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Dear SPM experts, When conducting PPI with event-related designs, I've found that the PPI regressors are dramatically affected by how trials are modeled, i.e. either with a delta (duration = 0) or boxcar (duration = stimulus offset minus onset) function. For example, the attached plots show PPI regressors from identical seeds in identical designs, where the only parameter varying is the use of a delta vs. boxcar function to model the stimulation (trials varied in length from about 3-5 seconds). The correlation between these regressors is only about .50. This seems surprisingly low. The reason I bring this up because, when one is merely concerned with modeling the psychological response, the difference between modeling trials as delta vs. boxcar functions is often small (although certainly not insignificant; Grinband et al., 2008, NeuroImage). However, this difference seems to be amplified when doing PPI with event-related designs. Is this a reasonable conclusion? Thanks in advance for any comments, Bob ------------------------------------------------------------------------------- Bob Spunt Postdoctoral Fellow Social Cognitive Neuroscience Lab - www.scn.ucla.edu Department of Psychology University of California, Los Angeles