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Hi Nicholas. I hope I can explain this in a clear and concise way. We've run into this problem before as well. The values of the numbers you use to specify groups really do not matter. What DOES matter, however, is the order of the groups. In your first example you used -1 for the HP group and +1 for the LP group. That means your first group was HP and your second group was LP (-1 comes before +1 numerically). In this case your -1 1 contrast for LP>HP and your 1 -1 for HP > LP were correct. However when you used the opposite specifications for LP (-1) and HP (1) then your group ordering was also switched. In that case your constrasts also reversed. -1 1 became LP < HP and 1 -1 became HP < LP. So, you need to be careful about assigning your groups to avoid confusion. I tend to choose my images in the order I want my groups to avoid any mixups. I also tend to use consecutive integers (1,2,3,etc) for specifying my groups as well. But, of course, everyone has there own "best" way. Hope this helps! Sincerly, John D. West Brain Imaging Lab Dartmouth Medical School