Involuntary laughter and inappropriate hilarity.

Laughter is a particularly human behavior. Neuropsychiatrists are faced with disorders of laughter, yet the nature of this behavior and its disturbances remains obscure. The authors report an unusual patient with involuntary and unremitting laughter for 20 years and review the literature. The nature of laughter suggests that it has a unique role in human communication, particularly in the social exploration of incongruous information. The disorders of laughter suggest a neuroanatomical circuitry that includes the anterior cingulate gyrus, caudal hypothalamus, temporal-amygdala structures, and a pontomedullary center. Treatment includes the use of antidepressant and antimanic agents for disorders of laughter.