The Role of the Brain in Laughter and Playfulness: A New Discovery by Scientists

The periaqueductal gray (PAG) region in a rat’s brain plays a significant role in play and laughter, as found by researchers. This part of the brain triggers strong neural reactions when the rats are tickled or engaged in play. However, when the PAG is suppressed or when the rats are exposed to a stressful environment, their playfulness and laughter decline, suggesting that the PAG governs these instinctive behaviors.

Studying animal play behaviors, such as in rats, necessitates that scientists can accurately recreate conditions conducive to play in the lab. If the rats experience anxiety or restriction, they are less inclined to play. The brain activity of freely playing rats remains largely unexplored.

The research team began by familiarizing rats with a human companion and tickling them under controlled settings. The team then recorded the rats’ vocal responses and brain activity. As reported in the Neuron journal on July 27, the team discovered the pivotal role of the periaqueductal gray region in laughter and play.

According to the study’s senior author, neuroscientist Michael Brecht from the Humboldt-Universität zu Berlin, vocalizations like laughter are fundamental during play, implying that some form of organizational signal exists in the brain that governs this behavior. He gives the example of children who cease their play-fight when their playmate stops laughing.

Understanding play as a behavior has been a challenge for scientists, with its neural pathways still largely unknown. To further their understanding of play’s neuroscience, the researchers ensured the rats could move freely during the experiment and took measures to help them adapt to their new surroundings.

The researchers engaged the rats in games of “chase the hand” and tickled them on their backs and bellies, once the rats were comfortable. Rats, although they do not laugh like humans, make high-pitched sounds that are inaudible to humans when amused. The researchers tracked these sounds to confirm the rats were enjoying the interaction.

Examining the brain activity of the rats, the researchers noted potent neural responses to both tickling and playing in the lateral column of the periaqueductal gray (PAG). When this brain area was suppressed, the rats displayed a decrease in playfulness and laughter. Similarly, when placed in an anxiety-inducing, unfamiliar setting, the rats’ laughter decreased, and the PAG’s tickling- and play-responsive cells reduced their activity.

The PAG, located in the midbrain, is known to control vocalizations and the fight-or-flight response. Play-fighting might also trigger a fight-or-flight response, suggesting a possible explanation for PAG’s role in play. Earlier studies have shown that playfulness continues even if the cortex, responsible for consciousness, is undeveloped, further indicating play is more of an instinctive behavior.

Brecht notes that play is often undervalued and dismissed as a trivial or childish activity, but in his view, play acts as a self-training behavior. While brains usually control behaviors, play seems to serve a role in brain development.

In their next steps, the researchers aim to see if similar activity is noticed in the lateral column of the PAG in other animals when they are engaged in play, which could provide a comparative measure of different species’ playfulness. They are also interested in investigating whether altering the play habits of younger rats could affect the development of the PAG’s lateral column.

The research was supported by multiple institutions, including the Humboldt-Universität zu Berlin, the Bernstein Center for Computational Neuroscience Berlin, NeuroCure Cluster of Excellence, Einstein Center for Neurosciences Berlin, Deutsche Forschungsgemeinschaft, and the European Research Council.

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More about Periaqueductal gray (PAG)

• Periaqueductal gray
• Neuroscience
• Humboldt-Universität zu Berlin
• Bernstein Center for Computational Neuroscience Berlin
• NeuroCure Cluster of Excellence
• Einstein Center for Neurosciences Berlin
• Deutsche Forschungsgemeinschaft
• European Research Council