[FieldTrip] Projection of simulated source activity to the surface EEG

[Nina Omejc]

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


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20230515/a6c91b78/attachment.htm>