Hi Francois,
not necessarily a Brainstorm question, but do you have any papers in mind that you could share with regards to advising against doing connectivity analysis at the source level?
I've often seen the Gross et al. 2013 ("Good Practice for Conducting and Reporting MEG Research") paper cited as why one would want to do connectivity at the source rather than the sensor level, along with a few other papers.
Thank you!