You can't tickle yourself

Go ahead and try: drag your own fingers across your ribs or the sole of your foot. Nothing. Hand the job to someone else and you’ll twist away laughing. The same touch, the same nerves, completely different result. Why?

The answer lies in prediction. Every time you move, your brain sends a copy of the motor command, an efference copy, to the cerebellum, the dense structure at the back of your skull. The cerebellum uses it to forecast exactly what that movement should feel like, then subtracts the expected sensation before it reaches your awareness.

A landmark 1998 study by Sarah-Jayne Blakemore and colleagues, published in Nature Neuroscience, put numbers to it. Using brain imaging, they showed that self-produced touch is perceived as less ticklish than identical touch delivered externally, and that the cerebellum supplies the signal that cancels the response.

The cerebellum predicts the specific sensory consequences of movements, providing the signal used to cancel the sensory response to self-generated stimulation.

In a clever twist, the researchers built a robot that let people “tickle themselves” with a delay. Add a fraction of a second of lag, and the prediction no longer matched reality, so the touch suddenly felt ticklish again. The brain isn’t reacting to the world; it’s constantly guessing it, and only the surprises get through.