Dear SPM'ers,
I have a few questions concerning a multi-subjects fMRI experiment.
For each subject we acquired 4 separate scan-sessions according to the
following experimental design:
session 1,2: Ce Cr Ce Cr Ce Cr Ce Cr Ae Ar Ae Ar Ae Ar Ae Ar
session 3,4: Ce Cr Ce Cr Ce Cr Ce Cr Be Br Be Br Be Br Be Br
where:
e = encoding
r = retrieval
C = baseline
A = condition1
B = condition2
Up to now, I have been able to analyze contrasts between conditions
belonging to sessions of the same type (e.g. Ae-Ce; Ar-Cr for sessions 1
and 2) both using a fixed-effect and a 2nd-level random effect model.
In addition, however, I would like to compare conditions belonging to
sessions of a different type - though I know it would have been much
better to include all conditions in one session. E.g. compare Ae-Be.
I have tried to do this on a random-effect basis, by computing the simple
main effects for each subject (Ae-Ce and Be-Ce) and the entering the
corresponding con*.images into a paired t-test (Ae-Ce vs Be-Ce).
My questions here are:what am I actually looking by this approach? At
interaction effects [of the type (Ae-Ce) - (Be-Ce)]? Do I have to mask
this contrast by another contrast (e.g. main effect) and how can I specify
the masking contrast?
Is there any way to look at direct comparisons (Ae-Be)?
In fact, I have tried with fixed-effect modelling, including all sessions
(1,2,3,4) and specifying only 4 'conditions' (Ce,Cr,Ae/Be,Ar/Br) and then
looking for direct comparisons by entering contrasts 'between sessions'
(e.g. 0 0 1 0 0 0 1 0 0 0 -1 0 0 0 -1 0, corresponding to: Ce Cr Ae Ar Ce
Cr Ae Ar Ce Cr Be Br Ce Cr Be Br), but I'm not at all sure whether
I'm actually allowed to do so.
Thanks a lot for your kind help!
Marco Tettamanti
Istituto Scientifico San Raffaele
Reparto Medicina Nucleare
Via Olgettina 60
I-20132 Milano
Italy
Tel. 0039 - 02 - 26 42 34 60
Fax: 0039 - 02 - 21 71 75 58
email: [log in to unmask]
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