Tickling is one of the most basic forms of playing, but it's also one of the weirdest and least understood of the body's physical reactions. Why are we ticklish at all? Why does it make us laugh? Why are some parts of the body more ticklish than others? And perhaps the weirdest of all: why can't we tickle ourselves? So how do scientists go about answering these questions? They tickle rats, of course.

The study by researchers at Humboldt University of Berlin builds on earlier work showing that when tickled, rats will actually "laugh" in the form of ultrasonic vocalizations at about 50 kHz, beyond the range of human hearing. Using a specialized microphone, the researchers tickled rats on their back, belly, feet and tail, and studied the calls they made in response, which are in a similar range to the happy sounds rats will make when socializing or eating. The vocalizations varied by body part too, indicating that rats don't have ticklish tails but are particularly sensitive on their belly and feet, just like humans.

The rats' behavior showed they were enjoying being tickled in other ways. When the researchers moved their hands away, the rats would often chase after them for more tickling, and in some cases they actually jumped for joy afterwards, a behavior known as Freudensprünge in German. However you say it, this is clearly a good gig for a lab rat.

"Ticklishness is one of the most mysterious forms of social touch, it's poorly understood," says Michael Brecht, lead researcher on the study. "And what we set out to do is try to understand what happens in the brain when animals and humans are being tickled."

The researchers focused on the somatosensory cortex, a region of the brain that maps out physical stimulation of different parts of the body(Credit: Shimpei Ishiyama & Michael Brecht)

To that end, the team focused on the somatosensory cortex, a region of the brain that processes physical stimulation such as touch, pressure, temperature and pain. That region effectively maps out the whole body, with different parts of it activating in response to touching different parts of the body. Since the back and belly elicited strong reactions from the rats, the researchers honed in on the part of the somatosensory cortex that corresponds to the rats' trunk. As expected, this area of the brain lit up in response to tickling on the belly or back, and matched the intensity of the rats' vocalizations.

The somatosensory cortex also activated when the rats were playing but weren't being tickled, and the researchers found that the animal's mood affected their reactions: anxious rats were much less ticklish than their more relaxed counterparts. To test if the sensation of ticklishness is all in your head, the team directly stimulated the trunk region of the somatosensory cortex, and lo and behold, the rats emitted the same laughter calls.

"The data show us that the somatosensory cortex, a brain structure which we thought was purely tactile, does much more than that," says Brecht. "In particular it seems to be very closely involved in generating the laughter itself. This was unexpected for us. Perhaps ticklishness is a trick of the brain that rewards interacting and playing."

The research is published in the journal Science. The team discusses the project in the video below.

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