[FieldTrip] Projection of simulated source activity to the surface EEG
Nina Omejc
nina.omejc at ijs.si
Mon May 15 14:01:02 CEST 2023
Dear Fieldtrip community,
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).
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.
Currently, I have followed your online examples to create headmodel
(BEM) and the forward model, with the important part of the code being:
headmodel = ft_read_headmodel('standard_bem.mat');
[headmodel, sens] = ft_prepare_vol_sens(headmodel, sens);
[lf] = ft_compute_leadfield(atlas_rois, sens, headmodel); where
atlas_rois is a 90x3 double, with 3d positions of source dipoles (at AAL
centers)
What I don't understand is the output of the function
/ft_compute_leadfield/, 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 /surface_time_series =
////forward_model * /source_time_serie//s//, meaning that the dimensions
should have been /[//number of electrodes) x (number of time points)] =
[//number of electrodes) x (number of sources)] * [(//number of sources)
x (number of time points], which comes down to: [128x3000] = [128x90] *
[90x3000].... /Is there a reason 3*90=270? Do source (dipole)
orientations play a role?
Additionally for clarification, to obtain the final EEG projection,
would it be then correct to directly apply the equation
/surface_time_series = ////forward_model * /source_time_serie//s, //with
forward model being the variable /"/lf/"/?
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.
Thank you very much!
Kind regards,
Nina Omejc
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