Dear FSL experts, I am working on a data-set which show some very interesting patterns but also something I fail to grasp:
I have 4 conditions, stimulus duration is identical for all 4 conditions and stimulus frequency is almost identical. At single subject level A, B, C en D are estimated and A is contrasted with B and C is contrasted with D. All stimuli are presented in the same run. At a grand-average level
A>B differes is a substantial (and expected number of) areas (B>A shows little effect).
C>D shows no differences (also not D>C).
So far everything is as expected. Now the strange thing (for me)
C>D has similar copes values (about 20%-40% higher) in terms of range but varcopes values that are about 10 times higher. An explanation would be that the variance is larger for C>D than A>B but as far as I can see this should not be possible since these are estimated in the same runs.
Finally a FIR analysis in some regions shows a smaller BOLD response differemce for the C>D contrast compared to the A>B contrast.
Anyone any idea?
Regards, Steven
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