Perspective and form ratio as determinants of relative slant judgments.

Abstract

Judgments of relative slant were elicited by a paired-comparison method from 24 Ss in each of three experiments. The stimuli were computergenerated slides representing regular dot patterns (Exp. I), regular line patterns (Exp. II), or random dot patterns (Exp. Ill) rotated about a horizontal axis. The ratio of horizontal to vertical separations (form ratio) and perspective were independently varied in Exp. I and II. Perspective clearly dominated slant judgments when in conflict with form ratio as an indicator of degree of slant. Perspective alone was varied in Exp. Ill and was found to be less effective in determining slant judgments for random dot patterns. The equivalence of perspective and optical theta as explanations of slant perception is discussed. The retinal image of a slanted surface pro- The form ratio, however, is independent of vides at least two potential sources of veridical slant information. The first is the proximal stimulus form, relative to an assumed original form. For a surface rotated about a horizontal axis, form is defined as the ratio of a vertical distance between identifiable features of the surface to a horizontal distance between identifiable features. The vertical distance is measured from the axis of rotation along a line perpendicular to that axis to a horizontal contour line or other identifiable surface feature. The horizontal distance is measured at that vertical distance along a line parallel to the axis of rotation. Then viewing distance when its components are computed in the manner specified. The second source of slant information may be expressed in several ways. Flock's (1962) optical theta is one such expression: