[FieldTrip] ft_volumelookup: Different devices different results same script

[Schoffelen, J.M. (Jan Mathijs)]

Dear Michelle,

Given the lack of responses so far, it seems that no one has encountered your issue before.

Yet, I was wondering whether this is not a user error indeed. The help of ft_volumelookup mentions that it should work on a ‘volume’ structure as input. You mention that you use (of course :) ) a warped grid, which is probably not what you want, for 2 reasons:

1) the atlas should be in the same coordinate space as the object that is used in ft_volumelookup, using something warped sounds as if this is expressed in a possibly ill-defined space. In your pasted code, you explicitly specify sourcemodel.coordsys as ‘mni’, but is this truly the case?
2) the geometric info in a sourcemodel structure is defined by a set of dipole positions in 3D (i.e. a Nx3 matrix), rather than by a 3-element dim-field, which combined with a transform-field implicitly define the positions in the volume.

Somehow, FT gives you the benefit of the doubt, and manages to convert the source(model) representation into a volume representation. It’s still surprising that the downstream effects of this depend on the matlab version + device.
Also, it might be informative to the readership of the list, to what extent the different matlab versions return different numbers of grid points. Have you inspected this in more detail?

Best wishes,
Jan-Mathijs


On 27 Oct 2021, at 13:44, Michelle via fieldtrip <fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>> wrote:


Dear Fieldtrip community,

I was wondering if you could help me with a problem occurring using the function ft_volumelookup. Depending on the computer I use, I get a different number of selected grid points. I use the same script (see scrip below) and the same FieldTrip Version (20210825), the only difference is that I used different MATLAB Versions on the different devices: MATLAB 2020b, MATLAB 2019b, MATLAB 2018a. But even when I used the same MATLAB Version (2019b) on different devices I received different results.

Has someone ever encountered a similar problem, or do I have some kind of error in my code (of course I always load the same warped grid I calculated from an MRI)?

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Thank you in advanced.

Best wishes,
Michelle
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