Playfulness and tickling aren't always thought of as "serious" topics, but a new study shows how they can help answer important questions about the brain.
neuroscience laboratory Subject 1 sat on a chair last year with their arms up and their bare toes pointed down. Subject 2 waited behind them, fingers curled, with full access to the soles of their feet. Subject 2 was instructed to take the open shot at their leisure: tickle the hell out of their partner.
A high-speed GoPro was pointed at Subject 1's face and body to capture the moment. Another is standing at their feet. A microphone was hanging nearby. Subject 1 couldn't help but laugh, as planned. The fact that they couldn't stop themselves is what drew Michael Brecht, the leader of the Humboldt University research group, to the neuroscience of tickling and play. It's amusing, but also mysterious—and understudied. "It's been a bit of a stepchild of scientific research," says Brecht. After all, depression, pain, and fear are common topics in brain and behavior research. "But," he adds, "I believe there are deeper prejudices against play—something it's for children."
Laughter, according to conventional wisdom, is a social behavior of certain mammals. It's a technique for disarming others, reducing social tensions, and bonding. It's done by chimps. Dogs and dolphins are also included. Tickling studies are typically conducted on rats. If you flip them over and squeak on their bellies, they'll squeak at more than twice the pitch of human ears. But there are many unanswered questions about tickling, whether in rats or humans. The most important is why we can't tickle ourselves.
"If you read the ancient Greeks, you'll notice that Aristotle was interested in ticklishness. "There's also Socrates, Galileo Galilei, and Francis Bacon," says Konstantina Kilteni, a cognitive neuroscientist at Sweden's Karolinska Institutet who studies touch and tickling but is not involved in Brecht's work. We don't know why touch can be so unpleasant, or what happens in the brain. We don't know why some people—or body parts—tickle more than others. "These are very old questions," she continues, "and we still don't have an answer after nearly 2,000 years."
Part of the problem is that collecting objective measures of how humans respond to tickling and correlating them with perceived ticklishness is difficult. That's why Brecht's group enticed 12 people to participate in a study that, despite its small sample size, was designed to observe the phenomenon using non-Aristotelian toys like GoPros and microphones. His team's footage would help them understand what happens when people are tickled and what changes when they tickle themselves. The team reports observations on reaction times, laughter, and breathing in Philosophical Transactions of the Royal Society B in September, and for the first time in a human study, they show that tickling oneself while being tickled suppresses ticklishness. "It's unusual. "Studies don't usually do that," Kilteni says. "It makes a significant contribution to the state of the art."
BRECHT DEFINES TICKLING AS "a very strange kind of touch and reaction to touch." He is intrigued by how basic these complex behaviors are. The authors of a paper titled "The Psychology of Tickling, Laughing, and the Comic" published in 1897 noted that everyone has the same ticklish spots. Feet come in first place. Armpits, necks, and chins are next. We tickle and play instinctively as children, and while some of that proclivity to play fades with age, we always understand this mysterious language. In the context of play fighting, Brecht believes it is a form of social signaling: "You signal with your giggles that it's okay to touch when normally it would be inappropriate to touch." (Your laugh signals may even precede the touch.) Consider a child about to be tickled by a parent: "They giggle like hell before you get there."
During the first phase of the new study, each subject had their turn in front of the GoPro cameras and microphone. Previous research has shown that tickling is mood-dependent—anxiety and unfamiliarity dampen it like a wet blanket. Because participants would have to take turns tickling each other, Brecht's team made sure each pair knew each other beforehand and felt comfortable—but the actual tickling attack caught everyone off guard. The tickler hid behind their partner at all times, watching a videoscreen that fed them randomized sequences of body parts to touch. Each spot received five quick tickles: the neck, armpit, lateral trunk, plantar foot, and crown of the head.
The first observation was that a person's facial expressions and breathing changed into a tickle after about 300 milliseconds. (The following is a description of the poetry captured on GoPro footage: "The occurrence of which in combination signals a joyous smile," the ticklee's cheeks raised and the corner of their lips pulled outward.
Then, surprisingly late, at around 500 milliseconds, came the vocalization. (The average vocal reaction time to being touched is approximately 320 milliseconds.) Laughter, according to the team, takes longer because it requires more complex emotional processing.
The subjects were also asked to rate how ticklish each touch was. Because the crown of the head is not normally ticklish, it served as a control for what happens when you tickle someone in an unresponsive area. Volunteers began laughing audibly after about 70% of the tickles, and the more intensely they felt the tickle, the louder and higher pitched their laughter became. In fact, the sound of their laughter was the best predictor of their subjective ratings of how intense each tickle felt.
The ticklers then did the same thing while their partners tickled themselves—either in the same spot on the opposite side of the body, just beside it, or in a hovering pretend tickle that never actually touched the skin.
Self-tickling went exactly as planned. But the team noticed something strange: tickling oneself made the tickle of the other person less intense. When self-tickling the same side, the occurrence of ticklee giggling decreased by 25% on average and was delayed by nearly 700 milliseconds. "It took us by surprise," Brecht says. "However, the data shows it."
Why could this be? It all comes back to why we can't tickle ourselves. The dominant theory holds that tickling causes laughter due to a brain prediction error. An unexpected touch confuses it, sending it into a frenzy. Because self-touch is always predictable, there is no need for a frenzy.
However, Brecht believes that it is not really about prediction. Instead, he proposes that when a person touches themselves, the brain sends a message throughout the body that inhibits touch sensitivity. "We believe what is happening is that the brain has a trick to know: don't listen as soon as you touch yourself," he says. He claims that if it didn't, we'd all be tickling ourselves every time we scratched our armpits or touched our toes.
According to Sophie Scott, a cognitive neuroscientist at University College London who was not involved in the study, this makes sense because our brains learn to suppress sensory perceptions when our actions contribute to them. "Right now, just by sitting, I'm generating a lot of physical sensations in my body." "And that's far less important to me to know than whether or not someone else entered the room and touched me," she says. In fact, the same dimming effect occurs with hearing, she continues. When you speak, the parts of your brain that listen to what others are saying are silenced. (As a result, she claims, "people are very bad at judging how loud they're speaking.") So, if the brain inhibits touch reactions while self-tickling, it will also inhibit reactions to being tickled by someone else.
Kilteni points out that it's still unclear what exactly happens in the nervous system when a person is tickled—even by themselves. And it will be difficult to know for sure unless muscle contractions are recorded, the study is expanded beyond 12 people, or the tickling process is standardized using robots or machines. Nonetheless, she is impressed by the information gathered by Brecht's team. For example, knowing that tickle intensity correlates best with the sound of laughter is useful—Kilteni now plans to incorporate giggle recordings and video into her own work.
Tickle sessions in the lab add more to science than just levity and technical write-up gems like "Ticklees were told to act as naturally as possible": They also debunk myths about understudied aspects of emotional processing. "People say we don't express emotion very intensely in the voice, that it's the job of the face to express emotion," Scott says. She couldn't be more opposed: Voices convey words, mood, identity, health, age, sex, gender, geographical origins, and socioeconomic status; they are simply more difficult to study than faces.
Scott adds that touch is also underappreciated. Touch communicates compassion and affection far more clearly than faces or words. "If you're with a friend who is upset, you could say, 'I really feel sorry for you,' or you could hug them," she suggests. "I suspect that touch, that kind of soothing, is very important."
With future studies, Brecht's team intends to continue deconstructing the neuroscience of playfulness. According to experts, your level of ticklishness reflects how playful you believe you are. While this appears to be true for other animals—a ticklish rat is also more playful—it is more debatable in humans. "My wife is more ticklish," says Brecht. "However, I am quite playful!"