Dealing with difference
Forgive this stub page, but...
Hemodynamic responses can differ from brain area to brain area. Of course neuronal response shapes can also differ. In either case, convolving your event onset with the canonical SPM HRF will give you a model that may be correct for some areas but not others. One classic citation on this is:
@article{aguirre1997eab,
title={{Empirical Analyses of BOLD fMRI Statistics}},
author={Aguirre, GK and Zarahn, E. and D'Esposito, M.},
journal={Neuroimage},
volume={5},
number={3},
pages={199--212},
year={1997},
publisher={Elsevier}
}Another problem that can occur with using the canonical HRF is when there are residual timing differences between slices - for example when you have not used some slice-time correction before statistics.
Using the temporal derivative
One standard way to deal with this is to add the HRF temporal derivative (TD) to your basis functions, along with the canonical HRF. You can also add the spatial derivative (SD). This creates further complexity for your second level analysis. Approaches are:
=== Dealing with the temporal derivative at the second level ==
- Ignore the TD at the second level, using only HRF contrasts. The TD, for short-event designs, will be more or less org