In humans, hippocampal−prefrontal oscillations in the theta band have also been linked to memory processes (34), and coordination between the prefrontal and temporoparietal DMN was shown to largely underlie theta-mediated oscillatory coupling (35). Congruently, seeding spontaneous fMRI activity fluctuations in the human hippocampus revealed signal reverberations in the major DMN nodes, which suggests an “ongoing functional relationship” (36). Further, quantitative metaanalyses of human neuroimaging tasks have established extensive spatial overlap in the DMN for neural activity increases during autobiographical memory retrieval, spatial navigation, anticipation of future events, and mind wandering (10)—all similar to findings from invasive experiments in the animal hippocampus. Finally, hippocampal functional connectivity with the medial prefrontal cortex was associated with patterns of thought that emphasize mental time travel (37). Our study builds on these findings by providing evidence at the level of the population that, compared with other tracts, individual differences in fornix white matter microstructure are particularly associated with gray matter differences within the DMN. One possibility is that this structural association reflects the underlying functional link between hippocampal systems and those within the DMN.
