It’s a well-known fact that you can’t tickle yourself. Now researchers at the Bernstein Center for Computational Neuroscience in Berlin have discovered why.
The study, published in the journal Philosophical Transactions of the Royal Society B, used facial expressions, vocalizations and subjective reports to measure ticklishness. Participants were subjected to two exercises: in the first, they were stimulated only by external tickling; in the second, they were asked to tickle themselves while someone else tickled them.
Tickling scores were compared between the two exercises. In all participants, self-tickling significantly attenuated the general tickle response.
“Tickling is the perfect naturalistic behavior where we can study the neural basis of learning and brain adaptation,” said Marlis Oostland, professor of neuroscience at the University of Amsterdam, who was not directly involved in the research. Newsweek. “This allows us to study how the brain deals with surprises and unexpected events.”
When you are tickled, your body sends a message to two different parts of your brain. The first, the somatosensory cortex, is responsible for analyzing and responding to touch. The second, the anterior cingulate, governs your body’s response to pleasure. And together they create the sensation of tickling.
There are two different types of tickling. The first, called knismesis, describes a touch that is light and soft, like a hair tickling your nose. The other, called gargalesis, describes the heavier, more rhythmic tickling seen in playful social interactions.
Marina Dávila-Ross, an expert in comparative psychology at the University of Portsmouth, who was also not involved in the study, told Newsweek that this type of tickling is not unique to humans: “Gargalesis … can be seen in all kinds of mammals. ” she said.
Michael Brecht of the Bernstein Center for Computational Neuroscience, who led the study published on September 21, told Newsweek: “We understand that tickling evolved in the context of a fighting game. The happy facial expression and laughter are signals to the interaction partner that it’s okay…to be touched.” This signaling helps both partners distinguish a play attack from a real fight.
Brecht’s study of self-tickling was part of a larger study of the play abilities of the mammalian brain. “We think that such playful abilities of the brain are understudied,” Brecht said.
Play fighting has been implicated in everything from muscle development to stress relief and even social skills.
“During playful behavior such as tickling, you can try out movements and sequences to unexpected events in a safe environment,” Oostland said.
In the latest study, scientists wanted to find out why we can be tickled by others but not by ourselves.
“We don’t think that the distinction between self and ‘other’ is made by the brain,” Brecht said. “Instead, the human and animal data show that as soon as you touch, the brain generates a lot of response suppression, regardless of whether you or someone else is touching you.”
“The neural and behavioral response to tickling occurs only when the tickling sensation comes as a surprise,” Oostland said.
Scientists think this is due to an area of your brain called the cerebellum. “It’s a brain region in the back of your brain important for movement, cognition and filtering relevant inputs,” Oostland said. “The cerebellum filters out the incoming sensation of self-tickling, which likely reduces activity in other parts of the brain [that usually] process the tickle response.”
Tickling also depends on mood and context. “Fear, anger, etc. suppress ticklishness,” said Brecht. A previous study he conducted in rats showed that anxiety-inducing situations suppress the activation of neurons involved in the tickle response.
“You’re only susceptible to tickling when you feel safe,” Oostland said.