Whay are we ticklish? Rats tell us!

Why we laugh when we are tickled?

In a recent study published in Neuron on Friday, researchers from the Bernstein Center for Computational Neuroscience Berlin embarked on a quest beneath the cerebral cortex to locate a central hub for play.

Shimpei Ishiyama and Michael Brecht, the researchers behind the study, opted to delve deeper into the rat brain. Their primary focus was directed towards the somatosensory cortex, an expansive brain area responsible for processing sensory details like touch, pain, temperature, and pressure. This cortex comprises distinct zones that correspond to various body parts.

Rats do not exhibit laughter as we do, but when they derive pleasure, they emit ultrasonic vocalizations beyond human perception. “We recognize that vocal cues akin to laughter hold substantial sway in shaping playful conduct, bolstering the idea that there exists a form of cerebral coordination responsible for regulating this behavior,” elucidates Michael Brecht, a neuroscientist leading the research at Humboldt University in Berlin. He further illustrates, “For instance, children ascertain that mutual laughter accompanies their play-fighting. If their playmate’s laughter subsides, they curtail their wrestling.”

Consequently, researchers fashioned an environment for the rodents where they enjoyed the freedom to roam and granted them several days to acclimatize. Subsequently, they embarked on traditional play interactions with the rats, instigating pursuits like attempting to grasp their hands or gently tickling their abdomen and dorsal area. The creatures unreservedly expressed elated vocalizations, intimating their relish for such diversions.

Why rats are ticklish as humans? The role of PAG

A specific neural domain, the periaqueductal gray (PAG), emerged notably active both during laughter and play. Inhibited PAG activity translated to diminished engagement in play and a decreased tendency to emit laughter in the rats. Inducing anxiety curtailed their “laughter,” coinciding with diminished activity in PAG cells. This cerebral area resides within the midbrain, a component of the brainstem controlling vocalizations and the fight-or-flight response, an instinctual physiological retort to perceived threats.

The PAG assumes roles in responding to danger and pain, as well as vocalization. Insight from rat tickling underscores the indispensability of this brain region in facilitating play. In rats, “laughter” manifests as high-pitched chirrups, and intensified tickling seems to correlate with heightened proclivity for play.

Parallel to prior investigations, mice emitted shrill chortles amid tickling, affirming their enjoyment even as they were tickled on their underbelly and dorsal surfaces. Subsequently, they gravitated toward the hand, seemingly seeking to rekindle the pleasurable sensation. Nevertheless, when the same actions were replicated on rodents experiencing heightened stress, laughter did not ensue, underscoring the pivotal role mood plays within this cerebral realm.

Concisely, akin to humans, mice are susceptible to tickling solely when in states of contentment and exuberance.

Brecht postulates, “While the evolutionary paths of rats and humans diverged around 100 million years ago, their response to tickling shares striking commonalities.”

The upcoming phase for the research team encompasses analogous experiments involving different animals, intending to ascertain if inhibiting the PAG yields comparable outcomes. This tickling-based investigation underscores the significance of play across diverse creatures, spanning mammals, insects, avians, and reptiles. Moreover, this exploration has only commenced its journey of discovery.