[FieldTrip] How to impove dics realfilter solution

[Gio Piantoni]

Well, the people disagreeing with me are indeed very smart!

Jan-Mathijs, your argument is very subtle and I agree with you that
the imaginary part of the filter contains the non-instantaneous
unmixing. However, for frequency-domain data it makes sense to use the
non-instantaneous information, for the very reason that
frequency-domain data are not instantaneous. If you take the fourier
transform of a signal at 10 Hz, you're not computing it at one
specific instant but over a period of time. I'd imagine that the
imaginary part contains information about the frequency-domain data
over that period of time.

I see Michael's point as well, but would it be possible for the
quasi-static approximation of Maxwell's equations to contain the
imaginary part only when applied to frequency-domain data instead of
time-domain data? In the end, the complex-value spatial filters are
specific to one frequency band over a period of time and maybe the
imaginary part accounts for the time-shift (phase-shift) implicit in
the frequency-domain representation.

Thanks for the interesting discussion! Happy to hear if you have
further thoughts on this.

Cheers,

Gio

On Thu, Sep 27, 2012 at 1:39 PM, jan-mathijs schoffelen
<jan.schoffelen at donders.ru.nl> 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
>
>
>
>
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>
> 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
> Telephone: +31-24-3614793
>
>
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