How does the brain react to being tickled?

Despite being the subject of research for the last fifteen years, tickling is still largely misunderstood and there is a certain mystery attached to this form of social contact. That’s why S .Ishiyama and M. Brecht, researchers at the Centre for Computational Neuroscience, Humboldt University Berlin, wanted to try to understand what happens in the brains of animals and humans when they are tickled. And to uncover part of the mystery of these cerebral mechanisms they started by… tickling rats!

Previous studies had shown that when rats are tickled they produce ultrasonic vocalization; in other words, they ‘laugh.’ These little cries of joy, inaudible to the human ear, are the same frequency (50kHz) as those produced when rodents eat or are in a context of socialization. With the help of a specialized microphone, the researchers found that these vocalizations varied depending on the part of the body that was tickled. The rats, just like us, were more sensitive to being tickled on their tummies and on their feet. So apart from this shared characteristic, what did the researchers observe?

The rodents evidently enjoyed being tickled because once tickling had stopped they rapidly approached and followed the microphone (which the researchers moved) to receive more. In some cases, they performed joy jumps after being tickled; a type of behavior also known by the German term ‘Freudensprünge’.

In order to find out more about the neural correlates of ticklishness, the team focused on the somatosensory cortex. This region of the brain processes physical sensations (such as touch), messages from all over the body are sent to a specific place in the somatosensory cortex. Consequently, the part corresponding to the rat’s trunk was activated when the rat was tickled on its tummy or on its back. The researchers also stimulated this region of the brain directly causing the rodents to emit the same laughs without being tickled. They also noted that the somatosensory cortex was activated when the rats were at play. Finally, anxious rats proved to be much less ticklish than their more relaxed counterparts.

So, the somatosensory cortex is not only a tactile region of the brain but seems to also be involved with the creation of laughter itself. Finally, because being tickled promotes social contact, let’s think about this appeal for tickling taken from a study published in 2004 (Laughing, Tickling and the Evolution of Speech and Self):

Tickle battles bind us together, in a laugh filled give-and-take that may be the basis of all human play. Consider the social choreography of tickle…. For young children who cannot yet talk, being tickled along with the associated laughter is an entry into a mutual social relationships with caregivers. Laughter signals ‘‘I like it; do it again!’’ Crying and fending off the other person signals the game has gone too far. (Robert R.Provine 2004)
Source: S. Ishiyama and M. Brecht, Neural correlates of ticklishness in the rat somatosensory cortex, in Sciences vol.34, Nov. 2016.

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