I'm investigating lh-to-rh asymmetry and am following instructions on this page :http://surfer.nmr.mgh.harvard.edu/fswiki/Xhemi.

My question is, what does the "--xhemi" flag do in the "mris_preproc" command? It seems to me that it calculates (lh-rh) value at each vertex for every subject. (I tried to use the same subject for each pair of input for "--paired-diff", and the result is not a all-zero output.) 

It would also be nice if someone can tell me how to realize my final goal, which is to get the (lh-rh)/[(lh+rh)/2] value at each vertex for each subject in a .mgh file.

Thanks,

Cherry