First of all, laughter is not the same as humor. Laughter is the physiological response to humor. Laughter consists of two parts -- a set of gestures and the production of a sound. When we laugh, the brain pressures us to conduct both those activities simultaneously. When we laugh heartily, changes occur in many parts of the body, even the arm, leg and trunk muscles.
Under certain conditions, our bodies perform what the Encyclopedia Britannica describes as "rhythmic, vocalized, expiratory and involuntary actions" -- better known as laughter. Fifteen facial muscles contract and stimulation of the zygomatic major muscle (the main lifting mechanism of your upper lip) occurs. Meanwhile, the respiratory system is upset by the epiglottis half-closing the larynx, so that air intake occurs irregularly, making you gasp. In extreme circumstances, the tear ducts are activated, so that while the mouth is opening and closing and the struggle for oxygen intake continues, the face becomes moist and often red (or purple). The noises that usually accompany this bizarre behavior range from sedate giggles to boisterous guffaws.
Behavioral neurobiologist and pioneering laughter researcher Robert Provine jokes that he has encountered one major problem in his study of laughter. The problem is that laughter disappears just when he is ready to observe it -- especially in the laboratory. One of his studies looked at the sonic structure of laughter. He discovered that all human laughter consists of variations on a basic form that consists of short, vowel-like notes repeated every 210 milliseconds. Laughter can be of the "ha-ha-ha" variety or the "ho-ho-ho" type but not a mixture of both, he says. Provine also suggests that humans have a "detector" that responds to laughter by triggering other neural circuits in the brain, which, in turn, generates more laughter. This explains why laughter is contagious.
Humor researcher Peter Derks describes laughter response as "a really quick, automatic type of behavior." "In fact, how quickly our brain recognizes the incongruity that lies at the heart of most humor and attaches an abstract meaning to it determines whether we laugh," he says.
In the next section, we'll learn why we laugh.