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Brightness and darkness
Basic Terms
If the light flux reaching the eye from a region of the visual field generates a sensation devoid of hue, the source of this flux is an achromatic stimulus. Hue is the attribute of a color perception denoted by red, blue, yellow, green, and so on. Achromatic stimuli may differ substantially from each other in terms of their power spectra. This chapter deals with achromatic perceptions independent of the spectral distribution of the stimuli generating them. Achromatic perceptions, however, may differ from each other in brightness. For instance, two self-luminous, achromatic objects, separated from each other in a dark surround, differ in brightness if one emits more luminous flux than the other. Brightness is the attribute of visual perception according to which an area of the visual field appears to emit more or less light.
Achromatic sensations may also be produced by the surface of objects illuminated by daylight or other achromatic sources. The photometric quantity most relevant to this case is luminance. This is defined as the light flux emitted per unit of solid angle from the unit area of the surface of an object projected perpendicular to the viewing direction. The reflectance of an object having a matte surface is the ratio between the light flux reflected by the surface and the incident flux, both relative to the unit area. The luminance of a uniformly illuminated diffusing surface is proportional to its reflectance.
A single achromatic object, illuminated in an otherwise dark surround, always appears bright, its brightness depending on whether it is more or less strongly illuminated. Variations in brightness range from bright to dim. If, however, several objects with reflectance independent of wavelength are simultaneously present in the visual field next to each other, and are illuminated by an achromatic source, their achromatic colors may vary according to their reflectance, between white (objects with very high reflectance) and black (objects with very low reflectance), among various shades of gray. Lightness is the attribute of visual sensation according to which an object's surface appears to reflect more or less light with respect to an object that appears white. Lightness, therefore, may be understood as a quality of sensation that arises from the comparison of objects with surfaces of different reflectance, and is an attribute that characterizes the perceived surface separately from the intensity of the illuminating light (see the section “Lightness Constancy”).
The objective contrast relative to two regions of luminance, L1 and L2, is the photometric quantity defined as the ratio between the luminance difference (L1 − L2) and the mean luminance (L1 + L2)/2.
The fact that a single illuminated object in a completely dark background always appears bright means that even a piece of coal would appear bright if illuminated in isolation, but the same piece of coal seen against a snow field would appear black (Gelb effect). This is an example of a large class of phenomena referred to as simultaneous contrast effects: an object of moderate reflectance may appear lighter or darker according to whether it is surrounded by a region that is considerably darker or brighter than the object itself (Fig. 56.1). More generally, if various objects of different reflectance are presented together and are illuminated from the same light source, the lightness of each object depends not only on the reflectance of the others, but also on their spatial distribution and relative distances (see the section “Spatial Contrast”). In particular, for an object of very low reflectance to appear black, objects of higher reflectance must be present close to it.
Figure 56.1..
Example of simultaneous contrast (area contrast). The inner squares all have the same luminance, but they seem to be different: the darker the surround, the lighter the inner square, and vice versa.
The sensation of blackness may also result from successive contrast, produced by rapid changes of luminance in a given area of the visual field: a surface of a given luminance may appear lighter or darker according to whether the preceding luminance was lower or higher.
Historical Background
The perception of blackness was interpreted differently by the two great visual physiologists of the nineteenth century. According to Helmholtz (1867), blackness results from the absence of light: “a spot in the visual field that sends no light to the eye is seen black” (Helmholtz, 1962, p. 131). He maintained that blackness is a real sensation, to be distinguished from the lack of stimulation relative to objects located outside the field of view. However, he did not realize that blackness required the simultaneous (or immediately successive) presence of objects of different luminance.
A practical demonstration of how simultaneous contrast may affect the sensations of both lightness and color of juxtaposed surfaces was given by Chevreul (1839). Later, Hering (1878) stated clearly that the sensation of blackness results from simultaneous or successive contrast. Hering interpreted achromatic contrast in terms of the existence of an achromatic opponent mechanism, coding brightness and darkness by an antagonistic process similar to the opponent mechanisms that he assumed to code for blue-yellow and red-green color information (Hering, 1920). His hypothesis of the existence of separate mechanisms for processing of the achromatic and chromatic components of a visual stimulus subsequently received support from anatomical and electrophysiological findings in animals (see Chapter 30) but is also in agreement with psychophysical findings in humans. Consistent with Hering's theory is the finding, by Shinomori et al. (1997) that the blackness of a color is inversely related to the luminance of its inducing field, independently of hue.
