2.6 Psychophysiological interactions (PPI) in the general linear model (GLM)
Psychophysiological interaction analysis reveals changes in the connectivity between brain regions as a function of psychological context. Specifically, PPI captures context-dependent connectivity between a source region with any possible target region(s). This means that PPI reveals which regions have similar activity patterns with the source region as a function of a specific contrast and thus shows task-dependent interactions between functional brain systems.
Source regions for PPI analysis were selected from our previous results (Kringelbach, 2005). Two spherical 5-mm regions of interest (ROIs) were drawn at left orbitofrontal cortex (left-OFC) and right orbitofrontal cortex (right-OFC) locations in brushing minus resting contrast to further delineate connectivity changes. The time-series for each participant was computed by using the first eigenvariate from all voxel time series in the defined ROIs, and de-convolved using the PPI-deconvolution parameter defaults in SPM12 (Gitelman et al., 2003). The PPI term was then calculated as the element-by-element product of the ROIs in “neuronal time-series” and a vector coding for the selected contrast (1 for brushing and -1 for resting tasks). This product was then re-convolved by the canonical hemodynamic response function (hrf).
First-level PPIs were run to generate SPM contrast images similar to the first level GLM model, and these contrast images were analyzed and thresholded in the second-level model. An a priori primary threshold for voxel-level statistical significance was set to p < 0.01 and results were FDR corrected at the cluster level (pFDR < 0.05), and a secondary threshold was set at p < 0.05, FDR corrected at the cluster level.