[FieldTrip] Be careful with nonlinear warping of your individual grids to MNI

[Stephen Whitmarsh]

Dear FieldTrip community,

I have encountered a little problem inverse-MNI-warping the grids of my
individual subjects for the purpose of averaging my beamformer results, in
line with:
http://www.fieldtriptoolbox.org/example/create_single-subject_grids_in_individual_head_space_that_are_all_aligned_in_mni_space?s[]=grid

When I visually checked the resultant individual grids, I found that 6 out
of 25 subjects had severely distorted grids. See attached examples. A bit
of distortion could be expected, but IMHO this is too much. It is the
result of nonlinear warping (done in the example script mentioned above)
and solved by using cfg.grid.nonlinear = 'no, in the following part of the
script:

cfg = [];
cfg.grid.warpmni = 'yes';
cfg.grid.template = template_scm;
cfg.grid.nonlinear = 'no';
cfg.mri = mri_segmented;
grid = ft_prepare_sourcemodel(cfg);

Ofcourse this is not a Fieldtrip bug in any way, but just something to be
aware of, and a warning I wanted to share. And it really shows that it is
important to check all of the warped individual grids!

I will redo only these subjects with linear instead of nonlinear warping.
That is, unless anyone has a better solution?

All the best,
Stephen Whitmarsh
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