[FieldTrip] Using fixed orientations for men-source estimation

[Sarang S. Dalal]

Hi Christian,

I had flagged your email to follow-up later but lost track of it -- it seems you didn't get a response yet, but I hope mine is still useful!

The strategy that the 'fixedori' implements for the beamformer variants (and sLORETA) are not based on the anatomical normal, but rather the direction that maximizes the theoretical SNR (adaptively determined from the signal characteristics). This optimal direction is dependent on the particular weight calculation formula for each source localization variant.

Therefore, a similar SNR optimization strategy for minimum norm would actually require a different formula than you see used for the others. It's simple enough to implement if you know what that formula is. :-) It is likely to be contained in the book by Sekihara & Nagarajan (2008), if you'd like to have a go at it yourself.

That said, min-norm is often (or perhaps usually) performed with the solution space constrained to gray matter voxels, and the orientations defined to be normal to the cortical surface. If you independently have a way to obtain anatomically derived orientations, then you can manually provide them in lf.ori. (Or maybe there is a FieldTrip function that could obtain these normals from the MRI segmentation procedure?)

Cheers,
Sarang



On Tue, 2017-08-01 at 11:11 +0000, christian.merkel at med.ovgu.de wrote:

Hello,


I am running ft_sourceanalysis and am wondering why I can restrict the parameter-estimation in LCMV and sLORETA by setting the parameter 'fixedori' but not when using MNE. Shouldn't one be able to also just use the normal direction of each source position here as well? Can I just apply the same logic in the script 'minimumnormestimate' to change the field 'lf.ori' as, for example, in 'ft_sloreta' or would this be problematic down the line?

Thank You,

Christian

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