Dear SPMers,
I have a rather basic question. I am confused by the results of one and
two sample ttests in SPM. I have done a one and two sample voxel-wise
ttest on the result of my resting state fMRI study. As I understand SPM
does a one-tail ttest, so in case of one-sample I used the contrast of 1
to look at the voxels whose mean is greater than zero, I though in this
case I am gonna get an image with all positive T-values (ie. activated
regions) but I get both positive and negative values! The same thing
with the two sample test, I have used a contrast of 1 -1 (ie. controls >
patients), but again I see both positive and negative values. This is
what confuses me! what do the negative T-values mean in case of a
one-tail ttest?
Thanks,
-Mojdeh
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