[FieldTrip] beamformer for EEG: mask the output for a specific atlas region

[Joram van Driel]

Hi Parinaz,

I'm working on similar atlas-based source analyses, and a few days back I
kind of asked the same question. In the mean time I solved it. There are
two possible approaches:

1) You can already focus your source analyses on ROIs at the level of the
lead field grid.
2) You can mask your output of ft_sourcegrandaverage with atlas-based ROIs.

For both you need to use ft_read_atlas (check the help, it's pretty self
explanatory, although I don't know if Fieldtrip has your HMAT atlas
shipped), and tf_volumelookup.

For option 1), you need subject-specific grids that are aligned (warped) to
an MNI template, and do the masking already at the grid level; then proceed
to your within-subject source analysis, and group stats; you'll now only
have source activity from a ROI. It's all explained here:
http://www.fieldtriptoolbox.org/example/create_single-subject_grids_in_individual_head_space_that_are_all_aligned_in_brain_atlas_based_mni_space,
although this example uses all combined ROIs from one atlas, to remove
ventricles etc. But you can apply the same trick to one specific ROI: for
the cfg of ft_volumelookup, instead of cfg.roi = atlas.tissuelabel; you
say, e.g.  cfg.roi = 'M1'; or whatever the atlas label of your motor area
is in the HMAT atlas you're using.

For option 2), you basically set the .inside voxels to be those of the ROI
mask, and additionally you could set the power of all voxels outside of the
mask to zero. Here, too, either the subject-specific source results should
be normalized to a common template, or you should have done the
subject-specific source analysis on pre-warped MNI-based grids. Below code
is from my analysis, where I select occipital voxels at the group level on
whole-brain source results:

atlas =
ft_read_atlas('Z:\Toolboxes\fieldtrip_github\fieldtrip\template\atlas\aal\ROI_MNI_V4.nii');
atlas = ft_convert_units(atlas,'cm');

cfg = [];
cfg.atlas = atlas;
cfg.inputcoord = 'mni';
cfg.roi = atlas.tissuelabel(43:54);

occ_mask = ft_volumelookup(cfg,template_grid);

grandavg_col_masked = grandavg_col;
grandavg_grey_masked = grandavg_grey;
for s=1:10
    grandavg_col_masked{s}.inside=occ_mask(:);
    grandavg_grey_masked{s}.inside=occ_mask(:);
    grandavg_col_masked{s}.avg.pow(occ_mask(:)==0)=0;
    grandavg_grey_masked{s}.avg.pow(occ_mask(:)==0)=0;
end

Hope this helps.
- Joram

On Mon, Nov 16, 2015 at 2:29 PM, Babaeeghazvini, P. <p.babaeeghazvini at vu.nl>
wrote:

> Hello
>
> I am doing beamformer on EEG data to find the source position of bimanual
> activity based on dics method. For that I want to see the activity only on
> motor area and remove the activities of other regions based on HMAT atlas (
> http://lrnlab.org/) which provides a binary mask of motor cortex. I need
> to do that because I want to calculate the position of maximum activity in
> motor region and I dont want this maximum activity appear  somewhere else
> due to any possible artifact. please tell me how and where should I apply
> HMAT binary mask on the output of source analysis.
>
> Regards,
> Parinaz.
>
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>



-- 
Joram van Driel, PhD
Postdoc @ Vrije Universiteit Amsterdam
Cognitive Psychology
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