The limbic system is a group of BRAIN STRUCTURES organized into a functional unit that is important in the expression of emotion and mood states. The term limbic lobe and associated terminology can be traced to the French neuroanatomist Paul Broca (1824-1880), who used it first to describe the forebrain structures that encircle the brain stem. The limbic system is a broader classification, composed of brain structures that form an integrated circuit surrounding the thalamus—an important relay station between higher brain centers and the hind brain and spinal cord.

The limbic system is thought to be important in emotional behaviors. This was hypothesized on the basis of neuropathological investigations of the brains of individuals displaying bizarre emotional disturbances. These initial clinical observations were followed by animal studies, in which the loss of these structures produced significant changes in emotional responsiveness. As research techniques and methodologies...