HEALTHY AGING AS INFORMATION DIVERGENCE IN THE MULTIPLEX BRAIN

Understanding the coevolution of the human brain's 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 to 88) from the Cambridge Centre for Ageing and Neuroscience (CamCAN) dataset. Using Jensen Shannon Divergence and relative entropy metrics, we identify a fundamental organizing principle of healthy aging characterized by 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 heterogeneous. 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 SC FC divergence. This topological shift toward functional independence in subcortical switchboards provides a mechanistic connectomic signature for the well documented decline in fluid intelligence and motor adaptation that accompanies aging. In striking contrast, the limbic core (hippocampus and entorhinal cortex) exhibits remarkable stability across the lifespan, suggesting a biological imperative to preserve high fidelity memory circuits amidst global communicative rewiring. By framing healthy aging as a systematic subcortical untethering alongside limbic resilience, our work provides a powerful new multiplex baseline to distinguish normative cognitive decline from the early topological signals of neurodegenerative disease.