Daniel knows movies are shown on a screen. He wants to know how his brain sees movement when he is really watching still pictures.

The answer is stranger than most people realize.

A film is a sequence of still photographs, typically around 24 per second for cinema. When they are shown fast enough, the brain does not see individual images flickering. It perceives continuous motion. But the motion itself does not exist on the screen. It is constructed by the visual system.

The traditional explanation is called persistence of vision. The idea is that an afterimage lingers briefly after a frame disappears, which has been offered as one reason the gaps between frames are not jarring. Researchers have questioned how much that explanation fully accounts for, particularly when it comes to perceiving motion itself.

Another concept  ...