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Hi Miklos I don't know if any one responded to your email of 29th April. Contrast 2 looks appropriate to me. The difference between contrast 2 and contrast 1 presumably reflects the fact that the condition effect on your electro-physiological parameter (p) was greater in some subjects than others. Contrast 2 allows the subjects with big condition effects on p to dominate the SPM whereas contrast 1 places even weights on each subject. How many of your subjects showed an effect of condition on p? The effect may just be coming from one or two subjects Best wishes Cathy At 08:52 PM 4/29/2001 +0200, you wrote: >Dear SPMers, > >I am working on an analysis of a PET activation study (8 subj, 2 cond, 3 >repl). >Multi-subject: conditions x subject interaction model was chosen >and two contrasts were used for generating SPM maps. >Contrast1: 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 >Contrast2: p1 -p1 p2 -p2 p3 -p3 p4 -p4 5 -p5 p6 -p6 p7 -p7 p8 -p8 > >Where pi the mean of an electro-physiological parameter of ith subject >(these parameters were measured during all PET scans) >These parameters were also used as indicators of the subjects in the >subject selection procedure. > >The result given by Contrast2 was better then Contrast1: >the t-values of cluster-maximums were higher and >the sizes of clusters were larger. > >My questions are: > >- Is the Conrast2 correct? (as factorial approach) >If it is: >- Can we say that the activation of these clusters have a parameter-dependent >augmentation? > >Many thanks in advance > > Miklos > >