[Freesurfer] Regressing out gender

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Hello,

I am currently attempting a paired t-test of subjects between two time points, regressing out the effects of age and gender. I am currently running the paired analysis tutorial, and have done the step of computing the differences between pairs using mris_preproc --paired-diff. However, I am having trouble with the FSGD and contrast files.

Currently, this is my FSGD file:

GroupDescriptorFile 1 Class M Class F Variables Age Input subject1pair M 80 Input subject2pair F 82 ...

I am not sure how to make the contrast file for this, I would like to answer the question of " does the mean paired difference differ from 0, regressing out the effect of age and gender?"

Any assistance would be greatly appreciated. William