Optical Illusions - Limitations of Visual Perception

In solving the ill-posed problem from recovering 3D form from 2D images our brain makes a priori assumptions about the world.
Rotating Mask
In the rotating mask illusion we have a hard time to see a hollow face mask because our everyday experience tells us that the nose is pointing outwards and not inwards.

Assumption 2: Light sources are stationary

Shadows - an import cue to indicate object movement:
Green Square
If the cast shadow of an object is moving, we assume that the object is moving and not the light source (In collaboration with Dan Kersten, Minnesota, USA).

A moving shadow induces a motion in depth of a stationary object (green square is not moving).
Kersten D Person, Knill DC , Mamassian P Person and Bülthoff I Person (January-1996) Illusory motion from shadows Nature 379(6560) 31-31.

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A ball moving
The following video shows how our our perception of object movements is manipulated by two types of prior knowledge: the fact that light sources usually are stationary and that there are only certain kinds of cast shadows objects normally create. The first part of the video shows a blue ball floating from the upper right corner to the lower left corner (and back). The following part apparently shows the ball rolling back and forth. Importantly, the ball moved in exactly the same way on the screen in these two parts; the only thing that changed was how the shadow moved on the ground. Accordingly, by removing the shadow, as shown in the next sequence, the movement of the ball becomes ambiguous: it could either be floating or rolling. Zigzagging shadow motion in the two following parts induces apparent zigzagging motion of an object. We do not see the correct linear trajectory of the blue ball although changes in shadowing on the ball and on the checker board  indicate that the light source is moving (in the last sequence of the video). The yellow line allows us to verify that the path of the ball is indeed linear.

Last updated: Tuesday, 08.10.2013