Abstract
Visual afterimages were initially thought to be solely caused by bleaching of photochemical pigments or neural adaptation in the retina. A number of recent results, however, require a contribution of cortical processing to after-image perception. Here, evidence is reported that afterimages are modulated by attention after the stage of visual adaptation. Observers adapted to overlapping colored, transparent horizontal and vertical bars. Upon offset of the adapting pattern, observers experienced an after-image that spontaneously alternates between three possible perceptual states before completely fading: 1. Only the vertical component after-image of the vertical-horizontal bar adapting stimulus; 2. Only the horizontal component of the after-image; or 3. The after-image of both horizontal and vertical components. That 1 and 2 dominate and alternate, as in cases of monocular rivalry, suggests that the after-image itself undergoes perceptual rivalry. If, upon offset of the adapting stimulus, two overlapping outline rectangles are presented, corresponding to the boundaries of the vertical and horizontal components of the adapting stimulus, an endogenous attentional influence on the perception of after-images is observed. When observers attended to either the horizontal or vertical outline rectangle of an overlapping horizontal-vertical rectangle pair of the same size as the adapting stimulus, they tended to report seeing an afterimage of horizontal bars if they attended to the horizontal outline rectangle, and reported seeing an afterimage of vertical bars if they attended to the vertical outline rectangle. Flashing either the vertical or horizontal outline rectangle of an overlapping horizontal/vertical outline rectangle pair modulated the visibility of either the vertical or horizontal component of the after-image in an analogous manipulation of exogenous attention. Because attention is a cortical process, this provides further evidence that there is a cortical contribution to afterimage formation.