The amygdala: historical and functional analysis.

The amygdala, part of the limbic system, is a small collection of neurons located in the ventral temporal lobe, and can be divided into two morphologically and functionally distinct nuclei - the corticomedial, a phylogenetically old group, and the basolateral, a phylogenetically more recent group. Both divisions of the amygdala have extensive interconnections with the hypothalamus, the corticomedial nucleus via the stria terminalis and the basolateral nucleus via the ventral amygdalohypothalamic pathway. The amygdala receives input from the orbital frontal cortex, the piriform cortex, the hypothalamus, the thalamus, and all sensory modalities as well as from the other structures of the limbic system. Early theories positing strictly emotional, olfactory, or visceral functions for the amygdala are at best incomplete. An intact amygdala seems to be essential for the successful performance of behavior patterns such as instinctive food getting and instinctive defensive reactions that are necessary for the survival of the individual organism and the species. Ablation of the amygdala produces many behavioral deficits that center around the inability to integrate changes in reinforcement contigencies into new behavior patterns.