The Secret Life of the Brain
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Episode 4: The Adult Brain - Laughter

Imagine that you are sitting with a friend and suddenly, your friend's face starts to convulse involuntary. Her facial muscles, particularly her lips, stretch and she has a peculiar expression in her eyes. Her vocal organs vibrate and she is making a sequence of rhythmic expiratory sounds. Oddly enough, nobody around you even turns his head to look as your friend chokes, trying to take broken, sudden inhalations, while her shoulders jerk and her entire body twists and shakes.

Call the paramedics? Not necessary. Actually, this has happened to you many times, and yes, it was probably something you said. This sequence of events is defined in medical dictionaries as laughter. And if the above description sounds like an account of a neurological disorder, you are right. Analyzing the physiological manifestation of laughter reveals striking similarities to seizures and even to some types of epilepsy.

Why do we laugh? What function does laughter have? Laughter is one of the most poorly understood of human behaviors. While we know, for example, that certain parts of the brain are responsible for certain functions and tasks, it seems that laughter cannot be traced to one specific area of the brain. Furthermore, the relationships between laughter and humor, or even laughter and mirth are not understood, despite their evident interconnection.

Some clues for the physiological basis of laughter have come from people who suffered brain injuries, strokes or neurological diseases. C.B., a landscaper in Iowa, is one of them. Three years ago, at the age of 48, C.B. suffered a stroke. Fortunately, he recovered quite well and was expected to return to his normal life. However, since the stroke, C.B. and those around him, have been perplexed by certain changes in his behavior. Though he seems healthy, and doesn't suffer any pain, occasionally, for no noticeable reason, he bursts out into uncontrollable, wild laughter. In other cases, out of the blue, he is swept into tears in a similar attack.

C.B. has joined a long list of clinical cases that are described in medical literature as pathological laughter and crying (PLC). All of these patients suffer from brain damage that has destroyed or impaired small areas in their brains. Usually, the lesions are no bigger than a few cubic millimeters. However, since the lesions do not always occur exactly in the same spot in the brain, it is hard to determine based on these cases, which brain areas are in charge of laughter. Nevertheless, PLC suggests an interesting linkage; the same tiny lesion can cause both laughter and crying. That means that the same brain regions are involved in both laughter and crying. But most surprisingly, these laughter and crying are not associated with mirth or sadness. PLC patients suffer from "mechanical laughter". The pleasant feelings, happiness, amusement or joy that usually accompany laughter are absent. Patients like C.B. often even suffer anxiety and fear with their laughter.

Photo laughing

Photo laughing
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