David: Interesting comments:
The average fiducial surface is particularly compressed in regions of high variability. [...]
How to reconcile the desire to plot data (or do anything) on a "representative brain surface" with the variability of individuals' surfaces is distractingly fascinating.
Good! The issue of whether the average fiducial surface intersects various blobs of activation is an important one. [...]
Right -- that all sounds good. What I'm not yet convinced about intuitively is that the average surface should have this relatively "in board" position. If that's the way it is, then so be it, of course! Further, my superficial understanding of MFM worries me that a weak activation caught by all surfaces might produce the same appearance as strong activation caught by only a small proportion of the MF surfaces.
But perhaps that's all dealt with in the math, and I know that there are also several mapping strategies in caret's fMRI-to-surface mapping procedure.
Anyhow, time to scrutinize your paper in more detail (not just the pictures :-).