[Mne_analysis] Calculate the planar gradient

[Narun Pornpattananangkul]

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Hi MNE users/developers,

I am trying to move away from Matlab, and I was wondering if MNE python has
a function to compute the planar gradient from the axial gradiometers. I
was told that it is more appropriate to use planar gradient for the
sensor-level type of analysis.

Fieldtrip has ft_megplanar and ft_combineplanar functions for this.
http://www.fieldtriptoolbox.org/reference/ft_megplanar/
http://www.fieldtriptoolbox.org/reference/ft_combineplanar/

From their tutorial
http://www.fieldtriptoolbox.org/tutorial/eventrelatedaveraging/:
"The planar gradient at a given sensor location can be approximated by
comparing the field at that sensor with its neighbors (i.e. finite
difference estimate of the derivative). The planar gradient at one location
is computed in both the horizontal and the vertical direction with the
FieldTrip function ft_megplanar. These two orthogonal gradients on a single
sensor location can be combined using Pythagoras rule with the FieldTrip
function ft_combineplanar."

Thank you so much,

Narun
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