I'm a newbie for freesurfer and would like to ask a naive question. 

I've just applied smoothing onto a single surface mgh file using mri_surf2surf (with fwhm ==4) and examined the values in the output map. 

As it turns out, values become 0 in a number of vertices after smoothing while those in many other vertices look normal (i.e., more or less similar to the values in the vertex of unsmoothed map). 

I'm not sure why this happened as smoothing average values locally within the kernel (4mm). 

Any helpful answers would be greatly appreciated. 

Glen 

p.s) fyi, the original EPI is a slap data covering only the temporal & occipital cortex.