[FieldTrip] Neighbors for Elekta Neuromag 306 gradiometers separately?

[Darinka Trübutschek]

Dear Fieldtrip community,

I am new to MEG/fieldtrip and have a question regarding the neighbor
structure necessary for computing cluster-based statistics. I am currently
analyzing data from a Neuromag 306 system (with 102 Mags and 204 Grads) and
would like to look separately at Mags, Grad1, and Grad2.
I assume that this means that I also need to compute the neighbors
separately for the different channel types.

My question therefore concerns fieldtrip's standard neighbor templates for
Neuromag. Is there a specific reason (theoretical or methodological), why
there are no separate templates for Grad1 and 2? All that I could find are
separate templates for Mag (neuromag306mag_neighb.mat), the combined planar
gradients (neuromag306cmb_neighb.mat), and the neuromag306planar_neighb.mat
template, which, if I understand correctly, does not combine the Grads, but
still lists sensors of one type as neighbors of sensors of another type
(e.g., for sensor 0713 - a gradiometer measuring the derivative along the
longitudinal component, the neighbors listed include 0432, 0723, but also
sensors that, if I interpret it correctly, should measure the derivative
along the latitudinal component, such as 0433, 0712, etc.) Is there a
specific reason, why sometimes, for a given sensor position, both Grad1 and
Grad2 are included in the neighbors (e.g., 0432 and 0433), but sometimes
only one of the two (e.g., 0742)?

Many thanks in advance for your help!

Best,
Darinka
-- 
Darinka Trübutschek (PhD Candidate)

Inserm-CEA Cognitive Neuroimaging Unit
CEA/SAC/DSV/DRM/Neurospin
Bât 145, Point Courier 156
F-91191 Gif-sur-Yvette

website: https://sites.google.com/site/dtruebutschek/
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