Gray Matter and White Matter Volume
Estimation of the gray matter and white matter volumes should be taken from the surfaces, and not from the aseg. The program mris_volume will compute the volume inside a surface. It should be run from within the <subjid>/surf directory with the following command:
where ?h.surface is replaced with the specific surface you are looking for the volume of.
To get the gray matter volume you should use the following formula:
gm volume = pial volume - white volume
To get the white matter volume the program mris_wm_volume will calculate the wm volume, using the command:
mris_wm_volume <subject> <hemi>