[Freesurfer] contrast matrix question

Hello,

I apologize if this is a very simple question, but how would you set up a contrast matrix to look at the interaction of 2 variables on a measure? For example, my fsgd file contains:

class=1 variables=2 Input Subjid class var1 var2

When I tried using a contrast of 0_1_1 the significance map looks very similar to the glm run only containing var1 (contrast_0_1). I thought 0_1_1 was correct, but now I'm not so sure and would like to double check. What I am interested in is how the interaction of var1 and var2 is correlated with the brain measure per voxel. Is this possible with mri_glmfit?

I've checked the tutorials and scoured the internet as not to ask any silly questions, but I'm having trouble finding anything helpful.

Thanks in advance! Tori