Source-density mapping of human visual receptive fields using scalp electrodes

A triangular array of 20 electrodes spaced by 1.5 cm was used to record the distribution of current source-densities at 12 locations over the occipital scalp, in response to tachistoscopic presentation of a 1/2 degrees X 1/2 degrees scanning pattern element which explored a 2 degrees X 2 degrees area of the visual field. Single scalp locations had visual receptive fields of the order of 1 degrees-2 degrees in diameter, their shape varying somewhat according to the response in question. Source density analysis can resolve details of the AEP scalp map to better than 1 cm. Even with a 1/2 degrees X 1/2 degrees stimulus, averages of only a few tens of sweeps gave adequate signal-to-noise ratios.