Distinct cortical networks associated with subregions of the hippocampal-entorhinal system in the first two years of human life

Episodic memory critically depends on the hippocampal-entorhinal system. Despite a rapid developmental change in human episodic memory during the first two years of life, little is known about the developmental trajectory of the hippocampal-entorhinal circuitry during this critical time period. In the current study, we collected high-quality precision fMRI data from five typically developing infants over the time period between 1 and 2 years of life and examined the cortical networks associated with subregions of the hippocampal-entorhinal system. We show that already at around 1 year of age, subdivisions of the canonical default network (DN) are dissociated at the neocortex but are differently associated with the hippocampal-entorhinal system, with tentatively differential developmental trajectories. While the DN-A, associated with episodic simulation, coupled with the anterior hippocampus and the medial part of the entorhinal cortex, the DN-B, associated with social cognition, showed weaker connectivity with the hippocampal-entorhinal system. The parietal memory network, in turn, coupled with the posterior hippocampus and the lateral part of the entorhinal cortex. Exploratory longitudinal results suggest that associations of the DN-B with the hippocampus and the intermediate part of the entorhinal cortex increase towards the end of the second postnatal year, and the associations of the parietal memory network become more topographically restricted to the posterior hippocampus. These networks and their differential maturation may provide the basis for important developments in episodic memory and social cognition in the second postnatal year and suggest that potentially unique properties of human hippocampal-entorhinal circuitry may emerge latest in development.