Dear DCM experts,
I have a question about extracting VOIs to be used in a DCM/PEB analysis (task fMRI data).
I have 6 conditions and several contrasts in my GLM. I am following the standard procedure for VOI extraction as explained in https://en.wikibooks.org/wiki/SPM/Timeseries_extraction , but I would like to use specific GLM contrasts depending on the ROI, e.g., using the GLM contrast 1 for finding the individual coordinates of ROI 1, 2, 3 and 4, using contrast 2 for ROI 5, 6, 7 and contrast 3 for ROI 8.
All VOIs would be adjusted for all 6 conditions anyway.
My first question is: is it ok to extract different VOIs from different contrasts (granted that they are adjusted with the same F-contrast), or would you advise against it?
Secondly: for the DCM analysis, I'd be interested in only including 4 of the 6 conditions in the design matrix (from my understanding the 2 remaining conditions would form the implicit baseline), and include all 4 together as driving input (a 'task' condition with cond1+cond2+cond3+cond4) and each separate condition as modulatory input.
The subsequent contrasts in the PEB will be different from the ones I used in the GLM to extract the VOIs. Is this ok too?
I would just like to make sure that it is correct to have different number of conditions/contrasts in the GLM for VOI extraction vs in the DCM design/PEB analysis.
Otherwise, I will appreciate alternative suggestions that you think might be better (e.g. including all 6 conditions in the DCM design, or extracting the VOIs from the same contrasts as the DCM/PEB using 4 conditions only)
Thank you in advance!
Best wishes,
Sara
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