Lesion studies with adult animals give researchers a unique ability to investigate the functional role of specific brain areas following normal development. However, such studies do not address the role these structures play in development itself.
For instance, amygdala damage results in markedly reduced sensitivity to novelty and threat signals in adult animals, but it is unknown whether compensatory development or brain reorganization mitigates these effects or their severity if damage occurs at a young age. To address this question, damage must be sustained early in life, and behavior tracked over time.
This approach was adopted in an ambitious project started in 2001 that chronicles the effects of neonatal amygdala or hippocampus damage on the development of social behavior. The latest installment of this longitudinal study is published in Behavioral Neuroscience.
Moadab, Bliss-Moreau, and Amaral (2015, Behavioral Neuroscience) (PDF, 136KB)opens in new window recorded the social behavior of adult rhesus macaque monkeys (8 years old) who received bilateral lesions to the amygdala or hippocampus or a sham control operation at approximately 2 weeks of age. This study is unique because unlike most experiments where animals are tested in a novel environment or with a novel social partner, Moadab and colleagues observed monkeys in their home cages interacting with the pair-mate animal they lived with. In other words, social behavior was observed in a familiar, everyday context.
Overall, the researchers characterized the observed social deficits as subtle. Amygdala-lesioned animals were less social than control animals, spending more time active but out of reach of their pair-mate. Although amygdala-lesioned animals did not differ from controls in total time spent socializing, they spent significantly less time grooming. Based on other work on the effect of amygdala damage, the authors speculate that social interactions were not rewarding for animals with amygdala lesions, so they were less motivated to engage in them. Amygdala-lesioned animals also displayed more stereotypies (repetitive or ritualistic movement) and stress-related behaviors when alone.
Comparing the results to the prior work with this cohort reveals that social impairments following early amygdala damage were becoming less pronounced as the animals matured. In contrast, animals with early hippocampus damage were more social than controls, spending more time in close social interactions that they were more likely to initiate. Compared to the previous studies with this cohort, this hyper-sociability was becoming more pronounced as the animals matured.
Given the novelty of this result and the fact that damage to the hippocampus is known to cause wide-spread changes to the brain, determining the cause of hypersociability in these animals is an important topic for further research.
Some of these results differ from previous findings, likely due to differences in how animals were reared and socialized across studies. In perhaps the most extreme example, in prior studies that showed increases in behavioral deficits with age following amygdala lesions, the animals were removed from their mothers on the day of birth, whereas in the current study, where social impairments associated with amygdala damage became more subtle over time, animals were reared with their mothers until they were 6 months old.
This illustrates the complexity of studying the development of social behavior, as social behavior is influenced by environmental factors that range from rearing and socialization history to the context in which behavior is observed.
Citation:
Moadab, G., Bliss-Moreau, E., & Amaral, D. G. (2015). Adult social behavior with familiar partners following neonatal amygdala or hippocampus damage. Behavioral Neuroscience, 129(3), 339–350. https://dx.doi.org/10.1037/bne0000062
Note: This article is in the Experimental Psychology topic area. View more articles in the Basic/Experimental Psychology topic area.