The primate amygdala and the neurobiology of social behavior: implications for understanding social anxiety.

The amygdala has long been implicated in the mediation of emotional and social behaviors. Because there are very few human subjects with selective bilateral damage of the amygdala, much of the evidence for these functional associations has come from studies employing animal subjects. Macaque monkeys live in complex, highly organized social groups that are characterized by stable and hierarchical relationships among individuals who engage in complex forms of social communication, such as facial expressions. Understanding the role of the amygdala in animals that display a level of social sophistication approaching that of humans will help in understanding the amygdala's role in human social behavior and in psychopathology such as social anxiety. Selective bilateral lesions of the amygdala in mature macaque monkeys result in a lack of fear responses to inanimate objects and a "socially uninhibited" pattern of behavior. These results imply that the amygdala functions as a protective "brake" on engagement of objects or organisms while an evaluation of potential threat is carried out. They also suggest that social anxiety may be a dysregulation or hyperactivity of the amygdala's evaluative process. Finally, recent data from developmental studies raise the possibility that, at least at some developmental stages, fear in social contexts may be subserved by different brain regions than fear of inanimate objects.