[Mne_analysis] EEG reference for covariance matrices

[Elisabeth Fonteneau]

Dear all,

We have a question regarding the computation of covariance matrices with 
EEG and MEG in MNE:

The inverse is always computed on the average reference data (for EEG).
Therefore the covariance matrices should be computed on the average 
reference too. Although the inverse solution, theoretically, does not 
depend on the choice of reference electrode, one has to make sure that 
the leadfield, covariance matrix, and data are referenced in the same 
way when performing the computations in MNE. Apparently, MNE always uses 
average reference for the leadfield and the data, but not necessarily 
for the covariance matrices.
This means that we have to use the option --projon when we are computing 
the covariance matrices with mne_process_raw.
If we use the --projoff option, then the covariance matrices are 
computed with the acquisition reference, and therefore, the inverse 
solution will be incorrect.

Are we right?

Thanks in advance for your answer,

Elisabeth

-- 
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Dr Elisabeth Fonteneau
MRC Cognition and Brain Sciences Unit
15 Chaucer Road
Cambridge CB2 7EF
England
Tel:  +44 (0)1223 355294 (extension 263)
Fax:  +44 (0)1223 359062 

Email: elisabeth.fonteneau at mrc-cbu.cam.ac.uk
Web: http://www.mrc-cbu.cam.ac.uk/people/elisabeth.fonteneau/
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