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.