THINGS TO DO AND NOTICE:
Carefully note the trajectory of the ball and its shadow. Does the ball appear to rise above the floor of the checkerboard pattern? Is there any ambiguity in the trajectory of the ball?
Change the shadow to a diagonal shadow. Does the diagonal shadow make the ball appear to roll across the floor and go to another depth plane. Is the trajectory of the ball any different from when there was a horizontal shadow?
What happens when you combine shadows? What is the effect on your perception of the ball's trajectory? Does the ball appear to bounce along the floor? Is this a natural movement for the ball?
What happens when there is no shadow?
So What ís Going On?
Without a shadow the ball's motion is ambiguous.
Your visual system can not decide between the two simplest possibilities for the movement of the ball. It can be perceived as either rolling diagonally across the floor or as rising diagonally along the same depth plane. This is because the trajectory of the ball, without a shadow, is consistant with a ball rising on the same depth plane as well as a ball receding into the distance against a level surface. A receding ball would rise in the visual plane relative to the level surface.
Actually, there are an infinite variety of possible paths for the ball to take, but your visual system selects only two simple paths.
Faced with an ambiguous situation, your visual system looks for any clue that can resolve the ambiguity.
A shadow can be enough of a visual clue to bias your perception of both the ball's trajectory and its depth. In addition, the shadow can define other possible trajectories for the ball - paths that your visual system rejected. For example, if you combine the diagonal and horizontal moving shadow, the ball will appear to move in a zig-zag motion, which causes the ball to appear to bounce along the floor. Without a shadow, your visual system rejected this motion, because it was too unusual, but here it is the only motion consistent with its shadow.
At the 1996 Cognitive Neuroscience meeting, François Michel and Marie-Anne Henaff reported the results of showing the cast shadow and ball demonstration to an occipito-parietal patient and an occipito-temporal patient. The occipito-parietal patient did not report seeing the "ball-in-the box" shadow illusion--the ball did not appear to "fly" above the floor of the box. In contrast to the parietal subject, the occipito-temporal patient reported seeing the ball fly above the floor of the box. This is what a normal observer would see.
The ball and shadow illusion was first developed in 1990 by Daniel Kersten, Pascal Mamassian, and David Knill at the University of Minnesota, and reported at the annual meeting of the Association for Research in Vision and Ophthalmology in 1991.
Kersten, D. J., Mamassian, P., & Knill, D. (1991) "Moving cast shadows generate illusory object trajectories," Investigative Ophthalmology and Visual Science, 32, 1179.
Kersten, D., Mamassian, P., & Knill, D. C. (1997) "Moving cast shadows induce apparent motion in depth," Perception, to be published.
Kersten, D., Mamassian, P. & Knill, D. (1994) "Moving cast shadows and the perception of relative depth," Max-Planck-Institut fuer biologische Kybernetik Technical Report, 6.
Knill, D. C., Kersten, D., & Mamassian, P. (1996) "Implications of a Bayesian formulation of visual information for psychophysics" in K. D.C. & R. W. eds., Perception as Bayesian Inference, Chap. 6, Cambridge, Cambridge University Press.
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