Thank you Francois. In fact, looking at the resting power on the cortical surface does make more sense. My original idea was to filter the resting scalp EEG in the delta band, and to then look at the RMS of each vertex element. But yes, of course, I can also just do source localization on the broadband signals and then do a frequency transform on the sources and look at delta power on the cortical surface.
Best wishes,
Joel