Tickling is not a laughing matter. Exactly why we have this mysterious response has baffled scientists until now. But research to be published next week indicates that although tickling may trigger smiling and giggling, it has as much to do with amusement as crying while cutting onions has with sadness. For the first time, psychologists have compared emotional reactions to tickling and to humour and found big differences. Tickling, it seems, is no joke; and may be an involuntary way of toughening up vulnerable parts of the body, or a way of encouraging rough-and –tumble play in developing children.
Tickling has long been a source of fascination: Socrates, Aristotle, Bacon and Darwin have all had views on its purpose and effects. “Darwin claimed that in order for laughter to be elicited, there must be an element of surprise, a pleasant hedonic state. In essence he argued that the tickle is a physical joke,” says Dr Christine Harris, an authority on tickling from the University of California and the lead author of new research being published in the Journal of Cognition and Emotion.
Much of the fascination with tickling lies in its inherent contradictions and riddles. Why do we smile at something we often don’t like and which can be painful? Why can we be tickled but not tickled ourselves? Is the reaction to tickling a reflex or a social behaviour? Why are some people more ticklish than others? Why are some parts of the body sensitive to tickling and others not? Can a machine tickle?
Tickling theorists have established there are two types of tickle, and whether it makes you laugh or not depends on the type of tickle.
“Knismesis tickling is a light or feather touch, an annoying sensation or movement across the skin. Gargalesis is the heavy or laughter-associated tickling,” says Dr Samuel Seldon, a dermatologist at Eastern Virginia Medical School. One of the areas of investigation is whether garagalesis tickling is a reaction or reflex or a behaviour. To shed light on this mystery, Dr Harris set out to compare emotional reactions to tickling, to jokes and to pain, to see whether there were any differences or similarities. “No work has tested for negative aspects of tickling or tried to determine what internal states drive ticklish laughter and smiling. Our work provides the first microanalysis of the facial movements that occur during tickling and is the first to examine the emotional states during tickling,” she says.
The idea that reactions to tickling are a sign of amusement is based on the assumption that the smiling that occurs during tickling is the same as that during humour. But, until now, no one has closely compared tickle-induced smiling to see whether it differs from that of humour. In the research, volunteers were filmed while they were tickled from behind by a researcher for ten seconds. They were also filmed reacting to jokes recorded by comedians and while they put their hand in icy water for as long as they could.
At the end of the experiment they answered questions about how they felt in the different conditions. The researchers then looked at the films to assess negative and positive facial expressions. In particular they looked for the so-called “Duchenne smile” which involves both the smile and a creasing of the skin around the eyes, and which is a response to humour. If volunteers were enjoying the tickle, they would show signs of a Duchenne smile.
The results indicate, that when tickled, people did show some Duchenne smiles but they also showed facial expressions associated with pain, including wrinkling of the nose and raising the upper lip. While they were being tickled, they also showed more emotions – pain and smiling – compared to when they were listening to the comedian.
That, say the researchers, implies that smiling is an automatic response to a stimulus rather than a sign of emotion. “This suggests that ticklish smiling need have no closer a connection to mirth and merriment than crying when cutting onions has to sorrow and sadness. We suggest that ticklish smiling can arise without any positive emotional state being present,” they say.
However the exact process that underlies the tickling phenomenon remains an open question. Why should such a reaction exist? One hypothesis put forward by other researchers is that it evolved to encourage us to instinctively protect soft body areas from attack, which would explain why we try to fend off the tickler, whilst appearing to enjoy it. Those who were ticklish had an evolutionary advantage because they were practised at defending vulnerable areas – and laughing gave them more advantage because it encouraged their tormentors to continue.
The tickling process starts simply enough, when receptors transmit the skin sensation along pathways to the brain. Just what the brain does with it is unclear, although research is helping to identify some of the brain areas involved.
Dr Marin Ingvar and researchers at the Karolinska Institute in Sweden compared brain images of what happened during an actual tickle with those of an anticipated tickle. The results showed that an anticipated and a real tickle lit up the same areas of the brain – the primary and secondary somatosensory cortexes.
Work at University College London, where researchers scanned volunteers’ brains while they were tickling themselves or having their palms tickled by someone else, is also shedding light on why we cannot tickle ourselves. Their results suggest that it can’t happen because the brain is aware that it’s going to happen.
Researchers in California have designed an automated tickling machine and compared its success rate with that of a human tickler. The results of the tests on 35 volunteers, which support the reflex theory, indicate that the machine was just as good a tickler as a human being.
This new knowledge about tickling serves a purpose. Knowing which pathways are involved in tickling helps researchers to understand just how the brain works. Information about how the brain is able to separate tickle from self-tickle, for example, may be of use to schizophrenia researchers. Schizophrenics can have difficulty distinguishing external events from self-generated ones, believing, say, that they are being touched when they are not.
“People with schizophrenia can tickle themselves because the produce realistic hallucinations. They can experience self-tickling with the same intensity as if it were produced by someone else,” says Dr Mark Blagrove, of the University of Wales, Swansea.
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