Hi SPM experts,
I have 12 subjects to perform 3 types of cognitive tasks (A,B,C) in an fMRI run, and subjects were asked to rate difficulty levels after each trial. My aim is to compare activations between the 3 tasks. However, I noticed that the difficulty levels were statistically different among the 3 tasks. I want to remove the effects of difficulty so as to clarify activation differences. My 1st-level model has set 6 contrasts (A,B,C, A-B, B-C, A-C). My 2nd-level model is actually group-level analysis by performing 1-sample t-test of these contrasts. When comparing activations between tasks, how can I remove the confounding effects of differences in difficulty (each subject have 3 difficulty scores for tasks A, B and C)?
(1). My idea is to remove it at group-level analysis. For example, if I want to see group map for A-B, in a group-level analysis, I can add a new EV representing each subject's difference in difficulty level between tasks A and B (i.e., A-B). So if I want to get all 3 between-task maps, I have to perform 3 group-level analyses in total. Am I correct?
(2). Is it possible and how to remove effects of difficulty in 1st-level model in SPM? (if I put difficulty scores in "parametric modulation", I will identify activity modulated by, or associated with, task difficulty, right? But this is not what I want.)
Thanks in advance.
Mike
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