Lesions to the central nucleus of the amygdala interfere with just about all of the manifestations of conditioned fear, including behavioural inhibition, autonomic nervous system responses, the suppression of pain, the release of stress hormones, and the potentiation of reflexes.

In addition, each of these responses is controlled by various bundles of nerve fibres projecting from the central nucleus. For example, the nerve fibres projecting to the periaqueductal grey matter interfere with behavioural inhibition, but not with blood pressure. Conversely, the fibres projecting to the lateral hypothalamus prevent blood pressure from increasing, but do not affect behavioural inhibition. Meanwhile, a lesion to the projection to the bed nucleus of the stria terminalis has no effect on blood pressure or behavioural inhibition, but does interfere with the release of stress hormones.

Neuron in the lateral nucleus of the amygdala Source: Thomas Heinbockel, University of Arizona.

These observations suggest that the information that enters the amygdala becomes represented at multiple locations within it. Moreover, there seems to be a point-to-point correspondence in the way this happens, so that the spatial organization of the groups of neurons is preserved.

The integration of these various representations, which originate in systems responsible for such widely differing functions as long-term memory, internal balance, and auditory perception, therefore depends on the numerous internal connections among the various nuclei of the amygdala. The evidence does seem to indicate that these connections are sufficiently numerous, complex, and diversified to play this role.