The white color of “white matter” is produced by myelin, fatty white sheaths formed by oligodendrocytes that wrap around axons and drastically increase the speed of neuronal signals. The volume of WM generally increases throughout childhood and adolescence (Lenroot et al., 2007), which may underlie greater connectivity and integration of disparate neural circuitry. An important feature that has only recently been appreciated is that myelin does not simply maximize speed of transmission but modulates the timing and synchrony of the neuronal firing patterns that create functional networks in the brain (Fields and Stevens-Graham, 2002). Consistent with this, a study using a measure of white matter density to map regional white matter growth found rapid localized increases between childhood and adolescence. Corticospinal tracts showed increases that were similar in magnitude on both sides, while tracts connecting the frontal and temporal regions showed more increase in the left-sided language-associated regions (Paus et al., 1999).
