I would like to down-sample the pial surface of individual subjects to have the same number of vertices as fsaverage6. I have done so using:

mri_surf2surf --hemi lh --srcsubject 'subjectID' --sval-xyz pial --trgsubject fsaverage6 --trgicoorder 6 --trgsurfval 'outputfilename'

This worked well and the pial output surface has 40962 vertices. I now have an overlay vector, which was computed on the down-sampled pial surface, which I would like to map back to fsaverage6 in order to perform a group comparison.

Please could you advise on how to do this as all scripts I am aware of (e.g. mris_preproc) use sphere.reg, which has ~150000 vertices.

Many thanks,

Christine