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here's some context to why mne treats the axial gradiometers as
magnetometers from a conversation I had with Matti four years ago. We
should probably have this documented somewhere:
This is on purpose. ?magnetometers? includes magnetometers and axial
gradiometers. The field is still computed correctly as defined by the coil
type. It is reasonable to have the units as Tesla in axial gradiometers,
since the compensation coil in this case is relatively far from the brain
so that the signal measured is still from the coil loop close to the brain.
In planar gradiometers both parts of the figure-of-eight loops are in the
cerebral magnetic field and, therefore, we use T/m: we measure a gradient
of the field of interest. Another way to say this is that the
figure-of-eight loop performs a dual purpose: the external homogeneous
disturbances are attenuated and, at the same time, the device measures a
derivative of the brain?s magnetic field.
I hope this helps,
Matti
Hi Matti,
I was looking through the code for the KIT reader in MNE and what I am
noticed is that although we have axial gradiometers, the channels are being
marked as magnetometers. I was wondering what is the reasoning behind this.
Also, the units are marked as Tesla instead of Tesla per meter, another
correspondence to the magnetometer. The same behavior also occurs for CTF
data.
Best,
-teon
Hope this helps,
Best,
Teon