[Freesurfer] Fwd: Problem about the fMRI Analysis?

Dear Sir, I get the correlation result between two condition of the functional data by using AFNI( I do not know how to do correlation by using freesurfer). And then I use the bbregister to aline the correlation result (produce by Afni) to the anatomical result produced by using freesurfer(recon-all). Next I using mri_vol2surf to aline the each one's functional result to the fsaverage offer by freesurfer. My question is : I have two group of subjects. I want to get the correlation different between them? What should I do next by using freesurfer? I want to know how to calculate the correlation between two group by using freesurfer based on the surface-based analysis? Thanks very much! Linda