A group of researchers in the Netherlands is using robotics and neural measurements to analyse why certain tactile stimuli cause laughter, discomfort or pain. This phenomenon, which is also studied in animals, raises new questions about perception, evolution and social relationships. In a unique laboratory in the city of Nijmegen, Netherlands, research into the phenomenon of tickling has taken on an unprecedented technological and scientific dimension. Using robots equipped with metal probes and volunteers wearing sensor-equipped helmets, the team led by Konstantina Kilteni of Radboud University investigates everything from the most basic tactile sensations to the neural complexities associated with tickling.
Participants, subjected to sessions during which their bare feet are stimulated by a robot, not only react with laughter and screams, but also demonstrate measurable physiological reactions: changes in heart rate, breathing and perspiration. While the sensors record this data, the researchers focus on unravelling mysteries that have existed since ancient times.
In this sense, the questions concern the meaning of tickling, its ambivalent nature between pleasure and revulsion, and its evolutionary purpose.
Kiltini and her team not only precisely regulate the speed and intensity of the robotic stimulus, but have also already discovered some key patterns in the electroencephalography records. A preliminary analysis shows that ‘the touch must be strong and very fast to be perceived as tickling,’ says the researcher. These stimuli elicit brain patterns that are different from other tactile sensations.
The team also intends to determine, through future functional magnetic resonance imaging tests, which areas of the brain process these specific stimuli, although adapting the robots to the scanner environment poses a technical challenge.
In addition to physical measurements, the laboratory investigates the subjective component: whether the tickling experience is pleasant or unpleasant. The comments collected show that individual perception varies significantly. ‘We see everything: both those who find it pleasant and those who find it unpleasant,’ explains Kilteni.
While some report initial positive sensations that become unpleasant over time, others express aversion from the outset. According to the scientist, some people anecdotally report that ‘it may be fun at first, but when it continues for some time, it becomes unpleasant and even painful.’
The question that interests scientists and philosophers is the impossibility of tickling oneself. It has been proven that the brain predicts and partially suppresses the sensations caused by its own movements, so that self-stimulation does not cause the same tickling effect as stimulation caused by another person.
However, previous studies have shown that this mechanism changes in people with certain mental disorders, such as schizophrenia, in which those who have auditory hallucinations or a sense of external control may feel tickling caused by their own movements.
In this regard, Kilteny notes: ‘This indicates that this brain mechanism, which allows us to predict how we will feel based on our movements, seems to have some flaws… It is also something we would like to test in clinical populations with schizophrenia.’
With regard to the evolutionary function of tickling, the debate remains open and multifaceted. Based on observations of primates, especially bonobos and chimpanzees, experts have discovered patterns that indicate the possible social nature of this phenomenon.
Studies conducted with bonobos show that tickling is mainly directed at young individuals and that social relationships determine the frequency of these episodes, with adults being the main ‘donors’ and young people the ‘recipients.’ Elise Demouret, from the University of Lyon, explains: ‘This is interesting because it is very similar to what happens with humans and means that this behaviour is mainly directed at babies.’
Furthermore, she emphasises: ‘The social connection has a very strong influence. Therefore, couples who participate more often in tickling sessions also have a very strong emotional connection.’
According to Demuro, the available data points to ‘clear evidence that tickling has evolved as a pro-social behaviour that strengthens the bonds between young people and other members of their group.’
In the case of bonobos at the Lola ya Bonobo reserve in the Democratic Republic of Congo, she adds: ‘It’s a very special behaviour and it’s always enjoyable because they laugh and it’s so cute!’
However, there are theories that suggest that laughter caused by tickling is simply a physiological reflex, independent of relationships or context. Others suggest that it may have a defensive function, helping young people to identify vulnerable areas of their bodies and train reactions to protect them.
The lack of consensus is evident in Kilteny’s words: ‘The reality is that there are arguments against all these theories, so we don’t really know.’
Studies on animals that do not belong to the primate group, especially rodents, bring unexpected nuances. Studies such as that by Marlis Oostland of the University of Amsterdam document that rats can react similarly to humans when they are in a safe and relaxed environment: when tickled, they emit sounds inaudible to the human ear, similar to laughter. Oostland explains it this way: ‘If we turn them over and they remain relaxed, we can start tickling them, and that’s when we hear sounds similar to laughter.’
He adds: ‘If we give rats the choice between a little house in the cage, which is completely safe and has its own smell, or if the researcher tickles them, the animals will choose tickling over shelter in the little house.’
The latest hypotheses suggest that tickling tests the brain’s prediction and response system. Oostland puts it this way: “Our brain is constantly making predictions about the world around us, making decisions about what might be a threat and what we should do to survive. Tickling involves stimulation that contradicts our predictions. However, if we feel safe, this surprise can be stimulating.”
Thus, this phenomenon can be understood as an adaptation strategy, in which the brain constantly calibrates its threshold of vigilance in the face of the unpredictable, as long as this occurs in a safe environment.