The global field power calculations are held in a temporary ’average’-like file that contains only one channel, the global field power. This file is normally automatically deleted after the plot has been displayed, but can be saved with the -save option.
Basically, the global field power
is the spatial standard deviation. So, for any given bin, the standard deviation
of the values across the channels is calculated at each point, and the
result is a waveform in time of the standard deviation of the values in
space for the entire epoch. The peaks in the waveform will indicate the
points in the epoch that have the greatest variability across channels.
A detailed description of the global field power calculation can be found
in the source code to gfp. The technique was presented in
"Lehmann, D., & Skrandies, W. (1980). Reference-free identification of components of checkerboard-evoked multichannel potential fields. Electroencephalography and Neurophysiology, 48, 609-621."
gfp xwin cclex12.avg 2 3
of the average file cclex12.avg
/usr/local/erp/lib/graphicdevs - list of available plot devices /usr/local/erp/lib/PLOTERPFILES/gfp - ploterps configuration file used by gfp
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