Afterimage
As after-image of the eye phantom images are called, which are also still felt when the original light stimulus has subsided. They are most noticeable as bright spots after looking into the sun or a light bulb . However, they also appear after weaker light impressions, if you then close your eyes or palmate.
The physiology of perception speaks of an entoptic impression, which results as an after-effect of the retinal image after a stimulus pattern (object) has been fixed for a long time.
Positive afterimage
In positive afterimages - if one has looked into a bright light source, for example - the brightness and color values correspond to those of the stimulus pattern. The reaction outlasts the stimulus for a short time.
Negative afterimage
In the case of negative afterimages, these values are reversed: light becomes too dark, and the colors of the stimulus pattern appear in their complementary colors . This observation led to the opposite color theory .
Negative afterimages are rarely perceived in everyday conditions. Frequent, also unconscious changes in the direction of view, combined with a mostly fragmented environment, exposes the individual photoreceptors to stimuli that are constantly changing. In addition, the smallest, unnoticed eye movements of high frequency - as microsaccades and even finer than micro-tremors - accompany the visual process, which causes a similar effect of changing the stimuli per receptor and prevents local adaptation even when viewing an object for a long time .
An afterimage is all the more clear the larger the homogeneous surfaces of the object, the sharper the brightness and / or color contrasts on their contours, and the longer and more rigidly the object has been fixed. The last-mentioned condition expresses the extent to which it has been possible to reduce the effect of the involuntary drift movements and subtle micro-movements of the eyes and thus to keep the irritation of individual receptors almost constant for a few seconds, i.e. to bring about local adaptation.
Two simple tests
Anyone can create a light-dark afterimage on the following pair of images. If you first fix the point in the middle on the left for about 30 seconds and immediately afterwards the point on the right, the black square appears light on a dark background, while the small white square appears dark on the light square.
Negative afterimages of sensory receptors undergo not only changes in brightness and color, but also changes in shape - exactly the same as the sensory percepts themselves when these are exposed to conditions that increase or decrease stimulus. These changes are known as actual geneses or updates. Anyone can experience an actualse, for example, in the afterimage of the above black and white pattern. Usually a “sharp” afterimage is first experienced, which contains all four contours that meet at right angles. Then the shape of the inner square gradually dissolves, that is: its squareness, straightness and contours vanish, so that it is only experienced as a circular disk and thus as a formless "figure in its environment" . Then the percept “figure in the surrounding area” is also broken down, in that the hitherto "sharp" border between figure and surrounding area gradually dissolves, and the darkness of the pane mixes with the lightness of the large square, and thus finally the earlier great difference in brightness between the Figure "small square" and its / its environment, the large square disappears completely. Only then does the large square dissolve, which is itself also a figure in relation to its own environment: the frame of the picture.
If you fix the point on the left in the second pair of images for just as long and then switch to the point on the right, the word "RED" appears in pale red color. The color-sensitive cones react similarly to the rods, although the color impression changes. After prolonged fixing of a z. B. blue-green object, the cones have become less sensitive to blue and green perception in the corresponding area of the retina, while those for red are not. When you then look at a white surface, which consists of the additive color mixture of red, blue and green, the area in question is perceived as red.
particularities
The optokinetic nystagmus as a result of the observation of a moving stripe pattern can also be understood as a form of the afterimage. However, the effect here is not based on an adaptation of the retina, but is related to the processing of the visual stimuli in the brain .
While the afterimages continuously overlap in daylight and are hardly noticed, they can be disturbing during astronomical observations. If you z. B. looks up from the telescope and into a street lamp, the afterimage can persist for 10 to 20 seconds. If the view wanders over bright celestial bodies (such as the moon or large planets ), their contours can continue to act as light shadows for a few seconds. Even with some measurements in geodesy - for example on pole signals or church towers in the backlight - a short-term disruption of the targeting process is possible.
In connection with bright areas, the afterimage can also change its brightness. So appears z. B. a negative (dark) afterimage of a fluorescent tube when you close your eyes. However, if you also hold your hand in front of your eyes, it suddenly becomes positive. Something similar occurs when changing light colors into their complementary colors.
A related effect from sunlight can be used to relax the eyes. If you let the sun shine on your closed eyelids for a while, the thin skin creates a pleasantly warm, orange-red visual impression. If you cover your eyes with your palms after about a minute (" Palmieren ", English palm "palm"), it soon turns into an even blue. Switching between the two states has a calming effect that can be intensified by conscious breathing. The superimposition of the afterimages can also give the impression that the retina moves like a restless surface of water after intense sun exposure.
Afterimages in ophthalmology
In ophthalmology there are two areas in which the phenomenon of afterimages is exploited. On the one hand, they serve to check and determine the retinal correspondence relationships (see also binocular vision ), and on the other hand, they are used in pleoptic training measures for the treatment of amblyopia .
See also
Web links
- Afterimages via JavaScript
- Rainer Zwisler - Visuelle Phenomene (1998) [1] historical research on the phenomenon at zwisler.de
Individual evidence
- ↑ Max Schneider: Introduction to Human Physiology , Springer, Berlin / Hamburg / New York 1971 (16), p. 556