An afterimage is a non-specific term that refers to an image continuing to
appear in one's vision after the exposure to the original image has
ceased. An afterimage may be a normal phenomenon or may be pathological.
Illusory palinopsia may be a pathological exaggeration of
physiological afterimages. The remainder of this article refers to physiological
afterimages. A common physiological afterimage is the dim area that seems to
float before one's eyes after briefly looking into a light source, such as a
camera flash. Negative afterimages
Negative afterimages are caused when the eye's photoreceptors, primarily known as
rods and cones, adapt to overstimulation and lose sensitivity. Newer evidence
suggests there is cortical contribution as well. Normally, the overstimulating
image is moved to a fresh area of the retina with small eye movements known as
microsaccades. However, if the image is large or the eye remains too steady,
these small movements are not enough to keep the image constantly moving to
fresh parts of the retina. The photoreceptors that are constantly
exposed to the same stimulus will eventually exhaust their supply of
photopigment, resulting in a decrease in signal to the brain. This phenomenon can
be seen when moving from a bright environment to a dim one, like walking
indoors on a bright snowy day. These effects are accompanied by neural
adaptations in the occipital lobe of the brain that function similar to color
balance adjustments in photography. These adaptations attempt to keep vision
consistent in dynamic lighting. Viewing a uniform background while these
adaptations are still occurring will allow an individual to see the after
image because localized areas of vision are still being processed by the brain
using adaptations that are no longer needed.
"When all wavelengths stimulate the retinal region adapted to green light,
the M and L cones contribute less to the resulting percept because their
photopigments absorb less light than the S cones. Thus, trichromatic theory can
not explain all afterimage phenomena, indicating the need for an
opponent-process theory such as that articulated by Ewald Hering and further
developed by Hurvich and Jameson. Afterimages are the complementary hue of
the adapting stimulus and trichromatic theory fails to account for this fact."
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 magenta afterimage. The green color fatigues the green
photoreceptors, so they produce a weaker signal. Anything resulting in less
green, is interpreted as its paired primary color, which is magenta.
Example movie which produces distortion illusion after one watches it and looks
away. See Motion aftereffect. 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. The
cause of positive afterimages is not well known, but possibly reflects
persisting activity in the brain when the retinal photoreceptor cells continue
to send neural impulses to the occipital lobe.
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. It is also possible to see afterimages of random
objects that are not bright, only these last for a split second and go unnoticed
by most people. Afterimage on empty shape
An afterimage in general is an optical illusion that refers to an image
continuing to appear after exposure to the original image has ceased. Prolonged
viewing of the colored patch induces an afterimage of the complementary color.
The "afterimage on empty shape" effect is related to a class of effects
referred to as contrast effects. In this effect, an empty shape is
presented on a colored background for several seconds. When the background
color disappears, an illusionary color similar to the original background is
perceived within the shape. The mechanism of the effect is still
unclear, and may be produced by one or two of the following mechanisms:
During the presentation of the empty shape on a colored background, the
colored background induces an illusory complementary color inside the empty
shape. After the disappearance of the colored background an afterimage of the
"induced color" might appear inside the "empty shape". Thus, the expected color
of the shape will be complementary to the "induced color", and therefore
similar to the color of the original background.
After the disappearance of the colored background, an afterimage of the
background is induced. This induced color has a complementary color to that
of the original background. It is possible that this background afterimage
induces simultaneous contrast on the "empty shape". Simultaneous contrast is
a psychophysical phenomenon of the change in the appearance of a color
caused by the presence of a surrounding average color.
Gallery See also
Palinopsia Book of Optics
Lilac chaser Motion aftereffect
Neural adaptation Persistence of vision
Screen burn-in Spectropia
Visual perception Notes
External links The Palinopsia Foundation is dedicated
to increasing awareness of palinopsia, to funding research into the causes,
prevention and treatments for palinopsia, and to advocating for the
needs of individuals with palinopsia and their families.
Eye On Vision Foundation raises money and awareness for persistent visual
conditions Afterimages, a small demonstration.
afterimage examples The Color Dove Illusion