Hello all.
I had some questions on using hrf+derivatives at the 1st level and carrying this to the second level. I have reviewed previous emails, but I am still unsure as how to proceed. For these questions, I will use the example of two task conditions A and B modeled using the HRF and its derivatives.
1) My understanding is if you apply the T-test for A>B as 1 0 0 -1 0 0 you get the same con*.nii file to carry to the 2nd level analysis as you would if you did not implement the derivatives and just used the T-test 1 -1. Is this correct?
2) To truly incorporate the derivatives into the second level anals you need to create F-tests which test A vs. B e.g. -1 -1 -1 1 1 1 or 1 1 1 -1 -1 -1 which are equivalent in an F-test. Then carry those contrast images to the second level. I understand this, but is 1 1 1 -1 -1 -1 the same as a 1 0 0 -1 0 0; 0 1 0 0 -1 0; 0 0 1 0 0 -1 matrix?
3) When you use the F contrast images at the second-level, can you then perform T-tests on those images? How would I interpret those results? For example if I had two groups, what would Group 1 (A vs. B) > Group 2 (A vs. B) tell me and how is this different from Group 1 (A>B) > Group 2 (A>B)?
Essentially, we have an event related task and we want to use the derivatives to try and see if we can improve our activation maps but are unsure of how to correctly go about it.
As an aside I was also wondering if there is a difference between averaging contrasts for 2 sessions vs. combining sessions at the first level. For example if I ran two 1st level anals for a subject on each of his/her task runs and then averaged the resulting contrasts is that different from just running a combined 1st level anal on both sessions and getting one contrast from that? If so, is one method better than the other?
Thank you very much for your help.
Sincerely,
John D. West
Imaging Analyst
IU Center for Neuroimaging
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