During adolescence, portions of the immature brain are pruned away to reveal a “leaner,” more efficient adult brain. This regionally specific developmental sculpting is seen terms of the relative amount of brain area devoted to cell bodies and their processes--termed “gray matter” (e.g., Sowell et al., 2002), and in the number of connections (synapses) between nerve cells (e.g., Rakic et al., 1994). In contrast, increases are seen during adolescence in the production of myelin, a fatty, sheathing material that insulates nerve processes (axons) and appears white in unstained tissue, leading to developmental increases in brain “white matter” (Benes et al., 1994). Myelination speeds the flow of information along axonal pathways, and thus, via selective myelination of pathways connecting distant brain regions (Salami et al., 2003), patterns of neural activity are shifted during adolescence from a bias toward a predominance of relatively local influences to the emergence of networks involving more distant brain regions (e.g., Rubia et al., 2007).
