To reveal how the brain creates joy, start by tickling rats

If you tickle a rat, it will jump for joy. It will squeak with glee. And like a kid, it will beg for more. A new study describes these reactions and identifies the brain cells that help turn rats into squirmy puddles of giggles.

The results are helping scientists begin to understand how the brain creates glee. It’s an emotion that’s not been studied much, notes Shimpei Ishiyama. He’s one of the study’s authors and a neurobiologist at Humboldt University of Berlin in Germany. “People really underrate the positive things — fun, happiness, joy,” he says.

Scientists knew that rats seemed to enjoy tickles from a human. But they didn’t know how the rodent brains created that joyful emotion. To start, Ishiyama simply stuck his hand in the cages and scribbled his fingers in the rats’ fur. It was “surprisingly easy,” Ishiyama says.

The rats seemed delighted. They laughed with an ultrasonic giggle that humans can’t hear. They spontaneously jumped for joy. And they chased Ishiyama’s hand around the cage to ask for more.

To find the rats’ most vulnerable spots, Ishiyama and the other researchers tickled bellies, backs and tails. Belly tickles elicited the most laughter overall. The scientists published their results November 11 in Science.

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The scientists suggest that the rats’ joyful response may be created in part by nerve cells in a specific part of the brain. It is called the somatosensory (So-MAT-oh-SEN-soh-ree) cortex. In people, this brain region responds to touch — including tickles.

Ishiyama and his colleagues placed electrodes into this region of the rats’ brains to see if it turned on. Nerve cells there that respond to the animals’ torsos were indeed active during tickling. Light stroking activated some of these same cells, but not as many.

It was not surprising that nerve cells responsive to touch became active during a tickle, Ishiyama says. What is surprising is what they found next. Some of the nerve cells in the same area also were active when the rats chased a tickling hand but weren’t actually being touched. That suggests there’s something specific about being tickled or anticipating a tickle that the cells are responding to.

And then the researchers went one step further. They used electrodes to stimulate the same touch-sensitive area of the rats’ brains. Even though the rats weren’t actually being touched, they giggled.

It turns out that ticklishness depends on many things. Just as in people, some rats simply like to be tickled more than others. “Rats have personalities,” Ishiyama says. “Some are very ticklish and some are very shy.” And even tickle-loving rats had off days on which they weren’t as playful. “If the rat’s mood is bad, it just doesn’t work,” Ishiyama says.

Anxious or stressed rats giggled less than calm ones. And nerve cells in the touch centers of the anxious animals’ brains were less likely to respond. Such results highlight how mood can affect a rat’s response to tickling.

Even the tickler’s mood can play a role. Ishiyama spent more than three years tickling rats. Over this time he noticed that his own mental state seemed to affect the rats’ playfulness. He tried to decrease its influence by meditating before tickle sessions.

The new study shows for the first time that stimulating the somatosensory cortex can cause laughter, says Elise Wattendorf. A neuroscientist at the University of Fribourg in Switzerland, she was not part of the new study. She and her colleagues had previously found that this part of the brain turns on when people laugh from being tickled. The fact that the area is involved in both touch and social aspects of tickling is “unexpected, and constitutes an outstanding result,” she wrote in an e-mail.

Many brain studies focus on disease, such as depression, Ishiyama says. He now hopes to reveal new details on how the brain creates and maintains happiness. This is important to understand, he says. What’s more, he adds: “It’s also fun to study fun.”