In the second edition of Rudolf Arnheim's Art and Visual Perception: A Psychology of the Creative Eye, published in 1974, the opening chapter on "balance" begins with a discussion of "perceptual forces" in static imagery, such as line drawings and paintings. For Arnheim, perceptual forces are literal forces that act on perceived objects, with a point of application, a direction, and a magnitude. They usually do not have direct counterparts in physical reality, but "Perceptually and artistically, they are quite real." On page 17, available here, Arnheim states that he doesn't know where these forces are represented in the brain:
… Where, then, are these forces?
In order to answer this question we must recall how an observer obtains his knowledge of the square and the disk. Light rays, emanating from the sun or some other source, hit the object and are partly absorbed and partly reflected by it. Some of the reflected rays reach the lenses of the eye and are projected on its sensitive background, the retina. Many of the small receptor organs situated in the retina combine in groups by means of ganglion cells. Through these groupings a first, elementary organization of visual shape is obtained very close to the level of retinal stimulation. As the electrochemical messages travel toward their final destination in the brain, they are subjected to further shaping at other way stations until the pattern is completed at the various levels of the visual cortex.
At which stages of this complex process the physiological counterpart of our perceptual forces originates, and by what particular mechanisms it comes about, is beyond our present knowledge. If, however, we make the reasonable assumption that every aspect of a visual experience has its physiological counterpart in the nervous system, we can anticipate, in a general way, the nature of these brain processes. We can assert, for instance, that they must be field processes. This means that whatever happens at any one place is determined by the interaction between the parts and the whole. If it were otherwise, the various inductions, attractions, and repulsions could not occur in the field of visual experience.
An observer sees the pushes and pulls in visual patterns as genuine properties of the perceived objects themselves. …
(Emphasis mine.) What scientific progress on this problem has been made in the 40 years since?
Have we confirmed the existence of a universal, involuntary process that generates perceptual forces? If so, what is its mechanism? Where and how early are the forces perceived? To what extent are they influenced by top-down attention? Does the mechanism share neural resources with the perception of motion, or with other senses?