It is a well-known fact that you can't tickle yourself. Now researchers from the Bernstein Center for Computational Neuroscience in Berlin have found out 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 underwent two exercises: in the first, they were stimulated by an external tickler only; in the second, they were asked to tickle themselves at the same time as being tickled by someone else.
Ticklishness scores were then compared between the two exercises. In all of the participants self-tickling significantly weakened the overall tickle response.
"Tickling is the perfect naturalistic behavior in which we can study the neuronal basis of learning and adaptation of the brain," Marlies Oostland, a professor of neuroscience at the University of Amsterdam who wasn't directly involved in the study, told Newsweek. "It 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 cingulated cortex, governs your body's response to pleasure. And together, they create the feeling of ticklishness.
There are two different types of tickling. The first, called knismesis, describes touch that is light and feathery, like a hair tickling your nose. The other, called gargalesis, describes the heavier, more rhythmic tickling that is observed in playful social interactions.
Marina Davila-Ross, an expert in comparative psychology from 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 across all kinds of mammals," she said.
Michael Brecht, from the Bernstein Center for Computational Neuroscience, who led the research published on September 21, told Newsweek: "We understand that ticklishness evolved in the context of play-fighting. The joyous facial expression and the laughter are signals to the interaction partner that it is ok...to be touched." This signaling helps both partners distinguish a play attack from real fighting.
Brecht's self-tickle study forms part of a larger investigation into the playful capacities of the mammalian brain. "We think such playful capacities of the brain are understudied," said Brecht.
Play fighting has been implicated in everything from muscle development to stress relief and even social skills.
"During playful behaviors, such as tickling, you can try out movements and sequences to unexpected events in a safe environment," said Oostland.
In the latest research, scientists wanted to establish why we can be tickled by others, but not by ourselves.
"We don't think the self vs 'other' distinction is made by the brain," said Brecht. "Instead, the human and animal data suggest that as soon as you touch yourself the brain generates a lot of response suppression irrespective of if you or somebody else touches you."
"The neuronal and behavioral response to being tickled only occurs when the sensation of tickling comes as a surprise," said Oostland.
Scientists think this is due to a region of your brain called the cerebellum. "This is a brain region in the back of your brain important for movement, cognition, and filtering of relevant inputs," said Oostland. "The cerebellum filters the incoming sensation of self-tickle, which is likely what reduces the activity in other parts of the brain [that usually] process the tickle response."
Tickling is also dependent 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 firing of neurons involved in the tickle response.
"You are only susceptible to tickling when you feel safe," said Oostland.