Here is an excerpt from
EEGLAB's location files website:
Code: Select all
Number labels theta radius X Y Z sph_theta sph_phi sph_radius
15 F3 -39 0.333 0.673 0.545 0.5 39 30 1
35 T7 -90 0.511 6.12e-17 0.999 -0.0349 90 -2 1
37 C3 -90 0.256 4.4e-17 0.719 0.695 90 44 1
39 Cz 90 0 3.75e-33 -6.12e-17 1 -90 90 1
48 CP3 -118 0.278 -0.36 0.676 0.643 118 40 1
There are 3 coordinate systems:
theta,radius are polar coordinates on a 2D projection. Cz is at the origin.
X,Y,Z are 3-D cartesian coordinates (not sure if on a sphere or on a template head). Cz is at 0,0,1. Every other electrode is below Cz in the Z direction.
sph_theta, sph_phi, sph_radius are 3-D polar coordinates on a sphere. Since it's a sphere, all electrodes have a radius of 1.
If you wanted to use X,Y,Z, you'd do the following in Matlab.
Code: Select all
locs = [0.673 0.545 0.5; 0 1 -0.0349; 0 0 1; 0.278 -0.36 0.676];
c3 = [0 0.719 0.695];
dists = nan(size(locs, 1), 1); % Will contain the distances between C3 and the other electrodes F3, T7, Cz, CP3
for el = 1:size(locs, 1)
dists(el) = sqrt(sum((c3 - locs(el, :)).^2)); % equivalent to norm(c3-locs(el,:), 2)
end
weights = 1./dists; % Instead of scaling by the inverse of the distance, you could scale by the square of the inverse or some other law.
weights = weights ./ sum(weights);
fprintf('C3 - %f*F3 - %f*T7 - %f*Cz - %f*CP3\n', weights)
C3 - 0.286097*F3 - 0.264091*T7 - 0.264465*Cz - 0.185348*CP3
