One day last year in a neuroscience lab in Berlin, subject 1 was sitting in a chair with his arms up and his bare toes pointing down. Hiding behind them, with full access to the soles of their feet, was Subject 2, waiting on tiptoes. At a time of their choosing, Subject 2 was instructed to take the opening shot: Tick the hell out of their partner.

To capture the moment, a high-speed GoPro camera was pointed at the face and body of Subject 1. Another at their feet. A microphone hung nearby. As planned, Subject 1 couldn’t help but laugh. The fact that they couldn’t help is what drew Michael Brecht, head of the Humboldt University research group, to the neuroscience of tickling and play. It’s funny, but it’s also deeply mysterious—and understudied. “He was a little stepchild of scientific research,” says Brecht. After all, brain and behavioral research tends to skew toward dark things, topics like depression, pain, and fear. “But,” he says, “I think there’s also a deeper prejudice against the game – it’s something for children.”

Prevailing wisdom holds that laughter is a social behavior among some mammals. It is a way of disarming others, easing social tension and bonding. Chimpanzees do it. Dogs and dolphins too. Rats are the usual objects in tickling research. If you flip them over and go to town on your stomach, they’ll screech at a pitch more than twice the limit of human ears. But there are many mysteries about tickling, whether among rats or humans. The biggest one of all: why we can’t tickle each other.

“If you read the ancient Greeks, Aristotle wondered about tickling. Also Socrates, Galileo Galilei and Francis Bacon,” says Constantina Kilteni, a cognitive neuroscientist who studies touch and tickling at Sweden’s Karolinska Institute and was not involved in Brecht’s work. We don’t know why touch can be ticklish, nor what happens in the brain. We don’t know why some people—or some parts of the body—are more ticklish than others. “These questions are very old,” she continues, “and after almost 2,000 years, we still really don’t have the answer.”

Part of the problem is that it is difficult to collect objective measures of how people respond to tickling and relate to the feeling of being tickled. That’s why Brecht’s group lured 12 people into a study that—albeit with a small sample size—was designed to observe the phenomenon with non-Aristotelian toys like GoPros and microphones. The footage collected by his team will help them uncover what happens when people are tickled, and what changes when they are tickled. Writing in Philosophical Transactions of the Royal Society B in September, the team report observations on reaction time, laughter, and breathing, and for the first time in a human study, they show that tickling during tickling suppresses tickling. “It’s rare. Studies don’t usually do that,” Kilteny says. “It really adds to the state of the art.”

Tickling, says Brecht, is “a very strange kind of touch and reaction to touch.” He is fascinated by how fundamental these complex behaviors are. In an 1897 paper called “The psychology of tickling, laughter and the comical,” the authors note that all people tend to have the same ticklish spots. The legs rank highest. Next come armpits, necks and chins. We instinctively tickle and play as children, and although some of this play bias fades with age, we always understand this mysterious language. Brecht believes this is a form of social signaling in the context of game combat: “You signal with your giggles that it’s okay to touch when it would normally be inappropriate to touch.” (Your laugh signals may even come before the touch. Consider a child about to be tickled by a parent: “They’re giggling like hell before you get there.”)

In the first phase of the new study, each subject had their moment in front of the GoPro and the microphone. Previous studies have found that tickling depends on mood – anxiety and unfamiliarity suppress it like a wet blanket. Because 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 actual tickling attack. The tickler always hid behind their partner while watching a video screen that fed them random sequences of body parts to touch. Neck, armpit, side of torso, plantar foot, top of head—each spot received five quick tickles.

The first observation: a person’s facial expression and breathing changed in about 300 milliseconds to a tickle. (The recording describes the poetry captured on GoPro footage: the tickler’s cheeks are raised, the corner of their lips pulled outward, “the appearance of which combined signals a joyous smile.”)

Then, about 500 milliseconds in, came the vocalization—surprisingly late. (The normal vocal response time to touch is about 320 milliseconds.) The team suspects that laughter takes longer because they require more complex emotional processing.

Subjects also rated how ticklish each touch felt. The crown of the head is not normally ticklish, so it serves as a control for what happens when you tickle someone in an unresponsive area. The volunteers laughed audibly after about 70 percent 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 correlated with their subjective ratings of how intense each tickle was.

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