Although total brain volume is about 95% of its adult size by the age of 5 years, various subcomponents of the brain do undergo age-related changes. In general, white matter—an indication of myelination—increases with age, while gray matter decreases with age. The brain’s most prominent white matter structure, the corpus callosum, increases about 1.8% per year between ages 3 and 18 years. The bottom figure demonstrates growth of the corpus callosum, the sideways C-shaped white structure in the center of the brain, by showing longitudinal scans of a healthy girl at ages 3 and 9 years. The corpus callosum integrates the activity of the left and right cerebral hemispheres, and increases in corpus callosum size may reflect increased aptitude for higher-order cognitive abilities.