In a theory called structuralism, that everyday perceptual experience is structured or synthesized from “sensations,” psychologists such as the English-U.S. introspectionistic psychologist Edward Bradford Titchener even devised a formal method of introspection for experimentally analyzing (or taking apart) percepts in an effort to reveal their constituent elements. The procedure required that the introspecting experimental subjects learn to avoid reporting on their experiences as they naıvely seemed. To establish this way of treating experience required careful training. One consequence of this training is that the observer’s introspective reports may be contaminated by his expectations and hence may, in all honesty, reflect little more than his theoretical biases.
But the problem remains interesting: If percepts are indeed syntheses of simpler elements, can those elements be made to appear in experience? If so, what will they turn out to be? Can this problem be investigated without recourse to the structuralist’s method of introspection and reliance on the reports of strongly biassed observers?
Evidence that percepts have constituent elements emerged serendipitously from research on stabilized retinal images. The image cast on the retina of the eye by a fixed object normally is continually moving because the perceiver himself is always in motion. Even when dampened by physical restraint, some residual movement will be left, attributable largely to high-frequency tremors (nystagmus) of the eyeballs. If the perceiver functioned as if he were a camera, the normal instability of the retinal image would produce a blurred percept and a concomitant impairment of visual acuity.
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.