An afterimage is a type of optical illusion in which an image continues to appear briefly even after exposure to the actual image has ended. There are two major types of afterimages: positive afterimages and negative afterimages.
- "An afterimage can retain the colors of the original stimulus (positive afterimage,) or the colors might be reverse in the afterimage, like a photographic negative (negative afterimage). The conditions favoring the production of afterimages are either brief exposures to intense or very bright stimuli, in otherwise dark conditions (a quick glance at the setting sun), or prolonged exposures to colored stimuli in well-lighted conditions (fixating steadily on a colored object for 60 sec and then averting the eyes to a gray or white background). With brief stimuli the first afterimage is usually positive (same colors as the visual stimulus), and when only a single stimulus is presented, the positive afterimage is difficult to distinguish from the initial image or sensation."
(Gustav Levine & Stanley Parkinson, Experimental Methods in Psychology, 1994)
In a positive afterimage, the colors of the original image are maintained. Essentially, the afterimage looks the same as the original image. You can experience a positive afterimage yourself by staring at a very brightly lit scene for a period of time and then closing your eyes. For the briefest of moments, you will continue to "see" the original scene in the same colors and brightness.
The exact mechanisms behind positive afterimages are not well understood, although researchers believe that the phenomenon might be related to retinal inertia. The original image stimulates nerve impulses, and these impulses continue for a small window of time after you close your eyes or look away from the scene. The cells in the retina take some time to respond to light, and once the cells have been excited it takes some time for that response to cease. While positive afterimages happen quite frequently, we are generally unaware of them because they are so brief, often lasting as little as 500 milliseconds.
In a negative afterimage, the colors you see are inverted from the original image. For example, if you stare for a long time at a red image, you will see a green afterimage. The appearance of negative afterimages can be explained by the opponent process theory of color vision.
You can see an example of how the opponent-process works by staring at the red shamrock on the above right for about one minute before shifting your gaze immediately to a white sheet of paper or a blank screen.
Here's how it works:
- "After staring at the red and blue shamrock, you saw a green and yellow afterimage. Opponent-process theory proposes that as you stared at the red and blue shamrock, you were using the red and blue portions of the opponent-process cells. After a period of 60 to 90 seconds of continuous staring, you expended these cells' capacity to fire action potentials. In a sense, you temporarily "wore out" the red and blue portions of these cells. Then you looked at a blank sheet of white paper. Under normal conditions, the while light would excite all of the opponent-process cells. Recall that white light contains all colors of light. But, given the exhausted state of your opponent-process cells, only parts of them were capable of firing action potentials. In this example, the green and yellow parts of the cells were ready to fire. The light reflected off of the white paper could excite only the yellow and green parts of the cells, so you saw a green and yellow shamrock."
(Ellen Pastorino & Susann Doyle-Portillo, What Is Psychology? Essentials, 2010)
You can also see an example of negative afterimages at work in an interesting visual illusion in the negative photo illusion. In this illusion, your brain and visual system essentially create a negative of an already negative image, resulting in a realistic, full-color afterimage.