The Development of White and Gray Matter: Adolescence as a Period of Transition

Abstract

In the last two decades advances in neuroimaging methods have allowed the study, in vivo, of the development of the brain. Cross-sectional and longitudinal studies using structural magnetic resonance imaging and diffusion tensor imaging have shown that gray matter volumes and cortical thickness decrease during childhood and adolescence, into adulthood, and that white matter tracts microstructure also continues to mature into the 1920s. Importantly, these changes are region-specific, with earlier maturation of sensory-motor regions, and later maturation of associative areas, which parallel the prolonged maturation of higher cognitive skills. These results cannot be directly linked to the synaptic proliferation, synaptic pruning and myelination studied post-mortem. However, decreases in cortical thickness are thought to reflect at least in part synaptic pruning and increases in white matter volume are thought to be driven by increases in myelination and axonal growth. These developmental changes in brain structure reflect the fine-tuning of neural connectivity to an individual's experiences and environment, and an increased speed of transmission of information between neurons, respectively. Adolescence is identified as a period of particularly rapid changes in brain structure, paralleling the considerable physical, cognitive and social changes individuals experience during this phase of development. While the focus has mostly been on general developmental trends, research has shown there is considerable variability in brain structure development between individuals, which associates with individual differences in cognition and mental health.