The dictionary of the Royal Spanish Academy says that tickling is a “nervous excitement accompanied by involuntary laughter, which is experienced in some parts of the body when they are lightly touched.”
Scientists already know that tickling usually produces a pleasurable sensation thanks to dopamine – a key neurotransmitter in the brain’s reward system – and that this fact causes laughter. However, in the Middle Ages it was considered a form of torture.
Today, its function and neurological mechanisms are still unknown. It is even a question to elucidate whether they have any evolutionary function.
Interestingly, the results of some tests suggest that the somatosensory cortex (where the “tickle neurons” are located) may play some role in mood. This is a surprising finding because traditionally this region of the brain has been primarily associated with the sensation of touch.
Brain imaging techniques also show that there is in parallel a lower activation of the anterior cingulate cortex in the case of trying to tickle oneself and it would seem that the cerebellum is responsible for this inhibition when the agent is oneself.
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In the Middle Ages it was considered a form of torture
Although all of us, or almost all of us, can be victims of tickling, we still have no idea why we have it. It is a strange mechanism that includes the participation of several things at the same time: the senses, various areas of the brain and, most importantly, someone who does them (you cannot wake them up yourself).
To begin with, there are two types of tickling. We have the first in common with many animals; It is a sensation of itching, intense and even unpleasant tingling, called “knismesis” (it is the reason why flies are hated the most when we are resting). The prevailing theory is that “knismesis” is a perfect evolutionary trait to react instantly to a situation of alarm or potential danger (scorpions, spiders, poisonous insects, flies, etc.).
Less widespread in nature are those that make us laugh and are called “gargalesis”, which are found only in humans and primates: this type suggests, according to one of the most widespread theories, that they serve to promote social bonds, something quite useful in the common context or when the outside world is hostile.
In humans, specifically, “gargalesis” stimulates laughter, another of our most complex and unknown responses, although we know that laughing stimulates all participants in a game or similar situation.
Tickling causes an extreme response in areas of our body that are not directly related to skin sensitivity. If this were the case, the palms of our hands would be our most vulnerable point; However, the armpits, the soles of the feet or the least expected parts are more so.
We know that the brains of children and adults are better able to track their own movement and it is useful to know where the hand or leg is at any given moment. That is why it is thought that tickling always includes the factor of surprise, that is, not knowing where the sensation is going to come from. For this reason, tickling yourself doesn’t work: you know where you’re going to touch yourself, and the brain activity related to tickling declines when you try to provoke it, which also shows that they are going on their own sensory highway.
The truth is that monitoring with electrodes implanted in rat brains has allowed a group of researchers to identify the region of the brain that activates the natural reflex to what is known as tickling. And although it is a sensation that everyone experienced (even Aristotle already reflected on it more than two thousand years ago and Darwin wondered about its reasons), today researchers still have a long list of unanswered questions about this strange behavior. .
The scientists, in addition to observing the behavior of the rats, were able to monitor what happens in the brain when they are tickled and the results showed that groups of neurons in the somatosensory cortex are activated. This is not surprising as it is the main region of the brain that integrates tactile stimuli.
“gargalesis” stimulates laughter, another of our most complex and unknown responses
However, I do draw attention to the fact that these same neurons are activated when the rat plays chasing the experimenter’s hand, but not when the animal is placed in a stressful situation, such as on top of a platform or under strong lighting. Under stress the tickle neurons do not activate and stop reacting with vocalizations when the abdomen is touched.
That is, just like in people, anxiety inhibits tickling. This observation demonstrates that the somatosensory cortex not only responds to tactile stimuli, but also to emotional states such as anxiety. Charles Darwin already noted, in this sense, that “the mind must be in a pleasant state” for tickling to make one laugh.
Currently, there is the idea that there is also a relationship from tactile stimulation to emotions and, as Professor Michael Brecht suggests, that in itself would explain the comforting effect of caresses and hugs.
As for why tickling has appeared throughout evolution, Brecht suggests that “it may have a relationship with play behaviors.” He even recalls in favor of this hypothesis that the groups of neurons that are activated by tickling are the same as when rats play chasing the experimenter’s hand. This would explain that perhaps the sensation is a trick of the brain to make playing and interacting with others rewarding.
At the Institute of Biology in London they studied why when you try to tickle yourself you don’t laugh. To do this, they analyzed a group of people using functional magnetic resonance imaging (fMRI) while they were tickled or when they provoked it themselves. It was found that the brain was capable of predicting tickling and that by recognizing that it was oneself who was trying to provoke it, both the tickling sensation and the laughter response were canceled because the somatosensory cortex was not activated.
The somatosensory cortex not only responds to tactile stimuli, but also to emotional states such as anxiety
More than a century ago, Darwin interpreted that the tickling response serves to create social bonds, an important fact to create or reinforce social relationships between equals. For this reason, it seems that the brain created the differentiation between the two types of tickling, since self-inflicted tickling would not have much meaning or purpose in socialization and even less in evolution.
At the level of the nervous system, tickling is just a sensation that involves the stimulation of nerve fibers related to touch and pain. But when they become something social, they imply that there is something more interesting and that is why at some point in evolution they became something fun.
The psychologist and neuroscientist Robert Provine has studied them in several animal species and believes that they are the primary stimulus of laughter, that is, its ancestor that is also present in a similar way in rodents, dogs and elephants. According to several observations, its origin dates back more than 80 million years, when the ancestor of all these animals, humans included, was still alive.
It is believed that the function of these social tickles is to create social bonds with other members obtaining pleasure. The first of all is the emotional bond with the mother, and, subsequently, that same happiness extends to other social relationships. That’s why, although some deny it, they are enjoyed a lot.
Another hypothesis compatible with the previous one is that during childhood and adolescence, a stage in which some mammals tickle themselves more while playing, they also help train defensive skills by protecting vulnerable areas of the body. The body language observed during a tickling session corresponds to defensive postures, which is consistent with this proposal.
But what can be said is that tickling is the most fantastic of toys and an excellent strategy to play with. Only two people who feel united can tickle each other and, once again, we are faced with another of those “magical” mechanisms with which evolution rewarded quality relationships.
Article previously published in , the online platform for neuroscientific training and transferred for publication in .
Author:
Luis M. Labath: Physician Specialist in Internal Medicine. Former Medical Director of the José M. Cullen Hospital. Honorary Member of the Argentine Medical Association. Designated as Master of Latin American Medicine by the Latin American Medical Association.
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