As these observations on the limbic system (and the amygdala in particular) make plain, understanding the neural basis of emotions requires understanding the role of the cerebral cortex. In animals like the rat, most behavioral responses are highly stereotyped. In more complex brains, however, individual experience is increasingly influential in determining responses to special and even idiosyncratic stimuli. Thus in humans, a stimulus that evokes fear or sadness in one person may have little or no effect on the emotions of another. Although the pathways underlying such responses are not well understood, the amygdala and its interconnections with an array of neocortical areas in the prefrontal cortex and several subcortical structures appear to be especially important in the higher order processing of emotion. In addition to its connections with the hypothalamus and brainstem centers that regulate autonomic function, the amygdala has significant connections with several cortical areas in the orbital and medial aspects of the frontal lobe (see Box B). These cortical fields associate information from every sensory modality (including information about visceral activities) and can thus integrate a variety of inputs pertinent to moment-to-moment experience. In addition, the amygdala projects to the thalamus (specifically, the mediodorsal nucleus), which projects in turn to these same cortical areas. Moreover, the amygdala innervates neurons in the ventral portions of the basal ganglia that receive the major cortico-striatal projections from the regions of the prefrontal cortex thought to process emotions. Considering all these seemingly arcane anatomical connections, the amygdala emerges as a nodal point in a network that links together the cortical (and subcortical) brain regions involved in emotional processing.

Clinical evidence concerning the significance of this circuitry linked through the amygdala has come from functional imaging studies of patients suffering from unipolar depression (Box E), in which this set of interrelated forebrain structures displayed abnormal patterns of cerebral blood flow, especially in the left hemisphere. More generally, the amygdala and its connections to the prefrontal cortex and basal ganglia are likely to influence the selection and initiation of behaviors aimed at obtaining rewards and avoiding punishments (recall that the process of motor program selection and initiation is an important function of basal ganglia circuitry; see Chapter 18). The parts of the prefrontal cortex interconnected with the amygdala are also involved in organizing and planning future behaviors; thus, the amygdala may provide emotional input to overt (and covert) deliberations of this sort (see “The Interplay of Emotion and Reason” below).

Box E

Affective Disorders.