Perceiving depth from reflection

As an object moves away from a shiny flat surface, the images of the object's reflection and shadow move away from the object's image, providing potential depth information. There are important differences, however, in how reflections and shadows constrain depth estimation. Unlike reflection, depth from shadows depends on an assumed light source direction. Depth from reflection is constrained by the line connecting the object and its reflection (in 3D) which is normal to and bisected by the surface. Does the visual system distinguish shadow and reflection constraints in judging depth? Observers viewed a simulated stereoscopic 3-D scene of a box containing a ball moving in a fronto-parallel trajectory from the lower left to the upper right. Observers judged the depth of the ball at the right-most peak of the trajectory by positioning a cursor on the right wall. Depth from stereo was perturbed by manipulations of the reflection and shadow in three conditions: 1) no-shadow and no-reflection; 2) the shadow followed the trajectory appropriate for a shadow (i.e. stereoscopically on the floor of the box) vs. reflection (beneath the floor) and; 3) the reflection followed the trajectory appropriate for reflection (below the floor) vs. shadow (on the floor). For all seven naïve observers, the addition of shadow or reflection resulted in significantly different depth judgments. For six of these, the depth of the ball was judged closer to the observer for the shadow than reflection case (same image location), qualitatively consistent with the natural constraints on reflection and shadows.