In the first phase of the new study, each subject had their moment in front of GoPros and microphones. Before learn determined that the feeling of tickling depends on mood — anxiety and unfamiliarity will suppress it like a wet blanket. Since the participants would have to take turns tickling each other, Brecht’s team made sure that each pair knew each other beforehand and felt comfortable – but each was still surprised by the actual tickling attack. The ticks always hide behind their mates, while watching a video screen that feeds them random body parts to touch. Neck, armpits, side trunks, base of trees, crown of head — 5 rapid ticks each.
First observation: a person’s facial expression and breathing change about 300 milliseconds to a tickling sensation. (The passage describes a poem caught on GoPro footage: Cheeks tickle upward, corners of lips pulled outward, “the presence of that combined to signal a cheerful smile.”)
Then, at about 500 milliseconds, the sound comes out – surprisingly late. (The normal response time of the voice to touch is about 320 milliseconds.) The team suspects that laughing takes longer because they demand more complex emotional processing.
Subjects also rated how ticklish each touch was. The top of the head doesn’t usually tickle, so it’s used as a control over what happens when you tickle someone in an insensitive spot. The volunteers laughed loudly after about 70% of the touches, and the more intense the tickling they felt, the louder and taller they laughed. In fact, the sound of their laughter was the measure that best correlated with their subjective ratings of how intense each tickling sensation was.
In the next phase of the experiment, the ticks did the same thing, while their partner tickled themselves simultaneously — in the same spot on the opposite side of the body, right next to it, or in a motion. pretending to hover without ever actually touching skin.
As expected, the feeling of self-tickling is not normal. But the team found something odd: Self-tickling made other people’s tickling feelings less intense. On average, the occurrence of giggles due to tickling was reduced by 25 percent and delayed to nearly 700 milliseconds when ticking on the same side itself. “It was a surprise for us,” Brecht said. “But it’s very clear in the data.”
Why could this be? It goes back to the question of why we can’t tickle ourselves. The leading theory is that tickling causes laughter thanks to a prediction error of the brain. An unpredictable touch will confuse it, turning it into a little frenzy. Self-touch is always predictable…so there’s nothing to worry about.
But it’s not so much about prediction, Brecht says. Instead, he suggests that when a person touches themselves, the brain sends out a whole-body message that inhibits touch sensitivity. “We think what’s happening is that the brain has a trick to tell: As soon as you touch yourself, don’t listen,” he says. Otherwise, he argues, we would all frequently feel a tickle every time we scratch our armpits or touch our toes.
This makes sense, says Sophie Scott, a cognitive neuroscientist from University College London, because our brains learn to reject sensory perceptions when our actions contribute to them. “Sitting right now, I am creating a lot of physical sensations in my body just by my movement. And that to me is much less important than whether someone has come into the room and touched me,” she said. In fact, she continued, the same blurring effect occurs with hearing. When you speak, the part of the brain that listens to the other person is inhibited. (That’s why, she says, “People are terrible at judging how loud they are.”) So if the brain inhibits the touch response when tickling itself, it will inhibit it, too. How to react to being tickled by others.
