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Dear Karl I thank you very much for your help. If I use these covariate will the contrast [1 -1] for cases in which the counts are higher during the traumatic event repetition than durin the baseline, but the counts are decreasing from one repetition to the next? If I want to use these covariates to compare between the group should I use the same covariates but within the Multi group*covariate model? Thank You Yoram Louzoun On Mon, 13 Mar 2000, Karl Friston wrote: > Dear Yoram > > > I am conducting a research on the response of PTSD and non PTSD patient > > to the repetition of their traumatic event.We have 2 group one of > > PTSD patients and one of non PTSD patients.For each patient we have 3 > > baselines and 4 repetition of the traumatic story. > > > > We want to check the difference in the reaction to the traumatic script > > between the PTSD and control group. > > > > We use three type of covariates > > > > constant response - 0 0 0 1 1 1 1 > > rise during the repetition - 0 0 0 1 2 3 4 > > decrease during the repetition - 0 0 0 3 2 1 0 > > > > These covariates are not orthogonal. If I want to check in what region > > of the brain the response correspond to each of the covariate how > > should I define my contrasts? > > The repetition-dependent increases and decreases are modelling the same > effect and you only need to specify one regressor: > > main effect of traumatic script - -1 -1 -1 1 1 1 1 > script x [linear] time interaction - 0 0 0 -3 -1 1 3 > > Note that these regressors are orthogonal (and orthogonal to the > constant term) and can be tested with contrasts [1 0], [-1 0], [0 1] > and [0 -1].The last two give you increases and decreases > respectively. > > I hope this helps - Karl > %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%