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.