Psychophysiological interaction (PPI) is a brain connectivity analysis method for functional brain imaging data, mainly functional magnetic resonance imaging (fMRI).
[1][2] Thus, PPI analysis identifies brain regions whose activity depends on an interaction between psychological context (the task) and physiological state (the time course of brain activity) of the seed region.
The PPI method is mainly applied to blood-oxygen-level dependent (BOLD) signals measured by functional MRI (fMRI).
Therefore, Gitelman and colleagues proposed to first deconvolve the BOLD time series of the seed with hemodynamic response function, so that the resulting "neuronal level" signals could match with the timing of the task design (the psychological variable).
That is, the PPI term is defined using a pre-defined seed, and a voxel-wise analysis is performed to identify regions in the whole brain that showed task modulated connectivity with the seed region.