the phenomenon with the turning wheels or ther spinning objects apearing to turn the wrontg way only apears at a specific speed this is when the wheel has turned almost completely since the last frame of the movie and so the brain percieves it as moveing backwards, it then apears when the wheel has done almost two rotations since the last frame and so on. it is because film is a seriese of pictures whereas in real life we see the continual stream of movement so dont notice it.