Hi Lars,

This pattern is because the faces in a geodesic sphere don't have all identical sizes, even if the original icosahedron was perfectly regular, i.e., a Platonic polyhedron. We describe this phenomenon here, and give a simple formula that addresses it (see Appendix A: Geodesic spheres and areal inequalities).

If you run the statistical analysis on the faces or vertices, this is never a problem, because the conventional stats, as t, F, or R^2 are all pivotal, i.e., don't depend on the scale of the original measurements. Nonetheless, it's something that needs to be addressed if smoothing is to be applied, as well as for analyses in which equal-area for each measurement unit is a (possibly implicit) assumption.

Hope this helps!

All the best,

Anderson

On 31 May 2014 15:54, Lars M. Rimol <larilin@gmail.com> wrote:

Hi,

I have generated surface area maps for a sample of subjects (using [mris_preproc --fsgd *.* --target fsaverage --hemi *h --meas area --out *.mgh). When I created a mean image across all subjects (163842 x 1) and looked at it in tksurfer, I noticed a pattern on the surface that's a little troubling. Please see the attached image file (patterninmeanareaimg_
allsubjs_unsmoothed.jpg).

I sampled the curv files to ico (with mris_preproc) for all subjects and looked at them individually (I have attached a few figures). I did find four resampled curv.mgh's that looked unusual, but removing them did not change the pattern seen in the attached figure. I also looked at some of the individual area files resampled to ico (using make_average_subject) and the outliers there were identical with the ones identified using the resampled curv-files (perhaps unsurprisingly). All these images are unsmoothed.

So I'm at a loss here. Any suggestions on what to do next in terms of trouble shooting?

(PS! I have attached the resampled curv and area files for a normal subject in lh_104_curvsjekk_lateral.jpg and lh_104_areasjekk_lateral.jpg, and for an unusual subject in lh_81_UNUSUAL_curvsjekk_lateral.jpg and lh_81_areasjekk_lateral.jpg. The three other subjects that seemed unusual looked like subject 81, and all the other subjects looked like 104.)

Thank you!

--
yours,

Lars M. Rimol, PhD
St. Olavs Hospital
Trondheim,
Norway

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