How tickling rats is revealing hidden mysteries of the brain

Yes, rats can laugh. But not in the same way that humans do; as an outburst of ultrasonic squeaks.

And now, scientists are using ratlaughterto map a part of the brain called the periaqueductal gray (PAG). Found in the midbrain, the PAG is a cell-dense region (also known rather aptly as the ‘midbrain central gray’), which is key for behavioural responses to threats, and in animals, this includeshunting behaviours in predators and defensive behaviours in prey.

We’ve known for a while now thatrats like to be tickled; they produce a high frequency (around 50 kHz) laugh in response to what scientists call ‘manual tickling sessions’. As human hearing tops out at around 20 kHz, special microphones were needed to hear them laughing.

Through measuring rats’ brain activity (via electrodes implanted into the brain), along with these giggles, researchers from Humboldt-Universitat zu Berlin have determined that the PAG is also essential for play and laughter, publishing their findings in the journalNeuron.

“We know that vocalisations such as laughter are very important in play, which supported the idea that there is some sort of organisation signal in the brain regulating this behaviour," says senior author ProfessorMichael Brecht, a neuroscientist at the Humboldt-Universität zu Berlin. "For example, children check for laughter when they play-fight with each other. If their playmate isn't laughing any more, they stop fighting."

Despite it being the universal language of childhood, and one which incorporates learning, experience, knowledge and imagination, play remains one of the least understood forms of behaviour. The neural pathways that control playfulness in either humans, or other animals have remained somewhat of a mystery.

“A lot of people think that play is childish or not a very decisive behaviour, but play is underrated,” says Brecht. “In my perception of play, it’s a self-training behaviour. Usually, brains serve for controlling behaviours. Play behaviour, however, seem to serve for growing brains.”

After making sure the rats were comfortable in their environment, researchers carried out dorsal tickling (tickling their backs), ventral tickling (tickling their bellies) and played games of ‘chase the hand’.

Strong neural responses in the lateral column of the PAG were observed in both tickle-time and playtime, which coincided with these ultrasonic chuckles; they were happy.

然而,当一个陌生的envir的老鼠onment designed to provoke anxiety, not only did they stop laughing, but those same cells in the PAG decreased their activity. Likewise, inactivation of the PAG lead to impaired playful behaviours and ticklishness.

这是一个强烈的暗示playfulness is instinctual, whichties in with previous researchthat has shown that playfulness persists, even if the cortex, the outer surface of the brain which controls consciousness, fails to develop.

Next, the researchers plan to compare the playfulness of other species, to see whether similar activity is also observed in the lateral column of the PAG. Another question they are keen on answering, is whether giving younger rats different play habits will change the way that this area of the brain develops as it matures.

Read more about rats:

• These search and rescue rats can find earthquake victims amongst the rubble
• Adorable landmine-detecting rat awarded tiny medal for bravery
• From dreaming to life-saving sniffs: 13 science-based rat facts