One of the cortical areas associated with the general emotional state has been given the name limbic (from limbus, Latin for "border") lobe (Fig. 2). Although not a true lobe, main elements of the limbic lobe—including the cingulate cortex, or cingulum, and the hippocampus— form a C-shaped structure that surrounds the rostral borders of the brain stem. Components of the limbic lobe are linked with two other structures, the anterior nuclei of the thalamus and the hypothalamus, in a pathway known as the Papez circuit. Based on experience with patients having lesions within these areas, it has been proposed that the Papez circuit represents an emotion system responsible for linking the experience and the expression of emotion. The proposed site of emotional experience is the cingulate cortex, which receives sensory information funneled directly through the thalamus and from sensory areas in the cerebral cortex. Output from the cingulate cortex is fed through the hippocampus and projects, via a band of fibers called the fornix, to the hypothalamus, where it is translated into the expression of emotion through autonomic and other responses. Because the hypothalamus also projects back to the anterior nuclei of the thalamus (mammillothalamic tract), the expression of emotion (through autonomic responses) could add an additional trigger or perhaps a positive-feedback loop for the emotional experience.
The hypothalamus and an associated area, the amygdala, play a direct role in mediating aggressive behavior. The amygdala is an almond-shaped nucleus situated at the anterior border of the hippocampus within the temporal lobe (Figs. 2 and 3). It has been demonstrated in several animal species that bilateral ablation of the amygdala reduces fear and decreases certain forms of aggression. Although a rare occurrence, selective damage to the amygdala has been observed in humans. Behavioral testing suggests that those whose amygdala has been damaged have a decreased ability to recognize fear. Consistent with these findings, direct electrical stimulation of the amygdala in humans has been reported to elicit anxiety and fear.
These effects of the amygdala are mediated through connections with the hypothalamus, a major staging area for generation of two distinct types of aggressive behavior in animals. The first, affective aggression, is expressed by assuming a threatening or defensive posture, as when a cat hisses and arches its back. The second, predatory aggression, involves attacks against a member of a different species for the purpose of obtaining food, as when a cat silently pounces on a mouse. Experimental application of a small stimulating current through electrodes implanted in the medial hypothalamus in cats causes immediate expression of affective aggression, whereas stimulation of lateral regions causes predatory aggression. It has also been demonstrated in primates that the amygdala provides excitatory input to the medial hypothalamus and plays an important role in maintaining an active level of affective aggression normally involved in maintaining a position in the social hierarchy. The amygdala has the opposite effect on the lateral hypothalamus, providing inhibitory input to this region and thus suppressing predatory aggression.
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