Global field power and topographic similarity.

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.