From: The World In Your Head by Steven Lehar

Fig. 4.14 Structure from motion of a random cluster of dots in rotation, computed by spatial reification. (A) Each dot of a random cluster of dots rotating in three dimensions projects an elliptical path in perspective. This ellipse is reified into the perceptual matrix as an elliptical cylinder in depth that represents every possible spatial interpretation of the two-dimensional projection. (B) Several possible paths (depicted in darker gray) represent ellipses in three dimensions with irregular rates of rotation. Of these, one special case (depicted in black) represents a circular rotation at uniform rate about the center. (C) This special case can actually emerge through a range of different depths, while (D) another set of circular paths corresponds to an alternative interpretation of the same stimulus as a circular rotation viewed from below instead of above. The final percept will therefore be bistable between the two states shown in (C and D).