[Freesurfer] converting fieldsign files into volume space

Hello Freesurfers,

I recently used sfa-sess and paint-sess to analyze some retinotopy data, so far every works great. I wonder if there is a way to convert fieldsign overlays from surface space into volume space? I tried mri_surf2vol but it doesn't seem to recognize that file format: hwang@kaih:~/Subjects/FSFAST/VM_1/bold/VM0/fieldsign> mri_surf2vol --surfval ./fieldsign --hemi lh --volreg ~/Subjects/FSFAST/VM_1/bold/register.dat gdiagno = -1 ERROR: cannot recognize the type of ./fieldsign

any suggestions how to make this work?

Also could someone explain to me whats the difference between fieldsign-lh fieldsign-lh.mgz fieldsignmask-lh fieldsignmask-lh.mgz

what is the difference of having a mgz or mask extension?

Thank you!

Kai