SPMers,
I have a couple of basic design questions: First, I am doing a 2-group
PET analysis, and my groups differ on one variable that might be
contributing to the difference.
Having individual subject *con.imgs already from a MultiSub PET model
without a covariate, can I test for group differences and include the
covariate in the 2nd level/Basic Models Section? For example, I noted that
the AnCOVA model appears to compare two groups and also requests a nuisance
variable.
If I am interpreting this 2nd model correctly, am I also correct in the
following? In this AnCOVA model, and given 2 groups and one nuisance
(covariate) variable, the contrast 1 -1 0 0 tells me where group 1 is higher
than group 2 in activation (adjusted for the covariate). The contrast 0 0 0
1 tells me where the effect of the nuisance (covariate) is found in the
activations, collapsing across groups.
Is it prefereable to instead include the covariate in the PET model that
generates the *con.imgs (i.e., MultiSub: Conditions x Subject Interaction &
Covariates). Including the covariate (nuisance) variable at the 2nd level
seems to permit me to see where in the brain the covariate is having its
effect, whereas I don't see how to do that in the first level analysis
(i.e., I don't see an estimable contrast for the covariate).
________________________________
David A. Kareken, Ph.D., ABPP
Board Certified Clinical Neuropsychologist
Assistant Professor & Director
Neuropsychology Section, Department of Neurology
RI 1773
Indiana University School of Medicine
Indianapolis, IN 46202
(317) 274-7327 (voice)
(317) 274-1337 (fax)
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