SPM5. Statistical parametric images were calculated individually for each subject and each task, using a general linear model (Friston et al., 1995a; Friston et al., 1995b). These images were subsequently entered into second level model, subjected to a voxel-wise contrast and t-test to assess statistical significance. Using the two-sample t-test, we made direct comparisons between the chronic, heavy marijuana smokers and the non-marijuana smoking controls. Contrast analyses were conducted for each region of interest and for each task condition, which consisted of the subtraction of one group map from the other; for example, anterior cingulate activity of marijuana smokers during the viewing of angry faces was subtracted from anterior cingulate activity of the healthy control subjects viewing angry faces to determine which areas showed increased activity in controls relative to smokers. The probability threshold was set at 0.005 uncorrected and a minimum cluster extent (k) of 20 contiguous voxels. Region of interest (ROI) masks were created using the Wake Forest University Pickatlas utility (Maldjian et al., 2003; 2004). These regions included the cingulate gyrus and the amygdala. The statistical threshold for the ROIs was set at 0.05, and k was set at 20 voxels. In addition, whole brain analyses were
