Thank you Anderson. That is clear.

The text I talked of was http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FIRST/UserGuide#Create_a_design_matrix:

"This is easiest done by using the "Higher-level" mode of the Glm gui. However, the design matrices are treated slightly differently by FIRST compared with FEAT. The main difference is that FIRST does not model separate group variances and so ignores the "Group" column in the Glm gui. Another difference is that only F-tests are needed, since we look for any change in vertex position. Therefore it is necessary to set up a least one F-contrast, which needs T-contrasts to be set up (since that is how F-contrasts are specified). "

I'm sure this is just me.