neural activity index (NAI) beamforming...

[Jan Mathijs Schoffelen]

Dear Ali,

The most likely reason for your finding is that the cross-spectral density matrix is rank-deficient. A solution would be to use some regularization by means of cfg.lambda = something.
A more or less identical problem has been covered some time ago (but then in EEG-data). You might want to search the mailing list's archive (reachable through the fieldtrip-wiki), and search for the subject: problems using dics.

Yours,

Jan-Mathijs

----- Original Message -----
From: Ali Mazaheri <ali.mazaheri at fcdonders.ru.nl>
Date: Thursday, October 18, 2007 6:46 pm
Subject: [FIELDTRIP] neural activity index (NAI) beamforming...

> Hello,
>
> I decided to  NAI to normalize my activations.....
>
> sourceNAI.avg.pow = sourcePost.avg.pow ./ sourcePost.avg.noise;
>
>
> but the avg.noise parameter is has zero values ( the only finite
> numbers
> in the matrice)
>
>
> my method to beamform has been straightforward
>
>
> cfg = [];
> cfg.grad = freqdata.grad;
> cfg.vol = vol; cfg.resolution = 1;
> cfg.reducerank = 2; cfg.channel = {'MEG'};
> cfg.xgrid = 'auto'; cfg.ygrid = 'auto';
> cfg.zgrid = 'auto';
> [grid] = prepare_leadfield(cfg);
>
>
>
> cfg = [];
> cfg.frequency = 18;
> cfg.method = 'dics';
> cfg.projectnoise = 'yes';
> cfg.grid = grid;
> cfg.vol = vol; cfg.lambda = 0;
> sourcePost = sourceanalysis(cfg,freqdata );
>
> My source.post.avg.pow has non-zero finite values......
> any suggestions on what I am doing wrong would be greatly
> appreciated.
>
>
>
> Ali
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users
> of the FieldTrip  toolbox, to share experiences and to discuss new
> ideas for MEG and EEG analysis. See also
> http://listserv.surfnet.nl/archives/fieldtrip.html and
> http://www.ru.nl/fcdonders/fieldtrip.
>

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.