Psychophysiological interaction (PPI) determines regions whose time-series of activation exhibit significant covariance with the seed region as a function of task manipulation, i.e. 2-back versus 0-back. Regressing out the contribution of the seed region time-series and the experimental context, the interaction is the contribution-dependent change in regional responses to the experimental factor (Friston et al., 1997), here working memory load. The clusters identified in the regression analysis (see Results), right pallidum and subthalamic nucleus (STN), were used for PPI seed regions of interest (ROIs). For these ROIs, the time-series data from the primary model was extracted for 5mm-diameter spheres centered at [MNI coordinates: 20, −2, 6] for right pallidum and [10, −14, −8] for STN. Anatomical validation of STN is based on work by Aron and Poldrack (2006). Each ROI time-series was deconvolved with the canonical HRF to create neuronal time-series (Gitelman et al., 2003). The PPI interaction term was the product of the neuronal time-series and a contrast vector coding for main effect of task (1 for 2-back; −1 for 0-back). This term was convolved with the HRF; PPI
