Dear Colline,

of course ... I think your approach is still valid ...

However, as Mr Will Penny wrote today in his answer to Peter Clerinx [Another contrast question], the first level concerns the effect of a condition ‘X’ for the subject analyzed. As you performed a fix-effect analysis (=1st level analysis), your results are only valid for the group studied. Maybe the reviewer asks you to do a random effect analysis … see  http://www.fil.ion.ucl.ac.uk/spm/doc/books/hbf2/pdfs/Ch12.pdf

Similarly, if you have a strong hypothesis, you can perform small volume correction (as proposed by Mr Matthew Brett) or use a tool dedicated to investigate a region of interest (MarBaR http://marsbar.sourceforge.net/ or WFU_pickatlas http://www.fmri.wfubmc.edu/download.htm)

Best,
   Cyril