Hippocampus (HPC) structure and its connectivity with frontoparietal regions continue to develop through adolescence, a developmental period associated with substantial gains in memory and reasoning. While such structural changes are well documented, we know less about the functions that HPC and frontoparietal development confer, fundamentally limiting our understanding of the mechanisms through which individuals learn and reason about the world at different ages. From very early in life, children can learn simple associations that they directly experience. However, with age, our memories become more complex, reflecting not only directly observed information, but also knowledge derived across multiple episodes. Such derived knowledge might represent commonalities among experiences while simultaneously exaggerating important differences between them, thus forming hierarchical knowledge structures that can support inference decisions about event relationships, while also preserving detailed memory for when and where those relationships vary by context. The overarching goal of this proposal is to test the hypothesis that representational capacity within HPC and frontoparietal cortex undergoes qualitative changes during adolescence. We will use a serial cohort design to collect data from adolescents (13-18 years) at three timepoints, each 1.5 years apart, as well as data from adults (19-25 years) at a single timepoint. This design will allow us to test longitudinal predictions about how changes in neural representation within individual adolescents, over time, predict corresponding changes in memory and inference behaviors, as well as cross-sectional predictions about how HPC and frontoparietal cortex representation differs between adolescents and adults. We will use high-resolution functional magnetic resonance imaging (fMRI), representational fMRI analyses, and computational modeling, to test three Aims. Aim 1 will test the prediction that HPC and ventromedial prefrontal (vmPFC) representations will transition from coding simple, individual associations to extracting hierarchical knowledge about the relationships among experiences. Aim 2 will test the prediction that lateral parietal cortex (LPC)-mediated memory reactivation during new learning and inference will have different consequences for HPC-vmPFC representation and inference behavior at distinct points in adolescence. Aim 3 will test the prediction that emerging vmPFC control will influence what memories are available in LPC during learning and inference, as well as directly mediate the impact reactivated memories have on the trajectory of HPC representation during adolescence. The results from this project will provide a key test of fundamental theories of cognitive development and substantially advance our knowledge of the representational capacities of the HPC-frontoparietal memory system in typically developing adolescents. In doing so, the findings may have important implications for our eventual understanding of how the onset of mental health disorders (e.g., affective disorders and schizophrenia) during adolescence impact neural representation as well as memory and reasoning ability.
