Why you can't tickle yourself

The classic question

Tickling from another person is unbearable. Tickling yourself does almost nothing. Researchers have tested why, and the answer says a lot about how the brain predicts its own movements.

Read also: Why do we dream? What science has found so far and Why is the sky blue (and the sunset orange)?.

The robot experiment

Sarah-Jayne Blakemore, Daniel Wolpert and Chris Frith asked participants to move a robotic arm that touched their own palm. When the touch followed the movement exactly, the sensation was weak. When the researchers added a short delay or changed the direction of the touch, the sensation grew stronger, closer to being touched by someone else.

Source: Blakemore, Wolpert and Frith, Central cancellation of self-produced tickle sensation, Nature Neuroscience, 1998.

Why the brain does that

The brain predicts the consequences of its own movements and turns down the response to them. That helps separate what you cause yourself from what comes from outside, which is useful for noticing an unexpected touch. Tickling depends precisely on surprise.

Tickling and humour are not the same

Charles Darwin suggested that laughing at tickling and laughing at a joke share the same origin. Christine Harris and Nicholas Christenfeld tested it: watching comedy first did not make people more ticklish, and those who laughed most at jokes were not the ones who laughed most at tickling.

In another experiment, blindfolded participants were told a machine would tickle them in some rounds. In practice the touch always came from the same source, and they laughed just as much either way, which suggests the laughter is largely a reflex.

Source: Harris and Christenfeld, Humour, Tickle, and the Darwin-Hecker Hypothesis, Cognition and Emotion, 1997.

Source: Harris and Christenfeld, Can a machine tickle?, Psychonomic Bulletin and Review, 1999.

Rats are ticklish too

In 2016, Berlin researchers tickled rats and recorded ultrasonic calls associated with positive states in those animals, while neurons in a touch-related brain area fired. When the rats were anxious, they responded far less. Context and mood change the response.

Source: Ishiyama and Brecht, Neural correlates of ticklishness in the rat somatosensory cortex, Science, 2016.