A physiological afterimage (or “ghost image”) is a concept in visual perception theory describing the phenomenon of an image remaining in one’s vision even after the source of the image is removed. A familiar example of an afterimage is the continuous bright light that one perceives after an intense flash – say, from a camera’s flashbulb. This is a positive afterimage – it appears in the same color as the original stimulus, although the stimulus in question is no longer present.
In contrast to a positive afterimage is a negative afterimage, a ghost image that results from photoreceptors within the eye losing visual acuity due to exhaustion or photoreceptors adapting to a particular color. Perception of an afterimage on an empty (colorless) shape is an example of a positive afterimage, whereas the color aftereffect illusion is a negative afterimage.
The most famous example of an afterimage appearing on an empty shape is an optical illusion by Yuval Barkan and Hedva Spitzer from Tel-Aviv University called the “color dove illusion.” Try staring at the black dot center of the colorless illustration of the dove while the background of the dove changes color. You should perceive the faint impression that the dove has changed color to one vaguely similar to the former background, although in reality the dove remains static and colorless.
This phenomenon occurs when the brain perceives of a complementary color within the bird that contrasts with the original background color, which appears white due to the color surrounding it. When the original color surrounding the bird is removed, the afterimage of the color in the bird remains.
The retina of the human eye is made up of four different photoreceptors, individually adapted cells that function for vision perception. Three of these photoreceptor types are three different types of cones, each representing a different color. Our perception of color is based on the opponency of these three colors – when something is “less red,” for example, our brain takes that as a cue that it must be “more blue.” Think of it as something like adding and subtracting paints from a mixture.
Consider the color dot illusion. In this illusion, if you fixate on the center cross while dots of three different colors alternate in quickly switching on and off, you should see the individual dots be replaced with dots of their opponent color.
This occurs because the cells of the eyes focused on that region of the image have adjusted to the visual of the color already appearing in the dot. When that color is removed, the brain cannot instantly adapt to the new grey color that has taken its place, and to substitute it the brain instead sees the dot’s opponent color.
In the case of the color dove illusion the brain is maintaining a color contrast, creating a positive afterimage. However, in the color dot illusion the brain is filling in a faulty color due to the photoreceptors “settling in” to one color perception.