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Socioemotional abnormalities, including decreased social interactions and increased self-directed activity, were reported when rhesus monkeys with neonatal ablations of either the medial temporal lobe (AH) or the inferior temporal cortex (TE) were paired with unoperated peers at two and six months of age, though these abnormalities were more severe in Group AH (Bachevalier et al., 2001). As adults (Experiment 1), the monkeys were re-evaluated in the same dyads and their reactivity to novel toys, social status, and reactions to separation were also assessed. Group TE now showed only few if any of the abnormal behaviors observed in infancy. In contrast, Group AH continued to display decreased social interactions and increased self-directed activity and showed also increased submission and reduced responses to separation, but normal reactivity to novel toys. To determine whether this degree of socioemotional impairment was less severe than that produced by the same damage in adulthood, we assessed dyadic social interactions of monkeys raised until adulthood in laboratory conditions similar to those in Experiment 1 and then given the AH ablations (Experiment 2). Two months postoperatively these monkeys showed a small reduction in social interactions that became more pronounced six months postoperatively, yet remained less severe than that seen in the infant-lesioned monkeys. No other socioemotional effects, except for an increase in food/water consumption, were observed. The finding that neonatal AH lesions produce more severe socioemotional disturbances than the same lesion in adulthood is the reverse of the effect commonly reported for other cognitive functions after cerebral damage. Keywords: amygdala, hippocampus, area TE, infants, social interactions DOI: 10.1037/a0021622

Nonhuman primate studies, using selective amygdala lesions that spare cortical areas and fibers of passage, have helped to clarify the amygdala's specific contribution to social and emotional behavior. M. D. Bauman, J. E. Toscano, W. A. Mason, P. Lavenex, and D. G. Amaral (2006) reported that macaque monkeys (Macaca mulatta) with neonatal neurotoxic amygdala lesions displayed lower rank in social dominance status, reduced aggressive gestures, and enhanced fearful reactions to social cues compared with normal controls and those with neonatal hippocampal lesions when tested as juveniles in a group of peers. These results are discussed in light of a recent study (C. J. Machado & J. Bachevalier, 2006) showing that the same selective amygdala damage in adolescent monkeys did not alter presurgical social dominance status. This variability in behavioral changes after selective amygdala lesions underscores the significant interplay between timing of the lesion, genetic traits, and environmental factors and suggests that the amygdala is not the generator of specific emotional responses, but acts as a modulator to ensure that emotional responses are appropriate to the external stimuli and social context. Keywords: social dominance, fear, anxiety, neurotoxic lesions, monkeys

Normal infant monkeys and infant monkeys with neonatal damage to either the medial temporal lobe or the inferior temporal visual area were assessed in dyadic social interactions at 2 and 6 months of age. Unlike the normal infant monkeys, which developed strong affiliative bonds and little or no behavioral disturbances, the lesioned monkeys (each of which was observed with an unoperated control) exhibited socioemotional abnormalities and aberrant behaviors. The socioemotional changes predominated at 6 months of age and were particularly severe in monkeys with medial temporal lesions. In both the pattern and time course, the socioemotional deficits produced by the neonatal medial temporal lesions bear a striking resemblance to the behavioral syndrome in children with autism. Further analysis of these lesion-induced abnormalities in nonhuman primates may therefore provide insight into this debilitating human developmental disorder.