In contrast to grey matter reductions, white matter across the adolescent years shows growth and enhancement of pathways (Giedd 2008; Yakovlev and Lecours 1967). This is reflected in white matter volume increase, particularly in fronto-parietal regions (Benes 1989; Huttenlocher 1990; Nagel et al. 2006; Yakovlev and Lecours 1967). Diffusion tensor imaging (DTI), a neuroimaging technique that has gained widespread use over the past decade, relies on the intrinsic diffusion properties of water molecules and has afforded a view into the more subtle microstructural changes that occur in white matter architecture. Two common scalar variables derived from DTI are fractional anisotropy (FA), which describes the directional variance of diffusional motion, and mean diffusivity (MD), an indicator of the overall magnitude of diffusional motion. These measures index relationships between signal intensity changes and underlying tissue structure, and provide descriptions of white matter quality and architecture (Conturo et al. 1999; Pierpaoli and Basser 1996; Shimony et al. 1999). High FA reflects greater fiber organization and coherence, myelination and/or other structural components of the axon, and low MD values suggest greater white matter density
