> - Created fid_eeg.mat, fid_mri.mat, sensors_orig.mat, and
> headshape_orig.mat with the script create_fid_files.m. (I have
> deleted the
> first 6 lines in sensors.pol and the first 3 lines in headshape.pol
> since
> these coordinates corresponded to the fiducials in the
> create_fid_files.m.
> Is it wrong to do this?)
>
I happened to look at the tex file used to generate this part of the
manual and found that there were some very helpful comments that are
not visible in the .pdf version, apparently because they were
preceded with percentage signs. Apparently the answer is that these
fiducial lines should indeed be deleted.
\section{Preparing the *.pol files}
\begin{quote}
% Just quick and dirty programming to extract the information from
the .pol
% files: fiducials, sensors & headshape in EEG space.
% Actually, I just opened the ascii files and copy the coord of the EEG
% fiducials here... Much easier.
% Order of the fiducials is: LE, RE, Na
fid\_eeg = ([-0.0587687 6.79448 -0.00636311 ; ...
0.0352661 -6.78906 -0.00369206 ; ...
9.3675 0.0260009 0.00481311] + ...
[-0.0328487 6.78991 0.00636288 ; ...
0.0563513 -6.79533 0.00369206 ; ...
9.45206 -0.0260009 -0.00481297])/2 ...
* 10 ; % To convert cm into mm
% These are coordinates picked by hand on the sMRI.
% So it's quite an approximation of where the fiducials are really...
fid\_mri = [-71.8 3.5 -58.8 ; ...
71.3 -6 -62.5 ; ...
0 90.6 -28.4] ;
% I edited the .pol files to REMOVE the first few lines with
% fiducial information.
sensors = load('sensors\_noFid.pol','-ASCII')*10;
headshape = load('headshape\_noFid.pol','-ASCII')*10;
% Again, multiply by 10 to get the measures in mm instead of cm
% ATTENTION !!!
% Polhemus, uses the axes in a different orientation!
% It's still a right handed system but axes are:
% - x: from back to front (versus left to right)
% - y: from right to left (versus back to front)
% - z: from bottom to top
%
% To make coord systems compatible, it is necessary to rotate
clockwise the
% coord by 90 degree around the z axis.
Rot = spm\_matrix([0 0 0 0 0 -pi/2]); Rot = Rot(1:3,1:3);
fid\_eeg = (Rot*fid\_eeg')';
sensors = (Rot*sensors')';
headshape = (Rot*headshape')';
save fid\_eeg fid\_eeg
save fid\_mri fid\_mri
save sensors\_orig sensors
save headshape\_orig headshape
\end{quote}
------------------------------------------------------------------------
--------
Joseph Dien
Assistant Professor of Psychology
Department of Psychology
419 Fraser Hall
1415 Jayhawk Blvd
University of Kansas
Lawrence, KS 66045-7556
E-mail: [log in to unmask]
Office: 785-864-9822 (note: no voicemail)
Fax: 785-864-5696
http://people.ku.edu/~jdien/Dien.html
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