[FieldTrip] Source analysis for grand averaged-TMS-evoked potentials

Agnese Zazio agnese.zazio at hotmail.it
Mon Jan 7 16:41:12 CET 2019

Dear Fieldtrip Community,

my name is Agnese Zazio and I would be grateful if you could help me with source analysis of TMS-EEG data.

Specifically, I am interested in source reconstruction of TMS-evoked potentials. To this am, I used the minimum-norm estimation following the Fieldtrip tutorial for source reconstruction of MEG event-related fields (http://www.fieldtriptoolbox.org/tutorial/minimumnormestimate/). I don't have individual MRIs, thus I am using templates for the headmodel and the sourcemodel. Importantly, I am working on the the grand average at the group level.
Here's the code I used.

% create a preprocessed structure
cfg =[];
cfg.channel = {'EEG'};
cfg.demean = 'yes';
cfg.baselinewindow  = [-0.01 0];
data_prepr = ft_preprocessing(cfg, data);

cfg =[];
cfg.covariance = 'yes';
cfg.channel ={'EEG'};
cfg.covariancewindow = [0 0.4];
data_tlck = ft_timelockanalysis (cfg, data_prepr);

% forward solution [prepare leadfield]
cfg = [];
cfg.elec = elec;
cfg.channel = {'EEG'};
cfg.headmodel = vol;   % volume conduction model
cfg.grid = ft_read_headshape('cortex_5124.surf.gii');
cfg.grid.pos = sourcespace.pos;
cfg.grid.inside = 1:size(sourcespace.pos,1); % all source points are inside of the brain
leadfield = ft_prepare_leadfield(cfg);

% inverse solution (method: mne)
cfg        = [];
cfg.method = 'mne'; %'lcmv';
cfg.elec = elec;
cfg.channel = {'EEG'};
cfg.grid   = leadfield;
cfg.headmodel     = vol;
cfg.mne.prewhiten = 'yes';
cfg.mne.lambda    = 3;
cfg.mne.scalesourcecov = 'yes';
source_bial_mne      = ft_sourceanalysis(cfg, data_tlck);

bnd.pos = sourcespace.pos;
bnd.tri = sourcespace.tri;
m=source_bial_mne.avg.pow(:,124); % point in time I want to plot
figure; ft_plot_mesh(bnd, 'vertexcolor', m);

I have two main questions:
First, is this approach correct to get sources for TMS-evoked potentials starting from the grandaverage?
Second, is it possible to use the same code but applying the beamformer method (lcmv) in "ft_sourceanalysis"? I have simply replaced the cfg.method field, but the output I get does not contain information about time.

Thanks in advance for your help.


Agnese Zazio, PhD Student
Cognitive Neuroscience Section,
IRCCS Saint John of God Clinical Research Centre
Via Pilastroni 4, 25125 Brescia, Italy
Site: http://www.cognitiveneuroscience.it<http://www.cognitiveneuroscience.it/>

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