[FieldTrip] Source Point Count & Atlas Point Count Discrepancy

[Sarah]

Hi all,


I have a question about mapping source points to atlas ROIs.


I have computed my source analysis using FT’s 3D grid of 7.5mm. I have also
interpolated the FT shipped AAL atlas (ROI_MNI_V4.nii) onto this 7.5mm
template grid.


I have now been mapping only some relevant source points to the atlas to
determine which ROI they exist in. However, I noticed that many of my
source points didn’t “exist” within the atlas. In fact, the # of my
subject’s source points that exist inside/outside are reported as: 6871
dipoles inside, 6791 dipoles outside brain, whereas I notice the atlas
interpolated source model only has 3475 points total.


I was wondering how to handle this apparent discrepancy (as I assume the
atlas point count should at least be somewhat closer to the total of source
points marked as inside the brain).


Thanks,

Sarah


P.S. I included some of my relevant code below to produce this, but it is
simply the code of the FT tutorials.


Subject Sourcemodel Code:

cfg           = [];

cfg.method    ='basedonmni';

cfg.template  = template_grid;

cfg.nonlinear = 'yes';

cfg.mri       = mri;

cfg.unit      ='mm';

grid          = ft_prepare_sourcemodel(cfg);


Interpolate Atlas Code:

cfg = [];

cfg.interpmethod = 'nearest';

cfg.parameter = 'tissue';

sourcemodel2 = ft_sourceinterpolate(cfg, atlas, template_grid);

sourcemodel2 = ft_convert_units(sourcemodel2, 'mm');
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