Dear Amy,

> I'm running an fMRI analysis with four different runs.  I'm concatenating the four runs into one long run in my design matrix (I realize this is not recommended).  I created four different regressors to model each of the four runs (using dummy coded 1s and 0s).
>
> Because these four regressors add up to the constant term (which is automatically created in SPM), they are not estimable. The boxes are grey below these regressors when reviewing the design matrix.
>
> I don't care about specific session effects, but I want to make sure that this is okay (i.e. my betas for the events of interest are valid).
>
> Do I need to remove the constant term?

If you leave out the regressor for one of the runs (e.g., the fourth),
then all will be fine (because the constant term won't be entirely
redundant).

Hope this helps!

Best regards,

Jonathan

--
Jonathan Peelle, PhD
Department of Otolaryngology
Washington University in St. Louis
Office: (314) 362-9044
http://peellelab.org || http://jonathanpeelle.net