[FieldTrip] Strange DICS Results

[Alexander Nakhnikian]

Hello all,

I'm hitting a wall in my analysis trying to source-localize EEG data in 
a visual task.  I'm trying to localize the 8 to 15 Hz power.

Unfortunately, I don't have individual electrode coordinates or sMRIs so 
I've had to create an electrode coordinate file and manually align it 
with the scalp in FT's template head.  I'm using the standard BEM as the 
head model and the standard MRI instead of individual structural scans.

I'm able to get reasonable results from ft_freqanalysis; importantly, 
they match the scalp topography we see using other programs, such as IDL.

The problem is that when I try to localize these data with DICS, I get 
absurd results.  The source localized power exceeds 3000 at some 
locations and the maxima are in strange places, such as behind the eye 
socket and near the base of the skull.

When I run ft_sourceinterpolate I get the following warning:

Warning: could not reshape freq to the expected dimensions
 > In ft_datatype_volume (line 136)
   In ft_checkdata (line 329)
   In ft_sourceinterpolate (line 163)

Otherwise, no errors or warning messages come up.  I'm not sure what's 
causing the odd results.  Am I not using the template files correctly?  
I've included analysis setup below.

Thanks in advance,

Alexander



cfg = [];
cfg.method = 'mtmfft';
cfg.output = 'powandcsd';
cfg.tapsmofrq = 4;
cfg.foilim = [12 12];
freqData = ft_freqanalysis(cfg,data);

cfg = [];
cfg.grid.xgrid = 'auto'; cfg.grid.ygrid = 'auto';cfg.grid.zgrid = 'auto';
cfg.resolution = 5; %units are mm
cfg.method = 'dics';
cfg.dics.normalize = 'yes';
cfg.headmodel = vol; %from loading 'standard_bem.mat'
cfg.elec = elec_aligned; %electrode coordinates (MNI) aligned to 
template head
sources = ft_sourceanalysis(cfg,freqData);

cfg = [];
cfg.downsample = 2;
cfg.parameter = 'pow';
sourcesInt = ft_sourceinterpolate(cfg,sources,mri); %mri from 
'standard_mri.mat' in FT's templates

cfg = [];
cfg.method = 'slice';
cfg.parameter = 'pow;
ft_sourceplot(cfg,sourcesInt)





-- 
Alexander Nakhnikian, Ph.D.
Postdoctoral Fellow
Clinical Research Training Program
VA Boston Healthcare System
Harvard Medical School