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Why You Can't Tickle Yourself

Your Body · 9 min

Why You Can't Tickle Yourself

Try to tickle your own ribs. Nothing. Now get somebody else to do exactly the same thing and you'll be on the floor. Same fingers, same ribs — so what changed?

Go on, try it

Find the spot on you that is most ticklish. Ribs, neck, the sole of a foot — whichever one makes you shriek when somebody finds it.

Now tickle it yourself. Properly. Same speed, same pressure, same fingers you'd complain about if they belonged to your brother.

Nothing happens.

This is genuinely strange, and it is worth stopping on for a second. The skin is the same skin. The nerve endings in it are the same nerve endings. The fingers are, if anything, better informed about where the ticklish spot actually is. Every physical part of the situation is identical — and the feeling is completely different.

The answer everybody gives

Ask around and you will get the same reply nearly every time: you can't tickle yourself because you know it's coming. No surprise, no tickle.

It sounds obviously right. It is also testable — and once somebody tested it properly, it fell over.

Here's how they did it. Researchers built a rig where you hold a lever in one hand, and the lever drives a small robot arm that strokes a piece of foam across your other palm. You are still the one doing the tickling. You know exactly what is about to happen, because you are the one making it happen.

Then they added one thing: a delay. The robot copied your movement, but a fraction of a second late.

The experiment, run by run

In every single row below, the person knows a touch is coming — they are the one causing it. The only thing that changes down the table is the gap between their movement and the robot's copy of it. Press to run the next version.

Somebody else works the leverVery ticklish

you sit still and another person drives the robot arm across your palm

0 of 5

Check yourself

In the third-of-a-second row, the person is working the lever themselves and knows perfectly well a touch is on its way. It tickles anyway. What does that tell you about the "no surprise, no tickle" explanation?

Your brain guesses before your skin reports

Here is what is actually happening, and it is a much better story than the surprise one.

Whenever your brain sends out a command to move — lift that finger, drag it across those ribs — it does not just send the command and wait to find out how it went. It keeps a copy of the command and uses it to work out, in advance, what that movement is going to feel like.

Then the real feeling arrives from your skin. And your brain compares the two. Whatever it already predicted gets turned down. Whatever it did not predict comes through at full volume.

So when you tickle yourself, the sensation arrives exactly where and exactly when your brain said it would. Almost all of it gets subtracted. Almost nothing is left over. That is why it feels so oddly flat.

And when the robot waits a third of a second, the prediction lands in the wrong moment. The subtraction misses. What's left over is a genuine tickle — from your own hand.

What you feel is not a reading of what happened to your skin. It is the difference between what happened and what your brain had already guessed would happen.

Once you know that, you start noticing it everywhere.

Think of a staircase in the dark, when you are sure there is one more step and there isn't. The jolt is enormous — far bigger than an ordinary step, even though your foot travelled a shorter distance and hit the floor more gently than usual. Your brain had already subtracted an ordinary step and got something else entirely.

Or pick up a cup you think is full and find it empty. Your arm flies up. Your muscles were set for a weight that never arrived.

None of that is your senses being fooled. That is your senses working exactly as designed — reporting the surprise, and quietly ignoring the rest.

Quiet or loud?

One question decides each of these: did your own brain order the movement that caused it? Sort them, and read what happens either way.

Tap an item, then tap where it belongs

Predicted — turned down

Not predicted — full volume

Check yourself

Nadia can rub her own ribs hard with no reaction whatsoever. Her little brother barely brushes the exact same spot and she yelps. What is the actual difference between the two touches?

What to keep

01It isn't about surprise. In the experiment people knew a touch was coming every single time, and it still tickled once the robot ran late.
02Your brain predicts its own movements. It keeps a copy of every command it sends and works out what that movement should feel like before the feeling arrives.
03Predicted sensations get turned down. Whatever the guess covered is subtracted. Whatever it missed comes through loud — which is why a third of a second of delay is enough to make you ticklish to your own hand.
04Feeling is a difference, not a reading. The missing stair, the empty cup, your own voice while you speak — all the same machinery, working exactly as designed.

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Where these figures come from

The delayed-robot experiment: Blakemore, Frith & Wolpert, "Spatio-temporal prediction modulates the perception of self-produced stimuli", Journal of Cognitive Neuroscience 11(5):551-559 (1999). Participants moved a lever that drove a robot arm to stroke their own palm. Delays of 0, 100, 200 and 300 milliseconds were introduced between the movement and the tactile stimulus; rated ticklishness rose with delay, and at the longest delay the self-produced stimulus was rated close to an externally produced one. Rotating the robot's trajectory relative to the participant's movement (up to 90 degrees) also increased ticklishness with no delay at all. Reduced brain response to self-produced touch: Blakemore, Wolpert & Frith, "Central cancellation of self-produced tickle sensation", Nature Neuroscience 1(7):635-640 (1998) — self-produced tactile stimulation produced less activity in somatosensory cortex than identical externally produced stimulation. The general principle — that a copy of a motor command is used to predict its own sensory consequences, and the predicted part is attenuated — is standard motor-control neuroscience; the cerebellum is strongly implicated in generating the prediction (Blakemore, Frith & Wolpert, NeuroReport, 2001). Attenuation of one's own voice during speech ("speaking-induced suppression" of auditory cortex responses) is the same principle in hearing rather than touch, and is widely reported in the speech-motor literature. Earlier observation in the same direction: Weiskrantz, Elliott & Darlington, "Preliminary observations on tickling oneself", Nature 230:598-599 (1971) — a machine tickling the sole of the foot was rated more ticklish when it was not under the participant's own control. No medical content: this episode describes ordinary sensation in an ordinary body and gives no advice of any kind.