Functional Inertia Index of Memory-Retaining Brain Dynamics: A Measure of Large-Scale Brain Adaptability

How does the brain retain enough dynamic memory to stabilize thought, yet remain flexible enough to adapt to new inputs? To address this, we introduce the functional inertia index (FII), the first quantitative measure of temporal momentum in large-scale brain dynamics. FII estimates the force required for new neural activity to deviate from a brain’s ongoing trajectory, providing a direct, time-resolved measure of adaptability versus rigidity. Applied to resting-state fMRI from a multisite schizophrenia cohort, FII revealed three recurrent dynamical regimes: a stabilizing regime with low inertia, a locked regime with high inertia, and a shifting regime with intermediate and rapidly changing inertia profiles. Prolonged residence in the high-inertia plateau predicted greater symptom severity, an effect mediated by whole-brain FII. Global inertia also showed opposite cognitive relationships across groups, improving performance in controls but impairing it in schizophrenia, revealing a separation between adaptive and maladaptive rigidity. At the regional level, FII unified two long-standing observations, specifically the stability of associative hubs and the volatility of sensory pathways, within a single mechanistic framework. These results establish functional inertia as a fundamental organizing principle of brain dynamics and identify a tractable biomarker of stability, adaptability, and clinical dysfunction.