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gfp - calculate and plot the global field power for ERP data


gfp plot_device avgfile bin [bin2 bin3 ...] [-save]


gfp is a program to calculate and plot the global field power for the electrical potential field on the scalp. The calculation is performed on the data contained in the ERP average file avgfile, and the results for the specified bin(s) are plotted to the device specified by plot_device. The available plot_devices are listed in the file /usr/local/erp/lib/graphicdevs. At least one bin must be specified.

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."


Save the temporary file that contains the global field power calculations, which is normally automatically deleted after the plot has been displayed. This file is similar to an average file, and contains only one channel of data, the global field power calculations. It may be desirable to save this file when the user wishes to create custom plots of the gfp waveforms. When the -save option has been specified, the program will indicate on exit the name of the temporary file that has been saved.


gfp xwin cclex12.avg 2 3 
Plot in an x-window the global field power calculations for bins 2 and 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

See Also



Paul Krewski

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