Afterimage

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For other uses, see Afterimage (disambiguation).

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.

If the viewer stares at this image for 20-60 seconds and stares at a white object a negative afterimage will appear (in this case being blue on maroon). This can also be achieved by the viewer closing his/her eyes and tilting their head up.

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.

[edit] 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.^[1] Normally the eye deals with this problem by rapidly moving small amounts (see: microsaccade), the motion later being "filtered out" so it is not noticeable. However if the colour 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 a weak signal and those colors remain muted. However, the surrounding cones that were not being excited by that colour 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 colour of an opponent channel are antagonistic to those to the other colour. 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 colour, which is purple.

Example movie which produces distortion illusion after one watches it and looks away. See Motion aftereffect.

[edit] Afterimage on empty shape

Afterimage on empty shape is a type of afterimage which result in "positive" afterimage. After viewing a colored background that contains a white shape (such as the color dove illusion^[2]).When the background color disappears (becomes white) an illusionary color, similar to the original background is perceived within the shape.

[edit] 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.