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... as clarification/elaboration, Glover 1999 found that the predicted blood flow response was closer to the actual blood flow response at some event durations as compared to others, with the predicted diverging from the actual duration quite a bit with longer durations. Hi Rik, Dorian, I've been looking into this a bit lately because i'd like to set up a design where events won't have the same duration, and is there a bit of a problem because the blood flow model created by SPM will be better for some durations than others? I.e., particularly when the durations are very different, you might get distorted results? E.g. Glover 1999 found marked discrepancy between blood flow response as predicted from a brief event, and actual blood flow (though maybe this no longer applies), and Zwart et al 2009 note that in this setting <If the GLM design matrix used describes the response to one stimulus type less accurately than the response to another, the residual after fit will be larger. Since the standard deviation of this residual is used to compute significance of the fitted amplitude, the computed t-score will be reduced. This not only reduces detection power, but will often be erroneously interpreted as decreased activation.> Zwart et al., 2009. p1656