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<td>Re: [FieldTrip] How to impove dics realfilter solution</td>
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<td>Thu, 27 Sep 2012 20:01:22 +0200</td>
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<td>wibral <a class="moz-txt-link-rfc2396E" href="mailto:wibral@em.uni-frankfurt.de"><wibral@em.uni-frankfurt.de></a></td>
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<td><a class="moz-txt-link-abbreviated" href="mailto:wibral@em.uni-frankfurt.de">wibral@em.uni-frankfurt.de</a></td>
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<div class="moz-cite-prefix"><br>
Dear Filedtrip users,<br>
<br>
I fully agree with jan Mathijs,<br>
<br>
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.<br>
<br>
Best,<br>
Michael<br>
<br>
On 09/27/2012 01:39 PM, jan-mathijs schoffelen wrote:<br>
</div>
<blockquote
cite="mid:ED1F2458-256C-4803-B744-F53F01F71CFF@donders.ru.nl"
type="cite">Dear Björn, caro Gio,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Cheers,</div>
<div><br>
</div>
<div>JM</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
<div>
<div>On Sep 25, 2012, at 12:21 PM, Gio Piantoni wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
<div>Hi Björn,<br>
<br>
It's not surprising that the results are pretty bad
after using real<br>
filters. The fourier transform and DICS are
intrinsically<br>
complex-valued, so using only the real part means
throwing away half<br>
of the information (and getting meaningless results).<br>
<br>
The realfilter option was implemented in order to allow
to pass the<br>
time-domain signal through the complex-value spatial
filter and obtain<br>
a "virtual electrode". A virtual electrode with complex
values is<br>
"weird to say the least", see:<br>
<a moz-do-not-send="true"
href="http://mailman.science.ru.nl/pipermail/fieldtrip/2006-May/000510.html">http://mailman.science.ru.nl/pipermail/fieldtrip/2006-May/000510.html</a><br>
Robert considered it an error in the algorithm and fixed
it by<br>
providing the realfilter option. However, to me it seems
more correct<br>
to think that complex spatial filters requires complex
input (i.e.<br>
frequency-domain signal), so don't use DICS spatial
filters on<br>
time-domain real-valued MEG recordings to generate
time-domain virtual<br>
electrodes. If you want time-domain virtual electrodes,
use LCMV<br>
spatial filters centered at the location of interest.
Other smart<br>
people might disagree though.<br>
<br>
I suggest not to use realfilter option at all, but I'd
be happy to<br>
hear other meaningful applications of this option.<br>
<br>
Hope this helps,<br>
<br>
Gio<br>
<br>
On Fri, Sep 14, 2012 at 10:23 AM, Björn Herrmann <a
moz-do-not-send="true" class="moz-txt-link-rfc2396E"
href="mailto:bherrmann@cbs.mpg.de"><bherrmann@cbs.mpg.de></a>
wrote:<br>
<blockquote type="cite">Dear fieldtrip user,<br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite">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).<br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite">Does someone have a suggestion
how to improve the solution using realfilter = 'yes'.
I use the following settings. They are rather standard
I guess.<br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite">cfg = [];<br>
</blockquote>
<blockquote type="cite">cfg.method = 'dics';<br>
</blockquote>
<blockquote type="cite">cfg.frequency = 3;<br>
</blockquote>
<blockquote type="cite">cfg.dics.projectnoise = 'yes';<br>
</blockquote>
<blockquote type="cite">cfg.dics.lambda = '5%';<br>
</blockquote>
<blockquote type="cite">cfg.dics.keepfilter = 'yes';<br>
</blockquote>
<blockquote type="cite">cfg.dics.realfilter = 'yes';<br>
</blockquote>
<blockquote type="cite">cfg.dics.fixedori = 'no';<br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite">If someone has a suggestion, I
would appreciate it.<br>
</blockquote>
<blockquote type="cite">Thanks in advance,<br>
</blockquote>
<blockquote type="cite">best,<br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite">Björn<br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite"><br>
</blockquote>
<blockquote type="cite">_______________________________________________<br>
</blockquote>
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<div>Jan-Mathijs Schoffelen, MD PhD </div>
<div><br>
</div>
<div>Donders Institute for Brain,
Cognition and Behaviour, <br>
Centre for Cognitive Neuroimaging,<br>
Radboud University Nijmegen, The
Netherlands</div>
<div><br>
</div>
<div>Max Planck Institute for
Psycholinguistics,</div>
<div>Nijmegen, The Netherlands</div>
<div><br>
</div>
<div><a moz-do-not-send="true"
href="mailto:J.Schoffelen@donders.ru.nl">J.Schoffelen@donders.ru.nl</a></div>
<div>Telephone: +31-24-3614793</div>
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