Why your fingers can't tickle you

Your brain refuses to be surprised by your own hand. Try to tickle yourself, and the sensation is cancelled before it arrives.

The cancellation happens in the cerebellum, the dense lobe at the back of the skull that, among other things, predicts the sensory consequences of your own movements. Every time you reach for something, the cerebellum issues a small forecast to the rest of the brain: *here is what your fingers are about to feel*. When the real signal comes in matching the forecast, the brain quietly subtracts it. This is why you don't feel the weight of your own clothes most of the day-the brain effectively filters out these expected sensations to focus on new information.

A tickle is a survival reflex. It evolved to flag the unexpected touch of something small crawling on you - an insect, a parasite, a snake. For that alarm to mean anything, it has to fire only when the touch is genuinely unpredicted.

A 1998 experiment at University College London proved the point with a robot arm. Volunteers used a lever to control a soft probe that tickled their own palm. When the robot moved in real time with the lever, the tickle vanished. When the researchers added a delay of as little as a fifth of a second, breaking the prediction, the tickle came back.

The feeling you cannot give yourself is the feeling your brain has decided is not worth your attention.