Dear Bruno

If you want to use F-stats (and that is typically the best choice for this sort of interaction analyses, unless you have an a priori hypothesis about the directionality of the expected interaction effect) simply switch the option in the results explorer window that reads 'one-sided (positive)' to 'two-sided', and that will be exactly equivalent to the standard F-test (which is blind to the directionality of the interaction effect) for this same interaction analysis. 

If in doubt, or if you prefer to have F-stat values directly, you could also simply enter in the 'between-subjects contrast' field [1 -1; 1 -1] (instead of just [1 -1]), and that will give you exactly the same results as the analysis above, but now all statistics will be reported using F-stats instead of T-stats. Another alternative would be, in the original analysis results explorer window, click on the 'display SPM' button and define there a new F-contrast with the [-1 1] values. Again, all of these options will produce exactly the same second-level results as your original analyses (using the 'two-sided' option), you will see exactly the same significant clusters, the same cluster-level statistics, etc. (they will only vary in the choice of voxel-level statistics being reported -either T(dof) or F(1,dof) stats-) 

Hope this helps
Alfonso

Originally posted by Bruno Baumann: