weird result for EEG visual N1 source localization using lcmv
Chen
ychen at BME.UFL.EDU
Tue Mar 20 15:32:19 CET 2007
Hi,
I am doing source localization on EEG biosemi 128 channels recordings and
get weird result. I am giving the details in the following to seek any
help:
KrisS = eeglab2fieldtrip(EEG,'preprocessing');
cfg=[];
cfg.latency=[.08 .12]; %latency of N1
cfg.covariancewindow=[.08 .12];
cfg.covariance='yes';
cfg.keeptrials='no';
datTest=timelockanalysis(cfg,KrisS);
clear KrisS
cfg = [];
cfg.showlabels = 'yes';
cfg.fontsize = 5;
figure(1);topoplotER(cfg,datTest);
datTest.elec.pnt = 1000 * datTest.elec.pnt*[0 -1 0; 1 0 0;0 0 1]; %convert
electrode coordinates from m to mm and make the x go to nose
load standard_vol.mat %use standard BEM vol from EEGLAB
load standard_mri.mat %use standard mri from EEGLAB
cfg=[];
cfg.vol=vol;
cfg.grid.xgrid='auto';
cfg.grid.ygrid='auto';
cfg.grid.zgrid='auto';
[grid] = prepare_leadfield(cfg, datTest);
cfg=[];
cfg.grid=source2sparse(grid);
cfg.method='lcmv';
cfg.vol=vol;
cfg.projectnoise='yes';
cfg.lambda=1;
[sourceN1] = sourceanalysis(cfg, datTest);
cfg=[];
cfg.powmethod='none'; %only option that works
[sourcenai] = sourcedescriptives(cfg, sourceN1);
cfg =[];
cfg.downsample = 2;
cfg.funparameter = 'nai';%or neural activity indes (nai)
sourceN1F=source2full(sourcenai);
[interp]=sourceinterpolate(cfg,sourceN1F,mri);
figure(3);sliceinterp(cfg,interp)
I have the following specific questions:
1. Should the units all be mm?
2. Is my coordinate rotation right?
3. How to choose the value of lambda? I just randomly choose 1 now since
without lamda I always get warnings of rank-deficient and divided by zeros.
4. Since I am using the standard BEM and mri, my electrodes are not on the
skin( see the attached figure), when I try to realign based on fiducials,
it is still the same. I check the function of electroderealign and find
out that norm.m is always identy matrix which make the realign doing
nothing. Why?
Appreciate to point out what I am wrong.
Chen
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