Afterimage

As an after-image of the eye phantom images are referred to, which are also still felt when the original light stimulus has subsided. The clearest they are as bright spots after looking at the sun or a light bulb. However, they also appear after weaker light impressions, if you then close your eyes or palmiert.

The perception physiology speaks of a entoptic impression that results in the aftermath of the retinal image after a stimulus pattern (object) has been fixed a long time.

Positive afterimage

In positive after-images - if you have, for example, looking into a bright light source - correspond to the brightness and color values ​​to those of the stimulus pattern. The reaction outlasted the stimulus for a short time.

A negative afterimage

At negative afterimages, these values ​​are reversed: Hell is too dark and the colors of the stimulus pattern appear in their complementary colors. This observation led to the opponent color theory.

Under everyday conditions, negative afterimages are rarely noticed. Through frequent, also unconscious, change of perspective, combined with a mostly fragmented structured environment, the individual photoreceptors are exposed to stimuli that are constantly changing. Besides accompany smallest unnoticed eye movements of high frequency - as microsaccades and even finer than micro- tremor - the process of seeing what even after prolonged viewing an object prevents a similar effect of the change of stimuli per receptor related and a local adaptation.

An afterimage is more evident the larger the homogeneous surfaces of the object, the sharper the brightness and / or color contrasts along the contour are, and the longer and the more rigid the object has been fixed. The latter condition expresses the extent to which it has been possible to reduce the effect of drift involuntary movements and feinschlägigen micro-movements of the eyes and so to keep the irritation of individual receptors for a few seconds or less constant, so bring a local adaptation.

Two simple tests

The next pair of images, anyone can produce a light-dark afterimage. When you first left the fixed point at the center for about 30 seconds and then immediately to the right point, the black square appears bright on a dark background, while the small white square appears dark on the bright square.

Negative afterimages of sensory receptors do not only brightness and color changes by, but also shape changes - exactly the same as the Sinnesperzepte even if these lovely or arousal enhancing or - diminishing conditions are exposed. These changes are known as actual genesis or Aktuallysen. A Aktuallyse anyone can experience, for example, the afterimage of the above black and white pattern. Usually a is first experienced " sharp " after-image, which contains all four right angles abutting contours. Then gradually dissolves the shape of the internal square, that is: it disappear its squareness, straightness and contour - tetrad, so that it is seen only as a circular disk, and thus as an informal "figure in their environment ." Then the percept is " figure in the environment " mined by the usual " sharp " boundary between figure and environment gradually dissolves, and the darkness of the disc mixes with the Hellen the large square, and so eventually the former big difference in brightness between the figure " small square " and his / her environment, the large square disappears. Only after that also the large square dissolves, which is also a figure in terms of his own environment itself: the frame of the picture.

Fixed to the left point in the second image pair for just as long, and then switches to the right, the word " RED " appears in pale red color. The color-sensitive cones respond similarly to the rods, which, however, changes the color impression. After prolonged fixation of eg a blue-green object, the pivot for the blue and green awareness have become tougher within the corresponding area of ​​the retina that are not for red. During the subsequent look at a white surface, which consists of the additive color mixture of red, blue and green, hence the area in question is perceived as red.

Special

The optokinetic nystagmus as a result of viewing a moving fringe pattern can also be understood as a form of the afterimage. However, here, the effect is based not on an adaptation of the retina, but is related to the processing of the visual stimuli in the brain.

As will superimpose the afterimages constantly and hardly noticed in daylight, they can be disruptive in astronomical observations. If, for example, from the telescope up and looking into a street lamp, the afterimage may persist for 10 to 20 seconds. Glancing bright celestial body (such as the moon or large planets), their contours can linger for a few seconds as a slight shadow. Even with some measurements in geodesy - about to rod signals or church towers against the light - is possible a short-term disruption of the target process.

In connection with bright surfaces, the residual image may also change its brightness. So, for example, appears a negative ( dark ) afterimage of a fluorescent tube, if you close your eyes. If one bears but also the hand in front of your eyes, it suddenly becomes positive. The same occurs when switchings of bright colors in their complementary colors.

A related effect of sunlight you can use to relax the eyes. If you leave some time the sun shine on the closed eyelids, caused by the thin skin stays warmer, orange - red visual impression. One discovers after about a minute the eyes with the palms from ( " palming ", engl. Palm ' palm ' ), it soon turns into a steady blue. The change between the two states has a calming effect that can be enhanced by a conscious breathing yet. The superposition of the afterimages may also give the impression that the retina moves like a restless water surface after intensive tanning.

Afterimages in ophthalmology

In ophthalmology, there are two areas in which one makes the phenomenon of afterimages advantage. Firstly, they are used to test and determination of retinal correspondence relations ( see also binocular vision ), on the other hand they are used in pleoptischen training for the treatment of amblyopia.