Why Can’t You Tickle Yourself? Your Brain’s Prediction Machine

The moment a friend’s fingers head for your ribs, you start to squirm — you may even laugh before they touch you. Yet make exactly the same movement with your own hand and nothing happens. That small mystery opens a door onto one of the cleverest systems in the brain.

What exactly is tickling?

Scientists divide tickling into two types:

• Knismesis: the light, itchy feeling of a feather or an insect walking across the skin. It is found in many animals and is probably an ancient warning system that helps us notice parasites and bugs.
• Gargalesis: the laughter-inducing tickle caused by firmer, repeated pressure on the armpits, ribs or soles. It is seen in humans and some other mammals, notably apes and rats.

It is the second type we cannot do to ourselves. Brush a feather across your own lip and you may feel a slight itch, but however much you poke your own ribs, you will not burst out laughing.

The brain predicts the future

The answer lies in a prediction mechanism in the brain’s movement system. When you decide to move your arm, the brain sends a command to your muscles. A copy of that command also goes elsewhere — mostly to the cerebellum, at the lower back of the brain.

The cerebellum uses the copy to work out: “What will this movement feel like?” Then, when the real sensation arrives from the skin, it compares the two:

• If the sensation matches the prediction, the brain treats it as unimportant and turns it down.
• If it differs, the brain directs attention there and feels it strongly.

When you try to tickle yourself, your brain knows exactly where, when and how hard your fingers will touch. There is no surprise. The sensation is suppressed and the tickle never happens.

The famous experiment: a tickling robot

To test the idea, in 1998 Sarah-Jayne Blakemore, Daniel Wolpert and Chris Frith at University College London built an ingenious set-up. Volunteers used their own hand to guide a robot arm that stroked their palm with soft foam.

The researchers introduced small mismatches between the movement and the touch:

• When the robot transmitted the movement instantly and in the same direction, people found the touch least ticklish.
• Adding a delay of just a couple of hundred milliseconds, or slightly changing the direction of the stroke, made it noticeably more ticklish.
• The bigger the mismatch, the more the touch felt like someone else’s.

Brain-imaging studies have also shown that the cerebellum and touch-processing regions are less active when we touch ourselves. The brain turns the volume down before the sensation fully registers.

Why does this system exist?

Not being able to tickle yourself might seem trivial. But the system behind it does something vital: it separates sensations we create ourselves from those coming from the outside world.

Without it:

• Every brush of our clothes as we walk would distract us.
• We would struggle to tell whether a touch was our own hand or an outside threat.
• Our own voice would constantly pull our attention while we talk.

By predicting and dampening the consequences of our own actions, the brain saves its attention for stimuli that are unexpected — and therefore potentially important. It is why you instantly notice an insect crawling up your leg.

The same system makes fights escalate

This prediction mechanism has a surprising side effect. In a 2003 experiment, pairs of volunteers took turns pressing on each other’s finger with a device. Each was asked to return exactly the same force they had just felt.

The result: with every round, the force increased. Because the brain dampens the force we produce ourselves, it feels weaker than it really is. Each person believed they were matching the force — but was actually pressing harder.

The researchers noted that this could explain why children’s squabbles of the “He hit me harder!” kind tend to escalate.

People who can tickle themselves

Some people can tickle themselves more easily. Research suggests that some people with schizophrenia — particularly those who feel their thoughts or actions are controlled from outside — dampen their own touch less.

The finding supports an important idea: in these people, the brain system that stamps a movement “made by me” may work differently. That could contribute to experiencing their own actions or inner voice as if they belonged to someone else.

Rats laugh too

Tickling is not unique to humans. In 2016 Shimpei Ishiyama and Michael Brecht at Humboldt University in Berlin showed that rats produce ultrasonic calls of around 50 kilohertz when tickled — far above what human ears can hear.

Tickled rats:

• chased the tickling hand, wanting the game to continue;
• “laughed” most when turned on their backs and tickled on the belly;
• responded far less to tickling when they were in a stressful or frightening setting.

That last finding will sound familiar. Being tickled when you are tense or uncomfortable is rarely fun. For tickling to be enjoyable, the setting needs to feel safe and playful.

Is tickling really fun?

Laughter during tickling does not always mean pleasure. Many people say they want to escape even while laughing. Tickling is therefore seen as a kind of mixed emotion: the laughter is a reflex, but the feeling itself can be unpleasant.

That is why, especially when playing with children, it matters to stop the moment someone says “stop”.

In short

You cannot tickle yourself because your brain predicts the sensations your own movements will cause and dampens them. This prediction system separates self-made sensations from those coming from outside, keeping our attention on stimuli that really matter. Someone else’s fingers, though, are always a bit of a surprise — and the brain’s answer to that surprise is often laughter.