The human response to being tickled is really danged weird. When someone pokes another someone just so, the result is often spontaneous, uncontrollable laughter.

This response is known as gargalesis, and here's the thing: neuroscientists just have no idea why it happens – especially because, for many people, the sensation of being tickled is not a pleasurable one. Nor do we know why some touches are ticklish and others are not, or why some parts of the body are more prone to evoking that gargalesis.

It might sound trivial – but to neuroscientist Konstantina Kilteni of the Karolinksa Institute and Radboud University, and many others, it's no laughing matter.

"Studying human ticklishness might sound like a humorous topic, but it has many important implications," she told ScienceAlert.

"From a purely neuroscientific perspective, tickling intersects with many branches of neuroscience – including clinical, developmental, motor, and affective neuroscience. I am fascinated by the fact that most of us can clearly recognize the sensation, that some of the greatest thinkers of human history, like Socrates, Aristotle, and Darwin, were intrigued by it, and yet, despite this long-standing curiosity, we still do not fully understand how it works."

In a comprehensive review of the published scientific literature on tickling, Kilteni has laid out the current state of research into the topic, identified the questions that still need to be answered, and suggested avenues and strategies for further research.

Tickling is a nearly universal human experience. It's one of the first ways parents play with their children, who, as they get older, begin to display different responses to tickling – they squeal and squirm and run away, but then come back for more.

It's been observed in non-human primates, and something very similar to gargalesis has been observed in rats, suggesting implications for evolutionary neuroscience.

In addition, neuroatypical individuals show different responses to tickling than neurotypical individuals. People with autism spectrum disorders perceive touches as more ticklish than neurotypical individuals, and people with schizotypal disorders perceive their own touches as just as ticklish as those of other people.

There is so much that we don't understand about tickling or how the gargalesis mechanism works, which Kilteni summarizes in five questions:

  • Why does touch feel more ticklish on certain areas of the body than on others?
  • Do we enjoy being tickled, and if not, why do we laugh?
  • Why are most people unable to tickle themselves?
  • Why are some people very ticklish while others are unresponsive?
  • Why do we experience tickle sensations?

Most of the research into tickling and gargalesis, Kilteni points out, focuses on the differences between gargalesis and genuinely joyful laughter, rather than the brain response. These studies also tend to rely on tickling by hand, which is difficult to replicate to ensure consistency from study to study. And the biggest problem: there is no standardized definition of what tickling actually is.

"In fact, there are relatively few studies on this topic," Kilteni said.

"Our approach is bottom-up: we focus on the physical mechanisms first, aiming to answer questions that are more straightforward to investigate experimentally, particularly those related to brain activity and the physical and physiological characteristics of ticklishness.

"Establishing scientific rigor is a prerequisite for eventually addressing broader questions about the evolutionary or anthropological explanations of ticklishness."

The aim of her review was to assess the current state of the gargalesis research landscape, and identify a path forward. At her own lab, Kilteni has a special apparatus: the participant sits in a chair, inserts their feet through holes in an attached plate, and a robotic hand applies the tickle. This keeps the tickle consistent from individual to individual.

"Our long-term question is: What purpose does ticklishness serve in humans and other species?" she explained. "But first, we need to establish a strong scientific foundation to answer that question."

The researchers plan to conduct tickling experiments on individuals with brain abnormalities to investigate in greater depth the brain regions involved in the inability to tickle oneself. There's a long way to go, but the goal, Kilteni believes, will be worth the giggles.

"It's possible that ticklishness once served a function but no longer does. But it might be that ticklishness serves a function we simply have not discovered yet," she said. "It remains an exciting mystery to solve. I think there is probably a lot more behind this phenomenon than we might think!"

Her review has been published in Science Advances.