Size as a cue to distance; radial motion.

The visual perception of motion has long been a subject of theoretical interest in experimental psychology. The question of the existence of a 'sensation of motion' may be almost as old as the concept of sensation itself. While the emphasis on specific theoretical considerations has shifted throughout the years with the gradual development of psychological concepts, the general area remains today, as almost a century ago, a fertile ground for speculative debate. Further incentives for the study of the perception of motion have arisen more recently in the widespread use of fast moving vehicles, with their operation dependent in the last analysis on some individual's perception of moving relationships. These highly practical considerations, underlined by the experiences of World War II and the continuing emphasis on military applications, have resulted in an intensification of the effort to gain an understanding of the basic phenomena involved in the visual perception of movement. A great volume of experimental work has been conducted upon many aspects of the problem. There are classical studies in the literature covering such aspects as the phi-phenomenon, perceived speed of motion, thresholds of movement, after-effect of seen movement, autokinetic movement, and numerous illusions of movement, to mention only a few of the areas which have been investigated. The history of this experimentation is too long and complex to be covered here, and a complete integration which should provide a working theory of all the perceptive facts of motion is still lacking. This paper is devoted to an experimental investigation of a phase of perceived movement which has not received the attention given to some of the above mentioned aspects; namely, movement directly toward or away from the observer (hereafter to be called radial motion). The specific experimental problem is that of size-change as a cue to radial motion. In view of its central importance for any complete understanding in this field, it is