Hi SPMers,
I have seen a few threads on covarying out the effects of RT, and I wanted to get some clarification on the difference between modeling an event as a non-zero duration regressor, and using RT as a parametric modulator.
As I understand it, modeling an event as a non-zero duration will account for any RT-related effects between the onset of the event and the end of the event (marked as the time when the participant makes a response). Using RT as a parametric modulator, on the other hand, will look for whether the amplitude of the BOLD response scales in any way with increases in RT. So, I presume that the approach you take depends on the question you want to answer: Whether there is a significant difference between modeling the duration of an event versus modeling it as a punctate response, or whether increasing RT leads to significant increases in the amplitude of the BOLD response.
However, when a reviewer asks that you covary out RT (without any further clarification), is one of these approaches better than the other to address that issue, or are both equally effective ways to account for RT-related effects?
Thanks,
-Andrew
