Perception of Slanted Surfaces

A satisfactory theory of the perception of surfaces is essential to an understanding of space perception. In particular, without considering the property of a surface called slant, we cannot deal fully with such important issues as perceived shape, distance along a surface, or even perceived size. Moreover, the study of slant perception provides an elegant illustration of the general nature of visual processes: as the present research suggests, a single impression of a slanted surface can only be accounted for by analysis of the entire pattern of retinal excitation. Hochberg and Smith (5) have made an interesting analysis of certain parts of this problem. In his experiments with the Gmzfeld, Metzger (9) drew attention to the importance of texture in the perception of surfaces. More recently, Gibson (3) has advanced a theory of space perception in which the perception of extended surfaces plays a fundamental role. Specifically, he suggests that a retinal gradient of texture density is the stimulus for slant, and his experimental results support this hypothesis (4). The purposes of the present paper are to examine the concept of gradients of texture density in more detail and to explore the influence of two other variables of stimulation which are inevitably associated with the presence of gradients of texture density. Let us consider some of the purely geometrical properties of the retinal image produced by a uniformly textured surface, as in Fig. 1. 1. For every orientation of a uniformly textured surface, SSP, there is not one retinal gradient of texture density but a uniqae pattern of gradients. The gradient at any point, p', in the retinal image is a function of the angle, VPS, formed by the stimulus surface and the optic radius to the corresponding point, P, on the surface. That angle, in turn, is a function of the particular optic radius chosen and of the angle, FSP, between the frontal-parallel plane and the stimulus surface. We will refer to the angle, FSP, as physical slant. 2. The pattern of retinal arousal has other variables, distinct from the pattern of gradients of texture density. For example, the same gradients can be prolThe research here reported formed pan of an M. A. thesis, Monocular Perception of Stnted Surfaces, by W. Crawford Clark, submitted to the Department of Psychology at Queen's University in 1953. Grateful acknowledgement is due Dr. A. H. Smith for his supervision of Mr. Clark's thesis and for his help in bringing the present paper to publication. Experiment I was reported by the present authors at the meeting of the American Psychological Association in 1952. The research for this paper was done under DRB Grant No. 370, Project No. D77-94-01-03.