Neonatal amygdala lesions result in globally blunted affect in adult rhesus macaques.

The amygdala has been implicated in affective and social processing for more than a century. Animals with damage to the amygdala have altered affective and social behavior patterns, though the precise nature of these behavioral changes depends on a number of factors including lesion technique, age of the subject at the time of lesion, rearing, and housing environments. Interpretations of amygdala lesion studies are further complicated by the potentially confounded nature of affective and social stimuli (e.g., social interactions with a conspecific partner that is consistently aggressive). In the present study, we evaluated affective responding to affectively and socially evocative video stimuli in a group of rhesus macaques that received bilateral amygdala lesions as neonates. The stimuli were produced to vary independently in terms of their affective and social content. The responses of the amygdala-lesioned animals were compared with a group of age-matched controls and a group of animals that had sustained bilateral hippocampus damage as neonates. As compared with control animals, amygdala-lesioned animals had blunted responding to both positive and negative stimuli, regardless of social content, but did differentiate between levels of social content. Taken together, these findings suggest that early amygdala damage permanently compromises affective processing while leaving intact the ability to distinguish between socially meaningful contexts.