[Marsbar-users] ROI extraction problem/question

[Mary S. <mf...@me...>]

Hello:

I have a question regarding data extracted from ROI's using the summary function in marsbar. After creating the ROI's, we extract the data using a script that calls marsy and returns the summarized data (we are looking at the mean of all voxels in a region). This works great except for the fact that if I choose to use a different number of subjects (i.e. 75 instead of 150) using the same images and same ROI, I get a different number for each subject. I also tried doing the extractions using the marsbar GUI within SPM and still get different values based on differences in number of subjects I use.  I can't figure out why this would be. Has any one else run into this or know why this is? 

Thanks for your help,

Mary




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