Longitudinal Change in Cognition and White Matter Integrity in Young Adult Cannabis Users

Abstract

Cross-sectional research indicates that cannabis use is associated with cognitive and neuroanatomical damage, particularly when used regularly during development. The timing of use-related impacts on cognition and brain structure remains unclear. This dissertation includes two studies to characterize the longitudinal (1) neurocognitive profile and (2) white matter microstructure of young adult cannabis users who initiated use during adolescence. Cannabis users were assessed on a comprehensive neurocognitive battery and Diffusion Tensor Imaging (DTI) protocol at baseline and at a 2-year follow-up. In Study 1, cannabis users had stable deficits in verbal learning and memory as well as planning ability, and a stable relative strength in processing speed at baseline and follow-up. Deficits in spatial working memory and motivated decision-making observed at baseline recovered to control-level performance at follow-up. Heavier and earlier use of cannabis during adolescence was associated with decline in verbal learning and memory performance over time. In Study 2, change in white matter microstructure between time points was observed. Cannabis users exhibited reduced white matter microstructure organization in the central and parietal regions of the superior longitudinal fasciculus, left superior frontal gyrus, corticospinal tract, right anterior thalamic radiation, and in the posterior cingulum; cannabis users demonstrated increased white matter microstructure in the left anterior corpus callosum and left thalamic white matter. The findings suggest that continued heavy cannabis use during adolescence and young adulthood disrupts ongoing development of white matter microstructure. White matter microstructure changes were generally unrelated to cognitive performance, and future research is needed to clarify their functional significance. Potential mechanisms and implications of the findings are discussed.