Healthy Aging As Information Divergence In The Multiplex Brain

AO_SCPLOWBSTRACTC_SCPLOWUnderstanding the co-evolution of the human brains structural scaffold and functional traffic across the adult lifespan remains a fundamental challenge in neuroscience. While age-related degradation in grey matter and functional activation is well-documented, the joint trajectory of the structural (SC) and functional (FC) connectomes is often overlooked due to the lack of an integrative framework. Here, we model the brain as a multiplex network to quantify the information-theoretic interdependencies between these two layers in a cross-sectional cohort of 589 healthy individuals (ages 18-88) from the Cam-CAN dataset. Using Jensen-Shannon Divergence and relative entropy metrics, we identify a fundamental organizing principle of healthy aging: a progressive information divergence where functional dynamics increasingly "untether" from their underlying structural constraints. Our results reveal that this decoupling follows a robust linear trajectory, yet is highly spatially hetero-geneous. Meso-scale community analysis using the Multiplex Map Equation identifies subcortical hubs--specifically the putamen, pallidum, caudate, and thalamus--as the primary epicenters of age-related divergence. This topological shift toward functional independence in subcortical "switch-boards" provides a mechanistic connectomic signature for the well-documented decline in fluid intelligence and motor adaptation. In striking contrast, the limbic core (hippocampus and entorhinal cortex) exhibits remarkable stability, suggesting a biological imperative to preserve high-fidelity memory circuits amidst global communicative rewiring. By framing healthy aging as a system-atic subcortical untethering alongside rigid limbic resilience, our work provides a powerful new multiplex baseline to distinguish normative cognitive decline from the early topological signals of neurodegenerative disease.