Gaussian distribution for statistics. Voxel-based inferential statistics were performed on the smoothed modulated gray matter images using a two-sample t-test embedded in the Random Effect framework of SPM8. We included age, total brain volume (TBV, the sum of gray matter and white matter), and alcohol consumption as regressors of no interest to control for the effects of these variables. Statistical thresholds were applied at p<0.05 after family-wise error (FWE) correction for multiple comparisons over the whole brain and k>60 for cluster extent (greater than the minimum voxels expected per cluster). Voxel-by-voxel correlation analysis was performed between the smoothed modulated gray matter images and the variable representative of the level of drug use (monthly frequency of joints used in the 3 months before inclusion in the study) after testing the presence of outliers.
