A team of researchers from Humboldt-Universität in Berlin has measured several aspects of the tickle response in humans and developed a theory for why humans cannot tickle themselves – that the brain sends signals inhibiting tickling when we touch ourselves.
Tickling, or being tickled, holds a special place in mammalian response – it doesn’t seem to serve any purpose at all. It also has proven to defy explanation.
Humans are not the only creatures to experience tickling: dolphins, chimps, dogs and rats all have a tickle response. And in all groups, the response is nearly the same – there is a moment of perception followed by a smile and then a laugh. In this new effort, the researchers set out to learn more about tickling, recruiting 12 volunteers for their study, reports MedicalXpress.
In their paper published in the journal Philosophical Transactions of the Royal Society B: Biological Sciences, the group describes studying the physical reactions to tickling in the volunteers.
Each person sat on a chair with their underarms and feet bared, ready for easy access by a partner who was the tickler. Before the tickling, the volunteers were given time to get to know one another a little bit. The researchers also filmed the action with GoPro cameras – one aimed at the face, the other the body part to be tickled.
In studying the videos, the researchers found that as expected, the first reaction to being tickled was a change in facial expression. A smile began forming at approximately 300 milliseconds, followed very closely by a change in breathing. Then, 200 milliseconds later, vocalisations began, generally in the form of a laugh. As the tickling ensued, the researchers asked the person being tickled to rate how ticklish the sensations were that they were feeling. The researchers found that, as expected, the volunteers found the feet to be the most ticklish.
They also found that as ratings of ticklishness rose, so, too, did the volume of laughter.
The researchers ran the same experiment a second time, but on the second go, asked the volunteers to tickle themselves while they were being tickled by the partner. And that, the researchers found, resulted in greatly reducing the ticklishness of the tickles administered by their partner.
The researchers acknowledge that they still have no idea what goes on with the nervous system when tickling occurs, but theorise that the reason people cannot tickle themselves is that the brain sends signals inhibiting tickling when we touch ourselves – otherwise, people would be giggling when putting on socks or scratching under their arms.
Study details
The human tickle response and mechanisms of self-tickle suppression
Sandra Proelss, Shimpei Ishiyama, Eduard Maier, Matthias Schultze-Kraft, and Michael Brecht.
Published in Philosophical Transactions of the Royal Society B: Biological Sciences on 21 September 2022
Abstract
A tickle is a complex sensation: it occurs in response to touch but not unequivocally so, and makes us laugh albeit not when we self-tickle. We quantified human ticklishness by means of physiological, visual and acoustic measures alongside subjective reports, and assessed mechanisms of self-tickle suppression. Tickle responses arose faster than previously reported as changes in thoracic circumference and joyous facial expressions co-emerge approximately 300 ms after tickle onset and are followed by vocalisations starting after an additional 200 ms. The timing and acoustic properties of vocalisations tightly correlated with subjective reports: the faster, louder and higher-pitched participants laughed, the stronger they rated the experienced ticklishness. Externally evoked ticklishness is reduced by simultaneous self-tickling, whereby self-touch evokes stronger suppression than sole self-tickle movement without touch. We suggest that self-tickle suppression can be understood as broad attenuation of sensory temporally coincident inputs. Our study provides new insight on the nature of human ticklishness and the attenuating effects of self-tickling.
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