HOW BRAIN MATURATION OF ADOLESCENT PRIMATES INFLUENCES REWARD PROCESSING

Abstract

Adolescence marks a pivotal period of development in brain structure and behavior. The changes in how our brains are wired directly influence how we respond to the world around us. The uncinate fasciculus (UF), which develops during adolescence, is a major white matter tract that connects the orbitofrontal cortex (OFC) and the amygdala. The OFC and amygdala are heavily involved in reward processing and social decision-making. To investigate how brain maturation influences these behaviors, we collected diffusion MR images and behavioral measurements (a non-social delay discounting task and a social reciprocation task) in four adolescent rhesus macaques (31 to 51 months old). Two MR images were taken about a year apart to detect any changes in diffusion indices such as fractional anisotropy (FA) and mean diffusivity (MD), which can inform us about myelination. We first analyzed whether these diffusion indices change over age, and then whether they predict behaviors in the two tasks during adolescence. We found that MD in the OFC and UF followed an inverted U curve across age, indicating ongoing brain development through adolescence. MD in the amygdala and FA in all three brain regions did not follow the same pattern across age. High MD was correlated with a higher willingness to wait for a reward in the non-social delay discounting task. Higher FA in the OFC and amygdala showed a correlation with higher prosocial decisions in the social reciprocation task. The changes in MD across age suggest naturalistic synaptic pruning followed by strengthening of myelination. A higher MD, associated with a decrease in myelination, predicts a higher tolerance for delayed reward. Higher FA, reflecting higher axonal integrity, predicts more prosocial decisions. This may be due to the strengthening of axons within the OFC and amygdala supporting the reward valuation of others, not only for themselves. Overall, we found certain anatomical features measured by diffusion MR images show changes with age and support both reward valuation and social decisions.