= Group studies: Subject co-registration =
In the typical Brainstorm workflow, you process each subject individually: you have access to the MRI of the subject, you extract the cortex envelope from it using the software of your choice (!FreeSurfer, BrainVISA, !BrainSuite or CIVET), you import the recordings and estimate the sources for each subject separately.

After importing and pre-processing all the individual information, you will probably be interested in calculating some group statistics, to extract the significant features across all the subjects in your study. in order to do this at the source level, all the source files must be using the same support cortex source (= the same source space). The problem you typically face is that the shape and number of vertices of the cortex surface varies across subject: one source index doesn't mean the same thing for all the subjects, hence it's impossible to compare them directly.

The solution is to reproject the sources of both subjects on  the default anatomy. The reprojected versions share the same  source space, the cortex from Colin27 anatomy, so they can be averaged or used to calculate any other statistics across subjects.

From the interface