I have data has been recon-all analyzed. I would like to compare the symmetry of function/structure property between LH and RH, such as cortical thickness. I already know that the vertices in the left hemisphere are not correspond to vertices in the right hemisphere. I would like to know the exactly symmetric vertex for both hemisphere. Thus the question is, how can I register both hemisphere to one hemisphere of fsaverage_sym, then compare them? 

I followed the instruction:

xhemireg --s sub001

surfreg --s sub001--t fsaverage_sym --lh

surfreg --s sub001/xhemi --t fsaverage_sym --lh --xhemi

then here is the error:

MRISread(/data/sub001/xhemi/xhemi/surf/lh.sphere): could not open file 

No such file or directory

mris_register: could not read surface file /data/sub001/xhemi/xhemi/surf/lh.sphere 

No such file or directory