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The relationship between scalp potentials evoked by visual stimulation and neuronal functioning is examined. A neuronal population model for the electrogenesis of the evoked surface potentials is developed which relies on the hypothesis that the surface potentials are a combination of excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs) which occur both at different depths and at different latencies. The model is used to determine the location (depth) of the sources and the approximate size of the neuronal population responsible for the potential. The spatio-temporal potential fields evoked by visual stimulation and monitored by an array of scalp electrodes over the occipital cortex are discussed along with our method for displaying these fields via contour maps. The experimental data are computer simulated via discrete spatially disparate sources. The waveform of each of these sources is determined and the various parameters of the waveform are related to the major characteristics of the spatio-temporal visual evoked potential fields as well as to specific features of the underlying neuronal population. These features include, e.g., the sequence of neuronal excitation, the magnitude of excitatory current, the depth of the neuronal population, and the period of activity of the EPSP and IPSP.