[Eeglablist] Electrode Positions
samnthaserban at gmail.com
Wed Aug 1 14:16:38 PDT 2007
In a similar situation to last weeks postings, here is my problem.
I am using an old system with an Electrocap. Our Polhemus is broken and we are
running studies just using the template coordinates in an *.XYZ file. Using the
template coordinates, things look decent. However, I need to reorient the nose
(nasion) along the -Y for it to show up the correct orientation. If I
opt to make
the head centre Cz or any other electrode, the electrodes are outside
of the head
template. I have removed all non scalp electrodes. So, I am not
doing this going
to use the centring option.
The main problem is, the display of the electrodes are still squished
too far back
from the front. Yet, the electrodes in the occ/parietal look fine. I
all the hanging electrodes as well to allow the spacing better for
As I have no way to collect new coordinates using our broken
Polhemus and I have
data without individual Polhemus location files for each subject, I am at a loss
what to do. I have found online the Standard-10-20-Cap81.ced which
for most of my electrodes, but there are 10 electrodes unique to the montage for
which I cannot find the coordinates, specifically: AF355, AF455, F155,
C255, CP155, CP255, PO355 and PO455. I do not want to drop these 10
use the Standard-10-20-Cap81.ced. If I find coordinates from
elsewhere, they may
not match be in proper relation to the other channels in this file. Does anyone
have any suggestions on how to transform the electrodes in my Polhemus standard
file? Or does anyone have the proper coordinates for the missing 10
in relation to
all the other electrodes in the Standard-10-20-Cap81.ced?
As I have the -Y towards the nasion and the electrodes are still too
far back on the
montage display, I think I need to transform toward the -Y direction
to spread them
out more X direction as well. Please let me know if you can help me.
I will send
you the montage as it is squished and my electrode file locations. I
think a simple
transform would do it, however, I am unsure how to do this even with the GUI
transform, i.e., what maths to use! Please let me know if you can help me with
this. I have 35 subject worth of data and we need to be able to use it!
Any help we would be greatly appreciated! I am no expert in MATLAB or
Alternatively, I have moved the electrodes in the X and Y directions
VISUALLY with the montage I am using on the template. However, is
this the correct
thing to do? When plotting in the 3D, the head shape is lost. Most
likely, I will
only be plotting in 2D. So, if things appear in 2D as I want them, is this an
acceptable thing to do?
Samantha Serban, Ph.D.
Postdoctoral Research Fellow/ Adjunct Professor of Electrophysiology
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