Dear FS experts，
I have two group and two times to do longitudinal cross analysis.
I looked your answer in  https://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg45870.html
and https://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg52032.html
Sorry, I can’t understand what you said” you can either subtract the two time points and then do a regular group analysis”
" subtract tp1 from tp2 manually and pass the result to mri_glmfit."
How to subtract the two time manually and how to do then ? Would you please tell me more details?
And in my analysis, if I use the paired analysis, how to design paired.fsgd and paired-diff.fsgd? 
In http://surfer.nmr.mgh.harvard.edu/fswiki/PairedAnalysis  I am sorry to only see how to do in one group.
 
Looking forward to reply, and thanks very much.
Kind regards,
Livia