Amygdala

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The amygdala is a key component of the LIMBIC SYSTEM and has an important role in EMOTION. This brain structure, which is named after its fanciful resemblance to an almond, lies in the anterior medial portion of each TEMPORAL LOBE. It is composed of approximately a dozen nuclei each with a distinctive pattern of cytoarchitectural, histo-chemical and connectional features. These nuclei are often divided into a basolateral group and a corticomedial group. The basolateral group contains many cortical like cells and its nuclei have dense connections with sensory association and limbic cortical areas. The corticomedial group is more closely linked with hypothalamic and olfactory regions (see HYPOTHALAMUS; OLFACTION). The central nucleus of the amygdala has features of both groups but is unique in having many connections with subcortical AUTONOMIC centres.

Other subcortical connections of the amygdala include dense projections to the NUCLEUS BASALIS OF MEYNERT, the ventral STRIATUM, and the DORSOMEDIAL NUCLEUS OF THE THALAMUS. The amygdala also has a complex array of intrinsic connections and is noteworthy for the enormous range of NEUROTRANSMITTERS and neuromodulators that are present within the structure.

The amygdala has a variety of functions (Aggleton, 2000), and while most attention has focused on its role in emotion it can also modulate different aspects of motivation (such as feeding, drinking, and sexual behaviour). Damage to this structure can decrease emotional reactivity, as seen in the KLÜVER-BUCY SYNDROME, and severely disrupt social behaviour. These changes, which are very dramatic in monkeys, are less evident in humans. Experimental studies indicate that the amygdala is involved in the formation of stimulus-affect associations, and the evidence for this is especially convincing for aversive stimuli (see FEAR). The structure is also important for the identification of affective states in others, and research on people with bilateral amygdala damage has revealed a selective loss of the ability to recognize expressions of fear. The structure may also facilitate learning about events that are associated with highly emotional or arousing experiences. Finally, the amygdala contributes more generally to LEARNING by enabling the formation of STIMULUS-REINFORCER ASSOCIATIONS.

Aggleton J.P. (ed.) (2000) The Amygdala: A Functional Analysis, Oxford University Press: Oxford.

JOHN P.AGGLETON

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