[FieldTrip] DICS with rawtrial='yes'

[jan-mathijs schoffelen]

Hi Nadia,

My guess here would be that you have used a vector beamformer (i.e. each of the common filters has size 3xM). Then, for each voxel the activity is computed by sandwiching the sensor level cross-spectral density matrix between the voxel's spatial filter: w'*C*w, which yields a 3x3 csd-matrix at the voxel level. Then, in order to get a single number as the power estimate for that voxel (and trial), some computation needs to be done. I believe that the default is to take the first singular value of this 3x3 matrix. Essentially, for each trial a PCA is performed, and the largest eigenvalue is taken to be the power. The PCA may yield a slightly different optimal orientation for each trial, and as a result the average across trials will be different from the first singular value estimated from the average-across-trials csd matrix.
If my guesses are incorrect (i.e. you didn't use a vector beamformer, OR the default is not to take the first singular value to represent the voxel power) then we need to dig into the FieldTrip-code.

Best wishes,

Jan-Mathijs



On Jan 17, 2013, at 11:11 AM, Muller, Nadia wrote:

> Dear all,
> 
> I am trying to beam single trials into source space using a DICS filter and the following code.
> 
> cfg=[];
> cfg.method='dics';
> cfg.grid=grid_ind;
> cfg.vol=hdm;
> cfg.grid.filter=alldics.avg.filter;% common filter
> cfg.dics.lambda='5%';
> cfg.channel=channs;
> cfg.frequency=13;
> cfg.grad=data.grad;
> cfg.keeptrials='yes';
> cfg.rawtrial='yes';
> dics=ft_sourceanalysis(cfg,pow);%pow contains csd of singletrials (only one condition)
> 
> I would suppose that the average of the rawtrials (obtained by ft_soucredescriptives([],dics) or by averaging power of single trials myselves) should be comparable to the output of dics with rawtrials='no' (and also keeptrials='no').
> 
> However this is not the case (the averaged raw trials have a 30-60% bigger amplitude compared to the average values obtained from dics without rawtrial='yes'). Am I misunderstanding something here? Or could it be that I use wrong parameters for beaming single trials?
> 
> Thanks for any advice!
> 
> Best,
> Nadia
> 
> Dr. Nadia Müller
> CIMeC - Center for Mind/Brain Sciences
> Università degli Studi di Trento
> via delle Regole, 101
> Mattarello (TN) - ITALY
> e-mail nadia.muller at unitn.it
> http://sites.google.com/site/obobcimec
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

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