Longitudinal developmental trajectories do not follow cross-sectional age associations in hippocampal subfield and memory development.

Cross-sectional findings suggest that volumes of specific hippocampal subfields increase in middle childhood and early adolescence. In contrast, a small number of available longitudinal studies reported decreased volumes in most subfields over this age range. Further, it remains unknown whether structural changes in development are associated with corresponding gains in children's memory. Here we report cross-sectional age differences in children's hippocampal subfield volumes together with longitudinal developmental trajectories and their relationships with memory performance. In two waves, 109 participants aged 6-10 years (wave 1: M=7.25, wave 2: M=9.27) underwent high-resolution magnetic resonance imaging to assess hippocampal subfield volumes (imaging data available at both waves for 65 participants) and completed tasks assessing hippocampus dependent memory processes. We found that cross-sectional age-associations and longitudinal developmental trends in hippocampal subfield volumes were discrepant, both by subfields and in direction. Further, volumetric changes were largely unrelated to changes in memory, with the exception that increase in subiculum volume was associated with gains in spatial memory. Longitudinal and cross-sectional patterns of brain-cognition couplings were also discrepant. We discuss potential sources of these discrepancies. This study underscores that children's structural brain development and its relationship to cognition cannot be inferred from cross-sectional age comparisons.