Early investigators focused on the role of olfaction in limbic function. Hence, the limbic system was formerly called the rhinencephalon, or "smell brain", because it is involved in the central processing of olfactory information. It was not until 1937, that emotion was link to the limbic brain, by Papez. Papez states that "It is proposed that the hypothalamus, the anterior thalamic nuclei, the gyrus cinguli, the hippocampus and their interconnections constitutes a harmonious mechanism which may elaborate the functions of central emotion, as well as participate in emotional expression" (Bromm & Desmedt, 1995). Then, in 1952, a researcher by the name of MacLean, introduced the term "limbic system" and characterized its functions. MacLean divides the limbic system into three main subdivisions: the amygdalar, septal, and thalamocingulate (see diagram 1). These three subdivisions represented sources of afferents that branched out and innervate different part of the limbic cortex. He further postulated that the limbic brain responds to two basic types of inputs: interoceptive and exteroceptive, which refers to sensory information from internal and external environments, respectively.
The closed circuit of information between the limbic system and the thalamus and hypothalamus proposed by Papez in now referred to as the Papez circuit. In the Papez circuit, a fiber tract called the fornix, connects the hippocampus to the mammillary bodies of the hypothalamus, which in turn project to the anterior nuclei of the thalamus (Fox, 1996). The nuclei of the thalamus, then completes the circuit by sending fibers to the hippocampus. Recent knowledge suggests that via these interconnections, the limbic system and the hypothalamus appears to cooperate in the neural basis of emotional states. In support of this, studies of the functions of the limbic system and the hypothalamus, shows that they are involved in the following feelings and behaviors (Fox, 1996): Aggression-Stimulation of certain areas of the amygdala produces rage and aggression, and lesion of the amygdala can produce docility in experimental animals. Stimulation of particular areas of the hypothalamus can produce similar effects. Fear-Fear can be produced by electrical stimulation of the amygdala and hypothalamus, and surgical removal of the limbic system results in an absence of fear. Feeding- The hypothalamus contains both a feeding center and a satiety center. Electrical stimulation of the former causes overeating, and stimulation of the ladder will stop feeding behavior in experimental animals. Sex-The hypothalamus and the limbic system are involved in the regulation of the sexual drive and sexual behavior, as shown by stimulation and ablation studies in experimental animals. (The cerebral cortex, however, is also critically important for the sex drive in lower animals, and the role of the cerebrum is even more important for the sex drive in humans). Goal-directed behavior (reward and punishment system). Electrodes placed in particular sites between the frontal cortex and the hypothalamus can deliver shocks that function as a reward. In rats, this reward is more powerful than food or sex in motivating behavior. Similar studies have been done in humans, who reported feelings of relaxation and relief from tension, but not of ecstasy. Electrodes placed in slightly different positions apparently stimulate a punishment system in experimental animals, who stop their behavior when stimulated in these regions.
As a result of these types of studies and their findings, it is believed that emotion is processed in the limbic system and the hypothalamus.