The electrical activity of the brain can be measured by placing electrodes on the scalp - a method call electroencephalography (EEG). A generic set of the coordinate positions of the electrodes were provided by the Psychology Department. To calculate a potential field over the skin surface, coordinates of virtual electrodes on the MRI head model are determined. The measured electrode positions have been orthogonally projected onto the outer surface of the model.

The UNEMAP signal eeg.signal is created using the program posdata2sig, to translate the EEG signal data produced by the Psychology Department into a format compatible with CMISS. The ouput for postdat2sig is given as

% posdat2sig
Position file name ? dataview.pos
129 positions
Data file name ? alldiff.dat
300 samples
Sampling frequency ? 1000
Signal file name ? eeg.signal
Created signal file: eeg.signal

where dataview.pos contains the 3-dimensional cartesian co-ordinates of the electrodes and alldiff.dat contains the signal. The input files and all relevant documentation can be obtained from here.

Shown below are the projections of the electrodes onto the torso surface. The red crosses are the measured electrode positions and the blue crosses represent the orthoganol projections on the model surface. The green lines show the error between the two distances. Also shown are the potential fields.

Front View Back View Front View with Field Back View with Field

A movie sequence can been seen in CMGUI by running the comfile view_potential.com. This also calls the comfile animate.com as a looping comfile to generate a sequence of frames for a movie. The electrode projections can be viewed by running the CMGUI comfile view_electrodes.com