Figure coherence in the kinetic depth effect.

When an observer views the twodimensional (2-D) projection, e.g., shadow, of a moving three-dimensional (3-D) object, he usually perceives the shadow pattern as a form with depth. This has been called the Kinetic Depth Effect (KDE). Wallach and O'Connell (1953) concluded that an essential condition for the occurrence of the KDE seemed to be contours or lines that change their direction and their length simultaneously. Wallach, O'Connell, and Neisser (1953) showed that experience with an unfamiliar figure undergoing the KDE led to later perception of depth in the stationary shadow of the object. Gibson and Gibson (1957) studied the apparent slant of a 2-D surface formed by a set of regular or irregular forms. They obtained accurate judgments of slant when these forms underwent the continuous perspective transformations associated with plane rotation. Their experiments could be considered instances of the KDE but the Gibsons emphasize the importance of perspective for the perception of rigid motion, while perspective was apparently not an important determinant in the studies of Wallach et al. In the KDE the relative motion of the various parts of a 3-D figure provides information not only about depth but also about the shape of the figure. The visual system in some way transforms the relative motions of the shadow's elements into the perception of a single rigid object, and this process appears to be a major factor in our normal perception of three-dimensional objects in the world around us. Johansson (1958) states the case as follows: "Mathematical relationships in the continuously changing energy distribution on the retina may be substituted for the classical static retinal picture, as the source of information from the external world. The substitution is viewed as an application of Gibson's gradient theory" (p. 3). It follows that a set of isolated dots or unconnected lines undergoing the appropriate changes can be perceived as a coherent rigid figure. White and Mueser (1960) studied the accuracy of the perception of arrangements of elements in a KDE setting. They found that all of their figures were perceived as rigid configurations in 3-D, but that reproducing the exact spatial relationships among the elements was a difficult task. This paper reports a series of experiments designed to isolate the effect of relative movement from all other cues to depth and coherence. The experiments explore some of the conditions under which the KDE imparts perceived rigidity (coherence) to elementary patterns of dots and lines. The Os viewed the changing 2-D projection associated with a rotating rigid 3-D configuration of dots or straight lines, and judged the apparent rigidity and coherence of the configurations, i.e., the extent to which the elements appeared to maintain their relative positions in the configuration. Independent variables were the num1 Operated with support from the United 2 Rigidity and coherence are treated as States Array, Navy, and Air Force. synonyms in this paper.