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.