Dear Indira,
the RFX will be based on those voxels which have non-NaN values in all of the single-subject models. The single-subject models are based on those voxels surpassing a certain intensity threshold specified in spm_defaults.m
So if the top of the head is excluded / has low intensity in one subject this region will not be part of the corresponding single-subject model, and not part of the RFX automatically. A mask is not required.
Hope this helps,
Helmut
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