Buried deep in the temporal lobes, amygdalas are neuronal computers involved in evaluating and generating emotional behavior. There are 23 nuclei in each amygdala. The expectation of reward or punishment is a strong determinant of human behavior. The amygdala is involved in assessing reward potential. The lateral nucleus of the amygdala is the sensory receptive area and the central nucleus provides the interface with motor systems that express conditioned responses.
In animal experiments, the amygdala is known to mediate the conditioned aspects of reward-related stimuli. Heroin is a strong stimulus, for example, whose condition effect is reduced by inactivating the basolateral nucleus. The lateral nucleus of the amygdala responds to amphetamines. The central nucleus of the amygdala receives dense reciprocal dopaminergic afferents from the ventral tegmental area.
Rats with lesions of the central nucleus exhibit less suppression of behavior elicited by fear, but were able to avoid further presentations of this aversive stimulus. Animals with lesions of the basolateral amygdala were unable to avoid the conditioned aversive stimulus by behavior, but exhibited normal conditioned suppression to this stimulus.
People with bilateral amygdala damage lose some or all of their avoidance behavior and approach strangers with ease; they fail to recognize untrustworthy individuals. In animal experiments, the amygdala is known to mediate the conditioned aspects of reward-related stimuli. Adolphs et al suggested that the amygdala recognizes and recalls fearful facial expressions.
Bilateral amygdala damage impairs the recognition of fear in facial expressions. LeDouxs et al suggested that projections from the posterior thalamus to the amygdala are involved in processing the emotional significance of sounds. Ueda et al monitored fifteen healthy subjects by fMRI and reported that the amygdala and anterior cingulate cortex play an important role in the expectancy of unpleasant stimuli. Expectation of pleasant stimuli produced activation in the left dorsolateral and left medial prefrontal cortex as well as in the right cerebellum.
The central amygdala processes conditioned fear. Two microcircuits have been
identified: one for fear acquisition and the other for fear responses. Haubensak
et al. identified GABAergic neurons that have a key role in gating learned fear.
Ciocchi et al. identified three distinct groups of neurons in a disinhibitory
network. They stated:" The central amygdala (CEA), a nucleus predominantly
composed of GABAergic inhibitory neurons, is essential for fear conditioning.
How the acquisition and expression of conditioned fear are encoded within CEA
inhibitory circuits is not understood. Using electrophysiological approaches in
mice, we show that neuronal activity in the lateral subdivision of the central
amygdala (CEl) is required for fear acquisition, whereas conditioned fear
responses are driven by output neurons in the medial subdivision. Functional
circuit analysis revealed that inhibitory CEA microcircuits are highly organized
and that cell-type-specific plasticity of phasic and tonic activity in the CEl
to CEM pathway may gate fear expression and regulate fear generalization. Our
results define the functional architecture of CEA microcircuits and their role
in the acquisition and regulation of conditioned fear behavior."
[i] Bickart et al reported: “We found that amygdala volume
correlates with the size and complexity of social networks in adult humans...
there were associations between social network variables and cortical thickness
in three cortical areas, two of them with amygdala connectivity. These findings
indicate that the amygdala is important in social behavior