Amygdalo-cortical Development and the Emergence of Primate Social Behavior

A number of studies in non-human primates have suggested that the amygdala is essential for the normal production and interpretation of social signals. Network researchers conducted a study in rhesus monkeys that challenges this long-held belief. Adult monkeys with amygdala lesions appear to demonstrate normal social behavior, while infant monkeys with amygdala lesions in previous studies have demonstrated impairments in social behavior.

However, these abnormalities may not have been attributable to the absence of the amygdala, but may rather have been attributable to abnormal rearing conditions (isolation or peer-rearing) or to additional damage to structures surrounding the amygdala due to less sophisticated lesioning methods than are currently available. This study found that infant rhesus monkeys with very precise lesions of the amygdala who were reared with their mothers did develop species-appropriate social behaviors, indicating that the amygdala is not necessary for normal social development.

However, these animals did produce more inappropriate fear behaviors (that is, fear behaviors that were not in response to behaviors that would be expected to elicit fear, such as aggression) during social encounters than normal animals or animals with hippocampal lesions. The animals with amygdala lesions also showed heightened affiliative behaviors. The production of more fear behaviors and more affiliative behaviors by the same animals may be due to hyper-responsivity or hyper-vigilance in the lesioned animals, leading to an over-production of all social signals in an attempt to appease other animals. It may also be due to an inability to interpret social signals from other animals.

This research is important for a number of reasons. First, previous research on amygdala-lesioned animals was used as the basis for a proposed “animal model” of autism due to the autistic-like behavior of the lesioned animals. This model proposed that dysfunction of the amygdala may have been responsible for some of the characteristics of autism. However, as this study found that the presence of a functional amygdala early in development is not necessary for the development of appropriate social behavior, it challenges the involvement of the amygdala in autism. It does, however, suggest that the amygdala may be involved in anxiety disorders. Secondly, in demonstrating that the amygdala is not necessary for the development of normal social development, it necessarily calls into question the role of other structures in such development.

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