Tickling is a mysterious phenomenon: this specialized form of touch is so powerful that it can send us into almost uncontrollable fits of shrieking, gasping laughter or defenseless pleas for mercy. Yet we still don’t understand how it works. Several decades ago the scientific community was surprised to discover that rats share our susceptibility to tickling, and a study published today in Science reveals more about what happens in a rat’s brain while being tickled.
Researchers at Humboldt University of Berlin tickled and played with rats under different circumstances, and were able to identify an area of the brain that could be key for how rats respond. But here is the catch: This brain region was activated during tickling and play in happy rats but did not respond to tickling or play in anxious rats. Perhaps most strikingly, when scientists stimulated this brain area, rats immediately began making the same high-pitched “giggles” (above the range of human hearing) they make during tickling and play.
Many mammal species are sensitive to the light, ticklish annoyance of an insect crawling across their skin, but fewer respond to gargalesis—intense, laughter-inducing tickling. Scientists once thought the effects of gargalesis were limited to highly intelligent, social species such as humans and other primates. In 1999, however, Jaak Panksepp and Jeffrey Burgdorf successfully demonstrated that tickling young rats spurs them into letting out the same ultrasonic giggles they make during play.
Shimpei Ishiyama and Michael Brecht, co-authors of the present study, decided to build on this research by taking a closer look at how rat brains respond to pleasurable activities like tickling and play. They focused their attention on the somatosensory cortex, a large brain region that responds to tactile information such as touch, pain, temperature and pressure, and has different areas that correspond to different body parts.
The researchers, who worked with young male rats (the most ticklish group), first wanted to confirm the previous findings. So they started tickling—the best job ever, according to Ishiyama, who carried out the experiments, and calls himself a “professional rat tickler.” The rats, as in previous studies, emitted high-pitched giggles during tickling on their bellies and backs. The researchers recorded the giggles with a special microphone and played them back at a lower, human-friendly register. They also observed that the rats began actively seeking out a tickling hand. The animals frolicked around their cages, making “joy jumps,” which, according to Brecht, is a “behavior that we see in many mammals when they’re very enthusiastic.” When the researchers made the rats anxious by placing them on high, exposed platforms under bright lights, however, the rats no longer giggled during tickling.
Next, the researchers looked at brain cells in an area of the somatosensory cortex that responds to touch in the animals’ “trunk” region (the belly and back areas where they were tickling the rats). They found that during tickling, happy rats exhibited activation of brain cells representing this trunk region just before lapsing into bursts of high-pitched giggles. Anxious rats did not have as much activation in this region, even though they were being tickled in the same way, which suggests that mood affects how this brain region responds to touch.
This brain region also activated while rats played with a friendly hand by chasing it around the cage. This result was particularly surprising because the somatosensory cortex is involved in processing touch. But as Ishiyama explains, during play, “nobody is touching the rat, so there is no tactile input to the body surface.” For Brecht, this finding suggests a link between tickling and play in the brain. “Maybe there are joint mechanisms or similarities between tickling and play,” he says “Maybe tickling is a trick of the brain to make animals interact with each other.”
In a final experiment the researchers used an electric current to artificially activate the trunk region of the somatosensory cortex, which led to what Ishiyama describes as a “eureka moment”: Even though the rats weren’t being tickled or playing, they immediately began making the same laughter sounds they naturally make during these activities.
Brecht explains that together these results create a more complete picture of the trunk region as an area of the somatosensory cortex that responds both to the physical touch of tickling and to the hands-off experience of play in that it is sensitive to mood, and may be the reason rats giggle during these activities.
Other researchers in the field who were not involved in the research are enthusiastic about the findings. Diana Roccaro and Elise Wattendorf, neuroscientists at the University of Fribourg, wrote in an e-mail that the study is the first to use brain stimulation to elicit ticklish laughter, which “constitutes an outstanding result.” They add that it is particularly remarkable play behavior activates the same brain region as tickling. Piotr Popik, a neuroscientist at the Polish Academy of Sciences, also praises the work, particularly for identifying a brain region that is a probable mechanism for the high-pitched giggles observed in rats. Fausto Caruana, a neuroscientist at the University of Parma, calls the study “extraordinary,” and notes that it advances earlier work by Panksepp and Burgdorf. He emphasizes that because studying laughter in humans is very difficult, “it is extremely important to find animal models for laughter.”
The study also raises new questions. Panksepp, considered a pioneer in the field, points out that we still don’t know whether this cognitive brain response involves an emotional component—what Panksepp called a “feeling message.” The big question is, “does that evoked sound in the cortex still feel good?”
But what does ticklishness in rats have to do with our own reactions to tickling? “Rats and humans [diverged] maybe 100 million years ago,” Brecht says. “But the phenomenon of ticklishness is remarkably similar.” The most telling observation was one he and Ishiyama didn’t include in the paper: Rats, like humans, aren’t ticklish on their hands, but are extremely ticklish on the bottoms of their feet, he says. “It was one of those observations that really made me think that, hey, we're looking at the same thing.”