[FieldTrip] Fwd: Re: How to impove dics realfilter solution

Michael Wibral michael.wibral at web.de
Thu Sep 27 20:03:43 CEST 2012

-------- Original Message --------
Subject: 	Re: [FieldTrip] How to impove dics realfilter solution
Date: 	Thu, 27 Sep 2012 20:01:22 +0200
From: 	wibral <wibral at em.uni-frankfurt.de>
Reply-To: 	wibral at em.uni-frankfurt.de
To: 	FieldTrip discussion list <fieldtrip at science.ru.nl>

Dear Filedtrip users,

I fully agree with jan Mathijs,

in principle when using DICS to reconstruct source power, the nature of
the physical processes (forward model) should guarantee that filters are
automatically real-valued, because this reflects the quasi-static
approximation of maxwell's equations which holds pretty well in MEG. In
practice filter coeffcients only deviate from being real because of
sensor noise, which is not a desirable effect.


On 09/27/2012 01:39 PM, jan-mathijs schoffelen wrote:
> Dear Björn, caro Gio,
> I don't agree with you, Gio, that one should use complex-valued
> filters. The reason for this is that they don't make sense from a
> biophysical point of view. The mixing from sources to channels (as
> represented in the leadfields) is strictly real-valued, because it is
> (near-)instantaneous. I think that the unmixing from channels to
> sources also should be real-valued, for the same argument. When you
> allow filter coefficients to be complex-valued, this is equivalent to
> allowing the sensor-level signals to be shifted in time with respect
> to one another. I consider this a bit odd. In Joachim's seminal paper
> (on which FieldTrip's initial DICS implementation is based), this was
> not really explicit in the methods description. However, based on
> personal communications with said Joachim, I at least distilled that
> the filters should be real-valued. The complex-valued filters were
> kept in (as default) for backward compatibilitiy reasons. Although
> mathematically possible (perhaps even more strictly answering to the
> minimum variance and unit gain constraints), and sometimes seemingly
> giving nicer subjective results, I'd advice against using them.
> Cheers,
> JM
> On Sep 25, 2012, at 12:21 PM, Gio Piantoni wrote:
>> 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
>>> _______________________________________________
>>> fieldtrip mailing list
>>> fieldtrip at donders.ru.nl
>>> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>> _______________________________________________
>> fieldtrip mailing list
>> fieldtrip at donders.ru.nl
>> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
> Jan-Mathijs Schoffelen, MD PhD 
> Donders Institute for Brain, Cognition and Behaviour, 
> Centre for Cognitive Neuroimaging,
> Radboud University Nijmegen, The Netherlands
> Max Planck Institute for Psycholinguistics,
> Nijmegen, The Netherlands
> J.Schoffelen at donders.ru.nl <mailto:J.Schoffelen at donders.ru.nl>
> Telephone: +31-24-3614793
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

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