The neuroscience of emotional disorders

Emotions can be defined as states elicited by rewards or punishments, and indeed the neurology of emotional disorders can be understood in terms of this foundation. The orbitofrontal cortex in humans and other primates is a critical area in emotion processing, determining the value of stimuli and whether they are rewarding or nonrewarding. The cortical processing that occurs before the orbitofrontal cortex primarily involves defining the identity of stimuli, i.e., "what" is present and not reward value. There is evidence that this holds true for taste, visual, somatosensory, and olfactory stimuli. The human medial orbitofrontal cortex is important in processing many different types of reward, and the lateral orbitofrontal cortex in processing nonreward and punishment. Humans with damage to the orbitofrontal cortex have an impaired ability to identify facial and voice expressions of emotions, and impaired subjective experience of emotion. They can have an altered personality and be impulsive because they are impaired at processing failures to receive expected rewards and at processing punishments. In humans, the role of the amygdala in the processing of emotions is reduced because of the great evolutionary development of the orbitofrontal cortex: amygdala damage has much less effect on emotion than does orbitofrontal cortex damage. The orbitofrontal cortex projects reward value information to the anterior cingulate cortex, which is involved in learning those actions required to obtain rewards and avoid punishments. The cingulate cortex thus provides an output route for emotional behavior. In depression, the medial orbitofrontal cortex has decreased connectivity and sensitivity to reward, and the lateral orbitofrontal cortex has increased connectivity and sensitivity to nonreward. The orbitofrontal cortex has major projections to the anterior cingulate cortex, including its subcommissural region, and the anterior cingulate cortex is also implicated in depression.