UC San Diego

White matter (WM) development is important for efficient communication between brain regions and higher order neurocognitive functioning. Adolescents have a higher propensity for engaging in risky behaviors such as substance misuse and delinquent acts, yet few studies have explored associations between WM integrity, neurocognitive functioning, and risk taking during adolescent development. This study evaluated baseline indices from diffusion tensor imaging (DTI) to examine the influence of WM microstructural integrity and executive functioning on subsequent real-world risk taking. Altered WM integrity in fiber tracts at baseline was suspected to be related to risk taking behaviors measured at 18-month follow-up, and neurocognitive functioning was proposed to mediate this relationship. Adolescent substance users (e.g., predominately marijuana and alcohol misuse; n=47) and controls (n=49) received DTI and comprehensive neuropsychological testing at baseline (ages 16-19), and risk taking measures at both baseline and an 18-month follow-up (i.e., ages 17-20). Brain regions of interest were: fronto-occipital fasciculus, superior longitudinal fasciculus, fornix, superior corona radiata, and genu of the corpus callosum. Regression analyses evaluating direct and indirect relationships were carried out in Mplus. In the user group (n=47), decreased WM integrity at baseline in the fornix and superior corona radiata predicted 12% of the variability in follow-up substance use, and fornix integrity predicted 7% of the variability in follow-up delinquent behaviors, above and beyond personality, emotional functioning, family history of an alcohol use disorder, and baseline risk taking behaviors, which were included as covariates (ps < .05). WM integrity was not significantly linked to executive functioning in users or controls above and beyond covariates, and executive functioning did not mediate the relationship between WM integrity and risk taking. Overall, findings suggest that poorer integrity or maturation in distinct WM pathways is linked to a greater propensity for increased risk taking behaviors into late adolescence, among those youth with heavy levels of substance use by mid-adolescence. Most notable were relationships between limbic system fibers and future substance use frequency. It is possible that an imbalance between the maturation levels in cognitive control and reward systems may disadvantage the resistance to engage in risk taking behaviors during adolescence