[Freesurfer] Vertex-wise thickness and mean thickness of clusters

Dear all,

I would like to ask you whether there should be an exact correspondence between vertex-wise thickness and mean thickness of clusters.

I'll explain you my problem. I did an analysis with 2 groups and 2 covariates. Then i run the mri_glmfit script and found clusters of greater thickness in controls compared to patients. When i extracted the mean thickness of these clusters and did a two sample t test i observed that the controls had greater thickness than patients, however not statistically significant, even after including in this analysis the 2 covariates.

Is this normal? How does freesurfer uses the covariates when performing an analysis?

Many thanks