Thanks, for the response! I see what you mean about individual subject variation. The reason for using a gray matter mask at the single subject stage is out of a necessity for the average time signal I extract per ROI per person to come specifically from gray matter. The end result of this analysis is outcome measures from the Brain Connectivity Toolbox, which involves inputting connectivity matrices built of each participants' time signal per ROI(so an ROIxROI matrix for every individual)..So there isn't a final group level image per-say at which I could apply a gray matter mask. Currently I plan to multiply a gray matter mask by my ROI Mask. I wonder if it wouldn't make the most sense to use an average of all my subjects' gray matter masks? Although averaging the masks could introduce it's own issues for accuracy of gray matter segmentation I suppose. I appreciate any feedback, thank you!
Best,
Sam DeWitt