[Freesurfer] visualization of statistical analysis

Valentina

I created a statistical parametric map and I visualized it on tksurfer. I didn't use a variable and I used a contrast vector (1 -1) because I was interested in knowing the difference between two classes. I'd like to know how I can interpreter the two colors, in other words do the two different colors means that there is a difference two classes? (i.e. the thickness of the cortex is greater in class one than class two: color red; the thickness of the cortex is lower in class one than class two: color blue). Another question is: why I need to create an average subject? Just to visualize the surface or it is used for the statistical analysis somhow?

Thanks Valentina