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<This is an updated repost>
Dear Freesurfers,
I have a simple design with patients and controls and would like to find regions showing group differences in cortical volume and thickness. I also remove the effect of age, sex and ICV. Running the GLM group analysis by the book I get a few clusters showing significant differences in intercept.
1. I interpret this like patients have smaller “predicted” regional volume at age 0, or at mean age of the group (if demeaned age covariate) compared to controls. How can this finding be described in a more clinically meaningful way?
2. Related to the above, how do I visualize what is going on in the significant clusters? The extracted values from these clusters (ocn.dat) show no group differences in volume when displaying the group means in SPSS (using either age/sex/ICV adjusted or non-adjusted values). Maybe I am misunderstanding something, how can there be differences in intercept but no differences between group means? (slopes not significantly different). How do I best describe and visualize this group difference in a paper?
Going through the studies I found on Pubmed that used Freesurfer with vertex-wise GLM, so far, I only found e.g. the “lower thickness” interpretation in one group vs the other and not “lower intercept”. In my case at least I cannot say this since the extracted values between groups show no group difference.
Julian