Hi freesurfer experts, It seems to me the pial surface should be the surface of gray matter, but when I overlay functional activations on it, it seems to be surface of white matter. The reason I think so is that when I load the same activation in both tksurfer and tkmedit, if I click on an activated area (or any point) on the pial surface in tksurfer, save it, and load it in tkmedit, the point will land on the yellow/green lines while the tksurfer and tkmedit show the same talairach coordinate for the point. If I click on a point that is on the red line in tkmedit, save it, and load it in tksurfer, they (tkmedit and tksurfer) show different talairach coordinates for the points. Question: what is the truth and what should I expect? Thanks, George