Confounder for categorical design

Dear all,

I have a question about controlling for confounding variables in SPM12. My task included 2 categorical conditions (A and B), which were modeled as separate condition regressors. I would like to control for the effects of RT. There are two options that came to my mind:

  1. Including RT as a nuisance regressor (like 6 head motion parameters). I have 200 scans (TR = 2s) for each subject. Can I add RTs to scans that cover the onsets of the corresponding trials, and leave other scans as zeros? For example, if the onset of trial 1 is 2.5s and the RT for that trial is 0.9, should I set scan 2 (2-4s) to 0.9?

  2. Including RT of each trial as a parametric modulator in condition A and B. I used to use this approach when I had multiple parametric modulators for each condition. In this case, however, I am interested in the difference between two conditions, so there will be only one parametric modulator (RT). I am not sure if this approach still works.

Thank you for your time and consideration!