Dear SPM experts,
I have made some reading on the issue of non-independent analyses and
"double dipping", and now I'm wondering about non independence -and thus
validity- of the following analyses:
1) Let's say I have an fMRI experiment with 3 conditions A, B and C. The
contrast A>B gives a significant cluster. Now I want to make sure that
the contrast A>C is also significant in this region, and to this aim I
extract the % signal change from the above cluster (separately for each
particpant and each condition) and compare the A and C conditions using
a paired t-test. Is such an ROI analysis valid or considered a case of
non independence ? (I could also use a whole-brain mask, but considering
I'm focusing on one particular cluster, I'm wondering whether the less
conservative ROI approach might not be sufficient)
2) Let's say I'm comparing condition A versus a control condition C and
find a significant cluster. Let's now imagine that condition A can
actually be split into 2 categories A1 and A2, and that I want to
determine whether activity in the previous cluster is modulated by these
categories. Similarly, is it valid to compare A1 versus A2 in this ROI,
or is it a case of non independence ?
Any insight will be greatly appreciated.
Best,
Elise
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