Dear SPM
users,
I would very much appreciate your help on a question regarding the use
of basic models on a VBM study: I am analyzing two sets of images aquired
in distinct in two samples (longitudinal or within subjects design). I would
like to use a covariate when comparing pre vs post gray matter volumes of these
subjects. I see that in the t.paired test or the ANOVA (within subjects) desing
there is not a possibility of including a covariate. I could then use the ANCOVA
model but the problem is that the vector of my covariate has the same lenght as
the number of images of the pre OR post conditions. This is to say, given that I
have 8 subjects for the pre and the same 8 subjects for the post analyses, my
covariate is also of 8 values, one single value for each pair of images
pre-post. Of course the ANCOVA model asks me for a 16-vector lenght of
covariates (because I have included 8 pre and 8 post images) and I have had the
same problem with other (fMRI and PET) models I have tried. I have also
analyzed the data with an ANCOVA but repeating twice the vector of the covariate
so that, for example the first and the ninth images corresponding to the
first subject would have the same value in the covariate and so on. However I am
not sure this approximation is appropiate.
I would be grateful if someone could help resolving this issue.
Blanca Ramirez
