The hippocampus is a structure that is critical for memory. Previous studies have shown that age-related differences in specialization along the longitudinal axis of this structure (i.e., subregions) and within its internal circuitry (i.e., subfields) relate to age-related improvements in memory in school-age children and adults. However, the influence of age on hippocampal development and its relations with memory ability earlier in life remains under-investigated. This study examined effects of age and sex on hippocampal subregion (i.e., head, body, tail) and subfield (i.e., subiculum, CA1, CA2-4/DG) volumes, and their relations with memory, using a large sample of 4- to 8-year-old children. Results examining hippocampal subregions suggest influences of both age and sex on the hippocampal head during early childhood. Results examining subfields within hippocampal head suggest these age effects may arise from CA1, whereas sex differences may arise from subiculum and CA2-4/DG. Memory ability was not associated with hippocampal subregion volume but was associated with subfield volume. Specifically, within the hippocampal head, relations between memory and CA1 were moderated by age; in younger children bigger was better, whereas in older children smaller was superior. Within the hippocampal body, smaller CA1 and larger CA2-4/DG contributed to better memory performance across all ages. Together, these results shed light on hippocampal development during early childhood and support claims that the prolonged developmental trajectory of the hippocampus contributes to memory development early in life.

The ability to keep similar experiences separate in memory is critical for forming unique and lasting memories, as many events share overlapping features (e.g., birthday parties, holidays). Research on memory in young children suggests their memories often lack high-resolution details, i.e., show impoverished pattern separation (PS). Recently developed assessments of PS suitable for children allow us to relate the formation of distinct, detailed memories for the development of the hippocampus, a neural structure critical for this ability in adults. The hippocampus displays a protracted developmental profile and underlies the ability to form detailed memories. This study examined age-related differences in hippocampal subfield volumes in 4- to 8-year-old children and relations with performance on a mnemonic similarity task (MST) designed to index memory specificity. Results revealed age-moderated associations between MST performance and cornu ammonis 2-4/dentate gyrus subfields. Specifically, age-related differences in the ability to form detailed memories tracked with normative patterns of volume increases followed by reductions over this age range. That is, greater volume correlated with better performance in younger children, whereas smaller volume correlated with better performance in older children. These findings support the hypothesis that developmental differences in hippocampal circuitry contribute to age-related improvements in detailed memory formation during this period.

Highlights During early- to mid-childhood, subfield development differs along the longitudinal axis of the hippocampus. CA1 subfield volume increases between 4−5 years in hippocampal head. CA2-4/DG and subiculum volume increases between 5−6 years in hippocampal body. Changes in CA1 and subiculum between 4−5 years relate to memory between 4−5 years.

Early childhood is characterized by vast changes in behaviors supported by the hippocampus and an increased susceptibility of the hippocampus to environmental influences. Thus, it is an important time to investigate the development of the hippocampus. Existing research suggests subregions of the hippocampus (i.e., head, body, tail) have dissociable functions and that the relations between subregions and cognitive abilities vary across development. However, longitudinal research examining age‐related changes in subregions in humans, particularly during early childhood (i.e., 4–6 years), is limited. Using a large sample of 184 healthy 4‐ to 8‐year‐old children, the present study is the first to characterize developmental changes in hippocampal subregion volume from early‐ to mid‐childhood. Results reveal differential developmental trajectories in hippocampal head, body, and tail during this period. Specifically, head volume showed a quadratic pattern of change, and both body and tail showed linear increases, resulting in a pattern of cubic change for total hippocampal volume. Further, main effects of sex on hippocampal volume (males > females) and hemispheric differences in developmental trajectories were observed. These findings provide an improved understanding of the development of the hippocampus and have important implications for research investigating a range of cognitive abilities and behaviors.

The hippocampus is a brain structure known to be important for memory. However, studies examining relations between hippocampal volume and memory across development yield mixed results. This may be due in part to the fact that volume is a coarser measure of hippocampal composition. Studies have begun to examine measures of diffusion, which capture characteristics of the microstructure of the hippocampus, and thus may provide additional information about the integrity of the underlying neural circuits. The present study applied this approach to a developmental period characterized by dramatic changes in both hippocampal microstructure and memory behavior-early childhood. Specifically, measures of hippocampal microstructural integrity were related to age and source memory performance in 93 children aged 4-8 years. Results revealed significant negative associations between hippocampal mean diffusivity and both age and memory, even after controlling for differences in hippocampal volume. These results suggest that hippocampal diffusion may provide additional, independent information about hippocampal integrity compared to volume, particularly during early childhood when important developmental changes have been proposed.