Electrophysiological correlates of positive and negative afterimages

We report here measurements of visual evoked potential (VEP) activity after induction of afterimages in human observers. Drifting gratings normally produce no measurable phase synchronized VEP. However, after an afterimage of a high contrast grating of the same orientation and spatial frequency of that of the drifting grating has been flashed on the retina. VEPs in synchrony with the drift frequency are produced. For up to about two minutes after the flash, the VEPs were all clustered in one phase, then declined for a minute or so, to reappear 180 degrees out of phase from the first VEPs. The first group of VEPs coincides with the percept of a positive afterimage, and the second with that of a negative afterimage. Possible explanations for the existence of positive and negative afterimages and the associated VEPs are considered.