[Eeglablist] Scripps MEG channel location issues

[Darren Weber]

Hi Greg,

the coordinate frame sounds like a CTF frame, with +x at the nasion, +y at
the left ear, and +z superior (vertex).  This coordinate frame was described
in a good paper:
Malmivuo JA, Wikswo JP, Barry WH, Harrison DC, Fairbank WM (1977):
Consistent system of rectangular and spherical coordinates for
electrocardiography and magnetocardiography. Med. & Biol. Eng. & Comput.
15:(4) 413-5.

See a description of the above online:

http://butler.cc.tut.fi/~malmivuo/bem/tutorial/coordina/index.htm

Maybe your sensors are tilted forward due to miscalculation of the
fiducials, especially the nasion.  If you are using a Polhemus digitiser,
you need to be VERY CAREFUL to operate in a METAL FREE environment (about
+/- 2 meters in XYZ from the center of the object to be digitised).  I have
noticed SIGNIFICANT DISTORTION of the fiducial locations when the Polhemus
is not calibrated and used correctly in a METAL FREE space.  For some
reason, some Polhemus FASTRAK data I've seen had OK the electrode/sensor
locations, but the fiducials were very bad.

Best, Darren


----- Original Message ----- 
From: "Greg Appelbaum" <lappelba at uci.edu>
To: <eeglablist at sccn.ucsd.edu>
Sent: Friday, December 12, 2003 11:05 AM
Subject: [Eeglablist] Scripps MEG channel location issues


> Hello all,
>
>     I have run into problems projecting my sensor locations from some data
I
> recently collected at the Scripps (LaJolla) MEG facility and am looking
for
> a little help.
>
> Imbedded in my data header files are coil locations for that subject on
that
> run.  Supposedly these coil locations are in XYZ space with the X axis
> running through the nasion and the Y axis running through the ears. I have
> extracted these locations and constructed an EEGLab .xyz format channel
> location file.  This file is read and accepted by EEGLab, however the
> channel locations are consistently registered as though the coil array
were
> rotated ~10-15 degrees forward on the head.  I.E. The coils look as though
> they are pushed down over the subjects forehead.  I have tried projecting
> these channels using a Polhemus system as well, and arrive at the same
> results.  Any help or advise would be greatly appreciated.
>
> Greg Appelbaum
>
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