problems using dics

[Nathan Weisz]

hi,

i am trying to do sourceanalysis on spontaneous activity data, i.e.  
there is no baseline condition (such as a pre-stimulus period). tried  
the approach using the "NAI" as described in the tutorial (Localizing  
oscillatory sources using beamformer techniques). however there  
apparently are problems. see more below.
data is 60 channel EEG, preprocessed in eeglab (ICA artefact  
correction + removal of bad data periods) and cut into non- 
overlapping "epochs" of 2 seconds. a fieldtrip structure was  
generated using eeglab2fieldtrip.

 >> data

data =

       label: {1x60 cell}
     fsample: 250
        elec: [1x1 struct]
       trial: {1x138 cell}
        time: {1x138 cell}
         cfg: [1x1 struct]

then estimate power with freqanalysis:

cfg=[];
cfg.method='mtmfft';
cfg.output='powandcsd';
cfg.foilim=[12 18];
cfg.tapsmofrq=1.5;

freq4dics=freqanalysis(cfg,data);

 >> freq4dics

freq4dics =

         label: {1x60 cell}
        dimord: 'chan_freq'
          freq: [12 12.5000 13 13.5000 14 14.5000 15 15.5000 16  
16.5000 17 17.5000 18]
     powspctrm: [60x13 double]
      labelcmb: {1770x2 cell}
     crsspctrm: [1770x13 double]
          elec: [1x1 struct]
           cfg: [1x1 struct]

so far so good. now i'd like to do some sourceanalysis.

cfg=[];
cfg.method='dics';
cfg.grid=grid;
cfg.vol=vol; %use of the standard BEM model
cfg.lambda =[];
cfg.frequency=15;
cfg.projectnoise='yes';
cfg.elec=newelec;%electrode positions adapted to BEM released on FT- 
FTP (in standard_BEM)

alphaS=sourceanalysis(cfg,freq4dics);

the problem here is that the noise is set to 0. while executing the  
code this warning message appears:
Warning: cross-spectral density matrix is rank deficient
 > In beamformer_dics at 161
   In sourceanalysis at 855

a look at the code shows that in this case that the noise is set to  
lambda (empty and set to 0 per default). I then tried different  
lamdas but as results varied quite a lot I have an uneasy feeling  
doing it this way.

question: is there something apparently wrong with the configs that  
leads to the problem of not being able to make a noise estimate from  
the CSD?

cheers,
nathan

--------------------------------
Dr. Nathan Weisz

INSERM - Unité 821
Dynamique cérébrale et cognition
Centre Hospitalier Le Vinatier, Bâtiment 452
95 Boulevard Pinel
69500 Bron, France

Tel: ++33 - (0)4 - 7213 8915
Email: weisz at lyon.inserm.fr
Chat-AV: nathanweisz at mac.com
Homepage: http://web.mac.com/nathanweisz
Neurotree: http://neurotree.org/neurotree/tree.php?pid=8692

Please avoid sending me Word or PowerPoint attachments.
See http://www.gnu.org/philosophy/no-word-attachments.html




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