The brain dynamics of intellectual development: waxing and waning white and gray matter.

Distributed brain areas support intellectual abilities in adults. How structural maturation of these areas in childhood enables development of intelligence is not established. Neuroimaging can be used to monitor brain development, but studies to date have typically considered single imaging modalities. To explore the impact of structural brain maturation on the development of intelligence, we used a combination of cortical thickness, white matter (WM) volume and WM microstructure in 168 healthy participants aged 8-30 years. Principal component analyses (PCAs) were conducted separately for cortical thickness, WM volume, fractional anisotropy (FA) and mean diffusivity (MD) in 64 different brain regions. For all four parameters, the PCAs revealed a general factor explaining between 40% and 53% of the variance across regions. When tested separately, negative age-independent relationships were found between intellectual abilities and cortical thickness and MD, respectively, while WM volume and FA were positively associated with intellectual abilities. The relationships between intellectual abilities and brain structure varied with age, with stronger relationships seen in children and adolescents than in young adults. Multiple regression analysis with the different imaging measures as simultaneous predictors, showed that cortical thickness, WM volume and MD all yielded unique information in explaining intellectual abilities in development. The present study demonstrates that different imaging modalities and measures give complementary information about the neural substrates of intellectual abilities in development, emphasizing the importance of multimodal imaging in investigations of neurocognitive development.