Hippocampal functional networking in wakefulness and sleep

Background: The hippocampal formation (HF+) is part of a system that plays a critical role in the encoding and retrieval of memory [1]. It has repeatedly been shown that consolidation of declarative memories occurs in human NREM sleep, supposedly involving transfer of information from the hippocampus to the neocortex [2,3]. The four major subregions of the HF+ are the cornu ammonis (CA), the dentate gyrus (DG), the subiculum (SUB) and the entorhinal cortex (EC). Anatomically, the SUB is the major source of efferent projections from the hippocampus to the orbitofrontal and infralimbic cortex, as well as to the retrosplenial cortex. The EC is interconnected with temporal, cingulate and insular cortices, whereas no direct neocortical connections have been reported for CA and DG. The goal of this work was to determine unique spontaneous connectivity maps of these hippocampal subregions and their alteration throughout NREM sleep.