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Hi David, Sorry for the delayed response. This makes sense. We actually used this approach in Humbert and McLaren to show the effect of habitation on swallow number. This should automatically be done if you are using the gPPI toolbox. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4002228/ As for multiple orthogonalized PM, including one or both would probably depend on your research question. I like to use the same comparisons in PPI as in my task models and results (e.g. include everything). Best, Donald Best Regards, Donald McLaren, PhD On Wed, May 31, 2017 at 12:41 PM, David <[log in to unmask]> wrote: > About my previous post, > I believe that performing PPI on orthogonalized modulators should make > sense. > The rationale is to test brain voxels that correlate with activity in one > VOI, as a function of its interaction with aspects of a parametric > modulator that cannot be accounted for by other regressors. > However, I would like to have the opinion of some expert on both PPI and > pmod (e.g., Donald McLaren). > > Best, > David >