The study of object recognition concerns itself with a two-fold problem. First, what is the form of visual object representation? Second, how do observers match object percepts to visual object representations? Unfortunately, the world isn't color coded or conveniently labeled for us. Many objects look similar (think about four-legged mammals, cars, or song birds) and most contain no single feature or mark that uniquely identifies them. Even worse, objects are rarely if ever seen under identical viewing conditions: objects change their size, position, orientation, and relations between parts, viewers move about, and sources of illumination turn on and off or move. Successful object recognition requires generalizing across such changes. Thus, even if an observer has never seen a bear outside of the zoo, on a walk in the woods they can tell that the big brown furry object with teeth 20 ft in front of them is an unfriendly bear and probably best avoided or that the orange-yellow blob hanging from a tree is a tasty papaya.Consider how walking around an object alters one's viewing direction. Unless the object is rotationally symmetric, for example, a cylinder, the visible shape of the object will change with observer movement -some surfaces will come into view, other surfaces will become occluded and the object's geometry will change both quantitatively and qualitatively (Tarr & Kriegman, 2001). Changes in the image as a consequence of object movement are even more dramatic -not only do the same alterations in shape occur, but the positions of light sources relative to the object also change. This alters both the pattern of shading on the object's surfaces and the shadows cast by some parts of the object on other parts. Transformations in size, position, and mean illumination also alter