[Freesurfer] Comparison of surface and euclidean distances of fsaverage

Dear FreeSurfer experts,

I computed distances on the cortex surface of fsaverage using mris_pmake. The exact syntax of the distance computation was:

mris_pmake --subject fsaverage --hemi lh/rh --surface0 smoothwm --curv0 sulc --curv1 sulc --mpmOverlay euclidean --mpmProg pathFind --mpmArgs startVertex:XXXXX,endVertex:XXXXX .

Subsequently, the euclidean distance was computed using the MNI coordinates displayed by tksurfer if the startVertex and the stopVertex are selected:

distance_euc = sqrt[(startMNI_x-stopMNI_x)^2+(startMNI_y-stopMNI_y)^2+(startMNI_z-stopMNI_z)^2] .

The in-surface distance must be larger or equal to the euclidean distance. However, in ~50% of the cases investigated the opposite is computed. Typical distances calculated are in the range of 7 mm. Do you have an explanation for this outcome and a possible solution? How can it be corrected? I am using FreeSurfer 5.3 and did not encounter any other problems in data evaluation. Any help us highly appreciated!

Sincerely yours

pfannmoe