[FieldTrip] Normalization of LCMV source power

[Gabriel Costa]

Hello Jan-Mathijs,

Thank you for your reply. After you made it clear that total variance and
the sum across the "full spectrum" are equivalent it clicked: the spectral
resolution to get both measures should be the same. Hence, using 2secs
epochs, cfg.freq with a 0.5Hz resolution should give identical results for
both estimates as seen in the Left image below (<3% difference), bar any
disparities in filters and bands of interest. If on the other hand,
freq_analysis is run with a 1Hz resolution, the sum of the spectrum is a
fraction of the total variance (Right image). It seems clear that that was
my initial issue and explains why values were highly correlated but still
different:

[image: image.png][image: image.png]
Thank you very much for your help.
Best regards,
Gabriel


On Wed, Sep 23, 2026 at 11:55 AM Schoffelen, J.M. (Jan Mathijs) via
fieldtrip <fieldtrip at science.ru.nl> wrote:

> Hi Gabriel,
>
> Given that according to Parseval’s theorem the sum across the full
> spectrum is equal to the total variance, and since you ask for theoretical
> justification, division by a measure of total variance ( ~source.avg.pow )
> would  be the way forward. Whether or not this is more or less
> interpretable than a division with an integral of the estimated power in a
> more constrained bandwidth I don’t know.
>
> Best wishes,
>
> Jan-Mathijs
>
>
>
>
> On 11 Sep 2026, at 11:54, Gabriel Costa via fieldtrip <
> fieldtrip at science.ru.nl> wrote:
>
> 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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