Why You Cannot Tickle Yourself

The story

Try a tiny experiment. Wiggle your fingers, reach out, and tickle your own tummy.

Not much, right? Maybe a small tingle, but almost never the squirming, giggling, proper tickle that gets you laughing. Yet if someone ELSE wiggled their fingers at your tummy, you might be laughing before they even touched you. Same tummy. Same wiggling fingers. Completely different feeling. Why should that be?

The answer is hiding inside your head, and scientists' best explanation is one of the cleverest tricks your brain does. Your brain is a prediction machine. When you choose to move, your brain sends the order to your muscles, and it can whisper a forecast to itself at the same moment: here is roughly what this movement will feel like. When you tickle yourself, your brain has a very good idea of where your fingers will land, and when, and how hard. When the touch arrives and closely matches the forecast, your brain seems to quietly turn the feeling down. Little surprise, little tickle.

But when someone else's fingers come wiggling in, your brain cannot be so sure what happens next. Where will they strike? Now? NOW? That not-knowing seems to be one ingredient in what makes tickling tickle, though scientists have not solved the whole recipe of a proper laughing tickle. It is not the whole story, because where you are touched matters too, and how, and even what mood you are in. But surprise is one of tickling's favourite ingredients.

Scientists have tested this in a laboratory, and they found a wonderful loophole. They built a machine where you move a handle, and the machine strokes your palm for you. When the machine copied your movement exactly, it felt dull, just like self-tickling. But when the scientists made the machine wait, just a fraction of a moment, or stroke in a slightly different direction than you moved, the touch felt more ticklish, people reported. The delay broke the brain's forecast, and the touch, no longer matching it so closely, began to feel more tickly.

And this little giggle-trick reveals something big. Your brain quietly predicts many of the feelings your own actions will cause, all day long. It forecasts the feel of your own footsteps, and even parts of the sound of your own voice, which helps it tell the difference between what YOU did and what the world did. And it can turn expected feelings down, so that new and important things stand out bright and clear. That is useful: the touch your body has already explained is dialled down, and an unexpected touch, like a bug landing on your arm, has a better chance of grabbing your attention fast.

So most people cannot get a proper laugh out of tickling themselves, and honestly, that is nothing to mind. It is an everyday glimpse of the forecast machine at work, smoothing away the expected so the unexpected can shine.

The next time someone's wiggling fingers make you shriek with laughter, you will know one likely ingredient: your own marvellous brain, less able to predict what comes next.