Psychophysiological interaction (PPI) analysis determines regions whose time series of activation exhibit significant covariance with the seed differently in two conditions, i.e. incentive versus neutral condition. Regressing out the contribution of the seed ROI time series and that of the experimental context, the interaction is the contribution-dependent change in regional responses to the experimental factor (Friston et al., 1997), here response to incentive anticipation. PPI analysis was seeded in the NAcc ROIs described above and implemented separately for reward and loss conditions for left and right NAcc, 4 analyses per subject. For each NAcc, time-series data from the primary model was extracted and deconvolved with the HRF to create a neuronal times-series (Gitelman et al., 2003). The interaction term was the product of the neuronal time-series and a contrast vector coding for main effect of task (1 for reward or loss; −1 for neutral), an A – B regressor of interest. This term was convolved with the HRF (Friston et al., 1997). Contrast vectors were also created for the A + B regressor (1 for reward or loss; 1 for
