Amygdala

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The amygdala is a collection of nuclei located along the medial wall of the temporal lobe. Amygdaloid nuclei are classified as either cortexlike or noncortexlike based on neuronal morphology. The cortexlike nuclei, which include the lateral, basal, accessory basal, periamygdaloid, amygdalo-hippocampal area, and cortical nuclei, possess pyramidal-

like neurons similar to the pyramidal neurons of the cortex. The noncortexlike nuclei, which include the central and medial nuclei, possess neurons similar to the medium spiny neurons of the striatum and do not possess pyramidal-like neurons. Each of the amygdaloid nuclei has distinct inputs and outputs, suggesting that they serve distinct functional roles. However, the amygdaloid nuclei are also interconnected, suggesting that circuitry within the amygdaloid nuclei allows the amygdala to function as a unit in processing information. In regard to information processing within the amygdala, sensory input is received primarily through the cortexlike nuclei, and output is relayed primarily through the noncortexlike nuclei.

The amygdala is a component of the limbic system, which is thought to be involved in learning, memory, emotion, and motivation. The amygdala receives highly integrated unimodal and polymodal sensory information and sends information to cortical, limbic, endocrine, autonomic, and motor areas. These anatomical connections suggest that the amygdala is ideally located for monitoring the environment and modifying physiological and behavioral responses accordingly. Indeed, the amygdala has been implicated in processing emotional stimuli, associative learning, memory, attention, arousal, and social behavior.

One of the first clues regarding the function of the amygdala was that symptoms of the Kluver-Bucy syndrome, including a loss of reactivity to emotional stimuli, were produced by amygdala lesions in monkeys. These monkeys willingly approached fear-inducing stimuli. This finding suggested that the amygdala is involved in processing the emotional significance of environmental stimuli. Several additional lines of evidence support this idea. Unilateral lesions of the amygdala along with cuts through the optic chi-asm and forebrain commissures produce a disconnection of visual input from one eye to the amygdala. Monkeys restricted to viewing threatening stimuli through the eye disconnected from the amygdala remain calm and fail to show defensive reactions to the stimuli. When the same monkeys are allowed to view the fearful stimuli through the other eye, however, they exhibit appropriate defensive reactions, which suggests that the intact amygdala processes the emotional significance of the stimuli. The amygdala also plays a role in processing reward, because animals will perform an operant response to obtain mild stimulation of the amygdala, and lesions of the amygdala disrupt appropriate responding to changes in reward magnitude. Electrophys-iological studies indicate that amygdala neurons are more responsive to complex emotional stimuli than simple neutral stimuli. In humans, imaging studies have demonstrated that the amygdala is activated by photographs of facial expressions and is more strongly activated by fearful faces than angry or happy faces. Furthermore, patients with amygdala damage have difficulty comprehending the emotional category and intensity of facial expressions. Moreover, stimulation of the amygdala in humans and an imals evokes emotional responses and species-specific defense reactions, respectively.

The amygdala is also involved in associative learning through which initially neutral stimuli gain biological significance (i.e., survival value). The best-documented example of this associative learning is fear conditioning, in which a fearful event or stimulus (unconditioned stimulus) is paired with an initially neutral stimulus (conditioned stimulus). Subsequently, the conditioned stimulus comes to elicit conditioned fear responses in the absence of the unconditioned stimulus. These responses include conditioned freezing behavior, startle reactivity, and autonomic responses. There are strong and converging lines of evidence that the amygdala is involved in fear conditioning.

Although the amygdala may play a prepotent role in information processing and associative learning involving aversive fearful stimuli, its function is not limited to aver-sively motivated learning, because it also plays a role in stimulus-reward associations. For instance, monkeys exhibit emotional reactions when presented with familiar palatable foods; however, they exhibit relatively little interest when presented with novel palatable foods. Upon tasting the novel foods, the monkeys learn to associate other sensory aspects of the foods with the taste of the foods. Subsequently, exposure to the foods elicits learned emotional responses and preferences for certain foods over other foods. Amygdala lesions disrupt acquisition of emotional responses and preferences for the novel palatable foods, suggesting that the animals are unable to associate the appearance of a new food with its palatability. Through stimulus-reward associations, initially neutral environmental stimuli gain incentive salience via their ability to predict reward. Consequently, these stimuli come to produce incentive motivation, reflected by approach behaviors, as well as secondary reinforcing effects. These behavioral effects are also disrupted in animals with amygdala lesions, particularly lesions of the basolateral amygdaloid nuclei. For instance, animals with basolateral amygdala lesions fail to acquire operant responding reinforced by presentation of a stimulus light (secondary reinforcer) that had been paired previously with delivery of a water reinforcer (primary reinforcer).

