What Can We Learn From Tickling Rats?

Rats will actually make a giggling noise when they're in a good mood and they're tickled. Shimpei Ishiyama and Michael Brecht/Science

Who would have figured that rats enjoy a good tickle.

Researchers found that if you tickle a rat, it will make a giggle-like squeak, jump for joy and even chase your hand, hoping to be tickled again. That's fun and cute, but there's science behind those sweet giggle noises, says the team from Humboldt University of Berlin, who wanted to learn what part of the brain controls tickling.

"Why does tickling induce laughter? Why are tickling effects so mood-dependent? Why do body parts differ in ticklishness? Why can't we tickle ourselves? Is ticklish laughter different from humorous laughter?" they wrote in the study, published in the journal Science. "To address such questions, we need a better understanding of the neural correlates of ticklishness."

Neuroscientists don't understand why people are ticklish, and the idea that touching could lead to laughter is confusing, study author Shimpei Ishiyama told NPR.

"Just a physical touch inducing such an emotional output — this is very mysterious," Ishiyama says. "This is weird."

Knowing from earlier research that rats appeared to enjoy being tickled, Ishiyama and his team inserted electrodes into the somatosensory cortex of rats' brains. In humans, that's the part of the brain that processes touch.

Ishiyama then stuck his hand into the rats' cages and tickled them. They emitted an ultrasonic 50-kilohertz giggle, the same laughing-like sounds they made when they were playing with other rats. They also jumped for joy, an activity called “Freudensprünge,” and Ishiyama noted that the neurons in the somatosensory cortex of the brain were firing. When Ishiyama stopped tickling them and just moved his hand in the cage, the rats chased it, as if hoping to get more attention. The researchers noted that the neurons in the region were firing, just in anticipation of tickling.

To test their theory, the researchers directly stimulated the somatosensory cortex with an electrical signal into the brain. The rats responded in the same way, suggesting this truly is the tickling center of the brain.

Rats have a serious side, too

But rats weren't always in the mood for a good laugh. When they were made anxious by putting them under bright lights or on a platform, they were less likely to be ticklish, reports ScienceNews. Nerve cells in the somatosensory cortex were also less likely to fire.

Although many studies look at depression, Ishiyama told ScienceNews that he hopes this one will reveal new insights into how the brain creates and maintains happiness.

Besides, he says, “it’s also fun to study fun.”

Here's a video that shows a rat being tickled and chasing the hand that does the tickling, asking for more: