The study by Lebel and Beaulieu examines the longitudinal development of white matter maturation in healthy subjects aged 5–32 years using diffusion tensor tractography. The researchers analyzed 221 scans from 103 subjects, assessing fractional anisotropy (FA) and mean diffusivity (MD) in 10 major white matter tracts. Key findings include: 1. **Nonlinear Development Trajectories**: All tracts showed significant nonlinear development trajectories for FA and MD, indicating that changes occur more rapidly at early ages and slow down during young adulthood. 2. **Within-Subject Changes**: Significant within-subject changes occurred in most children and early adolescents, with these changes mostly completing by late adolescence for projection and commissural tracts. However, association tracts continued to show postadolescent maturation. 3. **Diffusion Parameter Changes**: Changes were primarily driven by decreasing perpendicular diffusivity, although increasing parallel diffusivity contributed to prolonged increases in FA in association tracts. 4. **Volume Increases**: Volume increased significantly with age for most tracts, and several association tracts demonstrated postadolescent volume increases. 5. **Reliability and Within-Subject Changes**: The study also assessed the reliability of diffusion measurements and within-subject changes, finding that most subjects had consistent tracts across scans. 6. **Sex Differences**: Small sex differences were observed, with females having higher FA in the splenium of the corpus callosum and males having higher FA in the cingulum and corticospinal tracts. 7. **Implications for Mental Health**: The findings suggest that prolonged maturation of white matter fibers beyond adolescence may be relevant to mental health disorders that emerge during late adolescence and young adulthood. The study provides valuable insights into the continued development of brain wiring from childhood into young adulthood, highlighting the importance of understanding these changes for the prevention and treatment of psychiatric disorders.The study by Lebel and Beaulieu examines the longitudinal development of white matter maturation in healthy subjects aged 5–32 years using diffusion tensor tractography. The researchers analyzed 221 scans from 103 subjects, assessing fractional anisotropy (FA) and mean diffusivity (MD) in 10 major white matter tracts. Key findings include: 1. **Nonlinear Development Trajectories**: All tracts showed significant nonlinear development trajectories for FA and MD, indicating that changes occur more rapidly at early ages and slow down during young adulthood. 2. **Within-Subject Changes**: Significant within-subject changes occurred in most children and early adolescents, with these changes mostly completing by late adolescence for projection and commissural tracts. However, association tracts continued to show postadolescent maturation. 3. **Diffusion Parameter Changes**: Changes were primarily driven by decreasing perpendicular diffusivity, although increasing parallel diffusivity contributed to prolonged increases in FA in association tracts. 4. **Volume Increases**: Volume increased significantly with age for most tracts, and several association tracts demonstrated postadolescent volume increases. 5. **Reliability and Within-Subject Changes**: The study also assessed the reliability of diffusion measurements and within-subject changes, finding that most subjects had consistent tracts across scans. 6. **Sex Differences**: Small sex differences were observed, with females having higher FA in the splenium of the corpus callosum and males having higher FA in the cingulum and corticospinal tracts. 7. **Implications for Mental Health**: The findings suggest that prolonged maturation of white matter fibers beyond adolescence may be relevant to mental health disorders that emerge during late adolescence and young adulthood. The study provides valuable insights into the continued development of brain wiring from childhood into young adulthood, highlighting the importance of understanding these changes for the prevention and treatment of psychiatric disorders.