What do you think of this fact?
Go ahead and try it: run your fingers over the sole of your foot or under your armpit. Nothing? Then you’re perfectly normal. Let someone else do it and you’ll probably be squealing right away. Same touch, completely different feeling.
Your cerebellum is a spoiler
Neuroscientist Sarah-Jayne Blakemore of University College London looked into this. According to her, the answer sits at the back of your head, in the cerebellum, which monitors your movements. When you plan a movement, the cerebellum gets an internal copy of the command, known as an efference copy. That lets it predict pretty precisely what’s about to happen.
When you try to tickle yourself, the cerebellum predicts the sensation, and that prediction is used to turn down the response of other brain areas to the tickle. With someone else’s hand, that doesn’t work, because your brain can’t predict their movements. So a stranger gets the full tickle, and you get almost nothing.
The robot trick
In 2000, Blakemore, Daniel Wolpert and Chris Frith published a study on this in the journal NeuroReport. The 16 participants used their left hand to move a robot arm that stroked something soft across their right palm. The catch: the researchers could add a delay between movement and touch, of 100, 200 or 300 milliseconds. Or they could rotate the movement by 30, 60 or 90 degrees.
The result: as the delay grew from 0 to 200 milliseconds, the self-made touch felt more and more tickly. Same with rotating the direction from 0 to 90 degrees. Once prediction and reality stop matching, your brain treats you like a stranger. As Blakemore put it, it might be possible to tickle yourself if you’re willing to invest in a couple of robots.
There’s now even a dedicated tickle robot: Hektor is made of three electric motors, slides probes along people’s soles, and they rate each stroke on a scale of 1 to 10.
So your brain knows you so well that it ruins the fun. When it comes to tickling, you simply can’t beat yourself.