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Hello FSL list, I have a problem about orthogonalisation. I need to do correlation analysis for two groups (controls and patients). I created 4 EVs, EV1 for group 1 mean, EV2 for group 2 mean, EV3 for group 1's HPA response, EV4 for group 2's HPA response. Based on the example of FSL webiste, EV3 was orthogonalised wrt the EV1 (group 1 mean EV), and EV4 was orthogonalised wrt the EV2 (group 2 mean EV), and then in Contrasts 0 0 -1 0 means the negative correlation of group 1; 0 0 0 -1 means the negative correlation of group 2. Is this correct? I am a little confused about one thing. If wrt the group mean means exclude the effect of group mean for each group, then what will happen if I only make two EVs, EV3 and EV4 for HPA response, do not make EVs for group mean, do not orthogonalise them wrt the group means, will that influence my correlation results significantly? For I think demean the group mean maybe exclude all the areas activated in group mean, if some areas were activated in group mean, and also showed some correlation to HPA response, I will miss those areas. Is my concern reasonable? Thanks, Hongyu