Researchers at Humboldt University in Berlin are measuring rats' enjoyment of tickling under different conditions. (Gillian Brockell/The Washington Post)

A ticklish rat is an adorable sight to see.

The chubby little rodent darts toward a scientist's gloved hand, eager for the delightful agony of its next scratch. It emits rapid-fire, ultrasonic “giggles” — chirps so high pitched they're inaudible to human ears — and dashes around its enclosure in spontaneous leaps that researchers call Freudensprünge, or “joy jumps.”

Joy jumps. Really.

“It’s the weirdest job ever, tickling rats professionally,” animal physiologist Shimpei Ishiyama told Smithsonian magazine.

Ishiyama and Michael Brecht, a fellow researcher at Humboldt University in Berlin, co-authored a new study in the journal Science on what happens when a rat gets tickled. They learned that a rat only enjoys being tickled when it feels happy and safe. The scientists also pinpointed the ticklish spot in the rat's brain and discovered that they could induce behavior like squeaking and joy jumping just by activating that spot.

The results suggest that tickling could be intimately connected to mood — a finding that could have implications far beyond the world of rodent entertainment. It could help neuroscientists explain the brain circuits associated with mood and the influence of positive thinking on human behavior.

“This is the only deep scientific approach we currently have to understanding the evolutionary sources of our own emotions, which are very important for deepening psychiatric understanding and treatment of affective disorders,” neuroscientist Jaak Panksepp told Popular Science.

Nearly two decades ago, Panksepp was the first researcher to demonstrate that rats emit something like a giggle when tickled. Many scientists thought animals had to have more cognitive sophistication to be able to laugh; the only other creatures to do something similar are apes and dogs.


A rat gets tickled (Shimpei Ishiyama & Michael Brecht)

But it's evident in Ishiyama's study that the rats thoroughly enjoy being tickled. In a video of the experiment, a gray-and-white rat chases after the scientist's hand so he can gently ruffle its fur. It joy jumps. It emits the chirps that have been associated with pro-social behavior and expectations of rewards. And an electrode inserted into the rat's brain shows that neurons are enthusiastically firing in the somatosensory cortex — a part of the brain thought to process the physical sensation of being tickled.

The researchers also tested the rats' tickle response when they were in a stressful situations — perched on a high-up platform, blasted with bright light. Their tickling response was muted, their somatosensory cortex less active. This suggested that rats have to be in a good mood to enjoy being tickled, much the same way humans don't like being tickled by a stranger or when they feel unsafe.

After the rats had been trained (i.e., tickled by Ishiyama) for a few weeks, monitors showed that their somatosensory cortex was activated by running after Ishiyama's hand, even if he wasn't actively tickling them. The phenomenon is similar to most toddlers' joy at being chased around a room and the way they'll bubble with laughter at the mere anticipation of being tickled.

The finding bolsters the idea that the somatosensory cortex could be associated with emotion, not just the processing of touch, and that tickling may have evolved to promote playfulness, psychology researcher Carolyn McGettigan told Smithsonian.

“You get increased firing in a region where the animal is not being stimulated physically — they are anticipating the stimulation,” said McGettigan, who studies emotional vocalizations in humans at Royal Holloway, University of London, and was not involved in Ishiyama's research. “That’s really intriguing in terms of trying to link this as a behavior that is mood dependent.”

The “eureka moment” came when the researchers used an electric current to artificially stimulate the targeted area of the brain. The rats immediately began to emit their laughter-like ultrasonic chirps, even though they weren't playing or being touched.

The study is the first to use brain stimulation to elicit ticklish laughter, according to Scientific American. And it raises many questions about ways in which rat ticklishness can help us understand human emotions.

“Rats and humans [diverged] maybe 100 million years ago,” Brecht told the magazine. “But the phenomenon of ticklishness is remarkably similar.”

He noted that neither rats nor humans are ticklish on their hands, but both species are extremely sensitive to being tickled on their feet. “It was one of those observations that really made me think that, hey, we're looking at the same thing.”

Funny, isn't it?

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