New Delhi: Researchers have analysed reports of medical procedures in which the brains of awake people were electrically stimulated and found that distinct brain regions may be responsible for spontaneous and voluntary laughter.
Two types of laughter have been identified in humans.
"Think about the last time you were laughing and you could not stop," author Sophie Scott of University College London in the United Kingdom said.
"Something set you off and you are helpless with mirth," she said.
That is spontaneous, involuntary laughter, which can be uncontrollable and is sometimes associated with certain types of seizure disorders, mood disorders, Alzheimer's disease and schizophrenia, she said.
The second is voluntary laughter, which forms most of the laughter people encounter and is timed "incredibly precisely", according to Scott.
"If you look at people having a conversation, they will laugh together at the end of a sentence and then breathe together," she said.
"When people are talking to each other, voluntary laughter starts and stops really quickly," Scott said.
The coordination points to a degree of control that is lacking in spontaneous laughter, the author said.
The researchers turned to reports of pre-surgical brain stimulation in epilepsy patients to identify the brain circuits underlying the two distinct types of laughter.
During brain stimulation, clinicians identify brain regions to be targeted for surgery by electrically stimulating parts of the brain while patients are awake. The probes can often unintentionally evoke laughter, allowing patients to describe their feelings in real time, the team said.
The reports were analysed alongside clinical and animal studies, leading researchers to identify two distinct brain networks.
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 brain regions produces laughter accompanied by enhanced mood, euphoria and mirth, the researchers said.
The voluntary network comprises areas involved purely in the motor control of laughing and smiling, including the rolandic operculum, globus pallidus and presupplementary motor area. Stimulation of these regions was found to evoke laughter without positive emotions.
The team added that the spontaneous network is an evolutionarily older pathway that arose in animal rough-and-tumble play, with laughter-like vocalisations serving as a signal to prevent aggression and promote social bonding.
However, the voluntary network was found to overlap with brain regions involved in speech, supporting the idea that it controls more purposeful, conversational laughter.