Amygdala

A heterogeneous collection of nuclei and cortical areas in the temporal lobe, considered to subserve emotional and social behaviour, learning, and memory.

The amygdala (alias the amygdaloid or amygdalar complex), first described and named by the German anatomist Burdach in the early nineteenth century (Meyer 1971), is so called because in the primate brain its shape resembles an almond (amugdale in Greek). About a dozen different nuclei and specialized cortical areas are currently discerned in the amygdala, and many intra- and extra-amygdalar connections have been identified (Amaral et al. 1992; Pitkanen et al. 1997; Swanson and Petrovich 1998; Aggleton 2000). Indeed, the heterogeneity of the nuclei, areas, and pathways raised some doubts whether 'amygdala' as a whole is a discrete anatomical entity in situ, or only an artificial construct of the human mind (e.g. Kirkpatrick 1996; Swanson and Petrovich 1998; de Olmos and Heimer 1999). Whether a well-defined natural kind or merely a convenient concept, judging by its connectivity, the amygdaloid complex fits well to serve as a central processor for some facets of sensory and supramodal "representations. This is because sets of amygdaloid nuclei interconnect heavily with the unimodal and polymodal "cortex, as well as with subcortical structures. Some of these pathways are asymmetrical (more extensive in one direction, e.g. from amygdala to hippocampus), and the information flows into one amygdaloid nucleus but comes out at another.

The peculiar behavioural effects of bilateral lesions of the temporal lobe, including the amygdala, were noted over a century ago in "monkeys (Brown and Schafer 1888), and later further characterized (Kluver and Bucy 1938) and termed the 'Kluver-Bucy syndrome'. The overall impression was that the lesion produced 'a condition resembling idiocy' (Brown and Schafer 1888). A more detailed look described the lesioned animals as tamed, over-attentive but fearless, devoid of the ability to assess the significance of inanimate and animate objects, and indiscriminately phagic and sexual. A similar syndrome was shown to result from ablations confined to the amygdaloid complex and the medial temporal polar cortex (Weiskrantz 1956). It is indeed likely that many functions used to be attributed to the so-called "'limbic system', including control of phylogenetically primitive drives, emotions, and elementary social interactions, are carried out by the amygdala (LeDoux 1991).

Over the years, circumscribed lesions in monkeys and rodents, cases of diseased and injured amygdala in humans, and recently "functional neuroimaging, have all been employed to investigate the role of the amygdala in learning and behaviour. The effect of amygdala dysfunction on a number of "recognition tasks, including "delay tasks and visual and cross-modal associations, was first taken to imply that the amygdala plays a major part in these tasks; however, later studies indicated that the impairment was due to damage to the adjacent rhi-nal cortex, which was injured together with the amygdala in the original lesion experiments (Zola-Morgan et al. 1989a,b; Murray et al. 1993). In contrast, conclusive evidence for the involvement of amygdala in learning and memory was found in other types of tasks, which engage fear and emotional memory (Adolphs et al. 1995; Maren and Fanslow 1996; Rogan and LeDoux 1996; Scott et al. 1997; Walker and Davis 1997; Cahill and McGaugh 1998; Lamprecht and Dudai 2000; Parkinson et al. 2000). A most popular paradigm in this context is Pavlovian "fear conditioning, a ubiquitous form of "classical conditioning. In Pavlovian fear conditioning, a conditioned stimulus (e.g. tone) is associated with an aversive unconditioned stimulus (e.g. electric shock), to yield fear (e.g. freezing, increased blood pressure and heart rate) as the conditioned response. Amygdalar nuclei, including a subset dubbed the 'amygdalar basolateral complex', were specifically implicated in this simple type of conditioning (the identity of nuclei recruited in fear conditioning is probably also a function of the task complexity; Killcross et al. 1997; Nader and LeDoux 1997).

The meticulous analysis of fear conditioning in the amygdala had clearly paid off: it has yielded the first demonstration of "long-term potentiation induced by training in an identified pathway that subserves learning in the behaving rat (Rogan et al. 1997). The cellular analysis of fear conditioning also strengthened the assumption that the amygdala itself is a structure that stores information (see also Lamprecht et al. 1997). A different view is that the amygdala, occupying a strategic position in the network of widespread neuromodulatory systems in the brain, does not itself store memory, but rather modulates other circuits that store it (Cahill and McGaugh 1998). The clash between these opposing views has raised central issues concerning memory traces: is the evidence for the requirement for "protein synthesis and gene expression in training sufficient to prove that a certain brain area "consolidates a given memory? And if it is, will the memory be stored in that area forever after? And which parts of a circuit that subserves a memory should be considered as an integral part of the postulated "engram? On top of it all, there is actually no reason to assume that the 'storage' and the 'modulation' views are mutually exclusive. Moreover, even a close look at the Kluver-Bucy syndrome indicates that there is more to the amygdala than storage, and that it regulates "attention and additional facets of cognition (Gallagher and Holland 1994).

The study of the role of amygdala in fear conditioning is a beautiful example of a cross-"level analysis that has led from the behaving organism to circuits, "synapses, and molecules, and vice versa. An issue that deserves further emphasis is the ethological context of the findings. The amygdala fulfils an important role in navigating the individual in its species-specific milieu, enabling it to construe sign-"stimuli correctly, and react

Amygdaloid Nucleus

Fig. 3 The amygdaloid complex maintains extensive interconnections with multiple brain areas, including the hypothalamus, thalamus (MD, mediodorsal), *hippocampal formation, and temporal and frontal *cor-tex. This schematic diagram depicts the amygdala as a single area for simplicity, but in reality it is a collection of about a dozen main nuclei and cortical areas that interconnect differentially with targets over widely distributed brain areas, and subserve diverse functions, among them emotional behaviour and learning. (Adapted from Brodal 1998.)

Fig. 3 The amygdaloid complex maintains extensive interconnections with multiple brain areas, including the hypothalamus, thalamus (MD, mediodorsal), *hippocampal formation, and temporal and frontal *cor-tex. This schematic diagram depicts the amygdala as a single area for simplicity, but in reality it is a collection of about a dozen main nuclei and cortical areas that interconnect differentially with targets over widely distributed brain areas, and subserve diverse functions, among them emotional behaviour and learning. (Adapted from Brodal 1998.)

Anthropomorphism to them appropriately (e.g. see the role of amygdala in perception, memory, and judgement of facial as well as verbal expression in humans, Adolphs et al. 1998; Morris et al. 1998; Isenberg et al. 1999). This is definitely a place to look for brain defects that underlie some neurotic and affective disorders and asocial behaviours.

Selected associations: Fear conditioning, Functional neuroimaging, Limbic system, Long-term potentiation

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