How are brightness and contrast perceived?
Brightness is the perception of intensity of light. Roughly, the more intense a light is, the brighter it seems to be. Intensity refers to the physical energy of light, as measured by a photometer. Brightness, however, is a perceptual phenomenon: it cannot be measured by physical instruments. It is a basic perception, difficult if not impossible to describe; it must be experienced. Measurements of brightness are generally observers’ reports of their experience viewing lights of different intensities. Only in living systems—that is, only in the eye of the perceiver—is the term “brightness” relevant.
The brightness of a spot of light, although related to the intensity of light reflected from that spot, is also influenced by other factors. It varies with the intensity of light reflected from the immediately surrounding area at any given time and at immediately preceding times. In general, a spot appears brighter if the surrounding areas are dark or are stimulated with light perceived as complementary in color; it also appears brighter if the eye has become accustomed to the dark (dark-adapted). These factors contribute contrast, the perception of differences in light intensity, which enhances brightness. Brightness and contrast are perceptually linked.
A light of a given physical intensity may appear quite bright when viewed with an eye that has been dark-adapted, perhaps by being covered for ten to fifteen minutes. That same light may seem dim in comparison to an eye exposed to bright light for the same time period. This is largely attributable to the fact that a dark-adapted eye has more photopigment available to respond to incoming light; when this pigment has been exposed, it becomes bleached and needs time to regenerate. The enhancement of differences in brightness by an adapting light or other stimulus preceding the test light is called successive contrast and is primarily attributable to the state of adaptation of the retina.
Simultaneous contrast can also affect brightness perception. In this case, a spot of light at one place on the retina can be made to appear brighter or dimmer depending only on changes in the lighting of adjacent retinal locations. A small gray paper square placed on a sheet of black paper appears brighter than an identical square on a sheet of white paper. This is mostly a result of lateral inhibition, or photoreceptors stimulated by the white background inhibiting the receptors stimulated by the square so it appears less dazzling on white than on black. In general, differences are enhanced when the stimuli are side by side.
Sensitivity to contrast also varies with the detail of the object being viewed. Reading a book involves attending to high spatial frequencies, closely spaced lines, and minute detail. Recognizing a friend across the room or finding one’s car in a parking lot involves attention to much broader spatial frequencies; that is, the lines important for recognition are much farther apart. The visual system handles low, moderate, and high spatial frequencies, although not equally well. A contrast sensitivity function may be plotted to show which spatial frequencies are most easily detected—that is, to which frequencies the eye-brain system is most sensitive.
The peak of this function, the highest sensitivity to spatial frequency, is within the midrange of detectable frequencies. At this peak, it takes less physical contrast (a smaller intensity difference) for an observer to report seeing the border between areas of different frequency. At higher and lower spatial frequencies, sensitivity drops off, so greater intensity differences must be made for perception in those ranges.
While perceptual systems exaggerate physical contrast, they fail to notice lack of contrast, change, or movement. Changes in brightness, for example, can be made so gradually that no notice of them is taken at all. In fact, the visual system, which signals changes well, does not respond to seemingly constant stimulation. When an image, a bright pattern of light projected on the retina, is stabilized so it does not move at all, the observer reports first seeing the image and then, in a few seconds, its fading from view. The field does not turn gray or black or become empty; it simply ceases to exist. A border circumscribing a pattern within another pattern, perhaps a red-filled circle within a green-filled one, may be stabilized on the retina. In this case, the inner border disappears completely: The observer continues to see an unstabilized green-filled circle with no pattern in it. The area that formerly appeared red—and which indeed does reflect long-wavelength light—is perceived only as a part of the homogeneous green circle. Thus, while borders and movement creating physical contrast are exaggerated in perception, a stimulus signaling no change at all is simply not perceived.
Brightness and contrast are especially well illustrated in color perception. In the retina, three different cone pigments mediate color perception. Each pigment maximally absorbs light of certain wavelengths: One maximally absorbs the short lengths that are perceived as blue, one the medium wavelengths perceived as green, and one the red or long-wavelength region of the spectrum. The outputs of the cones interact with one another in the visual system in such a way that reds and greens stand in opposite or complementary roles, as do blues and yellows, and black and white. A gray square reflects light of all wavelengths equally. It has no hue, or color. Yet when it is placed on a red background, it appears greenish; if placed on a blue background, that same gray square appears yellowish. In each case, the neutral square moves toward the complement of its background color. The background has induced the perception of hue, tinting the gray with the color of its complement. Brightness of the background can also affect hue. A royal-blue square against a moderately white background can appear deep navy when the background intensity is increased or seem to be a powder blue when it is decreased. The same color in two different settings or under two different brightness conditions is not the same color.
The appearance of color is not a simple property of the color pigment itself but is defined in its relationship to others. Simultaneous color contrast can be quite startling, depending on the color relationships chosen. For example, if two squares of different hues but the same brightness are juxtaposed, colors appear very strong and exaggerated. One’s attention goes immediately to the contrast. If they are complementary colors such as red and green, the contrast is heightened. If they are close to the complements of each other, they are perceived in the direction of complementarity.