Memory for emotional events is superior to memory of nonemotional events. This phenomenon may be due, at least in part, to hormones that are released in response to stress that modulate effects on memory by binding to receptors in the amygdala. Stress hormones, such as epi-nephrine and corticosterone, produce a dose-dependent enhancement of memory when given after training. Posttraining administration corresponds to the time at which these hormones are naturally released in response to a stressful event and at which consolidation of the memory for the event occurs. Amygdala lesions abolish the memory-enhancing effects of these hormones, and direct injection of the hormones into the amygdala produces memory-

enhancing effects. Psychomotor stimulants, such as amphetamine, may also modulate learning and memory evident as enhancement of responding for secondary rein-forcers. Lesions of the central amygdala disrupt psy-chomotor stimulant-induced enhancement of responding for secondary reinforcers.

The amygdala, particularly the central nucleus, is involved in modulating attention and arousal. The central nucleus of the amygdala projects to several brain regions that are thought to be involved in attention and arousal, including cholinergic basal forebrain neurons, autonomic regulatory nuclei in the medulla, and the lateral tegmental area of the brainstem. In rabbits, a conditioned stimulus predictive of an aversive shock produces an increase in spontaneous firing of amygdala neurons that correlates with excitability of cortical neurons as measured by cortical electroencephalogram (EEG) activity. The cortical EEG activity is thought to reflect an increase in attention. Evidence from functional magnetic resonance imaging studies in humans suggests that the amygdala responds to stimuli processed at a subconscious level. Specifically, subjects given very brief presentations of happy or fearful faces followed immediately by longer presentations of neutral faces report seeing only the neutral faces, yet the amygdala is more strongly activated when the neutral faces are preceded by fearful faces rather than happy faces. These findings suggest that the amygdala constantly monitors the environment for biologically relevant stimuli and may modulate moment-to-moment levels of attention. Many conditioned responses mediated by the amygdala, including conditioned autonomic responses and an arrest of ongoing activity, may serve to enhance attention to environmental stimuli. Furthermore, lesion and brain stimulation studies across species suggest that the amygdala is involved in orienting responses to environmental stimuli.

The amygdala plays an important role in social behavior. In general, stimulation of the amygdala elicits rage and attack behaviors, whereas lesions of the amygdala decrease aggressive behaviors across species. Stimulation and lesion studies also suggest that the amygdala is involved in social rank and affiliation, as well as sexual and maternal behaviors. Radiotelemetry data from a social group suggest that electrical activity of the amygdala is strongest when animals are being chased or aggressed upon or given ambiguous social information.

Amygdala dysfunction has been implicated in a number of neurological and psychiatric disorders. The amygdala is among several structures in the temporal lobe that are involved in epileptic seizure disorders. Repeated electrical or pharmacological stimulation of the amygdala induces the development of seizures. This experimentally induced seizure activity is referred to as kindling and is used as an animal model of epilepsy. The amygdala has also been implicated in other disorders known to involve temporal lobe pathology, including Schizophrenia and Alzheimer's dis ease. Imaging studies have indicated that amygdala volume is reduced in patients presenting these disorders. The amygdala likely plays a role in depression, anxiety, and Post-Traumatic Stress Disorder. Most antidepressant and anxiolytic medications produce effects via either benzodi-azepine, norepinephrine, or serotonin receptors; the amygdala has a large population of these receptors. Furthermore, direct amygdaloid injection of benzodiazepine anxi-olytic drugs reduces behavioral reactions that are thought to reflect fear and anxiety. Moreover, imaging studies have found that depressed patients exhibit an increase in metabolic activity in the amygdala that correlates with measures of depressive symptoms and is reduced by anti-depressant treatments. The amygdala has also been implicated in the reinforcing effects of drugs of abuse. Furthermore, imaging studies suggest that the amygdala probably plays a role in the ability of drug-associated stimuli (e.g., drug paraphernalia) to elicit drug craving.

SUGGESTED READING

Aggleton, J. P. (Ed.). (2000). The amygdala: Afunctional analysis.

New York: Oxford University Press. McGinty, J. F. (Ed.). (1999). Advancing from the ventral striatum to the extended amygdala. Annals of the New York Academy of Sciences, 877.

Janet Neisewander Arizona State University

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