Could you help me with the following questions about group analysis, which I cannot solve even after spending two days in searching and reading relevant webpages (including SPM emaillist archives and CBU Imaging Wiki etc.)
I did 1st level F-test with informed basis functions on each subject's multi-sessions (each session has the same design matrix columns). My interested variables are two parametric ones. So, in each sesssion, the contrast has only one 1 like 1 0 0...; for informed bf, the contrast is like [1 0 0...; 0 1 0...; 0 0 1...] (RIGHT?). Now I move to 2nd level analysis and encounter the following questions. My SPM edition is SPM8.
i) The first level analysis produced image files prefixing with beta, ess and spmF (but NO con as in the example in the manual.) So, which kind of image files should I select for the 2nd level analysis? (A related question: for a contrast 1 -1, the con*.image is just a beta*.image - another one? or it also takes variance for each beta into account?)
ii) If the beta should be used, then a further question:
I have several sessions, and so have several beta images for each regressor per subject. Since the relationship of beta images within one subject (between sessions) should be different from that of between subjects, it seems unreasonable to simply pool these sessions as subjects. So, what is the reasonable solution?
a) firstly average the corresponding images across sessions within each subject and then only input the average images for the group analysis? Is this reasonable if not best? (note: session effect is not what I care about.)
b) take sessions as an independent varibale? In this case, how should I define the contrast (for the 2nd level analysis) for informed basis functions? As I know, the original form should be eye (n); but now what should it be?
c) any other better solutions?
Looking forward to your help.
Many thanks and best wishes,