[FieldTrip] PLV with DICS beamformer

[Jan.Hirschmann at med.uni-duesseldorf.de]

Hi Natalia,

How are you? I didn't know you were fieldtripin', too.
I can't really tell if your approach is correct, but I have some comments. First, to me it seems that the imaginary part of csd is not enough to calculate PLV. But then, I don't know the details of how you compute it. Second, the 3 orientations issue is commonly tackled by singular value decomposition rather than by averaging (see e.g. beamformer_dics.m). One can use svd to compute the orientation of maximum power, and I think one could adapt the code, e.g. to get the orientation of maximum phase difference.

One straight-forward alternative I know of, is to extract source level time courses using the LCMV beamfomer. You can set cfg.projectmom='yes' to only get the direction of max. power. Then you could use ft_connectivityanalysis with cfg.method='plv' to get the phase locking values.

Hope this helps,
Jan

Von: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-bounces at science.ru.nl] Im Auftrag von Natalia Zaretskaya
Gesendet: Dienstag, 1. April 2014 09:20
An: fieldtrip at science.ru.nl
Betreff: [FieldTrip] PLV with DICS beamformer

Dear FieldTrip experts,

I would like to be able to compute the phase-locking value (Lachaux, 1999) using source data derived from disc beamformer.

Right now I perform ft_sourceanalysis with
cfg.method = 'dics';
cfg.refdip = [x y z];
cfg.rawtrial = 'yes';
cfg.keepcsd = 'yes';
cfg.grid.filter = source.avg.filter; %  precomputed common filter for two conditions

and then use the imaginary part of the single-trial cross-spectral densities to compute PLV "by hand". Cross-spectra at each position have a 3x3 format, so I compute the PLV for each dipole orientation pair and then average.

I was wondering whether 1) the above procedure is technically correct and 2) whether there is a better, more efficient way to do it (I assume plv can be derived at the same step where source.avg.coh is computed?).

Many thanks in advance!

Best regards,
Natalia Zaretskaya

---
Dr. Natalia Zaretskaya
Centre for Integrative Neuroscience
Vision and Cognition Lab (AG Bartels)
72076 Tübingen, Germany
natalia.zaretskaya at cin.uni-tuebingen.de<mailto:natalia.zaretskaya at cin.uni-tuebingen.de>
phone: +49 7071 2989032



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