Hi Noah,

To be clear, do you want a single volume to represent
the average correlation of each voxel with all grey
matter voxels?

If so, afni_proc.py is almost doing this right now,
except that it is the average with all "brain" voxels.
You would just have to run the same commands starting
with a GM mask. But you would need to warp that GM
mask to match anything that happens to the anatomy.

Note that afni_proc.py does that to some datasets
right now (e.g. see -regress_ROI_PC), but I have not
yet added a direct -anat_followers option, say. In
any case, you would still have to modify the proc
script.

- rick