In a room adorned with gray walls in the Dutch city of Nijmegen, peculiar activities unfold beneath your feet. You find yourself seated in a chair, donning a hat covered with sensors, and your bare feet are placed in holes in the platform. Below, a robot equipped with a metal probe begins tickling the soles of your feet. Soon, the air fills with shrieks, laughter, and a certain painful mirth. Here at Radboud University’s Touch and Tickle Laboratory, volunteers are subjected to relentless tickling for the sake of science.

“We can monitor the intensity, speed, and specific areas of stimulation on the legs,” explains Constantina Kirteni, the lab’s director, regarding the robotic tickling experiment. Simultaneously, researchers document participants’ brain activity and physiological metrics such as heart rate, respiration, and sweating. Armed with these neurological and physiological insights, the researchers aim to tackle age-old questions that have intrigued philosophers from Socrates to René Descartes. Why do we experience ticklishness, what does it reveal about the boundary between pleasure and pain, and does this peculiar behavior serve any real purpose? The findings could illuminate areas such as infant brain development, clinical conditions like schizophrenia, and the structure of conscious experience in our brains.

Though the researchers have yet to publish their findings, Kirteni is willing to share some early insights. Regarding what triggers the tickling sensation, she states, “For us to recognize it as tickling, the contact must be both strong and rapid.” Preliminary analyses also indicate that electroencephalography (EEG) reveals distinct patterns of brain activity when experiencing ticklish feelings. To delve deeper into which brain regions process these sensations, the researchers intend to employ functional MRI, although the robot will require modifications to avoid interfering with the scanner. Moreover, scientists at the institute have initiated inquiries into the intriguing question of whether people actually enjoy being tickled.

“We observe a mix of responses, allowing us to see both those who find it pleasurable and those who find it distressing,” Kirteni notes. While people’s reactions may include smiles or laughter, these do not necessarily correlate with their enjoyment levels. Additionally, perceptions can shift over time. “Some individuals have reported that though it may be enjoyable initially, prolonged exposure can become uncomfortable and even painful,” she adds.

One of the enduring enigmas about tickling that Kirteni is eager to unravel is why self-tickling is impossible. This peculiar fact suggests that unpredictability in stimulation is crucial, a notion supported by contemporary studies. Numerous investigations indicate that our brains predict sensations triggered by our own actions, leading us to perceive our touch as less significant than that of others. This can become particularly perplexing in certain mental health conditions. Research suggests that individuals experiencing auditory hallucinations or sensations of being controlled by external forces find their own touch more ticklish. “This indicates a possible breakdown in how our brains forecast our feelings based on our movements,” Kirteni mentions. “We are keen to explore this further in clinical populations, especially those with schizophrenia.”

What Makes Us Ticklish?

Perhaps the most significant unanswered inquiry revolves around why we are ticklish. Known primarily among humans and their close relatives, tickling may have evolved from behaviors in great ape ancestors. For instance, chimpanzees and bonobos frequently tickle each other during play. In a study published this year, Elisa Demur and colleagues from the University of Lyon in France observed a bonobo colony for three months. They discovered a notable correlation between tickling and age, with older bonobos being tickled more often, while younger ones were tickled frequently.

Demur remarked, “This is intriguing because it aligns closely with human behavior, chiefly as an interaction for young children.” The researchers observed that social bonds significantly influenced the tickling interactions; pairs that primarily engaged in tickling sessions shared strong attachments.

For Demur, this suggests that tickling evolved as a prosocial behavior enhancing connections between youngsters and their group members. This is closely related to pretend play, she adds, since acts appearing aggressive and unpleasant from strangers can be enjoyable in the presence of friends or close acquaintances. In her studies of bonobos at the Lola ya Bonobo Sanctuary in the Democratic Republic of the Congo, she observes how orphaned infants respond to tickling by their human surrogate parents, highlighting the importance of familiarity. “It’s a fascinating behavior. It’s always joyful to see them laugh; they’re incredibly adorable!” she shares.

Regardless of one’s mental state or the relationship with the person (or machine) doing the tickling, even non-consensual tickling can elicit laughter. Some researchers argue that this indicates that tickling is a physiological reflex; however, this does not preclude the idea that its evolution served a social purpose. Another hypothesis suggests that this behavior could help young individuals learn to protect vulnerable areas of their body during play or combat. “The truth remains that we don’t have definitive answers because there are valid counterarguments for all these theories,” Kirteni states.

Nevertheless, focusing exclusively on tickling behaviors in great apes may overlook a significant aspect of this behavior. While rodents are not known to engage in tickling among themselves, they appear to enjoy human tickling. Though previously thought to be non-ticklish, mice have shown a fondness for tickling when they feel comfortable. Researcher Marlies Austrand from the University of Amsterdam found that if mice are relaxed and flipped over, they can delight in tickling, producing high-pitched sounds that resemble laughter.

Interestingly, these sounds are beyond human hearing range, and it’s uncertain whether mice can hear them as well, adding to the mystery of their laughter. While Austrand’s findings are not yet published, it’s evident that rodents respond positively to tickling. “If given the choice between a safe, scented hutch in their home cage and being tickled, mice will choose the latter,” she asserts.

Austrand speculates on why humans and animals react as they do under tickling. Our brains are constantly engaged in predicting external stimuli, evaluating potential threats and survival tactics. She proposes that tickling introduces surprises that contradict these expectations. Yet, if we feel secure, these unexpected sensations can be exhilarating. “This is more of a hypothesis; it remains unproven,” she admits. “But I believe tickling aids animals, especially young ones, in adapting to a fluid environment,” she concludes. Such peculiar behavior may well be an evolutionary quirk that we should embrace.