It’s an oddity of physiology — humans can’t tickle themselves. Studies suggest this stems from the brain’s ability to predict sensations caused by our own movements, removing the surprise necessary for a good tickle.

But George Van Doorn, a psychologist at Federation University in Australia, wanted to find out if the brain’s predictive power really explains why we can’t tickle ourselves. It’s a serious question, since tickling gives us a clue as to how the brain processes sensations, and can even teach us a bit about mental illness. (People with schizophrenia, for instance, actually can tickle themselves, for unknown reasons.)

Jay Smith

“The dominant theory also suggests that in highly surprising situations, a person will be able to tickle themselves,” Van Doorn says. The brain’s predictive abilities become too poor. To simulate this in the lab, Van Doorn and colleagues set up an experiment where the participant and experimenter would sit on either side of a rod with foam at both ends. Either person could move the rod, causing the foam to make light contact with both people’s left palms. The study’s participants would then don goggles connected to a camera on a helmet. When the experimenter wore the helmet, the participant saw from the experimenter’s perspective. This created what researchers call a body-swap illusion in the participant: “Their hand felt as though it was the experimenter’s,” Van Doorn explains, allowing the participant to surprise himself. “You’re tickling yourself, but feel another person is doing it.”

“According to the current thinking, people in this situation should be able to feel the tickle, but we found they couldn’t,” Van Doorn says. The self-tickling lost out every time, even when including visual delays to make sure the brain wasn’t just adapting to the illusion.

“Our results challenge this widely accepted theory about how our brains work,” he says. They also provide the first evidence supporting an alternative theory of tickling called active inference, which suggests that the brain simply dampens all sensory input during any movement, including tickling, to better react to new sensations. Further tests are necessary to determine exactly which theory gets the last laugh.

[This article originally appeared in print as "Tickle Yourself, Elmo."]