What the evolution of tickling tells us about being human

In a gray-walled room in the Dutch city of Nijmegen, strange activity is happening underfoot. After putting on a cap covered with sensors and sitting in a chair, a person places his bare feet on two holes in the platform. Underneath is a robot that, using a metal probe, begins to tickle the soles. Soon, squeals, squeals and painful laughter echo throughout the space. Here at Radboud University's Touch and Tickle Lab, volunteers are tickled mercilessly in the name of science.

“We can manipulate the strength of the stimulation, the speed and where it hits your leg,” says Konstantina Kilteni, head of the lab, about the robot tickling experiment. Meanwhile, the researchers record the participants' brain activity and physiological parameters such as heart rate, breathing and sweating. Using neural and physiological recordings, the researchers have one goal: to finally answer the questions that have troubled thinkers from Socrates to Rene Descartes. Why do we tickle, what does this tell us about the boundary between pleasure and pain, and does this strange behavior have any real function? The answers may shed light on neurodevelopment in infants, clinical conditions such as schizophrenia, and how our brains construct our conscious experiences.

The researchers have not yet published their results, but Kilteney can tell some of what they found. “The touch needs to be strong and very fast to be perceived as tickling,” says Kilteney when it comes to what constitutes tickling. Preliminary analysis also shows that electroencephalography (EEG) recordings capture different patterns of brain activity when someone experiences tickling sensations. The researchers plan to continue the study using functional MRI to determine which areas of the brain produce tickling sensations, although the robot will need to be adapted so that it does not interfere with the scanner. Scientists at the lab have also begun exploring the sticky question of whether people really enjoy being tickled.

“We see a little bit of everything, so both people who like it and people who don't like it,” says Kilteney. People may smile or laugh, but it doesn't always correspond to whether they say they liked it or not. Plus, impressions can change over time. “Some people tell us that it may be fun at first, but when it is applied to the body for a long time, it starts to become unpleasant and even painful,” she says.

One of the long-standing mysteries of tickling that Kilteney seeks to understand is why it is impossible to tickle yourself. This fact seems to suggest that stimulus unpredictability is important, which is supported by current research. Numerous studies have shown that our brain predicts the sensations generated by its own actions and suppresses them, which is why we usually perceive our own touches as less intense than those of others. This appears to be disrupted in some psychiatric illnesses: studies have shown that people with auditory hallucinations and a sense of being controlled by an external force find their own touch more ticklish. “What this tells us is that this mechanism by which the brain is supposed to predict how we will feel based on our movements seems to have some flaws,” says Kilteney. “This is also something we would like to test in clinical populations with schizophrenia.”

Why are we ticklish?

Perhaps the biggest unanswered question is why we get ticklish in the first place. Only humans and our close relatives are known to exhibit tickle behavior, suggesting that it may have evolved in one of our great primate ancestors. Take chimpanzees and bonobos, who often tickle each other while playing. IN study published earlier this year, Elisa Demuru from the University of Lyon, France, and her colleagues spent three months observing a group of bonobos in the Singou Valley in France. They found a strong correlation between tickling and age: older bonobos were more likely to be tickled, while younger bonobos were more likely to be tickled.

“It's interesting because it's the same thing as in humans, which means it's mostly infant-oriented behavior,” Demuru says. “We noticed that social connections have a very strong influence. [pairs] who mainly participate in tickling sessions are also [pairs] there is a very strong partnership between them.”

For Demuru, this is a strong sign that tickling has evolved into a prosocial behavior that strengthens bonds between children and other members of their group. This is closely related to play fighting, she says: actions that might seem aggressive or unpleasant if performed by a stranger can be enjoyable when performed by close family or friends. Demuru also studied bonobos at the Lola ya bonobo sanctuary in the Democratic Republic of the Congo, observing how orphaned babies react to being tickled by human surrogate parents and testing the importance of familiarity. “It's a very special behavior and it's always nice because they laugh and it's so cute!” she says.

Even unwanted tickling can cause laughter, regardless of the state of mind and attitude towards the person (or machine) doing the tickling. Some researchers argue that this proves that tickling is a physiological reflex, although this does not rule out the possibility that tickling evolved to serve a social function. The third hypothesis suggests that it helps young people learn to defend themselves in combat by protecting vulnerable areas of their body. “The reality is that there are arguments against all of these theories, so we really don't know,” Kilteney says.

However, focusing solely on the tickling behavior of great apes may miss an important part of the puzzle. Although they are not known to tickle each other, rodents seem to enjoy being tickled by humans. It was not previously believed that mice were ticklish. But Marlies Ostland from the University of Amsterdam in the Netherlands found that as long as mice are comfortable, they can enjoy being tickled. “If you turn them over and they stay relaxed, you can start tickling them and then we'll hear sounds like laughter,” she says.

These vocalizations are too high pitched for humans to hear. Interestingly, mice may not hear them either, so it remains a mystery why mice laugh at all. Ostland's research has not yet been published, but it is clear that rodents enjoy being tickled. “If we allow mice to choose between a house from their home cage, which is completely safe and has its own scent, or tickling from an experimenter, then the animals will choose to be tickled rather than hide in their house,” she says.

Ostland has her own opinion about why animals, including humans, react this way to tickling. Our brains are constantly making predictions about the world around us, making decisions about what might be a threat and what we need to do to survive. Tickling, she says, involves stimulation that disrupts our predictions. However, if we feel safe, this surprise can be uplifting. “This is a hypothesis that I don’t think has been confirmed. [proved] However, I see tickling as something that helps animals, especially young animals, prepare for an ever-changing environment,” she says. Whether we like it or not, perhaps this strange behavior is an evolutionary quirk for which we should be grateful.