Punchlines and Pathology: The Secrets of Laughter Explained

Knock-knock.Who’s there?Haha.Haha who? That depends — am I genuinely amused, or just trying to survive this conversation?New research on the evolution of laughter and the emergence of two distinct neural networks may help clinicians better understand, explain, and diagnose medical conditions where laughter goes awry.Long viewed as a byproduct of humor, laughter may have evolved not from punchlines but from mammalian play, according to a growing paradigm in neuroscience. Laughter is socio-communicative, evolutionary scientists say. A new review in the Cell Press journal Trends in Neuroscience builds on this knowledge with a unique approach: analyzing reports of presurgical brain stimulation in patients with epilepsy.The review describes the two networks — one for volitional laughter (produced for conversational reasons) and the other for spontaneous emotional laughter.“The two networks are quite independent. One can be preserved; the other one can be impaired in different populations,” said study co-author Fausto Caruana, PhD, senior research scientist at the Institute of Neuroscience of the National Research Council of Italy.The spontaneous laughter system is ancient and seems to exist in other mammals, Caruana said. Researchers posit that laughter emerged during rough-and-tumble play in animals such as monkeys and rats, to signal frisky attacks weren’t violent. In humans, voluntary laughter co-opted this meaning for conversation, Caruana said. A modern equivalent could be saying “shut up” followed by a chuckle (or smile emoji).“We are still using the same communicative code we were using in our evolutionary past,” Caruana said.This primitive system plays out in infants too: A baby’s first giggles, around 3-4 months, are involuntary. They begin to laugh voluntarily at about 6 months.Humans have a uniquely high degree of control over the muscles used for articulation, said Sophie Scott, PhD, the other study author and a professor of cognitive neuroscience at University College London, London, England. Laughter relies on that vocal control, which is “why it can live so happily within conversation,” she said. Humans may use laughter to show affiliation, affection, understanding, recognition, or agreement — or to diffuse a stressful situation or cover up an emotion like embarrassment or sadness.Beyond Comedy: The New Science of LaughterSpontaneous laughter is hard to study — it’s fleeting, it depends on making someone genuinely laugh, and it involves movement. “When you laugh, you move your body and your head,” Caruana said, “and this is not compatible with the large majority of techniques used in cognitive neuroscience.”Invasive intracranial recording is rare for data collection — but these patients with epilepsy had electrodes implanted for clinical purposes and gave permission for their results to be studied. The probes often unintentionally trigger laughter, offering researchers the opportunity to study brain circuitry during laughter and ask participants how they feel during laughing episodes.“They’d say, ‘Ok, I feel like I’m happy now. I’m not only laughing, I’m really happy,’” Caruana said. “Or maybe they’d say, ‘I’m just moving my head and my mouth without any kind of emotional content.’” Caruana analyzed the reports, and Scott classified the findings under the “spontaneous vs voluntary” framework.Spontaneous laughter is likely driven by an evolutionary pathway shared with other mammals that runs down the midline of the brain, from the anterior cingulate to the midbrain, Scott said. A lateral motor-opercular system is responsible for volitional laughter.Some of the same neural regions involved in laughter are also activated by pain, the authors noted. “An intriguing hypothesis is that when you’re in pain, part of this network starts to be activated for some kind of endogenous analgesia,” Caruana said. “Indeed, when you laugh, your pain threshold is elevated.”Why Humans Have BothCaruana and Scott’s conclusions align with the general pattern of human behavior, said Michiko Kimura Bruno, MD, a neurologist and medical director of the Parkinson’s and Movement Disorder Center at The Queen’s Medical Center, Honolulu, Hawaii.“Human behavior is very complex. Some parts are controllable, some parts are not, and both have benefits,” Bruno said. “The more primitive part makes things automatic, keeps us alive, keeps us surviving…But the more advanced, higher cortex gives us a little more option or richness.”The dual system could be applied to other emotional expressions, the authors said. Screaming can be a spontaneous response to fear, or a way to communicate happiness or surprise. A grimace could be a visceral reaction to the sour taste of spoiled milk — or a mother’s deliberate attempt to tell her child, “Don’t touch that!”“Humans, we have both,” Scott said. “And if you look at patients who’ve had strokes that mean they can’t talk anymore, they can still laugh and cry and shout.”Clinical ImplicationsLaughter can be a sign of pathologic conditions, Caruana said. People with gelastic seizures — or “laughing seizures” — laugh uncontrollably without feelings of happiness. Brain injuries or advanced neurologic conditions such as amyotrophic lateral sclerosis, multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease can sometimes cause sudden uncontrollable laughter (pseudobulbar affect).People with psychotic disorders such as schizophrenia may laugh at times that seem inappropriate. Patients with cataplexy, like those with narcolepsy, lose muscular tone when they laugh. People with autism may have a preserved spontaneous laughter network, but voluntary laughter — to, say, defuse an awkward situation or to connect during conversation — may not feel as automatic as it does for a neurotypical person.Ultimately, the new science helps to frame condition-related laughter not as a behavioral choice but rather a neurologic malfunction, which may offer researchers a target for treatment.It could also “help us move forward, not blaming, but understanding,” Bruno said.The experts cited in this article had no relevant disclosures.