Organization of cortico-hippocampal networks in the human brain

Episodic memory is thought to depend on interactions between the hippocampus and a set of closely interconnected regions that comprise the default mode network (DMN)--a large-scale network that has been identified with resting-state fMRI. Here, using data-driven analyses of resting-state fMRI data to characterize cortico-hippocampal network connectivity, we identified a discrete set of subnetworks that interact with the hippocampus. Specifically, we found that the hippocampus closely affiliated with the DMN and with a "Medial Temporal Network" (MTN) that included regions in the medial temporal lobe and retrosplenial cortex. The DMN could be further subdivided into three subnetworks: a "Posterior Medial" Subnetwork comprised of regions in the posterior cingulate, lateral parietal, and dorsal prefrontal cortex, an "Anterior Temporal: Subnetwork comprised of regions in the temporopolar, lateral orbitofrontal, and dorsal medial prefrontal cortex, and a "Medial Prefrontal" Subnetwork comprised of regions in the ventral medial prefrontal, and entorhinal cortex. These cortico-hippocampal networks vary in their functional connectivity along the hippocampal long-axis, and analyses of an independent task-fMRI dataset revealed that the three DMN subnetworks represent different kinds of information during memory-guided decision-making. Finally, a data-driven meta-analysis of functional imaging studies of cognition suggests new hypotheses regarding the functions of the MTN and DMN subnetworks, thus providing a framework to guide future research on the neural architecture of episodic memory.