Laughter is a universal social signal that connects us with others, but the brain regions underlying laughter are not well understood, in part because it's hard to elicit genuine laughter in the lab.
In a review publishing on June 23 in the Cell Press journal Trends in Neurosciences, researchers analyze reports from medical procedures in which the brain is electrically stimulated in awake patients. Laughter can be an unintentional byproduct of these stimulations, allowing scientists to pinpoint laughter-evoking brain areas. By examining these reports and other clinical and animal studies, the authors describe two distinct networks in the brain for laughter: one that elicits spontaneous outbursts, and another that produces voluntary, conversational laughter.
Researchers have long observed two types of laughter in healthy humans. "Think about the last time you were laughing and you could not stop," says author Sophie Scott of University College London, London, UK. "Something set of you off and you are helpless with mirth."
That, she says, is spontaneous, involuntary, and sometimes uncontrollable laughter, which can be associated with certain types of seizure disorders, mood disorders, Alzheimer's disease, and schizophrenia.
The second kind is volitional laughter. "That's most of the laughter you encounter," says Scott. "It's timed incredibly precisely. If you look at people having a conversation, they will laugh together at the end of a sentence and then breathe together."
"When people are talking to each other, volitional laughter starts and stops really quickly," says Scott. This type of coordination points to a degree of control that is lacking in spontaneous laughter.
To tease apart the brain circuitry underlying these two types of laughter, the team turned to reports of pre-surgical brain stimulation in epilepsy patients. During these procedures, clinicians identify brain regions to target for surgery by electrically stimulating parts of the brain while patients are awake. These probes often unintentionally evoke laughter, and patients are able to describe their feelings in real time.
The authors analyzed these reports, along with other clinical and animal studies, to propose two distinct networks underlying spontaneous and voluntary laughter. The spontaneous network consists of brain regions involved in motor control and emotional regulation, including the pregenual anterior cingulate cortex, nucleus accumbens, and the temporal pole. Stimulating these regions produces laughter accompanied by enhanced mood, euphoria, and mirth.
The voluntary network comprises areas involved purely in motor control of laughing and smiling, such as the rolandic operculum, globus pallidus, and presupplementary motor area. Stimulation of these regions evokes laughter without positive emotions.
The authors suggest that the spontaneous network is a more evolutionarily ancient pathway that arose in animal "rough-and-tumble" play, with laughter-like vocalizations serving as a signal to prevent aggression and promote social bonding. This hypothesis is consistent with recent discoveries that several mammalian species produce laughter-like vocalizations during social interactions.
The voluntary network, on the other hand, overlaps with brain regions that produce speech, supporting the idea that it controls more purpose-driven, conversational laughter.
In addition to shedding light on neurological and psychiatric disorders marked by altered laughter, the authors hope the findings can serve "as a kind of Rosetta stone for decoding multiple aspects of communication and the social use of vocalizations," particularly in the context of linguistics and conversation analysis, says author Fausto Caruana of the National Research Council of Italy, Parma, Italy.
"The role of these circuits in pain modulation is also intriguing," Caruana says. Studies have shown that laughter can act as a natural analgesic. And the anterior cingulate, identified in this literature review as part of the spontaneous laughter network, is also an important player in the brain's own pain-dampening system.
"We are interested in further investigating the analgesic role of laughter and the neural circuits that support it," Caruana says.
