What Is an Afterimage?

An eye seeing an afterimage
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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.

Types of Afterimages

In some instances, the colors of the original stimulus are retained. This is known as a positive afterimage. In other cases, the colors may be reversed.

This is known as a negative afterimage.

Research has shown that there are a number of situations that increase the likelihood of experiencing an afterimage:

  • Brief exposure to a very bright stimulus, particularly when the surrounding conditions are much darker that the stimulus. Glancing at the bright midday sun or the glare of bright headlights at night are two instances that might produce this type of afterimage. This brief exposure to an intense source often produces a positive afterimage.
  • Prolonged exposure to a colored stimulus, even if the surrounding conditions are equally well-lit. Staring at an image in a book for 60 seconds or so before turning to stare at a blank, light-colored wall can produce this type of afterimage. This prolonged exposure to a colored stimulus often results in a negative afterimage.

Positive Afterimages

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.

Negative Afterimages

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 opening this image of a red shamrock outlined in blue in a separate window.

Stare at the image for about one minute before shifting your gaze immediately to a white sheet of paper or a blank screen.

How exactly does this process work?

After staring at the shamrock, you probably experienced a green and yellow afterimage for a very brief moment of time. According to the opponent-process theory of color vision, staring at the original red and blue image involved using the red and blue parts of the opponent-process cells. After that minute of extended staring, the ability of these cells to fire action potential was exhausted. In others words, you basically wore out those red-blue cells.

When you shifted your focus to a blank, white screen, those cells were still unable to fire and only the green/yellow opponent process cells continued to fire action potentials. Since the light reflecting off your screen could only activate those green and yellow cells, you experienced a brief afterimage in green and yellow rather than in red and blue.

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.

References

Levin, G., & Parkinson, S. (1994). Experimental Methods in Psychology. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc.

Pastorino, E., & Doyle-Portillo, S. (2013). What is psychology? Essentials, (2nd ed.). Belmont, CA: Wadsworth, Cengage Learning.

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