Abstract
Rubin’s face/vase is an ambiguous figure/ground stimulus. One sees the vase (or faces) as the figure and the faces (or vase) as a meaningless shapeless ground. We used tessellations of oriented shapes such as triangles or arrows in which half the shapes pointed up and the other half pointed down. Result: Despite the up-down symmetry in the image, observers showed a strong preference for seeing the upward pointing shapes as figures and the downward pointing shapes as ground. Why? Objects pointing upwards from the ground (‘stalagmites’) are extremely common; for instance trees, people, furniture, and telephone poles. Objects hanging downwards (‘stalactites’) are uncommon; for instants skyhooks from construction cranes and lamps hanging from the ceiling. Thus, scene statistics and Bayesian probability favor ‘up’ perception. This ‘up preference’ can be compared with the ‘assumption of light from above’ in shape from shading