thanks for the answers,
I have a very related questions, instead of opening a new thread I hope someone will read it here:

How critical is the choice of duration for TENT(zero)/SPLIN(zero) etc. models?
What should keep me from setting the duration as long as the inter-stimulus interval? Because I would like to see how the hemodynamic response returns (and stays) to baseline, because if you set a very short duration (b..c) for SPLINzero of course you force it to return to zero but it would be biased/fake.
From my understanding beta values of zero are nothing bad if the real data is zero at these points, right? Or does this somehow mess up the F statistics because you're also modeling part of the data that is supposed to be random noise??