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Dear Stephen, > I have a group of subjects who have been scanned on two occasions, once > in the presence of drug and once on placebo (double-blind). On each > occasion they are performing a task. I want to explore the interaction > of drug with task in SPM99b but want to set up the design so that the > analysis reflects that these are the same individuals acting as their > own controls. In the multi-group conditions and covariates design, I > don't seem to be able to set this up. Is there another way of setting > up the design? There is no need to do this. By setting up a design "so that the analysis reflects that these are the same individuals acting as their own controls" simply means modelling subject-specific effects as a nuisance variable. However the design you are already using does this by modelling session and subject effects (not only are you modelling subject-specfic effects but also session-specfic effects). If you wanted a more parsimonious model, that discounted session effects (i.e. a mean effect of being brought back for the second activation study) then simply use multi-subject model with four conditions (the four cells of your psychopharmacological factorial design). I hope this helps - Karl %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%