Whereas WM increases during childhood and adolescence, the trajectories of GM volumes follow an inverted U shaped developmental trajectory. The different developmental curves of WM and GM belie the intimate connections among neurons, glial cells, and myelin, which are fellow components in neural circuits and are linked by lifelong reciprocal relationships. Cortical GM changes at the voxel level from ages 4 to 20 years derived from scans of 13 subjects who had each been scanned four times at ~2 year intervals are shown in Figure 2 (animation is available at http://www.nimh.nih.gov/videos/press/prbrainmaturing.mpeg) (Gogtay et al., 2004b). The age of peak GM density is earliest in primary sensorimotor areas and latest in higher-order association areas such as the dorsolateral prefrontal cortex, inferior parietal, and superior temporal gyrus. An unresolved question is the degree to which the cortical GM reductions are driven by synaptic pruning versus myelination along the GM/WM border (Sowell et al., 2001). The volume of the caudate nucleus, a subcortical GM structure, also follows an inverted U shaped developmental trajectory, with peaks similar to the frontal lobes with which they share extensive connections (Lenroot et al., 2007).
