To examine the interactions between NAcc and other regions of the brain and how these interactions relate to reward processing, we used resting-state functional connectivity MRI (rs-fcMRI). Rs-fcMRI assesses spontaneous intrinsic correlated blood oxygen level dependent (BOLD) activity while subjects are not performing a specific task (Biswal et al., 1995; Fair et al., 2007; Fox and Raichle, 2007). It is based on the discovery that, at rest, functionally connected brain regions display correlated spontaneous low-frequency (<~0.1 Hz) BOLD signal fluctuations (Biswal et al., 1995). Using rs-fcMRI it is possible to identify interactions between regions by cross correlating the time series of a region of interest (or seed region) with all other voxels in the brain. This technique allows the assessment of the intrinsic functional organization of the brain, by determining which voxels are `functionally connected' to the seed region, without many issues inherent to task-related fMRI, such as variability in subject's performance/effort (Fair et al., 2007). Rs-fcMRI has been used to study brain functional connectivity in ADHD (Castellanos et al., 2008; Fair et al., 2010), but studies focusing on cortical-subcortical connections remain infrequent.
