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When using the parametric modulation option for a 1st level analysis of an event-related design, our design matrix does not appear as we expected. We are using an HRF + temporal derivative to model brain activation in response to to 4 conditions with a parametric modulator (e.g., reaction time). For each onset time within a condition, there is an associated parametric modulator value. The resulting design matrix consisted of 4 columns per condition. Why are there 4 columns? I guess we might have expected 3 columns per condition. Does the 4th column correspond to the parametric modulation of the temporal derivative? Thanks.