Definition: What Are Afterimages?
Afterimages are common optical illusions created by your brain and visual system. In an afterimage, you continue to see an image even after your eyes’ exposure to the image has ended. Chances are that you have noticed this effect many times.
If you have ever stared at a fixed point for a long time and then finally shifted your gaze, you probably experienced an afterimage effect, continuing to see the original stimulus. This article will discuss two types of afterimages and the mechanisms in your brain that create them.
Different Types of Afterimages
Most afterimages fall into two categories, positive and negative. Positive afterimages retain the colors of the original image. Negative afterimages reverse the colors of the original image.
Laboratory experiments involving people staring at black and white images on paper have found that the creation of an afterimage results from phenomena both in the retina, the light-sensitive cells in the back of the eye, and in the brain.
Part of the process of generating an afterimage is attributable to a “retinal rebound effect.” The cones in the retina generate electrical signals in response to red, green, or blue light falling on them. When you fix your gaze on something and then suddenly look away, the electrical current from the cones doesn’t stop immediately.
However, your brain has the ability to “fill in the gaps” of visual images when the electrical signals from your retina are changing or unstable. Your brain turns visual signals from your retina into meaningful information, even if that information doesn’t match what your eyes actually see.
Just how much information your brain adds to the inputs from your retina depends on a variety of factors. Your alertness, your emotional state, your expectations, and your memories of similar objects are all called up, in just fractions of a second, to make an image from the fleeting electrical signals sent by your retina.
Moving afterimages can be even more vivid than reality. One of the first scientific descriptions of afterimages comes to us from a Scottish physician who described what he called “motion aftereffect” in 1834.
The afterimage Dr. Addams saw has been described by some as the Loch Ness monster, but Addams himself reached a more conservative conclusion:
During my recent tour of the Scottish Highlands, I visited the famed Falls of Foyers on the edge of Loch Ness. There I noticed the following phenomenon. After steadily gazing for a few seconds at an interesting part of the cascade, admiring the confluence and then the decussation of the boiling currents that formed the liquid drapery of water that create the falls to observe the vertical wall of worn rock immediately behind the falls, I saw the rocky wall as if it were moving upwards, moving with a velocity apparently equal to the speed of the descending torrents of water, which the moment before had trained my eye to behold this singular deception.
Cited by Peter Thompson and David Burr in the academic journal Cell
Dr. Addams referred to his experience as a motion aftereffect. Other observers looking closer to roiling waters at the base of the falls have seen Nessie the Loch Ness monster.
Common Sources of Afterimages
Afterimages can produce vivid, and incorrect, interpretations of the world around us. Most afterimages, however, are only an interesting optical illusion.
When you are not gazing at the Falls of Foyers or searching for monsters on Loch Ness, what triggers an afterimage?
- Brief exposure to a very bright image. This effect is strongest when the image has sharp edges, and when the area surrounding the image is much darker than the image itself. Two common examples of this effect are the afterimages from looking into oncoming bright headlights at night or glancing at the midday sun. Brief exposure to an intense light source often produces a positive afterimage.
- Prolonged exposure to a colorful image. Bright colors can produce an afterimage even if surrounding objects receive the same amount of lighting. Staring at a color photograph in a coffee table book for 60 seconds or so before turning the page or looking off into the distance can produce this kind of afterimage. This kind of prolonged exposure will usually produce a negative afterimage.
Positive Afterimages
As mentioned earlier, positive afterimages preserve the colors of the original image. A positive afterimage looks essentially the same as the original. It is easy to give yourself the experience of a positive afterimage. Just stare at a brightly lit scene – not so brightly lit that it can damage your eyes – for a full minute and then close your eyes or look away. For about half a second, you will continue to “see” the image.
You can also experience a positive afterimage by sitting in the dark for at least a minute, and then setting off a flash of bright light. Your visual experience of the surrounding room will continue even after the room goes dark again. If you are holding and moving an object as the light goes off, you will see your hand moving it even after the room has returned to darkness. Researchers Brian Stone and Jessica Tinker have confirmed that the brain integrates visual information with tactile information and proprioception of the location of the object to create a visual image even after there is no more light reaching your eyes.
Scientists believe that retinal inertia, the continued generation of electrical current in the rods and cones in the back of the eye, is the major reason for seeing a positive afterimage. There is also good reason to believe that tactile information goes into the brain’s recreation of the afterimage. Stone and Tinker found that their test subjects had afterimages of objects they dropped from their own hands when the light went off in a dark room, but they did not have afterimages of an object that fell from a mechanical hand when the light went off.
Auditory information does not appear to play a role in afterimages the way tactile information does. But the best current scientific findings support the idea that positive afterimages are actually a multisensory experience.
Negative Afterimages
Negative afterimages invert the colors of the original image. Red becomes green. Blue becomes yellow-orange. Green becomes red.
Here is an easy way to have the experience of a negative afterimage:
Draw a red shamrock with a blue border on a white sheet of paper. Stare at the image for 60 seconds and then shift your gaze to another unmarked white sheet of paper or a blank wall. You should see a green shamrock with a yellow border for about half a second.
Scientists explain this phenomenon with the opponent-process theory of color vision. Understanding the theory requires some additional information about three types of cones in the retina (long, medium, and short), and their different roles in color perception.
Cones are sensitive in bright-light conditions. They provide most of our daytime vision. There are three types of cones:
- Long cones have peak sensitivities around 560 nanometers, at red wavelengths. They respond to other wavelengths of light, but not as strongly.
- Medium cones have peak sensitivities around 530 nanometers, at green wavelengths. Like long cones, they respond to other wavelengths of light, but they produce the greatest electrical current in green light.
- Short cones have peak sensitivities around 420 nanometers, at blue wavelengths. They are maximally active when you are looking at a blue object, but they produce some current in any other kind of visible light.
The opponent-process theory of color vision explains negative afterimages as the contrast produced when a light source is suddenly removed. If you spend 60 seconds looking at a red object, for example, your long cones have been producing the strongest signals to be sent to the brain.
Your medium cones, however, haven’t been completely inactive. They also have been sending information to the brain. When stimulation of the long cones stops, the signals from the medium cones are stronger, so you briefly see a green afterimage.
It is also possible to see negative afterimages in an optical illusion known as the negative photo illusion. In this illusion, your brain creates a negative afterimage of a photo negative, giving you an image of the original full-color photo.
Who Is Susceptible to Afterimages?
Not everyone is equally likely to see afterimages.
People who are aware of how afterimages are formed, for example, are less likely to see them. People with generally slow reflexes are less likely to see afterimages, because slowly removing an image suppresses the formation of an afterimage. People seem to be primed to see, or not see, afterimages on the basis of whether they perceive objects as hard, soft, or squishy.
There are no significant differences between males and females or children and adults in how often people see negative and positive afterimages, but there are some people who see afterimages all the time.
People who have a condition called palinopsia see positive afterimages of objects immediately after looking away, and sometimes for several seconds afterward. They may see a series of images following a moving object, or a streak of images around a bright image on a dark background.