Yet not all colors are contrasting. A color configuration that does not move toward contrast moves toward assimilation—toward being united with the major color present. For example, a painting’s central blue feature may bring out subtly blue features elsewhere in the painting. Whenever colors show enough similarity to one another, they approach one another, emphasizing similarity rather than contrast. Both color contrast and assimilation are beautifully illustrated in Josef Albers’s book Interaction of Color (1987).
Another visual demonstration of brightness effects is the Pulfrich pendulum effect, or the Pulfrich phenomenon. To observe this, tie a pendulum bob to a two-foot length of string. Swing this in a plane normal to the line of sight, moving it back and forth as a pendulum. Then observe this continuing motion while wearing glasses, one lens of which is darkened or covered with a sunglass cover. Suddenly the pendulum appears to move in an ellipse instead of an arc. This illusion is a brightness effect. The shaded or sunglass-covered eye does not receive as much light as the other eye at any given time. It takes this eye longer to integrate the light information it does receive and so, by the time it sends location information to the brain, the other eye is sending its information of another location. The brain interprets disparity, this difference in the locations, as depth. Therefore the pendulum appears to move closer and farther away from the observer in elliptical depth and not constantly in a single plane. Intriguingly, switching the covered eye changes the elliptical path from clockwise rotation to counterclockwise or vice versa.
The Pulfrich phenomenon is a demonstration of changes in perception with changes in brightness; such changes have very practical effects. Driving at dusk, for example, can be dangerous, because light levels are suddenly lower than expected. Although the eye gathers the available light for form, distance, and depth perception, it takes a longer period of time to do so. Unaware of this, a driver may find reaction time to be longer than in the middle of the day and not allow enough braking distance. Similarly, an umpire may halt an evening soccer game earlier than the spectators think is necessary because of low light levels. The spectators can see well enough, as they gather the light needed to perceive what is happening. The players, on the other hand, notice that their reaction times are extended and that they are having trouble localizing the ball.
For a third application, the fact that contrast sensitivity shows peaks in particular spatial frequencies bears explanatory if not practical value. Robert Sekule, Lucinda Hutman, and Cynthia Owsley showed, in a 1980 study, that as one grows older, sensitivity to low spatial frequencies decreases. This may partly explain why older people may show greater difficulty recognizing faces or locating an automobile than the young, even though the two groups may be equally able to discriminate fine structural details. Making an older person aware of this change in sensitivity may be of assistance in defining the difficulty and in providing assurance that this is not a memory problem or a sign of decreasing cognitive ability.
In the late nineteenth century, much of the early development of psychology as a science came about through work in sensation and perception. As empirical evidence grew, theories of contrast perception took shape. Two of the most notable are those of Hermann von Helmholtz and Ewald Hering.
Helmholtz had a psychological theory—a cognitive theory that explained color and brightness changes with contrast as errors in judgment. Errors were attributed to lack of practice in making brightness judgments, not in any physiological change in the neural input. Something suddenly looked brighter simply because it was misinterpreted, probably because one was focusing on some other aspect.
At the same time, Hering insisted and provided convincing demonstrations that contrast involves no error in judgment but has a physiological base. The neural response of any region of the retina, he argued, is a function not only of that region but also of neighboring regions. These neighboring sensations were postulated as having an effect opposite in brightness, or in the complementary color, of the region being viewed. Hering showed with successive contrast and simultaneous contrast studies that the outputs of different places on the retina could be modified by one another.
In 1890, William James described this controversy and gave, in The Principles of Psychology , his support to Hering’s physiological position. With some modifications, it may be supported today. Yet the Helmholtz theory has some supportive evidence as well. For example, John Delk and Samuel Fillenbaum showed in 1956 that an object’s characteristic color influences an observer’s perception of that object’s color. In this way, for example, an apple cut out of red paper is identified as redder than it actually is. This line of evidence would support Helmholtz in his theory of errors in judgment.
Almost any modern consideration of contrast includes a discussion of brightness changes at borders, commonly called Mach bands. This dates to 1865, when Ernst Mach, an Austrian physicist, described borders as places where differences in brightness are shown side by side. One way to observe these is to create a shadow by holding a book or other object with a sharp edge between a light source and the surface it illuminates. The border of the shadow is not crisp; in fact, it seems to be made of several lines. On the inside there is a dark stripe, darker than the central shaded object that separates it from the unshaded region. Adjacent to this, on the bright side of the shadow, is another stripe that appears brighter than the rest of the illuminated surface. These additional bands are an example of brightness contrast at a border where the physical contrast between shadow and light is exaggerated in perception. As true brightness phenomena, Mach bands do not exist in the physics of the situation (that is, in the distribution of light intensity). They are purely a perceptual phenomenon, their brightness depending not only on the intensity of an area but also on the intensity of surrounding areas.
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