[The dependence of phase inversion of the visual evoked potential during half-field stimulation on the stimulation target and the pattern size].

In 10 young and healthy subjects visual evoked potentials were recorded to monocular whole-field and half-field (12 degrees radius) stimulation, with and without central (3 degrees radius) and peripheral occlusion of the target. The records were made from a horizontal array of 3 occipital electrodes, one placed in the midline 5 cm above the inion and the others 8 cm each lateral of the inion. The early and main components of the VEPs were analyzed and compared. We found that the N2-Peak has its origin in the macular parts of the retina and the P2-peak in the central and peripheral parts of the visual field. Inverted polarity of the P2-peaks contralateral to the stimulated half-field could be observed only when the VEP was recorded with small checks. When using large checks, only the early components inverted. The probable origin of the inverted main components is discussed, being either a simple inverted P2-peak because of the direction of the cortical dipoles, or a new, paramacular N105-peak.