Scientists built a tickle robot to solve one of biology’s strangest mysteries
When I enter my laboratory on weekday mornings, it is not uncommon to hear uncontrollable screams of joy or bursts of laughter. Laughter is not traditionally considered the noise of neuroscience, but my research group is somewhat unusual. In the Touch & Tickle Lab, we invite participants to experience Hektor, the tickling robot, in the name of scientific progress.
During a typical visit, participants remove their shoes and socks, then sit in what looks like a dentist’s chair. My colleagues and I place our feet on a platform under which rests Hektor, a robot made up of three electric motors. During a tickling session, Hektor slides probes along the soles of participants’ feet, and they report the intensity of each tickle on a scale of one to 10. Throughout this process, we track people’s facial expressions, heart rate, muscle activity, breathing, and skin conductance (which tells us if they’re sweating). Electrodes placed on participants’ scalps reveal their brain activity.
Being tickled is a sensation that most of us instantly recognize, and some of history’s greatest thinkers have been fascinated by this strange phenomenon. Socrates described this feeling as a mixture of pain and pleasure. Aristotle believed that tickling was a consequence of humans’ delicate skin. And Charles Darwin wrote extensively on the subject, hypothesizing that we may be more ticklish in places that are not frequently touched and only in certain psychological contexts.
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Today, I and other neuroscientists remain perplexed by many fundamental mysteries: Does tickling have any biological function – and if so, why did we evolve to have this experience? What is happening in our nervous system to cause this unique sensation? Why are some areas of the body more sensitive than others? Why do people react in different ways to potentially ticklish touches? Answering these questions could expand our knowledge of how the human body perceives and processes physical sensations. Tickling offers neuroscientists an opportunity to study the relationships between complex brain and body systems, including those involved in emotions, movements and sensations, while answering questions that could help us understand differences in human behavior and development.
Tickling – what we scientists sometimes call gargalesis – may be an evolutionarily ancient behavior.
Research so far suggests that tickling — what we scientists sometimes call gargalesis — might be an evolutionarily ancient behavior. Many primates, including chimpanzees, bonobos, gorillas and orangutans, also do this. And rodents respond to certain forms of touch that may be relevant to tickle research. For example, when a rat’s belly is stroked, it produces many more vocalizations than during gentle touch, and the stimulation appears to activate certain areas of the brain that also light up when humans are tickled.
Tickling also seems to transcend culture. In one study, participants from more than 20 cultural backgrounds, including people from the United Kingdom, Poland, India and Hong Kong, listened to recordings of German speakers laughing spontaneously and were able to recognize which laughs were caused by tickling, as opposed to joy or schadenfreude.
Studies have led to several theories about how and why tickling might have evolved. It could just be a reflex response with no clear function – perhaps it’s just a byproduct of how our touch perception systems developed. It could also help create social bonds between people, such as parents and children. Touch is an extremely important social signal for our species that can help us communicate with each other and increase our sense of closeness. Tickling might make us feel particularly connected to others because it makes us laugh, a response that often accompanies pleasure.
Another theory is that tickling is a behavior our ancestors used to teach their young where to attack others or how to defend themselves in a fight. This idea is based on observations that the playful back-and-forth interactions involved in tickling children and young monkeys resemble “battle simulation” and that some of the ticklish areas of our body (e.g., the armpit) would be vulnerable if attacked in a real fight.
However, not everyone reacts to tickling in the same way: people with certain neurodevelopmental or psychological conditions may experience it very differently. For example, in a study published in 2024, Japanese researchers observed that children with higher scores on tests of autistic traits were less sensitive to tickling and less likely to approach their parents with a positive emotional response to tickling than their peers with lower scores. Such studies show how scientists can use tickling as a tool to understand differences between people, which could one day illuminate important distinctions in how we process and perceive sensations.
People with schizophrenia may also experience tickling differently. When you make a movement, such as scratching your head, your brain can usually predict when and where your body parts will come into contact before it actually happens. This ability generally makes people experience their own touch on their body as less intense than when someone else touches them, and this is considered one reason why most people cannot tickle themselves. But some people with schizophrenia have difficulty predicting and processing the sensation of touching themselves. As a result, they perceive self-touch as more intense and more likely to tickle than people who process touch habitually. Something similar can be seen in individuals with high scores on schizotypal personality traits, who tend to show unusual patterns of thought and social behavior without meeting criteria for a mental disorder.
In our laboratory, we are currently studying how and why the brain can override certain self-generated touch sensations in some people. As with our work with the Hektor robot, we hope our experiments will help create a clearer picture of how people predict and perceive physical contact, thereby revealing some of the mysteries of tickling. Behind every laugh is a fascinating piece of neuroscience waiting to be discovered.
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