From bonobos and rats to tickling robots, research is finally cracking the secrets of why we're ticklish, and what that reveals about our brains In a grey-walled room in the Dutch city of Nijmegen, a strange activity is underfoot. Wearing a cap covered in sensors and positioning themselves into a chair, a person places their bare feet over two holes in a platform. Beneath this lies a robot, which uses a metal probe to begin to tickle their soles. Here, at Radboud University's Touch and Tickle lab, volunteers are being mercilessly tickled in the name of science. "We can manipulate how strong the stimulation is, how fast and where it is going to be applied on your foot," says Konstantina Kilteni, who runs the lab, of the robot tickling experiment.

ChatGPT knows how to use the word "tickle" in a sentence but it cannot feel the sensation. Can it then be said to understand the meaning of the word tickle the same way we humans do? In an ongoing debate, AI researchers are teasing apart whether large language models (LLMs) like ChatGPT and Google's PaLM understand language in any humanlike sense. The relationship between embodiment and understanding is one question, along with the nature of intelligence and understanding. Should concepts of meaning, understanding, and intelligence be revisited to create a distinction between how humans and machines understand the world?

As a child, my brother would frequently challenge me to a game he called punch-for-punch. He'd let me hit him in the arm if he could hit me back just as hard. It wasn't a long game; dull punches soon became bruising wallops. Being several years younger and many pounds lighter, I'd often concede quickly, fearing the next blow that, despite the game's equally-hard rule, always felt more forceful than the last. The thing is, my brother and I were both playing by the rules--at least, we thought we were.

Some of us are more ticklish than others, but nearly everyone is unable to tickle themselves. The answer is tied to how we see and how we perceive movement. To get to the bottom of why we can't tickle ourselves, let's first examine another phenomenon. Close one eye, and then carefully push against the side of your other (open) eye, moving the eyeball from side to side in its socket. It should appear as if the world is moving, even though you know it isn't.