Why don't you laugh when you tickle yourself?

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In fact, as early as 1872, Darwin studied the problem of tickling. For example, you will not laugh when you tickle yourself, but you will laugh when someone else tickles you. As a primitive defense response mechanism, tickling also has a pleasant side. However, there is a difference between "scratch it when you feel itchy" and "someone else tickles you and you feel itchy". The former is to eliminate the itch, while the latter is to induce laughter. So the question is, why can't you stand it when others tickle you, but you don't feel it when you tickle yourself?

One day last year, in a neuroscience lab in Berlin, Subject 1 sat in a chair, arms raised, bare feet pointed to the ground. Subject 2, lurking behind them, within reach of their soles, curled his fingers and waited.

Subject 2 was instructed to choose their own moment to strike: tickle their partner.

To capture the moment, a GoPro high-speed action camera was positioned in front of Subject 1, pointing at their face and body, and another at their feet, with a microphone hanging nearby. As planned, Subject 1 couldn't help laughing. The fact that they had trouble controlling this reaction intrigued Michael Brecht, the head of the Humboldt University research group, and led him to study the neuroscience of tickling and play. They are fun, but also very mysterious - and understudied.

“The subject has been treated a bit like the neglected stepchild in scientific inquiry,” Brecht says. After all, brain and behavioral research often tends to deal with darker topics, like depression, pain, and fear. “But,” he says, “I think there’s a deeper bias against play—that it’s something for kids.”

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The mainstream view is that laughter is a social behavior among certain mammals.[1] It is a way to let down one’s guard, relieve social tension, and enhance bonding. Chimpanzees laugh. So do dogs and dolphins. Rats are a common subject in tickling studies. If you turn them over, on their backs, and tickle their bellies as hard as you can, they produce a high-pitched squeak that is more than twice the pitch of human hearing.

But whether the subject is mice or humans, there are still many mysteries about tickling. The biggest one is: why can't we tickle ourselves?

“If you look at the ancient Greeks, Aristotle was curious about tickling, as were Socrates, Galileo, and Francis Bacon,” says Konstantina Kilteni, a cognitive neuroscientist who studies touch and tickling at the Karolinska Institutet in Sweden but was not involved in Brecht’s project.

We don’t know why touch causes itch, or what happens in the brain to make it tick. We don’t know why some people — or some parts of the body — are more ticklish than others. “These questions are very old,” she continued, “and almost 2,000 years later, we still don’t really have the answers.”

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Part of the trouble is that it's hard to collect objective measurements of how humans respond to tickling, and to correlate those measurements with perceived itch. That's why Brecht's team recruited 12 people for a study that, despite its small sample size, aims to observe the phenomenon using non-Aristotelian toys like GoPro action cameras and microphones. The footage his team collects will help them figure out what happens when people are tickled, and what changes when people tickle themselves.

Publishing in the September issue of Philosophical Transactions of the Royal Society B[2], the team reported observations on reaction time, laughter, and breathing, and showed that tickling oneself while being tickled by another person can suppress the itch sensation, the first such finding in humans. “This is rare. Studies don’t usually do this,” Kiltney says. “It really contributes to the current state of the art in this topic.”

Brecht said that tickling is “a very strange form of touching and reaction to touching.” He was fascinated by the fundamental nature and importance of these complex behaviors. In an 1897 paper titled “The Psychology of Tickling, Laughing, and the Comic,”[3] the author noted that, in general, the location of ticklish spots is the same for all people. The feet are the most ticklish. The armpits, neck, and chin are close behind.

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As children, we instinctively love tickling and games, and although some of our preferences for games fade as we get older, we always understand this mysterious language. Brecht believed that it was a form of social signaling in the context of play: "You use your giggles to indicate that touch is acceptable at the moment, when it would not be appropriate under normal circumstances." (Your laughter signal may even come before the touch. Imagine a child about to be tickled by a parent: "They laugh like crazy before you actually touch them.")

In the first phase of the new study, each subject experienced their own hilarious moment in front of a GoPro camera and microphone. Previous research[4] has shown that tickling is “emotionally dependent,” with anxiety and unfamiliarity acting like a wet blanket to dampen the itch. Because participants had to take turns tickling each other, Brecht’s team ensured that each pair knew each other beforehand and felt comfortable around each other—but everyone was still surprised when they were actually tickled.

The ticklers always hid behind their partners and looked at a screen where they were given a random sequence of body parts to touch: neck, armpits, sides, soles of feet, top of head - five quick tickles each.

The first thing we observed was that a person’s facial expression and breathing changed into a tickling response in about 300 milliseconds. (The research paper [5] described the beautiful moment captured on video: the tickled person’s cheek lifted and the corners of his mouth pulled outward, “a combination of movements that signaled the beginning of a happy smile.”)

