In the first phase of the new study, each subject had their moment in front of the GoPros and the microphone. Previous studies established that tickling is mood-dependent – anxiety and unawareness suppress it like a wet blanket. Since participants had to take turns tickling each other, Brecht’s team made sure each pair knew each other beforehand and felt comfortable, but each person was still surprised by the attack of real tickle. The tickler was still hiding behind his partner, while staring at a video screen that gave him random sequences of body parts to touch. Neck, armpit, lateral trunk, plantar foot, crown of the head – each point received five quick tickles.
The first observation: a person’s facial expressions and breathing turned into a tickle for about 300 milliseconds. (The article describes the poetry captured in GoPro footage: the tickling cheeks lifted, the corners of their lips drawn outward, “the occurrence of which in combination signals a cheerful smile.”)
Then, at about 500 milliseconds, came the surprisingly late vocalization. (A normal vocal reaction time to touch is about 320 milliseconds.) The team suspects that laughs take longer because They ask more complex emotional processing.
Subjects also rated how ticklish each touch was. The crown of the head isn’t usually ticklish, so it served as a control for what happens when you tickle someone in an unresponsive spot. The volunteers laughed audibly after about 70% of the touches, and the more intensely they felt the tickle, the louder and louder they laughed. In fact, the sound of their laughter was the measure that best matched their subjective ratings of how intense each tickle felt.
In the next phase of the experiment, the ticklers did the same, while their partners tickled each other simultaneously – either in the same spot on the opposite side of the body, right next to it, or in a hovering fake tickle that never really touched the skin.
As expected, the self-tickling was uneventful. But the team noticed something weird: self-tickling made the other person’s tickle less intense. On average, the onset of tickled laughter dropped by 25% and was delayed nearly 700 milliseconds when self-tickling the same side. “It was a surprise for us,” says Brecht. “But it’s very clear in the data.”
Why could this be? It comes down to the question of why we can’t tickle ourselves. The prevailing theory is that tickling induces laughter through a prediction error in the brain. An unpredictable touch confuses him, sending him into a mini frenzy. Self-touch is always predictable… so no frenzy.
But Brecht thinks it’s not really about prediction. Instead, he suggests that when a person touches themselves, the brain sends a body-wide message that inhibits sensitivity to touch. “We think what’s happening is that the brain has a trick: as soon as you touch yourself, don’t listen,” he says. If it weren’t for that, he argues, we’d all be constantly tickling ourselves every time we scratch our armpits or touch our toes.
That makes sense, says Sophie Scott, a cognitive neuroscientist at University College London who isn’t involved in the work, because our brains learn to reject sensory perceptions when our actions contribute to them. “Sitting right now, I generate a lot of physical sensations in my body just by my movement. And it’s much less important for me to know than if someone else came into the room and touched me,” she says. In fact, she continues, the same dimming effect occurs with hearing. When you talk, the parts of your brain that listen to other people talking are suppressed. (That’s why, she says, “people are very bad at judging how loud they talk.”) So if the brain inhibits reactions to touch while tickling, it also inhibits reactions to being tickled. to be tickled by someone else.