The hippocampus and cortical memory networks have an inflection point in middle childhood

Abstract

Decades of developmental memory research has mainly reported linear and protracted changes in both human hippocampal function and connectivity between the hippocampus and cortex. While foundational, very few studies have interrogated the reliability of hippocampal signals across age, and how this coincides with (or diverges from) age-related changes in connectivity to broader cortical networks supporting multiple memory systems. Here, utilizing movie-watching fMRI data in children 3-12 years and adults, we assessed hippocampal response stability using an inter-subject functional correlation (ISFC) approach, and then measured functional connectivity between the hippocampus and the Posterior Medial (PM) - Anterior Temporal (AT) cortical memory networks proposed to support episodic-like (PM) and semantic-like (AT) memory respectively. Results showed that hippocampal responses are stable in the youngest children, but bifurcate in 7-year-olds –– with half the subjects correlating most highly with younger and half with older age groups. Likewise, we found that while functional connectivity within the AT network is stable across development, connections between the anterior hippocampus and this network did not reach adult levels until around 7-years. Thus, while brain networks supporting semantic memory may be in place early, interactions with the hippocampus may not develop until after middle childhood –– with an inflection point around 7-years of age. Significance Statement The development of the hippocampus, a key-region in episodic memory behaviors, extends well into adolescence. However, little is known about the stability of hippocampal responses across age and how this might impact communication with cortical brain networks supporting episodic and semantic memory. Here, we show that connectivity within a network that supports semantic memory matures early, but connectivity between this network and hippocampus does not mature until at least 7-years of age. Unlike all other ages, 7-year-olds’ hippocampal responses are not stable: half of the subjects’ responses appear immature and half mature for their age. This work identifies 7-years as a developmental inflection point in both hippocampal signaling and hippocampal-cortical memory interactions that may support multiple memory systems.