For the ticklish among us, just the approach of wiggling fingers is enough to elicit squeals, if not screams. And it turns out we’re not the only ones. Now, a study in rats pinpoints the “tickle center” of the mammalian brain, showing for the first time that stimulating neurons in that region can elicit a paroxysm of ultrasonic squeaks, the rat version of human laughter.
Scientists have puzzled over the mysteries of tickling for millennia, with Aristotle famously asking why most people can’t tickle themselves. There are important neurological and psychological reasons to study tickling: One telltale symptom of schizophrenia, for example, is that people can tickle themselves. Tickling is also linked to our ability to laugh, play, and feel good, notes Shimpei Ishiyama, a neuroscientist at the Humboldt University of Berlin. “Neuroscientists are so obsessed with deficits such as depression and anxiety, it’s rare to find papers about positive emotions,” he says.
Previous studies of tickling in people reveal a mosaic of brain regions that orchestrate the tickle response. One of these regions, the somatosensory cortex, was thought to merely process the sensation of being tickled, but not trigger the laughter that follows. The new work, published today in Science, “shows for the first time that laughter can be elicited by stimulation” in that region, rather than through traditional emotional circuitry, says Elise Wattendorf, a neuroscientist at the University of Fribourg in Germany who was not involved in the research.
Rats weren’t always thought to be the chortling sort, but now a large body of literature backs up that they serve as a good model for tickling in people. “Although it was a very bold idea in the beginning, experiments show that rats are enjoying it,” Ishiyama says. Other studies suggest that the ultrasonic squeaks they make are expressions of pleasure. Not only do rats return over and over again to the place they were tickled, the handling triggers the neurotransmitter dopamine in key reward-related brain circuits in the rodents, he says. Best of all, rats display a classic expression of positive emotion, found across many species, including dogs, foxes, lambs, guinea pigs, and human children, called “joy jumps,” Ishiyama says. This involves leaping into the air with both legs together, he explains.
To tickle a rat effectively takes some practice, but Ishiyama is an expert. Starting with juvenile male rats, which tend to be most playful, he spends a week or two letting them get used to being tickled on the back and belly. “It’s pretty much like if you tickle kids or dogs or cats,” he says. Over time, the rats learn it is fun to play with this big hand, and they start chasing it and even recognizing it as a playmate, he says.
Once the animals were trained, the scientists inserted electrodes into their somatosensory cortex to record neural activity during tickling. Surprisingly, the cells increased their firing rates not only in response to the physical stimulus of being tickled, but also after, as the rodent chased the hand and “giggled.” That contradicts the traditional idea that the somatosensory cortex only processes sensory information, not triggers other behaviors, Ishiyama says.
Even more surprising was what happened next: When the research team applied a small amount of electrical current to the same cells, the stimulation caused the rats to vocalize in the same playful way they had while being tickled and chasing the hand, the team reports. “This is very important, clear evidence that the activity of those cells is responsible for ticklishness,” Ishiyama says.
One of the fascinating things about ticklishness is that it is so mood-dependent, Ishiyama says. “Even Darwin observed that children tickled by a stranger would rather scream than laugh.” To determine whether rats, too, are less ticklish when they are anxious, the researchers put them on an elevated platform and exposed the nocturnal animals to a bright light. The rats were distinctly less ticklish, and their brain activity showed a suppression of the cells that had fired so enthusiastically in the previous experiment, even when they were stimulated with the electrodes, he says.
The results suggests that hard-wired connections must form early in life between somatosensory cortical neurons and cells in other brain regions that process other aspects of tickling, from the motor neurons that trigger laughter to the socially attuned neurons that recognize whether a tickler is a friend or stranger, Wattendorf says. The study “constitutes an outstanding result,” she says.
That early development fits well with other observations: If rats and people aren’t tickled when they are young, they tend not to enjoy it as adults. That may explain another observation. Despite devoting much of his career to tickling rats, Ishiyama himself despises being tickled.