When rats giggle, which part of their brains light up? Scientists find out
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What happens in the brain when we are tickled? In 1999, scientists found that rats vocalize, just like humans, when they are tickled. Do they laugh or do they threaten to injure the person tickling them because it’s so annoying?
At the neurobiological level, previous research has been able to identify the area in the brain responsible for behaviors like fear, reward, sensory processing, and cognition, but play has been one of the least understood.
But scientists wanted to find out which part of the brain gets activated and how does tickling impact the rats’ gray matter. A research team in Germany found that a structure in rat brains called the periaqueductal gray (PAG) is essential for play and laughter.
The brain's 'play spot'
Located in the midbrain, the periaqueductal gray (PAG) is a key structure responsible for increase or decrease in pain, sympathetic responses, defensive and aversive behaviors.
“We know that vocalizations such as laughter are very important in play, which supported the idea that there is some sort of organization signal in the brain regulating this behavior,” said senior author Michael Brecht, a neuroscientist at the Humboldt-Universität zu Berlin and co-author of the study, in a press release.
“For example, children check for laughter when they play-fight with each other. If their playmate isn’t laughing anymore, they stop fighting,” he added.
But rats don’t laugh as easily as humans do. The researchers made sure that rats were given leeway to move around freely for e couple of days, and then eventually that they are accustomed to the humans interacting with them. When amused, rats squeak at a high-pitched tone that humans cannot hear. The researchers monitored this sound to ensure that the rats were having fun.
They found that the PAG lit up with neural responses when the rats were tickled. But if the PAG was inhibited or shut off, the rats played less and did not laugh as frequently. The same behavior was observed when rats were put in an anxious situation.
Rats giggle in relaxed environments
Laboratory rats are often used in animal model research. These rats have our sympathies. In a study conducted in Australia, researchers tickled rats every day for a month to see if it will improve their emotional well-being.
Previous research has proven that under stressful situations, rats stifle their laughter when tickled, but giggle away in a relaxed atmosphere. Brecht was a co-author in this study as well.
One of the limitations of the study, as pointed out by Margaret McCarthy, a neuroscientist at the University of Maryland School of Medicine, who spoke to Scientific American, the study used only male rats.
Since the behavioral response in male rats is different than female rats, which are known to have different rates of play, McCarthy said she hopes in the future follow-up studies will include female rats as well.
Study abstract:
The persistence of play after decortication points to a subcortical mechanism of play control. We found that global blockade of the rat periaqueductal gray with either muscimol or lidocaine interfered with ticklishness and play. We recorded vocalizations and neural activity from the periaqueductal gray of young, playful rats during interspecific touch, play, and tickling. Rats vocalized weakly to touch and more strongly to play and tickling. Periaqueductal gray units showed diverse but strong modulation to tickling and play. Hierarchical clustering based on neuronal responses to play and tickling revealed functional clusters mapping to different periaqueductal gray columns. Specifically, we observed play-neutral/tickling-inhibited and tickling/play-neutral units in dorsolateral and dorsomedial periaqueductal gray columns. In contrast, strongly play/tickling-excited units mapped to the lateral columns and were suppressed by anxiogenic conditions. Optogenetic inactivation of lateral periaqueductal columns disrupted ticklishness and play. We conclude that the lateral periaqueductal gray columns are decisive for play and laughter.
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