In conclusion, our results suggest spatial proximity between subregions in major DMN nodes that offer complementary structural and functional properties. Such topographic organization could provide a scaffold for communication between subregions that track unique aspects of whole-brain functional modes. We identified cortical subregions of the DMN that were closely allied to fornix microarchitecture, which, we speculate, may perhaps be related to processing information from the hippocampus in the medial temporal lobe as well as other DMN subregions that were important in explaining functional coupling shifts between major cortical networks. This mosaic biological design, we further speculate, may contribute to resolving competing requirements of modular functional specialization with between-network interplay via long-distance connections. While the DMN has repeatedly been shown to be placed at the heart of the brain network hierarchy (3, 7, 38), the DMN is itself composed of distributed modules, each of which embodies distinct submodules (8, 16, 24).
