Why Can't You Tickle Yourself?

Try it now, if nobody is watching. Run a fingertip lightly along the sole of your foot, or wiggle your fingers under your own ribs. Almost certainly, nothing happens. You feel the touch perfectly well, but the helpless, squirming laughter that arrives the moment someone else does exactly the same thing refuses to turn up. It is the same finger, the same skin and the same spot. The only thing that has changed is whose hand it is.

People have puzzled over this for a very long time. Aristotle raised the question more than two thousand years ago, and suggested that tickling makes us laugh because it catches us by surprise. Scientists now split the sensation into two kinds. Knismesis is the light, crawling feeling of a feather or an insect wandering across the skin, a sort of moving itch. Gargalesis is the harder, deeper prodding that makes people laugh, and it is the one that seems impossible to do to yourself.

Surprise turns out to be close to the heart of it. A tickle seems to need a little uncertainty: where the next poke will land, and when. When you are the one doing the poking, there is no uncertainty at all. Your brain knows the plan before your finger has even moved. Which raises a strangely philosophical question. Can you ever truly surprise yourself, or is some part of you always one step ahead, quietly reading the script?

There is a clue in an unexpected place. In a 2000 study, Blakemore and her colleagues found that psychiatric patients who hear voices or feel that their actions are controlled from outside rated their own touch as just as ticklish as someone else's. The difference went with those experiences, not with the diagnosis of schizophrenia as such, and the researchers linked it to a breakdown in the brain's tracking of its own actions. In other words, when the brain is less sure that a movement is its own, the tickle comes back.

The clearest answer comes from Sarah-Jayne Blakemore and her colleagues at the Institute of Cognitive Neuroscience at University College London. They built a machine that let volunteers tickle themselves at one remove. Pressing a button made a soft piece of foam stroke the palm of the hand, and the researchers could add a delay of up to two seconds between the press and the stroke. The longer the delay, the more tickly it felt. Brain scans told the same story. Two areas, the somatosensory cortex, which handles touch, and the anterior cingulate cortex, which processes pleasant information, were less active when people tickled themselves than when the touch came from someone else.

The reason lies in the cerebellum, at the back of the brain. Every time you plan a movement, your motor system sends it a copy of the instructions, known as an efference copy, so that it can predict exactly what the movement will feel like. That prediction is then used to turn down the response to the sensation when it arrives. A stranger's fingers are unpredictable, so nothing gets cancelled. As Blakemore once put it, it might be possible to tickle yourself after all, if you are willing to invest in a couple of robots.