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Hi again, Thanks a lot for the answer. First of all, since it wasn't correctly shown in my previous post, let me put the link for Jack Grinband's post I was referring to: https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=FSL;9d815b48.1006 Jack: thanks again for your comments. The part of the temporal derivative is now clear to me (I do use the optimal HRF function for each subject, and not the average). About the drug effect, although we're not sure, one possible side effect could be a change in hemodynamics. Anyway, I think the estimated HRF curves were very similar for both (placebo and drug) conditions, so that would confirm that there are not big changes and we can use the same one for the two of them. About the first issue (the actual way to proceed in the first level analysis)... What I do is: -Save the new estimated HRF in a txt file (it's actually a single column of values). -Define my EVs: I use the "3 columns format", since I need to specify the onset time, duration and a modulator (a different value for each trial). -For each EV, I set the "Optimal/custom HRF" (the same one where you select the FLOBS txt files by default) and, instead of the default txt file, I select the one mentioned in the first point (the new HRF estimated with your protocol). I hope that makes sense. I understand the way you do it in Matlab but, considering my limited understanding, I think it should be the same as the three steps I mentioned above. Could any FSL expert please confirm this? Again, thanks a lot! Javier