At very low light levels, when the eye is dark-adapted, the range of lightness variations is very restricted, and we perceive only faint variations of gray levels. With increasing illumination the range of lightness increases, and various shades of gray can be perceived, from white to black. Hering claimed that, with the increase in illumination, objects of higher reflectance appear increasingly whiter, but those of lower reflectance “appear increasingly darker, blacker, and finally dark black, even though their small light intensity has increased” (Hering, 1964, p. 75). This claim is disputable, as explained below. However, it was the increase in lightness differences with increased illumination that led Hering to assume the existence of two antagonistic mechanisms responsible for brightness and darkness sensations, respectively. Jung (1961) proposed that the two types of retinal ganglion cells, the ON-center and OFF-center cells, could have this role (see the section “Lightness Constancy” and Chapter 18).
A Dual System for the Perception of Brightness and Darkness
The possible role of ON-center and OFF-center cells in the perception of brightness and darkness, respectively, suggested by Jung (1961) and coworkers, was subsequently investigated both in animals and in humans (see Fiorentini et al., 1990, for review). It must be considered, however, that each retinal ganglion cell responds only to stimuli in a small area of the visual field, its receptive field, and that this field consists of two concentric regions with antagonistic responses to light, excitatory in the center and inhibitory in the surround, or vice versa (Fig. 56.2). Therefore, the optimal stimulus for a ganglion cell is not diffuse light, but a spot as large as the receptive field's center and either brighter (ON-center cells) or darker (OFF-center cells) than the background, that is, a contrast stimulus.
Figure 56.2..
Responses of an ON-center and an OFF-center ganglion cell of the cat to a stationary light spot located in the center of the receptive field and modulated sinusoidally in luminance (0.5 Hz, mean luminance 2 cd/m2) as a function of distance (in degrees of visual angle) from the receptive field center of a second spot of constant luminance (2 cd/m2). Spot diameter 34 minutes of arc. Background 0.1 cd/m2. The ordinate indicates the rate of discharge of the cell averaged over a stimulus period. The horizontal dotted lines indicate the average discharge recorded in the absence of the stationary spot. (Adapted from Maffei, 1968.)
The two populations of ganglion cells form two parallel mosaics in separate layers of the retina, and in the central fovea of the monkey the number of ganglion cells is three to four times the number of cones, indicating that the two ON and OFF systems can use independently all of the information sampled by the photoreceptors (Wässle et al., 1990). The ON and OFF channels are kept separate at the lateral geniculate nucleus (LGN) and at the early stages of processing in the visual cortex. In line with the properties of the two classes of cells and their specialization for responding to contrast stimuli of opposite sign, it has been found that pharmacological blocking of ON-cell responses in the retina, LGN, and visual cortex, having no effect on OFF-cell responses, severely impairs the behavioral response of monkeys to spots brighter than the background while leaving the response to dark spots unaffected (Schiller et al., 1986). Thus, ON and OFF channels provide independent processing of localized increments and decrements of a light stimulus. Psychophysical experiments suggest that in humans the perception of temporal increments and decrements of luminance in a spatially uniform light stimulus is subserved by two independent systems (Krauskopf, 1980).
But what is the advantage of having these two independent systems? One channel could well signal both increments and decrements of the stimulus by modulations of the rate of discharge. However, a dual system is much more efficient in signaling relatively large variations of either sign in the input, responding to both input increments and decrements with an increase of discharge and avoiding the effects of discharge saturation.
In conclusion, the ON and OFF channels seem to be the neural substrate accounting for the perception of increments and decrements of light, localized spatially or temporally. However, whether and how the two systems can account for brightness and darkness perception under more general conditions of illumination and contrast is still largely unknown.
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