Untitled

58 CHARACTERISTICS OF BRAIN AND BEHAVIORSince these early imaging reports, cortical gray matter volume decreases havebeen confirmed by other groups (Pfefferbaum et al., 1994; Reiss et al., 1996).Regionally, the most notable changes during childhood and adolescence occur inthe more dorsal cortices, as described by Jernigan and colleagues (1991). Duringadolescence, frontal and parietal lobes show highly significant increases in whitematter, along with concomitant decreases in gray matter (Giedd et al., 1999; Sowellet al., 2002c). Cortex in the more ventral temporal lobes appear to change lessdramatically between childhood and adolescence (Giedd et al., 1999; Jerniganet al., 1991; Sowell et al., 2002c). Notably, gray matter thinning in the frontal cortexis related to changing cognitive ability in normal children and adolescents. Wefound significant correlations between gray matter volume in the frontal lobe andchildren’s performance on a verbal learning task (Sowell et al., 2001a).From postmortem studies, we know that myelination begins near the end of the2nd trimester of fetal development and extends beyond the 2nd decade of life(Yakovlev & Lecours, 1967). Autopsy studies consistently reveal that myelinationoccurs in a systematic sequence progressing from inferior to superior brainregions and from posterior to anterior. In addition to continuing myelination, aregionally variable reduction in synaptic density also occurs during the adolescentage range (Huttenlocher, 1979; Huttenlocher & de Courten, 1987). Theseprocesses are thought to reflect the regional pattern of functional maturation ofthe brain. Findings from the volumetric MRI studies described above have tendedto concur with the postmortem studies given that cortical gray matter volume reductionsappear to be somewhat specific to the superior cortices of the frontal andparietal lobes relatively late in development (i.e., between childhood and adolescence;(Jernigan et al., 1991; Sowell & Jernigan, 1998)). One volumetric studywith a large sample and longitudinal data points has confirmed these results (Gieddet al., 1999), showing subtle increases in gray matter during childhood followedby significant declines during adolescence.Voxel-Based Morphometry of AdolescentBrain DevelopmentThe volumetric studies described above have provided invaluable informationabout continued brain development and are still thought to be a “gold standard”in quantitative brain imaging. This is largely because the method is quite easyto understand and to replicate, provided detailed protocols for defining anatomicallandmarks are established. However, they are limited because, typically, onlygross lobar structures can be reliably identified visually and manually defined.Newer methods have recently been employed to assess structural age effectsduring normal development on a voxel-by-voxel basis (Paus et al., 1999; Sowellet al., 1999a; Sowell et al., 1999b). This method allows assessment of anatomi-