It is not feasible to eliminate eye movements, but it is possible to stabilize or fix the location of the retinal image by coupling the source of the image to the eyeball itself. An optical lever system can be so adjusted that when the eye moves the image source moves with it, and potential motion in the retinal image is eliminated. As expected, visual acuity is slightly enhanced when the retinal image is kept motionless. A remarkable, unexpected finding, however, was that such stabilized images rapidly seem to disappear, the perceiver losing awareness of them. It would seem that some movement in retinal image is needed to maintain perception over extended periods of time.
One limitation of the optical lever system is that it permits the use of only very simple targets, such as straight, vertical lines. With a different device (in effect, a miniature projector attached to the eyeball), stabilized images of complex patterns may be presented. Complex patterns are found to produce percepts that are relatively slow to deteriorate; furthermore, they do not disappear in toto. The manner of the fragmentation is perhaps revealing of the way in which complex percepts are synthesized. Speaking metaphorically, observing how percepts “come apart” under retinal stabilization may be very much like discovering the structure of a rock by striking it with a powerful hammer blow.
Indeed, under retinal stabilization, single lines seem to disappear and reappear in a unitary (altogether) fashion. In a figure comprised of several lines (say, a square), percepts of parallel lines are likely to disappear and reappear together; proximity also affects the joint perceptual fate of pairs of lines. Retinally stabilized segments of such geometric figures as circles and triangles can seem to disappear and reappear without implicating the entire figure. In the disappearance of percepts of triangles, lines rather than angles are the functional units. (This finding is embarrassing to earlier theorizing about the crucial role of angles in the development of the neural network underlying the percept of a triangle.)
Clearly, with stabilized images, the constituent perceptual elements of complex geometric forms are lines, straight or curved; and lines with the same orientation are likely to have similar perceptual fates, as though forming a higher-order component of complex patterns than do individual lines. These conclusions are remarkably similar to those drawn from studies of the effect of visual stimuli on the electrical activity of single neurons in the cerebral cortex. A finding of major theoretical significance is the failure of percepts of circles, squares, and triangles to act as units. Such percepts are treated in classical Gestalt theory, however, as though they are basic and unitary and not readily decomposable. Primary tendencies in perceptual organization Gestalt principles
Gestalt theory was meant to have general applicability; its main tenets, however, were induced almost exclusively from observations on visual perception. Whatever their ultimate theoretical significance, these observations have been raised to the level of general principles. It is conventional to refer to them as Gestalt principles of perceptual organization.
The overriding theme of the theory is that stimulation is perceived in organized or configurational terms (Gestalt in German means “configuration”). Patterns take precedence over elements and have properties that are not inherent in the elements themselves. One does not merely perceive dots; he perceives a dotted line. This notion is captured in a phrase often used to characterize Gestalt theory: “The whole is more than the sum of its parts.”
Of the many principles of organization that have been enunciated by various Gestalt theorists, the most general is referred to as Prägnanz. In effect, according to the principle of Prägnanz, the particular perceptual configuration achieved, out of a myriad of potential configurations, will be as good as prevailing conditions permit. What constitutes a “good” configuration, or a poor one, is unfortunately not clearly specified, though several properties of good configurations can be listed, chief among them being simplicity, stability, regularity, symmetry, continuity, and unity. What happens when these properties of figures come into conflict is not specified, but should be possible to determine empirically.
The principle of closure often operates in the service of Prägnanz; for example, a circular figure with small gaps in it will be seen as a complete or closed circle. Similarly, if a portion of the image of a figure falls on the blind spot of the retina, a complete figure often will still be perceived. Some distortions from good configuration may be so large as to preclude closure; in those cases, the figures may be a source of tension for the observer.
Prägnanz may also be achieved through good continuation; this principle describes a tendency for smooth continuity of contour to be dominant over discrete, irregular, abruptly changing contours. Thus, a figure composed of the overlapping outlines of an ellipse and a rectangle will probably be seen as such rather than as three figures, each with irregular, noncontinuous borders.
Closure and good continuation represent two of the factors that are held to determine what percepts will emerge from a complex stimulus. Implicit in them (and in the general principle of Prägnanz) is the assumption that whenever possible some figure will be perceived; more specifically, that the visual field will be articulated into figures and patterns of figures. It is understood that such emerging patterns are not in the stimulus. Although they are permitted by the stimulus, they are created by the perceptual system; that is, by the perceiver himself.
In the illustrations in Figure 2, in the panel on the left, the vertical distance between elements is less than the horizontal distance. By virtue of this differential proximity, the elements become perceptually organized into columns. In the right-hand panel, similarity, another principle of organization, is operative. Here, by virtue of similarity in brightness, the visual field tends to be perceptually articulated into alternating sets of black and gray rows.
Figure 2: Examples of Gestalt principles of organization. (Left) Horizontal distance between dots is greater than vertical distance. (Right) Equal distance between horizontal and vertical.Encyclopædia Britannica, Inc.
It is not at all obvious why organization by similarity should occur; physical stimulation allows but does not demand it. Clearly in that case the articulation of the visual field into columns reflects a tendency in the perceptual system itself. Organization by proximity may not seem to reveal anything more than a close correspondence between perception and stimulation. (Though as argued by the Gestalt theorist Kurt Koffka, it is not an adequate explanation to say that “things look as they do because they are what they are.”) Yet, when a proximity pattern like the one shown in Figure 2 was briefly presented and subjects were asked (under guise of another task) to reproduce what they saw, many people failed to indicate a differentiated percept of columns. Instead, they reproduced a homogeneous matrix of elements. After repeated exposures, some of those subjects began to draw proximity-based columns of elements. Organization according to the principle of proximity seems to be neither universal nor, for those who achieve it, immediate.
In the latter experiment, people who failed to obtain the differentiated percept of columns scored significantly lower on a test of verbal intelligence than did those who succeeded at some point in the experiment. Perhaps the Gestalt principles of organization apply to perceivers (such as Gestalt theorists) whose intellectual development has reached a high degree of maturity. When organized percepts are easy to come by, gradations in intelligence do not seem to matter; when some barrier to organization is imposed (as by brief stimulus exposure), however, then the effect on perception of such differences among individuals may show up.