Afterimages are visual phenomena that occur after staring at an image for a prolonged period of time. They are the result of fatiguing certain color photoreceptors in the eye. Afterimages appear as an inverted or complementary color of the original image and provide evidence for the opponent process theory of color vision. The opponent process theory proposes that there are three opposing color channels in the visual system – red versus green, blue versus yellow, and black versus white. Afterimages demonstrate that fatiguing one color channel leads to perception of its opponent color. This article will explain what afterimages are, the physiology behind them, and how they support the opponent process theory.
What are Afterimages?
Afterimages, also known as palinopsia, are optical illusions that occur after viewing a strong visual stimulus and then looking away. They are caused by over-stimulating the photoreceptors for a particular color in the eye, which leads to temporary partial blindness in that color channel.
For example, staring at a bright green image for 30 seconds or longer and then looking at a white wall will produce an afterimage that appears pink or magenta. The reason this occurs is that the cones responsible for green light perception have become overworked and unable to function normally. Once you look away, those cones are unable to perceive the green components in the white wall, causing it to appear more pink.
Afterimages can last anywhere from a few seconds to several minutes before fading, depending on the intensity of the original image. They are usually negative in color, meaning they appear as the complementary color to the original image. For example, red will produce a green afterimage, blue a yellow one, and vice versa. However, afterimages can also be positive, appearing in the same color as the original image.
The Physiology Behind Afterimages
To understand how afterimages support the opponent process theory, it’s important to understand the physiology underlying their formation.
There are two types of photoreceptors in the retina responsible for color vision – rods and cones. The rods are responsible for low light and periphery vision. The cones on the other hand are concentrated in the fovea and central retina and are specialized for color perception.
There are three types of cones containing pigments that are maximally sensitive to short (blue), medium (green), and long (red) wavelengths of visible light. Fatiguing any one type of cone leads to an afterimage in the color that stimulates its opponent cone type.
For example, staring at a bright green image overstimulates the medium wavelength sensitive green cones. When you then look away at a neutral background, the fatigued green cones are unable to perceive the green components of that background. This causes an imbalance in signals being sent to the brain, with more red cone signals being sent relative to green. The brain thus perceives this imbalance as the opponent color to green, which is pink or magenta.
Opponent Process Theory
The opponent process theory of color vision proposes that there are three pairs of opponent colors – red versus green, blue versus yellow, and black versus white. The theory states that certain colors are never perceived together, but rather inhibit each other’s processing.
This theory helps explain various perceptual phenomena like afterimages. It posits that visual information is processed in two stages by the retina and lateral geniculate nucleus (LGN):
Stage 1: At the receptor level, information about wavelength and intensity is transduced and transmitted to the brain.
Stage 2: The neurons in the LGN compare and contrast signals from different photoreceptors in an antagonistic manner. This sets up opponent channels for processing color information.
For example, information from red and green cones is compared, and signals sent to red-green opponent neurons. One set of neurons increases firing to red and inhibits firing to green, while another population does the opposite.
Afterimages provide strong evidence for this opponent processing. Fatiguing one channel leads to perception of its opposite color because the equilibrium between the two channels is temporarily thrown off. The opponent theory elegantly accounts for this negative afterimage phenomenon.
Key Examples of Afterimages Demonstrating Opponent Process
Here are some key examples of afterimages and how they demonstrate the opponent process at work:
Red afterimage from green: Staring at an image of green for 30 seconds and then looking at a white wall causes an afterimage that appears red. This is because the red-green opponent neurons are thrown off balance. The green ones being fatigued leads to increased relative activity in the red channel.
Yellow afterimage from blue: Staring at bright blue then looking away produces a yellow afterimage. The blue cone receptors become fatigued, causing imbalance in blue-yellow channels and leading to perception of yellow.
Black afterimage from white: An all white stimulus leads to afterimages that appear black. This suggests the existence of black-white opponent neurons that work to maintain color constancy.
Reversed afterimages: After prolonged adaptation to a colored stimulus for 3-4 minutes, the afterimage can reverse to become positive instead of negative. This indicates there are multiple adaptation processes taking place as different neurons completely fatigue out.
| Original Stimulus | Resulting Afterimage | Opponent Process Demonstrated |
| Green image | Red afterimage | Red-Green opponent channels |
| Blue image | Yellow afterimage | Blue-Yellow opponent channels |
| White image | Black afterimage | Black-White opponent channels |
As seen from these examples, the negative afterimages provide evidence that there are opponent color processes in the visual system, just as theorized. Afterimages are a perceptual consequence of the fact that certain neurons are excited by some wavelengths and inhibited by others.
Neurophysiological Basis
Researchers have identified specific neurons in the LGN and visual cortex that demonstrate these opponent interactions. These provide a neurophysiological basis validating the theory:
– Type I neurons – ON-center cells that respond to red and OFF-surround cells that respond to green.
– Type II neurons – ON-center cells that respond to green and OFF-surround cells that respond to red.
– Type III neurons – ON-center cells that respond to blue and OFF-surround cells that respond to yellow.
– Type IV neurons – ON-center cells that respond to yellow and OFF-surround cells that respond to blue.
These center-surround neurons with opposite color responses provide clear neurophysiological evidence for opponent processing of visual information for color vision. Afterimages can be explained by the temporary saturation and unbalancing of signals from these populations of neurons.
Conclusion
Afterimages are visual illusions that appear as an inverted color of an original image after prolonged staring. They occur due to overstimulation and fatigue of photoreceptors for particular colors. Afterimages provide strong psychophysical evidence for the opponent process theory of color vision. This theory proposes that color information is processed by paired neurons that respond antagonistically to certain wavelengths. Afterimages result from temporary imbalance in the equilibrium between these opponent channels. Specific populations of center-surround neurons demonstrating these properties have been identified, providing neurophysiological validation of the theory. In summary, afterimages elegantly demonstrate the opponent interactions that underlie color perception.
