[FieldTrip] Fwd: single trials eLoreta

[David Pascucci]

Dear fieldtrip experts,
I am posting again my questions, hoping that someone has experience with a
similar pipeline and can give some feedback.

I am trying to use the eLoreta method to get single-trial estimates of
source activity from specific ROIs, in the following way:

% eLORETA
cfg = [];
cfg.method                          = 'eloreta';
cfg.grid                                  = leadfield;
cfg.headmodel                       = headmodel;
cfg.eloreta.keepfilter              = 'yes';
cfg.eloreta.normalize               = 'yes';
cfg.eloreta.lambda                  = 0.05;
     *(1)
cfg.eloreta.projectnoise            = 'yes';
eLO_source                          = ft_sourceanalysis(cfg,data);

% in the above line, "data" is the results of ft_timelockanalysis
% with cfg.covariance = 'yes';
     *(2)

% then I put the source positions from the MNI template I
% used for the sourcemodel (see:
% http://www.fieldtriptoolbox.org/tutorial/sourcemodel#subject-
% specific_grids_that_are_equivalent_across_subjects_in_normalized_space
eLO_source.pos                      = template_grid.pos;
iPOS                                = eLO_source.pos;
iPOS(eLO_source.inside==0,:)        = NaN; % only points inside gray matter

% Then I select ROIs (here only one for simplicity) to extract single-trial
source activity:
[v,I]      = min(pdist2(iPOS, ROIs_mni , 'euclidean'));

% And I multiply the spatial filter for the EEG data in each trial
W            = eLO_source.avg.filter{I}; % filter at my ROI of interest
for tr = 1:size(data.trial,1)
       % loop over trials
     trials{tr} = W * squeeze(data.trial(tr,:,:));
   *(3)
end

Is this approach correct?
My main questions are:

*(1) Is there a way to select the best lambda parameter (e.g., selecting
the one that best approximates the activity at the EEG channels level)?

*(2) I am confused about the role of the covariance, since it doesn't seem
to be used when source activity is estimated using the set of spatial
filters at the single trial

*(3) Is the "trials{tr} = W * squeeze(data.trial(tr,:,:)); " approach
correct to get time-series of source activity in a ROI?

Looking forward for your feedback.

Best,
David


-- 
---------------------
David Pascucci

Postdoctoral Fellow
University of Fribourg
Department of Psychology
Rue de Faucigny 2
1700 Fribourg
Switzerland



-- 
---------------------
David Pascucci

Postdoctoral Fellow
University of Fribourg
Department of Psychology
Rue de Faucigny 2
1700 Fribourg
Switzerland
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