[FieldTrip] Average referencing and demeaning prior to beamformer

[Maris Skujevskis]

Hi fieldtrip users,

I have a question of clarification regarding common average referencing
during beamformer source reconstruction. The yellow warning box in the
tutorial http://www.fieldtriptoolbox.org/tutorial/beamformer stresses the
importance of common average referencing but leaves me confused as to when
exactly it needs to be done.

Suppose I have an set of EEG trials (i.e., a set of multichannel time
series). The entire data set has already been demeaned and common average
referenced.
I then cut out two time windows of interest on which I wish to apply a
beamformer filter.

Question 1: after I combine the the two cut-out time windows to calculate
the common filter, do I need to average reference the combined data set
prior to calculating the beamformer filter?

Question 2: Do I need to separately average reference each of the two
cut-out windows prior to applying the beamformer on each of the cut-out
data segments?

Question 3 on demeaning: do the cut-out combined/separate time windows
should also be demeaned or demeaning doesn't make any difference for
beamformer source reconstruction?

Thanks!

Maris
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