Determining the structural integrity of white matter from diffusion weighted data is complicated by the existence of multiple fiber orientations within a voxel (Pierpaoli et al., 1996). To resolve regions of complex fiber distributions in characterizing local diffusion, measurements at higher angular resolution, in conjunction with higher order tensor analysis (Frank, 2002), will be considered in future work. The macrostructural correlates of increased and decreased diffusion anisotropy are the subject of ongoing research combining DTI with structural techniques. In addition, studies incorporating functional data will determine how indices of white matter compromise are linked to atypical neural networks and to what extent deviations in diffusion anisotropy contribute to altered connectivity. Integration of neuropsychological data will inform how changes in FA are associated with tasks involving different cognitive demands. These investigations are best conducted within a longitudinal framework where both the persistent and potentially reversible effects of marijuana and alcohol use over adolescence can be comprehensively assessed.
