[FieldTrip] center of sphere artefact

Frederic Roux fredericroux at hotmail.de
Tue May 8 17:29:39 CEST 2012

Dear all,

I was looking at my lcmv-beamformer maps of ~200 Sec of 
eyes closed resting state MEG activity and wondering if
what I was seeing is the center of sphere artefact.

The code I used is:

%filtering of the data
cfg = [];
cfg.bpfilter = 'yes';
cfg.bpfilttype = 'but';
cfg.bpfreq = [5 45];
cfg.bpfiltord = 4;
cfg.bpfiltdir = 'twopass';

[meg_data] = ft_preprocessing(cfg,meg_data);

%PCA to extract components with max explained variance
[cf,pcs,vexp] = princomp(meg_data.trial{1},'econ');
pexp = 100*vexp/sum(vexp);
indx = find(cumsum(vexp) <=90);
meg_data.trial{1} = (pcs(:,indx)*cv(:,indx)')';

% computing the covariance matrix
cfg = [];
cfg.channel = {'MEG'};
cfg.covariance = 'yes';
cfg.pad = 'maxperlen';
cfg.sgncmb = {'MEG' 'MEG'};
cfg.removemean = 'yes';

[cov_data] = ft_timelockanalysis(cfg,meg_data);

%LCMV beamformer
cfg = [];
cfg.channel = {'MEG'};
cfg.grid = grid_data;
cfg.vol = hdm;
cfg.method = 'lcmv';
cfg.grid.dim = [Nx Ny Nz];

cfg.lcmv.fixedori = 'no';
cfg.lcmv.lambda = '5%';
cfg.lcmv.projectnoise = 'yes';
cfg.lcmv.keepfilters = 'yes';
cfg.lcmv.projectmom = 'no';
cfg.lcmv.keepmom = 'no';
cfg.lcmv.reducerank = 2;
cfg.lcmv.normalize = 'yes';

[bf] = ft_sourceanalysis(cfg,cov_data);

%make power unit invariant
bf.avg.pow = bf.avg.pow./max(bf.avg.pow);

Since I don't have enough experience to judge about that
I wanted to ask if anybody out there with experience 
could tell me their opinion.

The grid was computed using an inwardshift of -0.5 and a grid
resolution of 2.5 mm. The head model using the ft_prepare_singleshell.

Any help,advice, comments or suggestions would be highly appreciated.


Frédéric Roux, PhD student
Department of Neurophysiology
Max Planck Institute for Brain Research
D-60529 Frankfurt am Main
Frederic.Roux at brain.mpg.de

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