How does the tickling work

Tickling - A Serious Thing: Pleasure and Pain

How to make our best friend scream, everyone knows. An unannounced tickle with a blade of grass on the neck or the rough kill-kill attack on the sofa. Laughing and giggling until the tears flow - tickling is pleasure and torture at the same time. But why can't we tickle ourselves? London researcher Sarah Blakemore used modern technology to research this question. It recorded the brain activity of subjects who were tickled or tickled themselves with the help of a functional magnetic resonance imaging (fMRI) scanner. A tickling robot was also used. And so Blakemore found an answer: Our brain thinks for itself. The nerves are so well networked that the brain knows exactly what feeling to expect when we touch ourselves. The surprise effect counts. The brain doesn't think it matters when we tickle ourselves and just ignores the feeling of tickling.

Researchers find this ignorance extremely exciting. In the journal "Nature Neuroscience" Blakemore showed her colleagues what happens in the nerve network during tickling and why we cannot tickle ourselves: the brain predicts how our own movements will feel. It expects certain feelings as soon as we move. If the expected touch does not match the touch actually felt, the brain sends a signal that something special is happening.

Seeing works in a similar way. So the picture in front of our eyes doesn't jump every time we move. The brain automatically compares what it sees with eye movements: the image in front of our eyes does not blur.

So as soon as the command "Tickle!" was radioed to the hand, the brain automatically hides the feeling of tickling. Blakemore provided evidence that this explanation was correct with her fMRI images. "Our results indicate that the cerebellum is involved in predicting the specific sensory consequences of movement," she says. If we tickle ourselves, the cerebellum is less active than when another person tickles us. The actual feedback from the touch is consistent with the predicted feedback.

The cerebellum then presumably sends a signal to the somatosensory region of the cerebral cortex, which is responsible for the body's perception of movement and touch. "If you feel tickled, this region of the brain is more active than if you don't notice it," says Blakemore. The cerebellum, in turn, receives its input for these calculations from the ascending nerve cells of the so-called olive nucleus. These nerve tracts react very reliably to unexpected stimuli, such as tickling. The tickle studies caused another surprise: Schizophrenics can tickle themselves. Regardless of whether they were tickled or tickled themselves, there was no difference in their brains.

But why do things as silly as tickling throw a sensible adult completely off track within seconds? As a first step on the way to knowledge, the researchers gave the various tickling feelings scientific names: Knismesis stands for the light tickling that you feel when a feather strokes your skin. From an evolutionary point of view, the uncontrolled twitching makes sense: Annoying insects are shaken off immediately. Anyone can do knismesis on themselves - here the brain cannot override the reflex. But for the screeching in a real tickle attack, only the so-called Gargalesis can take care of that. What's the point of the screeching laughter? Why was this reaction not discarded in the course of evolution, when most adults do not like tickling and screeching at all?

First of all, those who do not like tickle attacks are in good company. "Even Socrates noticed that the sensation of tickling is only pleasant to a certain extent, but at the same time also painful," says tickle researcher Christine Harris from the University of California in San Diego. And the researchers had two theories at their disposal as potential answers: One interprets the convulsive giggling as a pure reflex. The other assumes that tickle attacks usually arise in a fun context anyway and therefore laughter is the obvious answer.

To find out which of the two theories is the right one, the psychologist James Leuba started an experiment in the 1940s - with his two young children as test subjects. No one was allowed to tickle the babies for the first six months. Then the big day came: Leuba hid behind a mask and tickled the babies. And although they did not know tickling from happy contexts and the masked father was probably not very funny for the children, they burst into squeaky laughter. For Leuba it was clear: the screeching is nothing more than a reflex to tickle. People laugh even when the situation is anything but funny. The reflex cannot be switched off any more than the shock after a loud bang.

Even chimpanzees were tickled in the service of science. Unfortunately, they can't laugh because their voices are built differently, but the primate researchers also recognize fits of laughter in their throaty panting when the monkeys have been tickled. And the tickle reflex is not only reserved for primates: Even rats are said to be ticklish.