Distinct Brain Networks for Spontaneous and Voluntary Laughter Identified
Researchers have analyzed 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: spontaneous, involuntary laughter and volitional, controlled laughter.
Spontaneous Laughter: Uncontrollable and Emotional
"Think about the last time you were laughing and you could not stop," said author Sophie Scott of University College London, London, UK. "Something set you off and you are helpless with mirth." That is the spontaneous, involuntary laughter and can be uncontrollable, which can be sometimes associated with certain types of seizure disorders, mood disorders, Alzheimer's disease, and schizophrenia, she said.
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.
Voluntary Laughter: Controlled and Conversational
The second is the volitional kind, forming most of the laughter one encounters and 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, volitional laughter starts and stops really quickly." The coordination points to a degree of control that is lacking in spontaneous laughter.
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 was found to evoke laughter without positive emotions.
Methodology and Evolution
The researchers turned to reports of pre-surgical brain stimulation in epilepsy patients for discerning 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, and patients are able to describe their feelings in real time, the team said. The reports were looked together with clinical and animal studies and two distinct brain networks were identified.
The team added that the spontaneous network is a more evolutionarily ancient 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 that produce speech, supporting the idea that it controls more purpose-driven, conversational laughter.
Implications for Understanding Laughter Disorders
Understanding these distinct brain networks may help in diagnosing and treating conditions where laughter is abnormal, such as in certain seizure disorders, mood disorders, Alzheimer's disease, and schizophrenia. The findings provide insight into the neural basis of laughter and its role in social communication.
