second, and outside that band the illusion switches off. Change muscle and the direction changes: with the vibration on the triceps, the muscle that straightens the elbow, the brain deduces the opposite, that the arm is bending, and some people then feel their finger sinking into their face.
Close your eyes, cross your index and middle fingers right now and touch the tip of your nose once.