[FieldTrip] Normalization of LCMV source power

[Gabriel Costa]

Dear Fieldtrip community,

I came across an issue regarding normalization of resting-state EEG power
in source space using beamforming. I was hoping to get your opinion on this
. I am wondering if using the avg.pow output from LCMV source analysis is a
valid way of getting band-specific source power relative to total power. Using
the following LCMV configuration for the source analysis:


*     cfg = []; ...*

*    cfg.method = 'lcmv';*

*    cfg.lcmv.projectnoise = 'yes';*

*    cfg.lcmv.keepfilter   = 'yes';*

*    cfg.lcmv.weightnorm   = 'nai';*

*    source = ft_sourceanalysis(cfg, data_timelock);*



Fieldtrip returns "source" with a common filter and, I assume, an estimate
of broadband source power "source.avg.pow". After reconstructing source
time series (source_ts) using the common filter, I perform frequency
analysis for each source/dipole within a given frequency band, for example:


*    cfg.foi        = [8 12];*

    *cfg.taper      = 'hanning';*

*    Pow_alpha = ft_freqanalysis(cfg, source_ts);*



>From here, the two possible ways of normalizing are:

*(a)  Get a broadband source power estimate to normalize the alpha power.*

*    cfg.output     = 'pow';*

*    cfg.method     = 'mtmfft';*

*    cfg.foi        = [1 90]; % same as  high and low pass filters of data*

    *cfg.taper      = 'hanning';*

*    FA_source    = ft_freqanalysis(cfg, source_ts);*

    Pow_alphanorm  =  Pow_alpha.powspctrm / *sum(FA_source.powscptrm,2)*

*(b) * Use the estimate from LCMV ft_sourceanalysis

    Pow_alphanorm  =  Pow_alpha.powspctrm / *source.avg.pow*;


My question is whether *(b)* is a valid approach to normalizing the
spectrum/FOI power, or would normalization as in *(a)* be more
interpretable?

I noticed that the two approaches produce different results but not
dramatically so. I would greatly appreciate help on deciding which is more
theoretically justified.

Thanks,

Gabriel Costa
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