Then, around 500 milliseconds later, the sound occurred—unexpectedly late. (The normal reaction time to sound when being touched is about 320 milliseconds.) The team suspects that laughter takes longer because it requires more complex emotional processing.

The subjects also rated how itchy each touch was. The top of the head is not usually ticklish, so it serves as a control for what happens when you tickle someone in a non-ticklish area. The volunteers laughed after 70 percent of the touches, and the louder and higher-pitched the laughter was the more ticklish it was. In fact, their laughter was the best correlate of their subjective ratings of how itchy each tickle was.

In the next phase of the experiment, the ticklers continued to tickle their partners, but their partners also tickled themselves at the same time - either in the same spot on the opposite side of the body, next to the spot being tickled, or with their hands hanging free and pretending to tickle themselves without actually touching the skin.

As expected, tickling oneself did not produce any reaction. But the team noticed something odd: tickling oneself made tickling others feel less ticklish. On average, the tickled people laughed 25 percent less often, and the onset of laughter was delayed by nearly 700 milliseconds when the tickling site was on the same side as the person being tickled. “That was unexpected for us,” Brecht says, “but the data was very clear.”

Why is this? This goes back to why we don't feel tickled when we tickle ourselves. The leading theory is that tickling is hilarious thanks to a prediction error in the brain. Unpredictable touches confuse the brain and send it into a little frenzy. Touching yourself is always predictable... so the brain doesn't go crazy.

But Brecht thought it wasn't really about prediction.

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Instead, he thinks that when a person touches themselves, the brain sends a message throughout the body that suppresses sensitivity to touch. “We think what’s going on is that the brain has a trick of recognizing what you’re doing, and once you’re touching yourself, it tells your body to ignore it,” he says. If the brain didn’t do this, he points out, we’d be itching ourselves endlessly every time we scratch our armpits or touch our toes.

This makes sense, says Sophie Scott, a cognitive neuroscientist at University College London who was not involved in Brecht’s project, because our brains learn to shut down sensory perception when our actions have an impact on it. “I’m sitting here right now, and I’m generating a lot of physical sensations in my body just by my movements,” she says. “But to me, those sensations are irrelevant, compared to knowing if someone else comes into the room and touches me.”

In fact, she continues, the same “suppression effect” occurs with hearing. When you talk, the part of your brain that listens to other people talking is suppressed. (This is why, she says, “people are very bad at judging how loud their own voices are.”) So if the brain suppresses its response to touch when people tickle themselves, it would also suppress its response to being tickled by others.

Kiltney points out that it's still unclear what's going on in a person's nervous system when they're tickled, including when they tickle themselves. At a minimum, we'll have to consider recording muscle contractions, expanding the study to more than 12 people, or even using robots or machines to standardize the tickling process, before we know for sure.

Still, she was impressed by the data Brecht’s team had collected. For example, knowing that tickle intensity correlated most strongly with laughter volume was valuable—Kiltney now plans to include audio and video recordings of subjects laughing in her own studies.

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The contributions of lab-based tickling experiments to science go beyond much-needed liveliness and techno-stunts (like “the tickler is told to act as naturally as possible”) and shed light on an understudied area of ​​emotion processing.

“People often say that our voices don’t express emotion very strongly, that that’s the job of the face,” Scott says. She couldn’t disagree more: Voices convey words, emotions, identity, health, age, sex, gender, geographic origin and socioeconomic status — they’re just harder to study than facial expressions.

Touch is also underrated, Scott adds. Sympathy and affection are expressed more clearly through touch than through facial expressions or words. “If you’re with a friend and they’re having a really bad time, you can say ‘I feel really bad for you,’ but you can also give them a hug,” she says. “I think that touch — that comfort — is really important.”

Brecht's team plans to continue to unravel the neuroscience of playfulness through future studies. **Experts speculate that your level of ticklishness reflects how much you think you enjoy playing games. **While this theory seems to hold true in other animals—a very ticklish mouse may also enjoy playing games—it's still a little more debatable in humans. "My wife is more ticklish," Brecht says, "but I love playing games!"

References:

[1]www.tandfonline.com/doi/abs/10.1080/09524622.2021.1905065?journalCode=tbio20

[2]royalsocietypublishing.org/doi/10.1098/rstb.2021.0185#d1e532

[3]www.jstor.org/stable/1411471?origin=crossref

[4]www.science.org/doi/10.1126/science.aah5114

[5]academic.oup.com/cercor/article/23/6/1280/426218?login=false

By Max G. Levy

Translated by Kushan

Proofreading/Rabbit's Light Footsteps

Original article/www.wired.com/story/neuroscientists-unravel-the-mystery-of-why-you-cant-tickle-yourself/

This article is based on the Creative Commons License (BY-NC) and is published by Kushan on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan