Rats, much like human children, squeal, laugh and even jump for joy when they’re tickled.
Neuroscientists from Humboldt University in Berlin studied the phenomenon because they were motivated by the mystery of ticklishness, which they now believe could play an important role in social development and cognitive function.
“In general for humans, ticklishness is one of the most mysterious sensations despite its common appearance in human culture,” said study author Shimpei Ishiyama. “We really don’t understand why we laugh or why we have to be ticklish in terms of biology. Is there any biological meaning behind why we laugh [when tickled]? It’s very mysterious.”
So Ishiyama ventured out to pinpoint the area of the brain that gets activated when a rat is tickled.
What stood out for them was that when the rats were tickled in a relaxed, playful situation, they released high-pitched, ultrasonic vocalizations, almost like a laugh. When the scientists stimulated the same somatosensory neurons in the rat without the tickling, the rats would release the same laugh. But when the rat was made to feel anxious, their vocalizations were much softer when tickled and the neurons in the somatosensory cortex didn’t activate.
This showed that the somatosensory cortex, traditionally thought to only activate when physical touch was used on a subject, can be affected by emotions or mood.
“It wasn’t known that it was influenced by mood. We’ve always thought mood was handled somewhere else in the brain,” he said.
Ishiyama’s rodent subjects also hopped when they were in a playful mood and being tickled – a new observation among the scientific community. The researchers dubbed this rare behaviour “freudensprünge,” the German term for jumping for joy.
The happy hops were just one similarity between humans and rats in this study.
Other similarities included the mood-dependent response to being tickled and the similar response the rats had when they anticipated the tickling (that feeling of already being ticklish when you see a friend playfully approach you with wiggling hands, even if they’re not touching you).
Hate being tickled? Well rats showed another similarity there.
“When you were five years old, I’m sure you enjoyed being tickled. I think this is a universal thing in children, while most adults claim that it’s quite annoying. That’s also the case in rats,” he explained, adding that his study subjects were young rats, while adult rats he’s studied expressed annoyance when tickled.
“[Co-author] Michael Brecht said that maybe the ticklish sensation is a brain trick to make us play more or to have fun,” he said. “It’s known that play behaviour with a lot of social touch is very important to develop normal social cognitive function. So the tickling sensation is important for social development as well.”
Ishiyama said the next mystery of tickling that he’d like to tackle is why we can’t tickle ourselves.