For visualization purpose (i.e., displaying group maps), I applied a mask for medial wall regions using the files (e.g., rh.Medial_wall.label or lh.Medial.wall.label), which I found in fsaverage/label/ directory. 

However, I noticed a somewhat imperfect medial wall masking in both hemispheres, such that 

transversing lines are seen in unthresholded t maps for both hemispheres (see attached ). 

Those lines eventually disappear when I increased threshold, but I wonder why this happened. 

Any ideas? 

best,

yune