The other main part of our project is determining how reward valuation for self and other develops over adolescence. For macaques and humans, adolescence is a time in which important social bonds are developed between peers, and placement within the social hierarchy may solidify. To integrate into a social group, it is necessary to be able to represent rewards in a self-other framework, or one that takes into account not only the individual’s reward valuations, but also how social partners may value rewards.

Monkeys perform a social task developed in collaboration with Dr. Steve Chang's lab at Yale University. The monkey must choose between options on two separate conditions: give a treat only to themselves or both themselves and a bonded partner; or give a treat to only their partner or to neither animal, meaning the treat drops into a clear container both monkeys can see instead.

We consider monkeys that give treats to their partners more prosocial. We predict that prosociality increases over puberty as their reward representations begin to include rewards given to others, not only rewards they receive. We will test if this is influenced by dominance rank of the participants, or by familiarity with the social partner.

To determine  how the brain changes throughout adolescence, we took yearly MRI scans to evaluate structural and diffusion metrics. We have evaluated the cortical thickness across the brain, primarily focusing in the PFC, a region that is believed to be involved in the emergence of adult-like behaviors. Current results revealed a relationship of cortical thinning and increased prosociality using the task outlined above.

To evaluate white matter connections in the brain, we completed diffusion MR imaging, or DTI, which measures the directionality of water movement which occurs along axons (directionally encoded colored tensor weighted imaging). We are currently in the process of analyzing this data and processing the scans further to create tractography and evaluate how the amygdala connects to the PFC. 

To determine the monkey's stage of puberty, monthly physical and hormonal measurements are completed. While the monkey's birthdate and age are known, these are not good measures for the stage of puberty that the monkey is in. Our current cohort is made up of male monkeys with the goal of expanding to females in the future to evaluate how sex differences may affect social and nonsocial decision making. 

Our monkeys’ physical growth has been consistent with previously reported studies and can further elucidate how correlated hormonal changes are with bodily growth and behavior. Current hormones of interest include testosterone, estradiol, follicular stimulating hormone (FSH), and luteinizing hormone (LH). With future possible evaluations of inhibin-B, cortisol, CRP, DHEA-S, and kisspeptin.