Afterimage

An afterimage is an optical illusion that occurs after looking away from a direct gaze at an image. This is closely related to the phenomenon called the persistence of vision, which is used in animation and cinema. 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.

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, 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 and stop responding. The rod cells can also be affected by this.

When the eyes are then diverted to a blank space, the tired 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. For example, 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.

To see an afterimage of the picture below, stare at it for thirty seconds, then immediately look at a blank wall or piece of white paper to see the afterimage. In the afterimage, the colors of the United States flag will be corrected.



In a visual disturbance called palinopsia, patients have an increased propensity for seeing afterimages, but color inversion does not necessarily take place. Palinopsia can be a persistent condition, but it is also experienced periodically by migraine sufferers.