Topographical evolution of spike-wave complexes.

Computer-generated 3-dimensional field potential maps of spike-wave complexes from two 4 X 4 electrode grids on the scalp were studied. A visual analysis of these field maps throughout the spike-wave evolution permitted quantification of the spike, trough and slow wave components in terms of distribution, origin and propagation. In addition, a more objective morphological analyzer also quantified the discriptive parameters of distribution, origin and propagation for the spike component of the patients' spike-wave complexes. We found that field distributions of spikes differed from that of waves. Succeeding positive troughs evolved more symmetrically than did spikes but less than ensuing negative waves. Negative waves were more diffuse, more symmetrical in evolution, and more posteriorly centred than either spikes or troughs. Unlike the troughs and slow waves whose fields tended to remain stationary during their evolution, spikes always moved from their points of origin. Spikes originated at the most lateral points of the grids and propagated laterally and anteriorly in one of two ways: a simultaneous origin at both lateral positions, then propagation toward the midline, and then usually anteriorly, a clearly unilateral origin with spread contralaterally to the homologous electrode position of the contralateral hemisphere followed again by anterior propagation. Interhemispheric lag times of spikes ranged from 0 to 25 ms with an average of 10.5 ms. Inter- and intrapatient variability was considerable. This type of analysis reveals properties of spike-wave complexes which may not be appreciated by standard paper writeout.