AfterImage

Afterimage is a non-specific term that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased. An afterimage may be a normal phenomenon (physiological afterimage) or may be pathological (palinopsia). Illusory palinopsia may be a pathological exaggeration of physiological afterimages. After images occur because photochemical activity in the retina continues even when you 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. Afterimages are 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 after image because localized areas of vision are still being processed by the brain using adaptations that are no longer needed.

"When all wavelengths stimulate the retinal region adapted to green light, the M and L cones contribute less to the resulting percept because their photopigments absorb less light than the S cones. Thus, trichromatic theory can not explain all afterimage phenomena, indicating the need for an opponent-process theory such as that articulated by Ewald Hering (1878) and further developed by Hurvich and Jameson (1957). Afterimages are the complementary hue of the adapting stimulus and trichromatic theory fails to account for this fact." (David T. Horner, Demonstrations of Color Perception and the Importance of Contours, Handbook for Teaching Introductory Psychology, Volume 2, page 217. Psychology Press, Texas, 2000)

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