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Hello Maybe if you look at the controls at a slightly reduced threshold you would see bilateral activations. That is, the right-sided activations in this group might be pretty strong but might not quite survive the corrected thresholding. In effect, they may not be that much less than the right-sided activations in the patients, hence the lack of difference at the stringent threshold when you do the groupwise comparison. Best wishes Paul Fletcher On Apr 19 2007, Sophie Lafaille wrote: >using SPM2 I'm looking at some random effects analyses. > >One sample T-test >when looking at task X within a group of 15 controls, we get very >significant left hemisphere activation with T-values of 10-14 (0.05 FWE) >when looking at task X within a group of 15 patients, we get very >significant bilateral activations with T-values of 9-15 (0.05 FWE) > > when I enter all these con files into a 2 sample t-test to look at > controls vs. patients for task X I get nothing at a corrected level and > only start seeing the pattern of activation that makes sense at a 0.05 > uncorrected see attached, I've shown one sample data on top, and then the > subtraction on bottom. > > am I doing something wrong or is the subtraction of either controls from > patients or vice-versa removing a lot of the right hemisphere activation > that I'm looking for and can only been seen at a much much lower > uncorrected p-value. > > >thank you, sophie > >