Long-lasting afterimages caused by neural adaptation

Abstract

If the retinal location of a sinusoidal grating alternates back and forth in half-period steps, the temporal average stimulation is equivalent to that produced by a homogeneous field of light. Adaptation to this stimulus causes strong patterned afterimages, however, and they last longer than 2 min in the dark. The afterimages appear as gratings but at twice the spatial frequency of the adaptation gratings even though only the fundamental frequency is seen during adaptation. The appearance of this illusory grating afterimage indicates that a nonlinear intensity scaling precedes the site of some afterimages, the origin of which is neural and not photochemical. The neural origin was confirmed by the results of various experiments in which the eye was pressure blinded during adaptation. Blinding eliminated all negative afterimages otherwise caused by weakly bleaching primary stimuli but left positive afterimages intact. Two classes of long-lasting afterimages, the “sensitivity images” and “bleaching images”, are required to explain our results.