Why can’t you tickle yourself?Neuroscientists Unravel Mystery

In the first phase of the new study, each subject spent their time in front of a GoPro and a microphone. Previous study It has been established that scratching is emotionally dependent – anxiety and unfamiliarity suppress it like a wet blanket. Since the participants had to take turns tickling each other, Brecht’s team made sure each pair knew the other beforehand and felt comfortable — but everyone was still surprised by the actual tickling attacks. Ticklers are always hiding behind their partners, watching a video screen and giving them random body parts to touch. Neck, armpits, side torso, soles, top of head – five rapid itches in each area.

First observation: A person’s facial expressions and breathing become ticklish in about 300 milliseconds. (The article describes the verse captured in the GoPro footage: tickled cheeks lifted, mouth corners pulled outward, “the combination of the two suggests a happy smile.”)

Then, at about 500 milliseconds, the sound came out — unexpectedly late. (The normal sound response time to being touched is about 320 milliseconds.) The team suspects that laughter takes longer because they need More complex emotional processing.

The subjects also rated the itchiness with each touch. The top of the head is usually not itchy, so it can control what happens when you scratch someone in an unresponsive area. After about 70 percent of the touches, the volunteers produced audible laughter, and the more intense the itch they felt, the louder and higher-pitched they laughed. In fact, their laughter was the measure most correlated with their subjective assessment of how intense each tickling felt.

In the next phase of the experiment, the ticklers did the same thing while their partner tickled themselves at the same time—either in the same place on the other side of the body, right next to it, or in a circle that never really touched Pretend to tickle. skin.

Self-tickling was calming, as expected. But the team noticed something odd: Self-tickling made each other’s tickling less intense. On average, the incidence of tickling decreased by 25%, and the delay was nearly 700 ms when self-tickling on the same side. “It was a surprise for us,” Brecht said. “But it’s very clear in the data.”

Why is this happening? This goes back to the question of why we can’t tickle ourselves. The leading theory is that tickling causes laughter due to the brain’s faulty predictions. An unpredictable touch confuses it, sending it into a state of mini-madness. Self-touching is always predictable…so, don’t go crazy.

But Brecht thinks this has nothing to do with predictions. Instead, he suggests that when a person touches themselves, the brain sends out a whole-body message that dampens touch sensitivity. “What we think is happening is that the brain knows a trick: Once you touch yourself, don’t listen,” he said. If not, he argues, we’re constantly scratching ourselves every time we scratch our armpits or touch our toes.

Cognitive neuroscientist Sophie Scott of University College London, who was not involved in the work, says it makes sense because our brains learn to reject sensory perception when our actions contribute to it. “Sitting now, I have a lot of physical sensations in my body, just through my movements. It doesn’t matter to me to know if someone else came into the room and touched me,” she said. In fact, she continued, the same darkening effect occurs in hearing. When you speak, the part of your brain that listens to others is suppressed. (That’s why, she says, “people have a hard time judging the volume at which they speak.”) So if the brain suppresses its response to touch when it tickles itself, it also suppresses its response to being tickled by others.

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