Whole-brain image analysis was conducted using SPM8 (http://www.fil.ion.ucl.ac.uk/spm). For each scan, images for each participant were realigned to correct for head motion. Data sets were then selected for quality based on our standard small-motion correction (<2 mm) and for signal coverage in the striatal region of interest >80%. Realigned images were co-registered to structural images that had been segmented to select grey matter, then spatially normalized into standard stereotactic space (Montreal Neurological Institute template) using a 12-parameter affine model. Normalized images were smoothed with a 6 mm full-width at half-maximum Gaussian filter. Voxel-wise signal intensities were ratio normalized to the whole-brain global mean.
