The intensity of substance use in relation to diffusion indices also revealed unexpected trends. More years of regular drinking was linked to lower FA in the right inferior frontal gyrus, whereas greater marijuana and alcohol use were linked to higher left SLF and occipito-frontal FA. Low FA values in the left SLF and occipito-frontal fasciculi are found to be associated with complex mental illness such as schizophrenia (Cheung et al., 2008; Karlsgodt et al., 2008; Xu et al., 2008), where altered fronto-parietal connectivity is also evident. While it is surprising that increased duration and intensity of use was associated with higher FA in these regions, it is possible that these differences may have predated the onset of use, and may be linked to increased risk for use or altered attentional processes that lead to more frequent use. Importantly, this finding did not hold in very heavy substance users, suggesting that differences in white matter composition may be dependent on the extent of use. Among heavy MJ+ALC users, greater lifetime alcohol use was linked to higher FA in the anterior limb
