The Tickle Your Brain Cancels
Everyone at the table can test this with their own hands in seconds, and the answer reveals something deep: your brain is constantly predicting your own body and editing what you feel.
- • In 1998-1999, Sarah-Jayne Blakemore and colleagues in London used a robot that people controlled with one hand to stroke their other palm; adding a tiny delay or a change of direction made the self-made touch feel more tickly, and the bigger the mismatch, the stronger the tickle.
- • Brain scans showed the touch-feeling area, the somatosensory cortex, was more active when the touch came from outside; the cerebellum at the back of the brain seems to make the prediction that cancels your own touch.
- • The same prediction trick may be why we barely notice many sensations our own movements make, like our vocal cords buzzing as we talk. Scientists still don't know every detail of how the brain switches the feeling off.
Why can your sister make you shriek with one finger, but you can't make yourself giggle at all? Try it right now. Tickle your own palm. Nothing, right?
People have puzzled over this for more than 2,000 years. The Greek thinker Aristotle guessed it was because you can't surprise yourself. He was close.
Here's how it works. Step one: every time you move, your brain makes a guess about what the move will feel like. Step two: the real feeling arrives. Step three: if it matches the guess, your brain turns the volume down. Your own touch shrinks to a whisper. Someone else's finger? No guess, no shrinking. Full blast.
In London in the late 1990s, scientists proved it with a tickle robot. People moved a robot arm with their left hand, and it stroked their right palm. Boring. Then the scientists sneaked in a tiny delay, or made the robot wobble off in a different direction. Suddenly it tickled. The bigger the gap between the guess and the feeling, the more it tickled.
A part at the back of your brain, the cerebellum, seems to make that guess. It may be why you barely notice your own throat buzzing when you talk. Try it: hum and listen.