In this study, we explored the relationship between adolescent marijuana and alcohol use, WM quality, and neurocognitive performance. The present results link performance on standardized neurocognitive measures with WM diffusion properties in substance users. We found decreased FA in temporal brain areas in adolescents with histories of marijuana and alcohol use that was related to poorer attention, working memory, and speeded processing. These findings implicate substance-related alterations in WM with corresponding functional weakness. Conversely, in occipital brain regions, we found evidence for possible compensatory mechanisms in the user group. Specifically, users displayed higher FA that was associated with better working memory and complex sequencing performance. Higher FA and lower MD in this region did not correlate with enhanced cognitive performance in the control group, suggesting that changes in this region may optimize performance in the user group only. Another area of increased FA in users, in the internal capsule, was associated with poorer verbal memory performance indicating that neuroadaptive processes may not simply be additive or subtractive, but may be complicated by interfering or competing maturational processes.
