Functional Connectivity (FC) in resting-state or task conditions is not frozen but inherently dynamic. Yet, there is no consensus on whether fluctuations in FC resemble isolated transitions between discrete FC states rather than continuous changes. This quarrel hampered advancing the study of dynamic FC. This is unfortunate as the structure of fluctuations in FC can provide crucial information about developmental changes, aging, or progression of pathologies. We merge the two perspectives and consider dynamic FC as continuous network reconfiguration, including a stochastic exploration of the space of possible steady FC states. The statistical properties of this random walk deviate both from an “order-driven” dynamics, in which the mean FC is preserved, and from a “randomness-driven” scenario, in which fluctuations of FC remain uncorrelated over time. Instead, dynamic FC turns out to have a complex structure endowed with long-range sequential correlations giving rise to transient slowing and acceleration epochs in the continuous flow of reconfiguration. When applying our analysis to an fMRI dataset in healthy elderly, we find that the dynamic FC tends to slow down, becomes less complex and more random with increasing age. All these effects are strongly associated with age-related changes in cognitive performance.Highlights *   rs     :   resting-state    RSN     :   resting-state network    fMRI     :   functional magnetic resonance imaging    BOLD     :   blood oxygen level dependent    SC     :   structural connectivity    FC     :   functional connectivity    d FC     :   dynamic functional connectivity    DFA     :   detrended fluctuation analysis    MoCA     :   Montreal Cognitive Assessment    SO     :   spectral overlap

Dynamic Functional Connectivity between Order and Randomness and its Evolution across the Human Adult Lifespan