[Freesurfer] WM segmentation

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Hi, I would like to create a mask of the WM for fMRI analysis. I read a previous post in which it was suggested to get the wm segmentation and erode it, since the result will be exclusively WM. My question is why is this so? Why can I assume that getting the WM mask from the aseg file will almost certainly get me high probability WM, >0.9 for example? In most segmentation algorithms we get posterior probabilities at each voxel, so that we can then select voxels with posterior probabilities >0.90 for example. Why is freesurfer's WM segmentation so accurate that we do not need posterior probabilities? Are these very high probabilities somehow already coded in freesurfer's segmentation algorithm? Thanks, Ben