There’s no sensation quite like being tickled. Whether it’s done playfully by a romantic partner or mischievously by an annoying sibling, you can’t help but giggle — even if the laughter is against your will. So why can’t you tickle yourself? The answer says something much larger about how your brain knows where you stop and everything else begins.
The Brain’s the Boss
You take it for granted that the hand that’s touching the device you’re using to read this is part of your body and the device itself isn’t. But that’s not a given: Your brain is wired with mechanisms that help it differentiate between you and the world around you. When those mechanisms go haywire — as they can in disorders like schizophrenia — it can make people think that their arms or legs are not their own, or that some outside force is guiding their actions.
Your brain keeps tabs on your body with all of its senses, not least of all the sense of touch. Every time you stroke a cat or type on a keyboard, a flurry of messages are zipping from your brain to your hand and back. When your brain tells your hand to move, it makes two copies of the command: one to move your hand, the other as a reference to help it make predictions for what will happen next. Once that movement happens, your hand sends a signal back to your brain to let it know how things went — the cat was soft; I typed “peeface” instead of “preface” — which your brain compares to its reference copy. If all goes well, there will be no surprises: It predicted that the cat would be soft, and it was. End of story.
You can see just how central your brain’s motor commands are to it knowing what your body parts are doing with a little experiment. First, look up and move your eyes around. You’ll probably see different parts of one still scene. Because your brain is telling your eyes to move, it knows that the movement is coming from your eyes, not the world. Now take two (clean!) fingers and gently press on both of your eyelids in the same direction. The scene moves, doesn’t it? That’s because your brain didn’t tell your eye muscles to move, so it didn’t make the same predictions about what you’d see.
You can see where this is going: Your brain knows what to expect when you move your hand to tickle yourself. But while this might sound similar to the way knowing the punchline to a joke makes it less funny, that’s not the only phenomenon at play here; there’s something else going on when you tickle yourself that makes it feel different than someone else doing it.
In a classic study from the year 2000, neuroscientist Sarah-Jayne Blackmore scanned people’s brains to see how they reacted when tickled by a robot versus their own hands. But there was an extra twist: A third condition had the participants control the robot, but with various amounts of electronic delay or movement discrepancies. That is, they might remote-control the robot hand to tickle themselves, but they’d actually feel it a few hundred milliseconds late or from a slightly different angle.
As you might expect, the participants rated the original robot tickle (the one they didn’t control) as most ticklish and the self-tickling as least. But what was most fascinating was that they reported a gradual increase in the ticklishness of the robot hand they controlled as the delay and movement changes increased. According to Blackmore and her team, that shows that it was the discrepancy between what the participants’ brain expected and what actually happened that led to the ticklish feeling.
Brain scans showed why this prediction is important. When people tickled themselves, there was a stark deactivation in three brain regions: the somatosensory cortex, which controls the feeling of touch; the anterior cingulate gyrus (ACG), which has roles in emotion and positive reinforcement; and the cerebellum, which is responsible for movement. Interestingly, there was no deactivation in the cerebellum when participants moved their hands without tickling themselves, suggesting that predicting a sensation results in a different brain response than just predicting movement. When someone else tickled them, on the other hand, the participants’ somatosensory cortex and ACG lit right up.
It’s important to note that all of the tickling in that study was done with objects (soft foam, to be precise), not via skin-to-skin contact. In January, Swedish scientists made an important update to this nearly 20-year-old discovery by having experimenters touch participants with their own hands instead of with objects as the participants had their brains scanned. They found that when participants touched themselves versus being touched by experimenters, a wave of deactivation could be detected even before the sensory messages reached the brain.
“We saw a very clear difference between being touched by someone else and self-touch,” said principal author Rebecca Böhme in a statement. “In the latter case, activity in several parts of the brain was reduced. We can see evidence that this difference arises as early as in the spinal cord, before the perceptions are processed in the brain.”
When you try to tickle yourself, it’s not just your brain that puts a stop to the fun — the whole nervous system rains on your parade, too. But don’t despair; it’s all a sign that your brain knows which parts are you and which parts are the outside world. There are people who can tickle themselves, but it’s usually because of a psychotic disorder and generally comes with side effects like auditory hallucinations and feeling as if they’re not in control of their own bodies. Not being able to tickle yourself is a small price to pay for feeling that sense of control. And anyway, we’re sure a friend is willing to do you the favor.