Spatiotemporal Patterns and Structural Substrates of Individual Functional Variability in Youth

Youth is a period of emerging individuality and extensive neural remodeling, yet how functional brain individuality is organized across development remains unclear. Prior work has often conflated variability in functional topography and connectivity, highlighting the need to examine them separately to better understand how functional individuality relates to brain structure and cognition. Here we used individualized functional parcellation in a large multimodal developmental cohort to separately quantify variability in individualized functional parcellation (vIFP) and variability in functional connectivity (vFC). Both forms of variability followed the sensorimotor-association axis and were greatest in the association cortex. vIFP increased significantly with age, whereas vFC showed regionally specific maturation without a significant whole-brain increase. Both trajectories showed a common mid-adolescent inflection at 14-16 years, marking a window of accelerated functional individualization. Despite shared spatial and temporal organization, vIFP and vFC showed dissociable links to structure and cognition. vIFP was more strongly coupled to structural variability, whereas vFC was more strongly associated with cognitive variability. These findings reveal convergent and divergent developmental principles of topographic and connectional functional variability, highlighting their complementary roles in structural constraints and cognitive specialization.