Sarah-Jayne Blakemore, a neuroscientist at University College London, has hooked people up to fMRI machines and monitored how their response to a tickle from a scientist compared with attempts to tickle themselves. When being tickled by someone else, the participants' somatosensory cortex (the part of the brain responsible for sense of touch) and anterior cingulate cortex (which deals with emotion, rewards and impulse control) light up like a Christmas tree. But when they try to tickle themselves, that area stays relatively dull while the cerebellum — which is involved in coordinating and regulating muscular activity — gets all the action.
This led her to conclude that the cerebellum is somehow involved in predicting the specific sensations of certain motion — and tamping down the brain's response ahead of time. In other words, the cerebellum knows where the hand is heading, determines, "That's gonna feel funny! Shut it down!" and cuts off the somatosensory cortex's response before the participant even starts to giggle.
But why? To understand the reason for this response, you have to think about why humans are ticklish in the first place.
That question has a long history — even Darwin wondered about it. We'll start in 1897, when psychologists G. Stanley Hall and Arthur Allin coined the first technical terms for tickles (say that 10 times fast). There's "knismesis," the prickly feeling you get when something lightly brushes across your skin. Then there's the far more mysterious "gargalesis," the laughter-inducing sensation that only another person can cause.
"It seems easy to imagine an evolutionary function for knismesis," Christine Harris, a psychologist at the University of California at San Diego, wrote in a book chapter on the subject. "The annoying sensation prompts one to scratch or rub the tickled spot, thereby removing insects or parasites that might be crawling on one's body."
Lots of mammals exhibit this response to a light touch, but only primates are known to get "gargalesis." Primatologist Marina Davila Ross has conducted experiments in which she tickled young orangutans, gorillas, chimps and bonobos and recorded laughter-like vocalizations. Chimps even display "laugh faces", smiling with their teeth bared, while playing. Ross believes that humans inherited our ability to laugh from our last common ancestor with great apes.
Interestingly, a 2000 study found that lab rats respond to being tickled by scientists with the same chirping noises they use while playing with each other.
It's not entirely clear what the evolutionary benefit of this sort of ticklishness might be. Some argue that laughter — including the ticklish kind — is linked to social intelligence. If you're an animal that has to live and work in groups, it pays to be able to pal around. Robert Provine, a psychologist at the University of Maryland, has suggested that tickling provides a means of communication between parents and infants before the babies are able to talk. Laughing in response to a playful touch could provide positive feedback for parents, making the adults more attentive and their babies more likely to survive.
Another theory suggests that ticklishness promotes protection of vulnerable areas — on the stomach, under the armpits, the soles of the feet — resulting in an adaptive advantage during hand-to-hand combat. (Harris pointed out that this theory doesn't explain why you should laugh and smile while someone tickles your belly.) Alternatively, perhaps tickling makes us both giggly and squeamish in order to encourage other people to tickle us while motivating us to defend ourselves. "This would promote rough-and-tumble play that may help the development and acquisition of combat or other skills that have survival benefits," Harris wrote.
Harris's own research suggests that ticklishness is a low-level, automatic physiological response, a biological behavior that we don't have conscious control over. In this, it's a lot like being startled, she said — we can't control it or induce it, because if our brain knows what's coming based on signals about muscle movement in the cerebellum, it cuts off the physiological response. From an evolutionary standpoint, there's an obvious advantage to this system: it keeps us alert to possible predators, but prevents us from wasting time being startled by our own movements, which aren't going to do us harm.
But there are ways to get around it. In her studies on self-tickling, Blakemore has used machines to create a small delay between the movement of the participant's hand and the actual tickling motion. The longer she delayed the reaction, the more "tickly" the participant's felt. "So it might be possible to tickle yourself, if you are willing to invest in a couple of robots," she wrote in Scientific American.
Monash University professor Jakob Hohwy has achieved the self-tickle by tricking participants into thinking they were in someone else's body (participants wore goggles hooked up to a camera on another person's head). He's also found that people with schizophrenia are more likely to be able to tickle themselves.
This research sounds silly, but Hohwy says it helps scientists understand how humans make sense of the world. He believes that our brains are "hypothesis testers" that learn by comparing expectations about our environment to our actual perceptions.
“It’s a constant, ceaseless function — predictions being tested against what actually happens, and then error-corrected,” Hohwy said in a statement. “It may explain the basis of perception, learning and action, from birth onward.”