how to plot the direction and amplitude of the planar gradient

[Jim Li]

Dear all,

I want to view the direction and amplitude of the planar gradient  near each 
sensor location and create a plot like what's shown in the  attached file (more 
or less).  Is it possible to do it? If so, how?

Our MEG sensors are 248 axial gradiometers. After running "megplanar" I got 
A1_dH...A248_dH,A1_dV...A248_dV for my channels. If, say, in the plane 
tangential to a given channel we have the Horizontal (x axis) and Vertical (y 
axis) directions. Then for the channel of interest, say A1,  I can run sqrt
(A1_dH^2+A1_dV^2) to get the amplitude of the planar gradient, and I can 
run atan2(A1_dV,A1_dH)*180/pi) to get the angle between the field vector 
and x-axis. What I'm not sure  about is how fieldtrip defines the Horizontal (x 
axis) and Vertical (y axis) for each channel (in a plane tangential to that 
channel). So it's hard to create the plot I want...

Thanks a lot.

Jim

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