Hello,
I have what seems a trivial question but could not find an answer in the archives, so here it is...
I have several runs per subject, each containing all of my conditions. I set up all the contrasts of interest at the first level, and then just do a cross-session average at the second level for each subject. In the contrasts I chose, I usually have a contrast and it inverse: A-B and B-A. As expected, at the first level tese give me inverse statistical maps. However, when I run the cross session average at the second level, this is not true any more, some supposedly inverse contrasts even giving sometimes very similar maps...
Is there a reason why this should happen? Would it be better to approch this by averaging only the A-B contrasts at the second level, and defining the B-A contrast as its inverse at the second level (rather than average the first level B-A)?
Thanks for any help or advice!
Stephane
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