Brain Topography

, Volume 3, Issue 1, pp 137–141 | Cite as

Global field power and topographic similarity

  • Wolfgang Skrandies


Multichannel recordings are commonly presented as topographic maps series displaying the change of the potential distribution over time. When reviewing a sequence of potential maps it becomes obvious that there are epochs with only little activity (few field lines; small extrema values) while at other times the fields display high peaks and deep troughs with steep gradients. The measure of global field power (GFP) corresponds to the spatial standard deviation, and it quantifies the amount of activity at each time point in the field considering the data from all recording electrodes simultaneously resulting in a reference-independent descriptor of the potential field. Global field power is plotted as a function of time, and the occurrence times of GFP maxima are used to determine the latencies of evoked potential components. The topographical change occurring in subsequent potential field distributions may also be quantified by computing an index of global dissimilarity. Global field power and global dissimilarity show a complementary behavior over time: in general, high GFP is associated with similar fields while during periods between GFP peaks the topographic patterns of successive field distributions change rapidly accompanied by high dissimilarity values. The topographic changes, however, are best recognized by a segmentation procedure that considers field structure independent of GFP and global dissimilarity. The principles and practical applications of GFP computation, component latency determination and global dissimilarity of potential field distributions as well as a topographical time segmentation procedure will be illustrated with multichannel data evoked by visual stimuli.

Key words

GFP Global dissimilarity VEP topography Ssgmentation Human electrophysiology 


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  1. Brandeis, D. and Lehmann, D. Segments of event-related potential map series reveal landscape changes with visual attention and subjective contours. Electroenceph. Clin. Neurophysiol., 1989, 73: 507–519.PubMedGoogle Scholar
  2. Duffy, F.H., Bartels, P.H. and Burchfiel, J.L. Significance probability mapping: an aid in the topographical analyses of brain electrical activity. Electroenceph. Clin. Neurophysiol., 1981, 51: 455–462.PubMedGoogle Scholar
  3. Harner, R. and Riggio, S. Application of singular value decomposition to topographic analysis of flash evoked potentials. Brain Topography, 1989, 2, 91–98.PubMedGoogle Scholar
  4. Lehmann, D. and Skrandies, W. Reference-free identification of components of checkerboard-evoked multichannel potential fields. Electroenceph. Clin. Neurophysiol., 1980a, 48: 609–621.PubMedGoogle Scholar
  5. Lehmann, D. and Skrandies, W. Visually evoked scalp potential fields in hemiretinal stimulation. Docum. Ophthalmol., 1980b, 23, 237–243.Google Scholar
  6. Lehmann, D. and Skrandies, W. Time segmentation of evoked potentials (EPs) based on spatial scalp field configuration in multichannel recordings. Electroenceph. Clin. Neurophysiol., 1986, Suppl. 38: 27–29.Google Scholar
  7. Michael, D. and Houchin, J. Automatic EEG analysis: a segmentation procedure based on the autocorrelation function. Electroenceph. Clin. Neurophysiol., 1979, 46: 232–235.PubMedGoogle Scholar
  8. Skrandies, W. The upper and lower visual field of man: electrophysiological and functional differences. Progress in Sensory Physiology, 1987, 8: 1–93.Google Scholar
  9. Skrandies, W. Time range analysis of evoked potential fields. Brain Topography, 1988, 1: 107–116.PubMedGoogle Scholar
  10. Skrandies, W. Data reduction of multichannel fields: global field power and principal components. Brain Topography, 1989, 2: 73–80.PubMedGoogle Scholar
  11. Skrandies, W. and Lehmann, D. Occurrence time and scalp location of components of evoked EEG potential fields. In: W.M. Herrmann (Ed.), Electroencephalography in Drug Research, Fischer, Stuttgart, 1982a: 183–192.Google Scholar
  12. Skrandies, W. and Lehmann, D. Spatial principal components of multi-channel maps evoked by lateral visual half-field stimuli. Electroenceph. Clin. Neurophysiol., 1982b, 54: 662–667.PubMedGoogle Scholar
  13. Skrandies, W., Dodt, E., Kofmel, B.A. and Michel, Ch. Scalp potential field topography evoked by lateralized dynamic random-dot stereograms. Invest. Ophthalmol. Vis. Scie., 1989, Suppl. 30: 515.Google Scholar

Copyright information

© Human Sciences Press, Inc 1990

Authors and Affiliations

  • Wolfgang Skrandies
    • 1
  1. 1.Max-Planck-Institute for Physiological and Clinical ResearchBah NauheimF.R.G.

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