I have scanned 24 subjects with fMRI twice per person. I am trying to
examine a paired t-test while controlling for effects of two covariates. I
am able to generate a paired t-test in SPM5 without the covariates, and the
activation looks proper.
However, the results (glass brain, cluster analysis) do not change when I
add 1 or 2 covariates to the model. When I made the second model (paired
t-test that has the two covariates), I used all the same choices (same
pairs, no change in Grand mean scaling choices, etc) for the paired t-test,
and note too that the covariates are entered without error (vector entered
ok, variable named OK, no interactions, etc). In the second model (with 2
covariates), I entered a zero for both of these covariates in the contrast
manager.
Thus, it appears that in SPM5, a paired t-test with no covariates produces
identical results as a paired t-test with 2 covariates properly specified.
I would expect that the second model, with the two covariates that have a
zero in contrast manager, would have a different result than the first
model, with the difference reflecting removal of the signal accounted for by
these two covariates. What am I missing, or doing wrong ? Thanks--Steve
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