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Afterimages are a class of visual phenomena in which some aspect of an image persists after the visual stimulus for the image ceases. In simpler words, we continue to see something that is not there anymore. When the afterimages are strong and persistent enough to be considered a medical problem, they are called palinopsia. There are basically two types of true afterimage, negative and positive.
The positive afterimage is like a positive photograph; it has the same coloration as the original image. The familiar example is when you happen to be staring at a flashbulb when it goes off and the intense white image remains even when you close your eyes or look away. Positive afterimages are usually very brief and often not even noticed. As for all afterimages, how long the effect lasts depends on the strength of the light and the length of exposure. Positive afterimages are caused by the slowness of the chemical reaction in the cells of the retina that allows those cells to sense light (see rhodopsin), a kind of hysteresis effect. Because of that property, the effect of light stimulation lingers ever so briefly after the stimulus has been removed.
The negative afterimage is like a negative photograph, where the complementary colors are reversed so that red becomes green and green becomes red, blue becomes yellow and yellow becomes blue, black becomes white, and white becomes black. In the flashbulb example, the positive afterimage of a white spot will be quickly replaced by a negative afterimage of a black spot. The negative afterimage usually persists longer than the fleeting positive afterimage, but it also depends on the intensity and length of the stimulus light.
The explanation for negative afterimages is somewhat complex, but the simplest and most direct factor is a desensitization of the cells of the retina. What makes those cells sensitive to light is a chemical called rhodopsin. A cell's supply of rhodopsin is gradually depleted by chemical reaction during exposure to light. The brighter the light, the faster the chemical is used up. So if a certain part of your retina is exposed to bright light, by staring at a white disk on a black background for example, that part of the retina will become less sensitive to light. If you then move your gaze to a white surface, you will see a negative afterimage in the form of a relatively dark disk. That happens because the cells within the disk image on the retina are now less sensitive to light than those outside the disk image, and so they don't respond as well to the same level of light and you see the dark disk, even though the white background is uniform in brightness (luminance).
If you stare at a red spot, however, you will see a greenish negative afterimage against the white background. This reversed color effect comes from a similar desensitization of the cone cells that sense colors. Exposure to red light desensitizes the cells that detect that color, but it doesn't affect the cells that are sensitive to green so much.
There is more to it than that, however.There are important neurological mechanisms at work in negative afterimages as well. Humans have three kinds of cone cells, which are respectively sensitive to red, green and blue light, but the output of those cone cells enters a layer of neural processing in the retina which results in what is called color opponency. The output of that processing is information about color differences, specifically about red vs. green and blue vs. yellow. Because of that processing, a desensitization of red cones will result in a stronger green signal being sent to the brain. Thus a red stimulus produces a green afterimage. A nice illustration of the complementary color effect in an afterimage is the old American flag illusion, where you stare an image of Old Glory in reversed colors for a while and then look at white surface to see the image in normal color.
Afterimages are also caused by drugs and damage to nerve in the optical pathway and certain areas of the brain.
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