[Freesurfer] Question about group analysis

Hello,

I've got a very basic question related to group analysis. If I want run a correlation of voxels in the cortex with a variable *controlling *for another continous variable, how do I do this. I know how to run them both in a GLM simultaneously by coding the matrix 0 1 1, however, how do I look at the first variable after controlling for the other? Here is an example of my current fsgd input (I want to control for income):

GroupDescriptorFile 1 Title MyTitle Class Class1 plus blue CLASS Class2 circle green SomeTag Variables impulsivity income Input subjid1 Main 10 100 Input subjid2 Main 20 200 etc.