Brain mapping of visual evoked activity--topographical and functional components.

Electroencephalographic (EEG) and evoked activity can be recorded non-invasively in order to monitor human brain activation online. Electrical fields are generated by large intracranial neural populations and spread to the scalp through volume conduction. All measured signals depend on the location of the recording and reference electrodes. Simultaneous recordings from many scalp positions allow for a topographical assessment of the complete electrical fields of the brain and avoid problems typically seen with time series analysis. The present contribution will illustrate the fundamentals of topographic mapping of human electrophysiological brain activity as well as quantitative analysis methods. Visual evoked potential data obtained in a group of 12 healthy adults are presented. We will focus on the definition and identification of evoked components that represent steps in visual information processing. In addition, the application of statistical data reduction techniques are described, and the results of functional components which are related to experimental variations are discussed.