Product Code Database
An afterimage, or after-image, is an image that continues to appear in the eyes after a period of exposure to the original image. An afterimage may be a normal phenomenon (physiological afterimage) or may be pathological (palinopsia). Illusory palinopsia may be a pathological exaggeration of physiological afterimages. Afterimages occur because Photochemistry activity in the retina continues even when the eyes are no longer experiencing the original stimulus.
The remainder of this article refers to physiological afterimages. A common physiological afterimage is the dim area that seems to float before one's human eye after briefly looking into a light source, such as a camera flash. Palinopsia is a common symptom of visual snow.
Normally, any image is moved over the retina by small eye movements known as before much adaptation can occur. However, if the image is very intense and brief, or if the image is large, or if the eye remains very steady, these small movements cannot keep the image on unadapted parts of the retina.
Afterimages can be seen when moving from a bright environment to a dim one, like walking indoors on a bright snowy day. They are accompanied by neural adaptation in the occipital lobe of the brain that function similar to color balance adjustments in photography. These adaptations attempt to keep vision consistent in dynamic lighting. Viewing a uniform background while adaptation is still occurring will allow an individual to see the afterimage because localized areas of vision are still being processed by the brain using adaptations that are no longer needed.
The Young-Helmholtz trichromatic theory of color vision postulated that there were three types of photoreceptors in the eye, each sensitive to a particular range of visible light: short-wavelength cones, medium-wavelength cones, and long-wavelength cones. Trichromatic theory, however, cannot explain all afterimage phenomena. Specifically, afterimages are the complementary hue of the adapting stimulus, and trichromatic theory fails to account for this fact. Originally published as:
The failure of trichromatic theory to account for afterimages indicates the need for an opponent process such as that articulated by Ewald Hering (1878) and further developed by Hurvich and Jameson (1957). The opponent process theory states that the human visual system interprets color information by processing signals from cones and rods in an antagonistic manner. The opponent color theory is that there are four opponent channels: red versus cyan, green vs magenta, blue versus yellow, and black versus white. Responses to one color of an opponent channel are antagonistic to those of the other color. Therefore, a green image will produce a magenta afterimage. The green color adapts the green channel, so they produce a weaker signal. Anything resulting in less green is interpreted as its paired primary color, which is magenta (an equal mixture of red and blue).
A stimulus which elicits a positive image will usually trigger a negative afterimage quickly via the adaptation process. To experience this phenomenon, one can look at a bright source of light and then look away to a dark area, such as by closing the eyes. At first one should see a fading positive afterimage, likely followed by a negative afterimage that may last for much longer. It is also possible to see afterimages of random objects that are not bright, only these last for a split second and go unnoticed by most people.
In this effect, an empty (white) shape is presented on a colored background for several seconds. When the background color disappears (becomes white), an illusionary color similar to the original background is perceived within the shape. The mechanism of the effect is still unclear, and may be produced by one or two of the following mechanisms:
==Gallery==
Positive afterimages
Positive afterimages, by contrast, appear the same color as the original image. They are often very brief, lasting less than half a second. The cause of positive afterimages is not well known, but possibly reflects persisting activity in the visual cortex when the photoreceptor cells continue to send neural impulses to the occipital lobe.
On empty shape
An afterimage in general is an optical illusion that refers to an image continuing to appear after exposure to the original image has ceased. Prolonged viewing of the colored patch induces an afterimage of the complementary color (for example, yellow color induces a bluish afterimage). The "afterimage on empty shape" effect is related to a class of effects referred to as .
See also
Notes
External links
|
|
