DICS Localization Problem

[Nathan Dees]

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!!

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