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Dear Fieldtrip community,<br>
<br>
I am a second year PhD student from Slovenia, looking for some help.
As the title suggests, I am trying to project simulated source
activity to the surface (EEG data simulation from sources).<br>
<br>
Up to now, I have simulated 90 timeseries at the source level using
Jansen-Rit neuronal mass models, based on the AAL volumetric atlas
(activity is placed in each center of the 90 regions), so I have
90x3000 matrix (sources x time points). I have decided to project
the source activity to 128 channel standard Biosemi 128 EEG setup.
Now I have issues with defining forward model from the AAL sources
to the EEG channels. <br>
<br>
<p>Currently, I have followed your online examples to create
headmodel (BEM) and the forward model, with the important part of
the code being: <br>
</p>
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<div class="lineNode"><span style="white-space: pre"><span>headmodel = ft_read_headmodel(</span><span style="color: rgb(167, 9, 245);">'standard_bem.mat'</span><span>);</span><span style="color: rgb(0, 128, 19);">
</span></span></div>
<div class="lineNode"><span style="white-space: pre"><span>[headmodel, sens] = ft_prepare_vol_sens(headmodel, sens);</span><span style="color: rgb(0, 128, 19);">
</span></span></div>
<div class="lineNode"><span style="white-space: pre"><span>[lf] = ft_compute_leadfield(atlas_rois, sens, headmodel); where atlas_rois is a 90x3 double, with 3d positions of source dipoles (at AAL centers)</span></span></div>
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<p>
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<p>What I don't understand is the output of the function <i><span style="white-space: pre"><span>ft_compute_leadfield</span></span></i>,
which is a 128x270 double. Could you please explain what it
represents? As far as I understand, the surface EEG should be
obtained by the equation <i>surface_time_series = </i><i><i><i>forward_model
* </i>source_time_serie</i><i>s</i></i>, meaning that the
dimensions should have been <i>[</i><i><span style="color: rgb(55, 65, 81); font-family: Söhne, ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, Ubuntu, Cantarell, "Noto Sans", sans-serif, "Helvetica Neue", Arial, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji"; font-size: 16px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; white-space: pre-wrap; background-color: rgb(247, 247, 248); text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;">number of electrodes) x (number of time points)</span>]
= [</i><i><span style="color: rgb(55, 65, 81); font-family: Söhne, ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, Ubuntu, Cantarell, "Noto Sans", sans-serif, "Helvetica Neue", Arial, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji"; font-size: 16px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; white-space: pre-wrap; background-color: rgb(247, 247, 248); text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;">number of electrodes) x (number of sources)</span>]
* [(</i><i><span style="color: rgb(55, 65, 81); font-family: Söhne, ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, Ubuntu, Cantarell, "Noto Sans", sans-serif, "Helvetica Neue", Arial, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji"; font-size: 16px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; white-space: pre-wrap; background-color: rgb(247, 247, 248); text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;">number of sources) x (number of time points</span>],
which comes down to: [128x3000] = [128x90] * [90x3000].... </i>Is
there a reason 3*90=270? Do source (dipole) orientations play a
role?<br>
</p>
<p>Additionally for clarification, to obtain the final EEG
projection, would it be then correct to directly apply the
equation <i>surface_time_series = </i><i><i><i>forward_model * </i>source_time_serie</i><i>s,
</i></i>with forward model being the variable <i>"</i><span style="white-space: pre"><span>lf</span></span><i>"</i>?<br>
</p>
<p>I have tried various toolboxes for this simulated sources to EEG
projection (TVB, MNE, SPM, Fieldtrip) and had issues everywhere,
but came the furthest with Fieldtrip, so I would really appreciate
your help. General recommendations on this topic would also be
greatly appreciated.</p>
<p>Thank you very much!<br>
</p>
<p>Kind regards,</p>
<p>Nina Omejc<br>
</p>
<br>
<br>
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