Another comment: I assume you're especially interested in the decision period. 6 seconds between stimulus and decision period might not be sufficient to effectively control for the stimulus activaiton (as the stimulus is always followed by the decision). In that case it might be useful to add some "half trials" in which you present the visual stimulus only, but no cue follows. If you want to go with a FIR model you could then plot the time course for "full trials" including stimulus, cue and response and subtract the time course for "half trials", which should eliminate activation due to the stimulus. A study by Brown et al., 2008, Neuroimage, "Isolation of saccade inhibition processes: rapid event-related fMRI of saccades and nogo trials." might give you an idea (it focuses on eye movements, but the principle should be the same for any task).
Best,
Helmut
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