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

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: PlotGoal.TIF
Type: image/tiff
Size: 5718 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20091002/10af403b/attachment.tif>