Rats Giggle When Tickled — But Only When The Mood Is Right

At parties and bars, he introduces himself as a “rat tickler.” The title makes Shimpei Ishiyama sound like he belongs in some forgotten guild of yore, with the Victorian “pure-finders,” who collected dog dung for a living, and the “flankers and flaggers,” who kept partridges in the range of hunters’ guns. But he is, in actual fact, a neuroscientist, and his rat-tickling is anything but antiquated. By trying to titillate these rodents — and recording how their neurons respond — Ishiyama and his adviser are unraveling a mystery that has puzzled thinkers ever since Aristotle posited that humans, given their thin skin and unique ability to laugh, were the only ticklish animals. Aristotle was wrong, it turns out. In a study published Thursday in Science, Ishiyama and his adviser, Michael Brecht, not only found that rats squeaked and jumped with pleasure when tickled on their backs and bellies, but also that these signs of joy changed according to the rodents’ moods. And, for the first time, they pinpointed a cluster of neurons that makes this sensation so powerful that it causes an individual being tickled to lose control. “It’s truly innovative and groundbreaking,” said Jeffrey Burgdorf, a neuroscientist at Northwestern University who reviewed the paper. “It takes the study of emotion to a new level.” Burgdorf has played a central role in our understanding of animal tickling. He was part of a team that first noticed, in the late 1990s, that rats make a symphony of noises when they are experiencing social pleasure. Others had already noted that they trill and yip and sing during sex and meals — all above the range of human hearing — but the lab where Burgdorf worked noticed that the rodents emitted similar sounds while playing. And so one day, the senior scientist in the lab said, “Let’s go tickle some rats.” They quickly found that those cries of pleasure doubled. But other researchers didn’t share the rats’ joy. Prominent scientists of emotion tried to impede the publication, accusing the team of “the sin of anthropomorphism,” Burgdorf and his colleague Jaak Panksepp wrote in a review paper in 2003. Tickling — and why it has such a powerful effect on us — has remained largely mysterious. “Here’s the problem in a nutshell, and it’s a little philosophical,” Burgdorf told STAT. “In order for us to function, we have to ignore about 90 percent of our sensory information. We have to process only the important stimuli. What the brain is doing is saying this tickling is important, and I’m going to be able to discriminate this kind of stimulation from other kinds of stimulation.” Ishiyama, a postdoc at Humboldt University in Berlin, wanted to figure out how that worked. Everyone knows how to titillate an ocelot — you oscillate its tit a lot. But designing a rigorous experiment on how tickling is processed by rat brains isn’t as obvious, and is hardly mainstream in neuroscience. What Ishiyama did was to drill tiny holes into the […]

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