[FieldTrip] How to understand the channel location of ctf275 MEG data

[Zhi Li]

Hi Jan-Mathijs,

Thanks for your kind suggestion. The x-coordinates of channels of the third
subject are much bigger than others and I do not know why. May I ask if
there is any method to fix problematic channel location after data
collection? Many thanks.

Best,

Li Zhi


On Thu, 4 Aug 2022 at 14:31, Schoffelen, J.M. (Jan Mathijs) via fieldtrip <
fieldtrip at science.ru.nl> wrote:

> Hi Li Zhi,
>
> Sensor coordinates are registered to a coordinate system that is
> determined on a per measurement basis. This coordinate system is determined
> based on how the head positioning coils have been positioned on the
> participant’s head (typically on the nasion, and on the left/right ear).
> The positions of the sensors then also depend on how the participant was
> positioned in the MEG device, which may cause slight (but not too big)
> differences across participants. The first two sets of numbers that you
> report make sense to me, the third set of numbers (with an x-coordinate of
> 20 cm(?)) seems off to me. This FAQ
> https://www.fieldtriptoolbox.org/faq/coordsys/ explains a bit more about
> coordinate systems.
>
> In general, for sensor level analyses no standardisation is done. I would
> double check your third subject though, because something might have gone
> wrong with the head localization procedure.
>
> Best wishes,
> Jan-Mathijs
>
>
>
> > On 14 Jul 2022, at 18:41, Zhi Li via fieldtrip <fieldtrip at science.ru.nl>
> wrote:
> >
> > Hi,
> >
> > I am new to MEG data and have no experience in collecting them. When I
> processed the ctf275 MEG data, I found that the channel location in
> hdr.grad.chanpos were not the same across subjects. For example, the
> Cartesian coordinates of MLC11 from the data of 3 subjects were [7.96,
> 1.67, 13.94], [8.9, 1.1, 13.68] and [20.04, -0.03, 14.33] respectively.
> Hence both the 3D channel location and their 2D projections are different
> across subjects. May I ask how these Cartesian coordinates were calculated
> and should we standardize them before data processing, i.e., make the
> channel location the same across subjects? Any suggestions would be much
> appreciated.
> >
> > Best,
> >
> > Li Zhi
> >
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