Shape constancy measured by a canonical-shape method

Shape constancy is the ability to perceive that a shape remains the same when seen in different orientations. It has usually been measured by asking subjects to match a shape in the frontal plane with an inclined shape. But this method is subject to ambiguity. In Experiment 1 we used a canonical-shape method, which is not subject to ambiguity. Observers selected from a set of inclined trapezoids the one that most resembled a rectangle (the canonical shape). This task requires subjects to register the linear perspective of the image, and the distance and inclination of the stimulus. For inclinations of 30° and 60° and distances up to 1m, subjects were able to distinguish between a rectangle and a trapezoid tapered 0.4°. As the distance of the stimulus increased to 3m, linear perspective became increasingly perceived as taper. In Experiment 2 subjects matched the perceived inclination of an inclined rectangle, in which the only cue to inclination was disparity, to the perceived inclination of a rectangle with all depth cues present. As the distance of the stimulus increased, subjects increasingly underestimated the inclination of the rectangle. We show that this pattern of inclination underestimation explains the distance-dependent bias in taper judgments found in Experiment 1.