[Mne_analysis] Group dSPMs

[Marina Papoutsi]

Dear MNE users,

I have been recently trying to figure out how to do group level,
whole-brain statistical comparisons in MNE and I would be really 
grateful if someone could clarify some of my (perhaps naive) questions.

I had a look at D. Goldenholz's slides about MNE and dSPM and have been
wondering about how to create group dSPMs. (see
http://www.nmr.mgh.harvard.edu/~daniel/links/presentation/stats_on_roi.pdf)

In the slides (page 46) it mentions that one first computes the MNE
signal and then computes a kind of dSPM. My limited understanding is 
that the value "sum(s_i)" would come from the mne*.stc file. Since, I 
notice that the MNE estimates are sums of squared dipole component 
strengths. Is that so?

My next question is then how to calculate sum(var(s_i))? My 
understanding is that I will need to calculate this by hand using the 
noise covariance matrix and the inverse operator, unless these values 
are stored somewhere.

A final question (and perhaps the most ignorant one), is why calculate 
group dSPMs vs doing an ANOVA on the single-subject MNE estimates (or 
even on contrasts of MNE estimates)?

Sorry for my ignorance and thanks for reading.
Marina