If people see a pattern of columns in the left panel of Figure 2 because that is how the stimulus is constructed, then why do some people not see it that way? Both achieving and failing to achieve an organized percept must be explained. Surely, part of the explanation must lie in the nature of the perceptual process itself. Thus, the experimental results indicate that perceptual organization is not universal and immediate; rather, they support the major tenet of Gestalt theory that things look as they do because of the organization imposed by the perceptual process (e.g., by the perceiver).
One Gestalt principle, that of common fate, depends on movement and is quite striking when observed. According to the principle of common fate, stimulus elements are likely to be perceived as a unit if they move together. An illustration of this principle is provided by a well-camouflaged object, such as a military vehicle; when stationary, the elements of the vehicle are integrated, through proximity, similarity, and so on, into patterns of background elements, and the object is difficult to detect. But it is easy to see it once it starts moving; with all of its elements moving in unison, the vehicle is readily perceived as a unitary figure, clearly segregated from its background.
Movement is also at the heart of a set of observations of considerable significance in the historical development of Gestalt theory. These observations concern circumstances in which people perceive movement in the absence of actual physical motion of the stimulus. One familiar instance of this class of events is referred to as the phi phenomenon. In simplest form, the phi phenomenon can be demonstrated by successively turning two adjacent lights on and off. Given appropriate temporal and spatial relations between the two lights, an observer will perceive the first light as if it were moving from its location to that of the second light. The phi phenomenon is basic to the eye-catching displays used on theatre marquees and to cinematic and television presentations. The motion-picture screen, for example, presents a series of briefly flashed, still images; the movement people see is a creation of their own perceptual systems.
It is the lack of one-to-one correspondence between stimulation and perception, as dramatically illustrated in the phi phenomenon, that underscores the Gestaltists’ dissatisfaction with stimulus-bound models of perception and their insistence on the priority of patterns and relations. What people perceive is determined not only by what is present at the point under direct observation but also by what is occurring in the total stimulus context or display. Context effects
One of the simplest instance of relational (or context) effects in perception is that of brightness contrast. Thus, the apparent brightness of a stimulus depends not only on its own luminance but also on that of the surrounding stimulation. The same gray square looks whiter against a dark background and blacker when placed in a bright surround. Similarly, a white or gray patch will take on an apparent hue that is complementary to the colour of the surround (e.g., the patch will seem tinged with yellow when it is placed against a blue background).
Analogous context effects are evident in many commonplace experiences. A man of average height seems to be a runt when he is on a basketball court with much taller players; yet the same man looms like a giant when refereeing a game played by little boys. It is known that a typical winter’s day seems delightfully balmy when temperatures rise after a week of subfreezing weather.
To the Gestaltist, contrast effects dramatize the relational nature of perception. They also play a significant role in a more recently developed adaptation-level theory, which also provides a general perceptual model. At the core of the model is the notion that the manner in which a stimulus is perceived depends not only on its own physical characteristics but also on those of surrounding stimuli and of stimuli previously experienced by the observer. In other words, the perceiver is said to be perceptually adapted to past sensory stimuli; his adaptation level forms a kind of zero point against which any new stimulus is perceived. An example is provided by the almost overwhelming silence one experiences when the sound of an air conditioner (to which he has adapted) suddenly ceases.
Gestalt theorists also attached significance to the observer’s history of stimulation; indeed, some of them interpreted so-called figural aftereffects within a Gestaltist model of brain functioning. Figural aftereffects refer to changes in the perceived shape or location of a figure following its inspection; for example, a curved line will appear to get straighter after prolonged inspection. Or the distance between two parallel lines seems to change as an aftereffect of previous inspection.
In a typical experiment one looks at a point adjacent to a dark vertical bar (the inspection figure) on a screen. Following this inspection period, the dark bar is replaced by two identical pairs of vertical lines, one pair on either side of the region where the bar had been, the second pair alongside in a region not previously exposed to the inspection figure. The subject again fixates the same point. A figural aftereffect shows up as a greater apparent distance between the pair of lines surrounding the region of the inspection figure even though the other pair is actually identical. This distortion is not simply a generalized contrast effect because it occurs only in the small area along the borders of the inspection figure; that is, the effect is localized and restricted.
It thus has been speculated that visual exposure to a figure induces in the brain a condition of localized satiation. The passage of electrical activity is assumed to be impeded in satiated areas of the brain. Moreover, it is postulated that the perceived distance between two borders of a figure is directly related to the time it takes for electrical currents to pass between them. Thus, it is held that one effect of satiation is to increase the apparent distance between the borders of a figure that straddles a satiated region. Whatever the merits of such physiological speculations, they have stimulated a vast amount of research on figural aftereffects. Good evidence for similar effects in other senses, such as touch, also has been obtained. Clearly, perception can be influenced not only by the context of current background but also by the residues (after-effects) of previous stimulation.
Concurrent visual stimulation may modify one’s acuity in detecting auditory stimuli. Similar interactions are claimed to occur for other combinations of senses. Some dentists report success in using audioanalgesia, in which stimulation with sound waves is said to reduce the experience of pain in the mouth. The high specificity of some of the reported sensory interactions seems to preclude an explanation that concurrent stimulation works by changing the subject’s general level of alertness. However these intersensory effects might be mediated, they do suggest that the brain does not function as a collection of entirely independent sensory channels. As a physical system, the brain follows physical principles; thus overlapping and spreading or waning fields of neural excitation in the brain have been theorized to underlie such phenomena as closure and audioanalgesia. Köhler referred to these models of neural analogues of perceptual phenomena as physical Gestalten; unfortunately, there is little direct physiological evidence for them.
An alternative to field effects in brain functioning is the assumption that local stimulation gives rise, in one-to-one fashion, to a mosaic of local responses. Implicit in the mosaic hypothesis is a kind of telephone switchboard model of the brain as a machine in which the electrical activity is strictly confined to separate pathways of neurons that are well insulated (isolated) from one another. The Gestaltists rejected this model because in its early formulations it did not explain intersensory and intrasensory perceptual phenomena. A more sophisticated machine model, however, provides for fieldlike effects through the operation of complex networks of neural elements. It is held that electrical activity remains confined to discrete pathways, but that these pathways do not simply travel straight through the system; that they also interconnect, with both excitatory and inhibitory consequences. Supporting evidence comes from records of the electrical activity in single neurons in the cat brain; when the cat’s eye is probed by a small spot of light, a specific area on the retina can be found that serves to excite a given brain neuron.