This study investigated the integrity of white matter microstructure in adolescent marijuana and alcohol users. Using tract-based spatial analysis, we compared FA and MD within white matter structures throughout the brain. Based on neurobiological evidence of cannabis receptors within myelin precursors, persistently impaired myelination in alcoholism, and findings of altered neural networks in MJ+ALC-using teens, we predicted that substance users would show lower FA than non-users with predominant alterations in fronto-parietal white matter pathways. Consistent with our hypothesis, users evidenced diminutions in mean FA relative to controls, notable in frontal-parietal circuitry comprising fibers of the inferior frontal region, splenium of the corpus callosum, postcentral gyrus, and left SLF. Contrary to expectations, areas of increased FA were observed among MJ+ALC teens in the occipital lobe, internal capsule, and arcuate portion of the right SLF. MD was similar between groups within the regions of significant FA discrepancy. However, white matter adjacent the lingual gyrus showed higher MD among MJ+ALC users, whereas the posterior aspect of the left inferior longitudinal fasciculus demonstrated lower MD in users than controls. Together, these findings suggest that selective aberrancies in cerebral white matter are evident in early-onset adolescent marijuana and alcohol use.
