DICS Localization Problem

[Nathan Weisz]

dear nathan,

we see nice mu desynchronizations nicely localized to motor cortex
when subjects prepare responding to a target using a 4-D 2500 WH (the
one with 148 sensors).
Have you already done localizations with your MEG system using a
segmented brain? If not, then I suggest to check if you get the same
results using a more 'standard' headmodel, e.g. multisphere (like in
the tutorial). this assures you always are in the MEG-coordinate
system, and have no weird mixture between coordinate systems. you need
a headshape for that (pretty standard with 4D; i can provide you with
one if you don't have one). also you might want to check if your
volume / grid makes sense relative to the sensor position using
'headmodelplot'.

good luck,
yet-another-nathan

On 05.08.2008, at 02:45, Nathan Dees wrote:

> Dear Fieldtrippers,
>
> Have been struggling a long time to produce a viable image of
> beamforming
> analysis.   We are using a 4D-Magnes3600 Magnetometer MEG unit to
> record a
> prompted finger tapping motor task with variable ISI of 2.5 - 3.0
> seconds.
> Our time-frequency plots  show evident beta and alpha
> desynchronization at movement onset.  We are using the beamforming
> tutorial
> as a guide.  After calculating dataPre and dataPost, and verifying
> our mri
> has been properly segmented by SPM, our code is:
>
> cfg = [];
> cfg.method    = 'mtmfft';
> cfg.output    = 'powandcsd';
> cfg.tapsmofrq = 2;
> cfg.foilim    = [18 20];
> cfg.pad       = 0.5;
> freqPre = freqanalysis(cfg,dataPre);
>
> cfg = [];
> cfg.method    = 'mtmfft';
> cfg.output    = 'powandcsd';
> cfg.tapsmofrq = 2;
> cfg.foilim    = [18 20];segmentedmriF.transform = mri.transform;
> segmentedmriF.anatomy   = mri.anatomy;
> cfg.pad       = 0.5;
> freqPost = freqanalysis(cfg,dataPost);
>
> segmentedmri.transform = mri.transform;
> segmentedmri.anatomy   = mri.anatomy;
> [vol, cfg] = prepare_singleshell([], segmentedmri);
>
> cfg                       = [];
> cfg.grad                  = freqPre.grad;
> cfg.vol                   = vol;
> cfg.reducerank            = 2;
> cfg.channel               = {'MEG'};
> cfg.xgrid                 = -30:1:30;   % 'AUTO' does not work!
> cfg.ygrid                 = -30:1:30;   % 'AUTO' does not work!
> cfg.zgrid                 = -40:1:40;    % 'AUTO' does not work!
> [grid]                    = prepare_leadfield(cfg);
>
> cfg = [];
> cfg.frequency    = 20;
> cfg.method       = 'dics';
> cfg.projectnoise = 'yes';
> cfg.grid         = grid;
> cfg.vol          = vol;
> cfg.lambda       = 0;
>
> sourcePre  = sourceanalysis(cfg,freqPre );
> sourcePost = sourceanalysis(cfg,freqPost);
>
> %Plot of deep sources in the center of the brain looks reasonable.
>
> sourceDiff = sourcePost;
> sourceDiff.avg.pow = (sourcePre.avg.pow - sourcePost.avg.pow) ./
> sourcePost.avg.pow;
> cfg = [];
> cfg.downsample = 2;
> sourceDiffInt = sourceinterpolate(cfg, sourceDiff , mri);
>
> cfg = [];
> cfg.method = 'slice';
> cfg.funparameter = 'avg.pow';
> cfg.maskparameter = cfg.funparameter;
> cfg.funcolorlim = [0.0 .2];
> cfg.opacitylim = [0.0 .2];
> cfg.opacitymap = 'rampup';
> figure
> sourceplot(cfg,sourceDiffInt);
>
> We expect localization of desynchronization to be in the motor cortex,
> predominantly on the left side.  The result we get is diffusely
> localized,
> and with weak signal.  I have images to possibly help diagnose the
> problem,
> but for now, can anyone spot the errors?
>
> Thank you!!
>
> ----------------------------------
> 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.

--------------------------------------------
Attention: New Contact Details
--------------------------------------------
Dr. Nathan Weisz

OBOB-Lab
University of Konstanz
Department of Psychology
P.O. Box D23
78457 Konstanz
Germany

Tel: ++49 - (0)7531 - 88 45 84
Email: nathan.weisz at uni-konstanz.de
Homepage: coming soon

Bob-wisdom: It's better to do one job well than two jobs... not so well.


----------------------------------
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.
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