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

Schoffelen, J.M. (Jan Mathijs) janmathijs.schoffelen at donders.ru.nl
Thu Aug 4 20:31:07 CEST 2022

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,


> 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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