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 photochemical 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 eyes after briefly looking into a light source, such as a camera flash. Palinopsia is a common symptom of visual snow.
Negative afterimages are caused when the eye’s photoreceptors, primarily known as rods and cones, adapt to overstimulation and lose sensitivity. Newer evidence suggests there is cortical contribution as well. Normally, the overstimulating image is moved to a fresh area of the retina with small eye movements known as microsaccades. However, if the image is large or the eye remains too steady, these small movements are not enough to keep the image constantly moving to fresh parts of the retina. The photoreceptors that are constantly exposed to the same stimulus will eventually exhaust their supply of photopigment, resulting in a decrease in signal to the brain. This phenomenon can be seen when moving from a bright environment to a dim one, like walking indoors on a bright snowy day. These effects are accompanied by neural adaptations 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 these adaptations are 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.
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 brain when the retinal photoreceptor cells continue to send neural impulses to the occipital lobe.
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.