[FieldTrip] How to impove dics realfilter solution

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

Hi Gio,

What about the application of projecting frequency domain data through a common filter? This is described in the beamformer tutorial on the fieldtrip website and has worked fine for me. After commenting out the realfilter line the group statistical results looked worse in my case (although I also changed lambda on this second try). 

>From the tutorial:
"Then we compute the inverse filter based on both conditions. Note the use of cfg.keepfilter so that the output saves this computed filter.

cfg              = [];
cfg.method       = 'dics';
cfg.frequency    = 18;
cfg.grid         = grid;
cfg.vol          = vol;
cfg.dics.projectnoise = 'yes';
cfg.dics.lambda       = '5%';
cfg.dics.keepfilter   = 'yes';
cfg.dics.realfilter   = 'yes';
sourceAll = ft_sourceanalysis(cfg, freqAll);

By placing this pre-computed filter inside cfg.grid.filter, it can now be applied to each condition separately.

cfg.grid.filter = sourceAll.avg.filter;
sourcePre_con  = ft_sourceanalysis(cfg, freqPre );
sourcePost_con = ft_sourceanalysis(cfg, freqPost);"

Best,
Jan



-----Ursprüngliche Nachricht-----
Von: fieldtrip-bounces at science.ru.nl [mailto:fieldtrip-bounces at science.ru.nl] Im Auftrag von Gio Piantoni
Gesendet: Dienstag, 25. September 2012 12:22
An: FieldTrip discussion list
Betreff: Re: [FieldTrip] How to impove dics realfilter solution

Hi Björn,

It's not surprising that the results are pretty bad after using real filters. The fourier transform and DICS are intrinsically complex-valued, so using only the real part means throwing away half of the information (and getting meaningless results).

The realfilter option was implemented in order to allow to pass the time-domain signal through the complex-value spatial filter and obtain a "virtual electrode". A virtual electrode with complex values is "weird to say the least", see:
http://mailman.science.ru.nl/pipermail/fieldtrip/2006-May/000510.html
Robert considered it an error in the algorithm and fixed it by providing the realfilter option. However, to me it seems more correct to think that complex spatial filters requires complex input (i.e.
frequency-domain signal), so don't use DICS spatial filters on time-domain real-valued MEG recordings to generate time-domain virtual electrodes. If you want time-domain virtual electrodes, use LCMV spatial filters centered at the location of interest. Other smart people might disagree though.

I suggest not to use realfilter option at all, but I'd be happy to hear other meaningful applications of this option.

Hope this helps,

Gio

On Fri, Sep 14, 2012 at 10:23 AM, Björn Herrmann <bherrmann at cbs.mpg.de> wrote:
> Dear fieldtrip user,
>
> I am using the DICS beamformer to localize a 3Hz FFT power topographical distribution (using a neuromag system). The topographies are very clear auditory in single subjects, thus it shouldn't be a problem finding the "right" solution. When I use the beamformer to obtain complex-value spatial filters, the solution looks very good (using NAI). No problem here. However, using cfg.dics.realfilter = 'yes', i.e. the spatial filter will have real values, the localization completely breaks down, sometimes even finding a strong peak in the opposite hemisphere than visible in the sensor topography (the head is centered in the sensor array).
>
> Does someone have a suggestion how to improve the solution using realfilter = 'yes'. I use the following settings. They are rather standard I guess.
>
> cfg = [];
> cfg.method            = 'dics';
> cfg.frequency         = 3;
> cfg.dics.projectnoise = 'yes';
> cfg.dics.lambda       = '5%';
> cfg.dics.keepfilter   = 'yes';
> cfg.dics.realfilter   = 'yes';
> cfg.dics.fixedori     = 'no';
>
>
> If someone has a suggestion, I would appreciate it.
> Thanks in advance,
> best,
>
> Björn
>
>
>
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> fieldtrip at donders.ru.nl
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