Abstract
Psychophysiological interaction (PPI) and beta series correlations (BSC) are two commonly used methods for studying task modulated connectivity using functional MRI (fMRI) data. So far there are no comprehensive tutorials to explain these two methods, and the relationships between these two have not been established. In the current paper, we explain in detail what the two methods measure, and how these two methods are related. We demonstrate that the PPI approach always measures connectivity differences as coded in the psychological variable. We further establish that putting some conditions of no-interest as 0 does not mean to “zero out” those conditions, but introduces arbitrary effects regarding to these conditions. However, if modeled correctly, direct contrast PPI with conditions of no-interest modeled as 0 can generate the same results as the “generalized PPI” approach. In contrast to PPI, the BSC approach can measure absolute connectivity in a specific condition. When comparing different conditions, PPI and BSC methods could in principle generate similar results. We also report PPI and BSC analyses on empirical fMRI data of a stop signal task to illustrate our points.