Afterimage

An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased. One of the most common afterimages is the bright glow that seems to float before one's eyes after staring at a light bulb or a headlight for a few seconds. The phenomenon of afterimages may be closely related to persistence of vision, which allows a rapid series of pictures to portray motion, which is the basis of animation and cinema.

Afterimages come in two forms, negative (inverted) and positive (retaining original color). The process behind positive afterimages is unknown, though thought to be related to neural adaptation. On the other hand, negative afterimages are a retinal phenomenon and are well understood.

Negative afterimages
Negative afterimages are caused when the eye's photoreceptors, primarily those known as cone cells, adapt from the over stimulation and lose sensitivity. Normally the eye deals with this problem by rapidly moving the eye small amounts (see: microsaccade), the motion later being "filtered out" so it is not noticeable. However if the color image is large enough that the small movements are not enough to change the color under one area of the retina, those cones will eventually tire or adapt and stop responding. The rod cells can also be affected by this.

When the eyes are then diverted to a blank space, the adapted photoreceptors send out little signal and those colors remain muted. However, the surrounding cones that were not being excited by that color are still "fresh", and send out a strong signal. The signal is exactly the same as if looking at the opposite color, which is how the brain interprets it.

Ewald Hering explained how the brain sees afterimages, in terms of three pairs of primary colors. This 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 suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white. Responses to one color of an opponent channel are antagonistic to those to the other color. Therefore, a green image will produce a red afterimage. The green color tires out the green photoreceptors, so they produce a weaker signal. Anything resulting in less green, is interpreted as its paired primary color, which is red.

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, and may not occur unless the stimulus is very bright. The cause of positive afterimages is not well known, but possibly reflects persisting activity in the visual system where the retinal photoreceptor cells continue to send neural impulses to the occipital lobe. , suggesting that the experience of a stimulus can vary with the intensity of the stimulus.

As in most circumstances only very bright stimuli such as the sun produce positive afterimages, and 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.

Medical conditions
In a visual disturbance called palinopsia, patients have an increased propensity for seeing afterimages, having both a reduced amount of time required to form an afterimage, and an increased duration of the afterimage. Positive afterimages are particularly noticeable, such that even routine eye movement is often accompanied by flickers of what the eye has scanned over (called "tracers"). However, increased negative afterimages are also experienced by palinopsia sufferers. It is unknown if the negative afterimages encountered in palinopsia are formed by the same process described above, although what little research that exists regarding the phenomena suggests that it is brain-related, and not eye-related. Palinopsia can be a persistent condition, but it is also experienced periodically by migraine sufferers.