Psychophysiological Interaction

Psycho-physiological interaction (PPI) is a brain connectivity analysis method for functional brain imaging data. It estimates context-specific changes in effective connectivity (coupling) between a region of interest (seed) and other brain regions. 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 can also be applied to every pair of regions in the brain, so that the whole-brain task modulated connectivity, i.e. task connectome, can be mapped. Psycho-physiological interaction (PPI) is a brain connectivity analysis method for functional brain imaging data. It estimates context-specific changes in effective connectivity (coupling) between a region of interest (seed) and other brain regions. 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 can also be applied to every pair of regions in the brain, so that the whole-brain task modulated connectivity, i.e. task connectome, can be mapped. The PPI method is mainly applied to blood-oxygen-level dependent (BOLD) signals measured by functional MRI (fMRI). The hemodynamic response is slow compared with fast neuronal activities. Therefore, when calculating the PPI term, the BOLD time series of the seed should be deconvolved with hemodynamic response function, and then multiplied with the psychological function (task design). Before the multiplication, both the psychological and physioilogical variables should be centered.