This may be a fairly simple issue, but since I'm new to fMRI and SPM, I cannot get my head around it.
I simply want to compare experimental condition A to the zero level (which I understand is the mean of the implicit rest from all subjects). I have already calculated the contrast A against rest in the 1st level analysis, and I have seen clear activation clusters that are topographically similar across subjects and are significant after FWE-correction and with a p-value threshold of <0.05. However, when I enter the con* images (1per subject) into a one-sample t-test in 2nd level analysis, using the same p-value threshold does not give me significant activation clusters. The activation clusters are present only if I do not correct for multiple comparisons. If the data looks similar across subjects, wouldn't averaging just increase the effect observed in the data and the results should be even more prominent in the 2nd level analysis? Or is intersubject variability in absolute BOLD-signal the issue? Should I somehow normalize the amplitude of BOLD signal before 2nd level analysis? Or should I be looking at individually defined ROIs instead of whole brain analysis at the second level?
Thank you so much for your help!
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