DICS, complex filter coeffcients and Virtual Electrodes

[Vladimir Litvak]

Hi Michael,

I'm doing this kind of things and I use LCMV beamformer (where the
locations of the sources might come from your DICS analysis). This was
Robert's suggestion. The reason is that DICS is only optimized for the
particular frequency you are looking at and if you want to extract
source data that contains other frequencies you need to look at the
full frequency range. There are options in lcmv (cfg.lcmv.fixedori =
'yes') that reduces the two orthogonal sources (in MEG case) to one
based on the direction of maximal variance. That's something similar
to what SAM does (although I'm not sure exactly the same).

Best,

Vladimir


On Mon, Feb 23, 2009 at 11:45 AM, Michael Wibral
<wibral at bic.uni-frankfurt.de> wrote:
> Dear Fieldtrippers,
>
> I was wondering about the correct way to get SAM-like virtual electrodes using DICS. I naively tried to compute some virtual electrodes using the DICS filter coeffcients on the raw timecourses, but that obviously doesn't work because DICS filter coefficients are complex (at least in Fieldtrip 20081208 ?). Next thing I thought about was to FFT the single trial, filter it and then inverse FFT it, because the DICS filters are supposed to work in the frequency domain. But that shouldn't work either, because we have iFFT(filter(FFT(trial))) = iFFT(FFT(filter(trial))) = filter(trial), because all operation are linear.
>
> Next thing I am puzzled about is that the Filters (A) times the leadfield matrix (L) should be the identity matrix: AL=1 (see the Gross, 2001, PNAS paper). If A is complex - as it is in Fieldtrip -, then we would also have a complex leadfield, which seems odd to me.
>
> Any suggestion would be appreciated,
> Michael
>
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.
>

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The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.