Eyewear
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Visual after effects
Visual display terminals have the ability to add motion, fixed patterns and color combinations that may produce visual after-effects. The most common is the after-image. After viewing a surface of one
color for a period of time and then shifting to a neutral-colored surface, a faint complementary color may be seen briefly. This after-image is a product of the retina adapting to one set of color receptors
more than to another. Then, when looking at a neutral-colored surface, the adapted receptors may not respond as much as do the other color receptors, causing a slight shift in the perceived color of the
surface. Most, if not all, of these effects, however, are not specific to VDTs.
There are other visual after-effects that have been studied in an attempt to understand how the brain processes information. One is called the waterfall illusion and may be produced by a display
that has a scrolling capability. After viewing a pattern in motion, the pattern may appear to move in the opposite direction when the motion stops.
The McCollough (1965) effect, which may sometimes be of longer duration, may produce a similar type of response, but with color. For example, if after viewing a pattern of yellow bars on a black
background for a few minutes and you look away at some other object, no after-image may be seen.
If, however, you look at a similar pattern of white bars of the same angular width (i.e. spatial frequency) on a black background, the white bars may appear as pale blue. The effect becomes much
less pronounced or is absent if the size of the bars is different.
Like other visual after-images, the McCollough effect is physiologically harmless and is easily neutralized by any of a number of relatively simple procedures, such as viewing orthogonal gratings
of the same color (Hajos, 1967; Stroymeyer, 1969; Lockhead, 1983: Greenwald, Greenwald and Blake, 1983).
A motion after-effect may also be produced from a still pattern. Stare at the center of Figure 32 for a few seconds, then look at one of the dots at either side.
Figure 32: An after-image can be created by looking at the center of the figure and then glancing at one of the dots at either side (Davson, 1976).
Do this a few times and you should perceive a faint swirling pattern.
At one time, it was thought that there was a rather simple connection
from the retinal receptors through the optic nerve to the brain. Some
image processing, however, actually takes place at the retinal level
(Ratliff, 1972; Werblin, 1973). The visual system appears to be an
edge-detecting system that concentrates on the information-carrying
parts of the image rather than on uniform surfaces. This reduces the
amount of processing required to produce a mental image of what is
being viewed. Some portions of the visual system appear to be selectively
tuned to be sensitive to lines or edges of one orientation, to motion
or a specific direction of motion and so on. When one of these groups
is stimulated, it adapts slightly by becoming less sensitive. The groups
tuned to the opposite stimulus may then slightly over-respond under
appropriate conditions and produce a visual after-effect. These after-effects
are natural and happen constantly. Some situations, however, may make
them more apparent than others.
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