Studies examining brain morphology in marijuana users, particularly within white matter, offer equivocal findings. Although some report no changes in white matter volume and composition in adult users (Block et al., 2000; Gruber and Yurgelun-Todd, 2005), reduced white matter volumes in the left parietal lobe and increased tissue density surrounding the left parahippocampal and fusiform gyri have been documented (Matochik et al., 2005). Whether the reported changes in brain structure among marijuana using adolescents persist into adulthood remains tentative. Increased mean diffusivity in the prefrontal fiber bundles of the corpus callosum in adults who initiated use during early adolescence suggests long-term changes to white matter quality as a result of adolescent marijuana use (Arnone et al., 2008). In conflict with these findings, a DTI study using whole-brain voxelwise analysis of 10 young adults who used moderately as adolescents suggested no loss of white matter integrity relative to non-users (Delisi et al., 2006). Still, another report shows higher fractional anisotropy in the rostral body and isthmus of the corpus callosum in adolescent substance users compared to controls (De Bellis et al., 2008).
