Dear Tina,

Kitty is right - if you want to use a cluster ROI, MarsBaR is a nice option: http://marsbar.sourceforge.net/
In MarsBaR, you would choose ROI definition -> Get SPM cluster(s)... Then choose your analysis SPM.mat file, threshold appropriately, and finally click on Write ROI(s) -> Write one (or all) clusters.
The ROI will be in .mat format, so you will need to transform it into a .nii file. To do this, you can do ROI definition -> Export.. -> Export ROI(s) to... -> image. Then pick the .mat file for your cluster. As space, I would choose 'From image' and choose an image that is in the same space as your gPPI analysis. In this way, you ensure that the ROI will be in the right space.

To use an independent atlas ROI, you can simply use any .nii file where all voxels in the ROI are 1, and all others are 0. For example, the Anatomy toolbox allows you to export such anatomical ROIs easily: https://www.fz-juelich.de/inm/inm-1/DE/F...

I hope this helps!

All the best,
Phil
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Philipp Kuhnke, M.Sc.
Ph.D. Student
Max Planck Institute for Human Cognitive and Brain Sciences,
Leipzig, Germany