I want to make sure I understand the fast prior probability map
fast -P needs a user supplied prior probability map while --Prior uses
the probability map supplied by FSL in the standard space (I assume).
If that's the case, I'm a little surprised that --Prior does not
require <standard2input.mat> as in the case of -a. Does that mean
---Prior performs a FLIRT transform to standard space under the hood
by itself or am I misinterpreting the similarity between --Prior and