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Hi Philip,
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<div class="">Yes, indeed. </div>
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<div class="">Best wishes,</div>
<div class="">Jan-Mathijs</div>
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<blockquote type="cite" class="">
<div class="">On 12 Apr 2021, at 13:40, philip Joadavi <<a href="mailto:p.joadavi@gmail.com" class="">p.joadavi@gmail.com</a>> wrote:</div>
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<div dir="ltr" class="">Dear Jan-Mathijs,
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<div class="">Thanks for your quick reply. </div>
<div class="">so if I sum over the channels on the selected dipole, I get these results :</div>
<div class="">so </div>
<div class="">for leadfield computed with 64 channels:</div>
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<div class=""><font color="#ff0000" class="">1.0e-14 *<br class="">
-0.1818 -0.6828 0.5815</font><br class="">
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</div>
<div class="">for leadfield computed with 32 channels:</div>
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<div class=""><font color="#ff0000" class="">1.0e-13 <br class="">
0.0046 -0.1538 0.0872</font><br class="">
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<div class="">and for the subset of 32 extracted from 64 channels leadfield :</div>
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<div class=""><font color="#ff0000" class="">-0.3738 -0.4471 -0.2351</font><br class="">
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</font></div>
<div class=""><font class="">is that what you are referring to?</font></div>
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<div class=""><font class="">Best,</font></div>
<div class=""><font class="">Philip </font></div>
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<div dir="ltr" class="gmail_attr">On Sun, Apr 11, 2021 at 6:59 PM Schoffelen, J.M. (Jan Mathijs) <<a href="mailto:jan.schoffelen@donders.ru.nl" class="">jan.schoffelen@donders.ru.nl</a>> wrote:<br class="">
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<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div style="overflow-wrap: break-word;" class="">Hi Philip,
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<div class="">If I understand you well, the 32 channel array is a subset of the 64 channel array?</div>
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<div class="">Perhaps it may be useful to you know that FieldTrip computes EEG leadfields as if the channel array is average referenced. This means that - for a ‘leadfield’ topography for a given dipole - the average along the rows is 0, by construction. If
you construct a 64-channel leadfield, which will have an average of 0 along its rows, a selection of 32 rows (i.e. your 32 channel subset) will not by default have a zero mean across the rows. However, had you pre-selected those 32 channels and computed the
leadfield, the average would be 0.</div>
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<div class="">Could this explain the difference you describe?</div>
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</div>
<div class="">Best wishes,</div>
<div class="">Jan-Mathijs</div>
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</div>
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<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On 11 Apr 2021, at 16:14, philip Joadavi <<a href="mailto:p.joadavi@gmail.com" target="_blank" class="">p.joadavi@gmail.com</a>> wrote:</div>
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<div dir="ltr" class="">Dear all,
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<div class="">I'm trying to compute the leadfield for my EEG data. </div>
<div class="">I've done all the steps in the tutorial, <a href="https://www.fieldtriptoolbox.org/workshop/oslo2019/forward_modeling/" target="_blank" class="">
here</a>.</div>
<div class="">I run the <b class=""><u class="">bemcp</u></b>, for computing the headmodel. Since in my data I have 2 electrode sets (64 channels and 32 channels), I tried to compute the leadfield for 64 and then extract the leadfield from that file for 32
channels. However, when I compute the leadfield for 32 channels <u class="">separately</u>, the results are different from when I
<u class="">extract </u>the 32 channels from the 64 channel leadfield. </div>
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</div>
<div class="">I have checked everything, the patterns that I get are exactly the same, but the values from the leadfield computed separately from 32 channels are higher when I plot the potentials (using the simulation in the tutorial). </div>
<div class="">Can someone explain why is this the case and where it happens in the code for computing the leadfield? why the number of channels is important for leadfield computation?</div>
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<div class="">Unfortunately, I can not run the 'simbio' to compare. </div>
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</div>
<div class="">Thank you very much!</div>
<div class="">Best,</div>
<div class="">Philip </div>
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