Imaging studies using lobar measures first documented the non-linear changes in regional gray matter (GM) volumes during childhood and adolescence. The total volume of GM in each lobe, and in the brain overall, exhibits a pre-pubertal increase followed by post-pubertal loss (J. N. Giedd, J. Blumenthal, N. O. Jeffries, F. X. Castellanos et al., 1999; Jernigan & Tallal, 1990; Jernigan, Trauner, Hesselink, & Tallal, 1991; E. R. Sowell, Thompson, Tessner, & Toga, 2001). These findings showed that gray matter volume followed an inverted U-shaped trajectory in frontal, parietal and temporal lobes (J. N. Giedd et al., 1999). The post-pubertal GM loss was seen as consistent with post mortem observations of increased synaptic pruning during adolescence and early adulthood (Bourgeois, Goldman-Rakic, & Rakic, 1994; P.R. Huttenlocher, 1994; Rakic, 1996). Because the GM reductions were greater than synaptic pruning would explain, other investigators suggested that continued intra-cortical myelination might also partly explain the progressive reduction in the amount of tissue with a gray matter appearance on MRI (T. Paus, 2005). The developmental trajectories for each lobe were also surprising, because for the
