Figure 8. Erroneous or ambiguous interpretations of depth can arise when lighting does not meet our expectations.  In Figure 8.1, computer-rendered spheres lit from different directions are perceived as either convex “pimples” or concave “dimples” (after Ramachandran 1988).  Spheres lit from above tend to be interpreted as protruding, whereas those lit from below are seen as hollow.  Figure 8.2 shows the same pattern rotated 180 degrees.  Spheres previously seen as convex appear concave, and vice versa.  Such flipping occurs with photographs of dinosaur footprints.  Spheres lit from the side as in Figure 8.3 remain ambiguous.