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

Summary 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° × 1/2° scanning pattern element which explored a 2° × 2° area of the visual field. Single scalp locations had visual receptive fields of the order of 1°–2° 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° × 1/2° stimulus, averages of only a few tens of sweeps gave adequate signal-to-noise ratios.