Go ahead and try it. Wiggle your fingers under your own arm or across the sole of your foot. Chances are you'll feel the touch, but you won't giggle. Yet when a friend does the exact same thing, you may squirm and laugh until you can barely breathe.
The short answer is that your brain sees it coming. When you move your own hand, your brain predicts exactly what the touch will feel like and turns the sensation down. Tickling seems to need an element of surprise, and you simply can't surprise yourself.
Your Brain's Built-In Forecast
Every time you send a command to move, your brain keeps a kind of carbon copy of that command. Scientists call it an efference copy. The brain uses it to forecast what your body is about to feel.
When the actual feeling matches the forecast, the brain dials it down. That's handy in everyday life, since it helps you tell the difference between touching something and being touched. But it also means your own fingers can't deliver the shock that makes a tickle work.
Brain scans have pointed to a key player. In 1998, a team led by researcher Blakemore used MRI to study self-tickling. Their work suggests that the cerebellum, a region at the back of the brain, sends precise information about where the touch will land and what it should feel like, and then the sensation is reduced.
The Tickling Robot Experiment
To test the idea, researchers had people tickle their own feet using a robotic arm controlled by their own movements. As expected, the self-made tickle felt much less tickly than one produced by an outside source.
Then came the clever twist. When the scientists added a short delay between a person's movement and the robot's touch, the tickle started to feel stronger. The delay broke the brain's forecast, so the sensation felt more like it came from someone else.
In other words, a tickle isn't just about the touch. It's about whether your brain saw it coming.
Two Kinds of Tickle
Scientists actually describe two different tickles. One is a light, crawling feeling, like an insect walking across your arm. It's called knismesis, and it usually makes you twitch or scratch rather than laugh. Many animals feel it, which is why a dog's leg may start kicking when you scratch the right spot.
The other, called gargalesis, is the laugh-out-loud kind caused by firmer pressure in sensitive spots. It's thought to be mostly limited to humans and other primates, though some research suggests rats can be tickled this way too.
Surprisingly, the most sensitive skin isn't always the most ticklish. Your palm is far more sensitive to touch, but most people find the soles of their feet the most ticklish. The armpits, neck, under the chin, and ribs follow behind.
Darwin Noticed It First
Long before brain scanners, Charles Darwin was puzzling over tickling. He noticed that for a tickle to work, the person being tickled must not know exactly where the touch will land. He reasoned that this is why most people can't tickle themselves, and modern science has largely backed him up.
There are rare exceptions. Researchers have found that some people with schizophrenia can tickle themselves, possibly because their brains track their own actions differently.
So if you've ever been frustrated that you can't tickle yourself, take it as a compliment to your brain. It's simply too good at predicting your next move to be fooled.
