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 with variability in functional connectivity, highlighting the need to examine them separately to better understand how functional individuality relates to brain structure and cognition. Here, using individualized functional parcellation in two large multimodal developmental cohorts (total N = 1211), we separately quantified variability in individualized functional parcellation (vIFP) and in functional connectivity (vFC). Both forms of variability were organized along the sensorimotor-association axis and were greatest in the association cortex. During development, vIFP increased significantly with age at both regional and whole-brain level, whereas vFC showed regionally specific age effects but no significant whole-brain increase. Both trajectories exhibited a common mid-adolescent inflection (14-16 years), marking a window of accelerated functional individualization. Despite shared spatial and temporal organization, vIFP and vFC showed dissociable associations with brain 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.