Time range analysis of evoked potential fields.

Potentials evoked by contrast reversing grating stimuli of different spatial frequency and orientation were recorded in 16 channels from twelve healthy adults. The amount of electrical brain activity was quantified independent of the reference electrode by the computation of global field power (GFP). Maxima of the GFP function over times, determined component latencies which turned out to be influenced by spatial frequency and orientation. Both effects were statistically significant. Analysis of GFP at component latencies demonstrated the significant influence of spatial frequency on the amount of activity in the potential fields, whereas different stimulus orientations yielded brain activity of similar strength. Component location on the scalp determined at P100 latency showed no systematic variation with spatial frequency or orientation of the grating pattern. A reference-free topographic segmentation procedure based on the statistical recognition of stable potential field configurations disregarding amplitude characteristics, was used for all subjects and stimulus conditions. Segments were identified over the whole recording epoch and were interpreted as time epochs with identical potential field configurations, that were compatible with identical neuronal generators. The sequence of such segments was further analysed and compared statistically between subjects and stimulus conditions. The results showed topographical differences that were not observed when only selected time points at component latency were considered.