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<p>hi,</p>
<p>i would suggest updating to a recent fieldtrip version. combining
mags and grads has been implemented there and you basically do not
need to do any extra steps. there are just these three pitfalls
that you need to pay attention to because it might lead to wrong
results:</p>
<ol>
<li>make sure that you put in the original data. i.e. no
prescaling etc. also make sure that you have not modified the
.tra matrix of your grad structure.</li>
<li>always compute your own leadfield. i.e. always use
ft_prepare_leadfield to compute the forward model and don't have
it done by ft_sourceanalysis. i don't know if this is absolutely
necessary but as the ft_prepare_leadfield step is the one in
which the actual pitfalls are, i would strongly suggest you do
it separately to have maximum control!<br>
</li>
<li>all the data going into ft_prepare_leadfield MUST BE in
meters! i.e.:</li>
<ol>
<li>the grid mus be either created directly in meters of
converted to meters by using ft_convert_unit</li>
<li>the grad structure of your data must have been converted to
meters. this is something you must do manually as according to
my experience, fieldtrip does not take care of it.
additionally, the default grad returned by ft_read_sens (and
thus the one placed in your data structure) is in centimeters!
so, it is a good idea to do something like this: data.grad =
ft_convert_data(data.grad, 'm'); right after getting your data
via ft_preprocessing!</li>
<li>if you provide your grad structure in the cfg structure,
also make sure that it is in meters.</li>
</ol>
</ol>
<p>the meters stuff is really extremely important!!! if your units
are wrong, you are going to end up with wrong source projections
and will not get an error message!!!</p>
<p>if you see some fieldtrip output like: "converting XXX to cm",
your results will be wrong!</p>
<p>if you take care of all that, fieldtrip does the scaling
automatically based on the distance between the two parts of each
gradiometer.</p>
<p>best,<br>
thomas<br>
</p>
<br>
<div class="moz-cite-prefix">Am 10.05.2017 um 09:33 schrieb Klink-3,
N.E.C. van:<br>
</div>
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US">Hi Andria,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US">We have been struggling with the same problem,
also using a 306-channel Neuromag system. We ended up
correcting for the scale difference between magnetometer and
gradiometer signals (i.e. noise 5e-15T vs 5e-13T/m). So we
multiplied the signals of the magnetometers and the
leadfield values of the magnetometers by a factor 100. This
gave us the best result.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US">Kind regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US">Nicole<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">Van:</span></b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
<a class="moz-txt-link-abbreviated" href="mailto:fieldtrip-bounces@science.ru.nl">fieldtrip-bounces@science.ru.nl</a>
[<a class="moz-txt-link-freetext" href="mailto:fieldtrip-bounces@science.ru.nl">mailto:fieldtrip-bounces@science.ru.nl</a>]
<b>Namens </b>Andria Pelentritou<br>
<b>Verzonden:</b> 10 mei 17 1:52<br>
<b>Aan:</b> <a class="moz-txt-link-abbreviated" href="mailto:fieldtrip@science.ru.nl">fieldtrip@science.ru.nl</a><br>
<b>Onderwerp:</b> [FieldTrip] combining
gradiometer/magnetometer sensors - neuromag data<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p>Greetings from Melbourne!<o:p></o:p></p>
<p>We've been using fieldtrip (v-20160801) for a while now and
we are having issues when it comes to combining magnetometer
and gradiometer MEG sensors types during (LCMV) beamforming
for the Elekta Neuromag Triux 306-channel system.
Beamforming the magnetometers or gradiometers separately on
auditory evoked data works well, but we cannot successfully
combine the two sensor types. We've played with the
cfg.coilaccuracy variable as advised in one of the tutorials
with no luck as there seem to be no obvious effects on the
output. The documentation provides very little information
on the topic.<o:p></o:p></p>
<p>What strategies would you recommend when trying to combine
the Neuromag 306-channel system magnetometer/gradiometer
data?<o:p></o:p></p>
<div>
<p>Many thanks in advance for your help and many more thanks
for releasing such a valuable toolbox!<br>
Warm regards<br>
Andria<o:p></o:p></p>
</div>
</div>
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<pre wrap="">_______________________________________________
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<a class="moz-txt-link-freetext" href="https://mailman.science.ru.nl/mailman/listinfo/fieldtrip">https://mailman.science.ru.nl/mailman/listinfo/fieldtrip</a></pre>
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<pre class="moz-signature" cols="72">--
Dr. Thomas Hartmann
Centre for Cognitive Neuroscience
FB Psychologie
Universität Salzburg
Hellbrunnerstraße 34/II
5020 Salzburg
Tel: +43 662 8044 5109
Email: <a class="moz-txt-link-abbreviated" href="mailto:thomas.hartmann@th-ht.de">thomas.hartmann@th-ht.de</a>
"I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan Doyle)
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