The Laughing Brain
Scientists have been studying the play behavior of young animals to identify its neural basis. Some are also evaluating whether and to what extent early social experiences such as play or lack of it influence neural structure and function. Although the neural bases of laughter are not fully understood, we are slowly putting together a picture of the various brain regions involved in producing laughter.
 
 

The ancient limbic system of the brain, including two structures involved with emotions - the hippocampus and the amygdala - is involved in laughter, suggesting that it is deep in our animal nture. But our cortex, too, appears to have a role, relating laughter and speech and bringing humor into the equation. A. Primitive brain (archipallium) - Self preservation, aggression. B. Intermediate brain (paleopallium) - Limbic system, emotions. C. Rational brain (neopallium) - Neocortex, intellectual tasks.

The evolving human brain acquired three major parts that developed in succession and became superimposed (see illustration above). The archipallium, or primitive (reptilian) brain comprises the structures of the brain stem; the paleopallium, or intermediate (old mammalian) brain comprises the structures of the limbic system. The neopallium, also known as the superior or rational (new mammalian) brain, includes almost the whole of the hemispheres (made of a more recent type of tissue called the neocortex) and some subcortical neuronal groups. These three make up the brain of the superior mammals, including primates and humans.

When we examine brain areas involved with laughter, the limbic system (part of the old mammalian brain) seems central. It controls behaviors essential to the survival of all mammals—for example, defending oneself and finding food. The same structures that are found in the human limbic system are found in the brains of much earlier animals such as lizards, snakes, and toads. In them, the limbic system, linked to the sense of smell, is involved in hunting, eating prey, and defending territory. In humans, the limbic system is more important in motivation and emotional behaviors. Two limbic structures involved with emotions are the hippocampus (a seahorse-shaped structure) and the amygdala, a small, almond-shaped structure deep in the brain. The amygdala connects with the hippocampus and the medial dorsal nucleus of the thalamus, enabling the amygdala to have an important role in mediating and controlling mood expression and emotion-laden activities like friendship, love, and affection. The hypothalamus, particularly its median part, has been identified as a key contributor to the production of loud, uncontrollable laughter.

If laughter is such a primitive feature of the human primate, with origins in lower mammals, it stands to reason that its neural substrate would be in areas of the brain we share with other animals, probably the same places aggressive and emotional behaviors are organized.

Neurobiologist Provine believes that laughter emerges not from our conscious mind (in other words, the cortex) but from a more primitive part that he calls the “pre-cognitive brain.” He says, “We’re talking about something that’s very deep in our animal nature.” Panksepp, too, sees evidence that laughter is triggered by genetically older structures of the brain. He writes that “it is not exactly a cerebral activity. Then laughter must be an ancient response, part of our earliest evolution into mammals, and is triggered by brain regions that evolved from ancient times.”

Recently, however, scientists have discovered that the brain’s cortex also plays a critical role in creating laughter. Studying a young epileptic woman, researchers electrically stimulated a part of the cortex called the supplemental motor area (SMA), and found that low-intensity current induced her to smile (5). A higher-intensity current transformed her smile into a contagious fit of laughter. The SMA, associated with the control of speech, lies just behind the pre-supplemental motor area, which is also found in nonhuman primates. Neurophysiologists believe that the SMA is part of a larger brain circuit responsible for processing emotions related to humorous things: the cognitive part (“getting it”) as well as the motor part (smiling and laughing), which involves muscles of the face and respiratory system.

Based on those observations, researchers conclude that there is a link among the motor, emotional, and cognitive components of laughter. They propose that the SMA spot they found “is part of a further development in humans to accommodate the specialized functions of speech, manual dexterity and laughter.” So, not surprisingly, there is a relationship in the brain between speech and laughter. After all, both are communicative activities, for which we use the same muscles of the face and need to control the way we breathe. Primates have highly mobile facial expressions, and we rely on our faces more than any other body part for communication, including speech.