Persistence of vision

Persistence of vision or visual persistence is the optical illusion that occurs when the visual perception of an object continues for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been described as "retinal persistence" when attributed strictly to the retina, and "persistence of impressions" as a more general term that can also apply to the other senses. An example of the phenomenon is the fiery trail from a glowing coal or burning stick that is whirled around in the dark.

Some explanations of the illusion describe either positive afterimages or motion smear (comparable to motion blur in photography, film and video).

Recent theories about visual sensory memory differentiate between higher-level psychological 'informational persistence' and lower-level phenomenal 'visible persistence'.

The human visual system depends on temporal integration in general; it continuously processes visual inputs over time.

"Persistence of vision" can be understood to be the same as "flicker fusion", the effect that images persist when the light that enters the eyes is interrupted at short, regular intervals. When the frequency is too high for the visual system to discern differences between moments, light and dark impressions fuse together into a continuous image of the scene with intermediate brightness (defined by the Talbot-Plateau law).

Since its introduction, the term "persistence of vision" has often been claimed to be the explanation for motion perception in optical toys like the phenakistiscope and the zoetrope, praxinoscope, mutoscope, theatre optique (optical theater) and later in cinema. This theory has been disputed since long before cinematography's breakthrough in 1895. The illusion of motion as a result of fast intermittent presentations of sequential images is a stroboscopic effect, as explained in 1833 by Simon Stampfer, one of the inventors of the stroboscopic disc (phenakistiscope). If the retention of images lasts longer than the interruptions, it does explain the apparent continuity of the image, which looks weaker, depending on the ratio between exposures and interruptions.

Early descriptions of the illusion attributed the effect purely to the physiology of the eye, particularly the retina. The nervous system later became accepted as an important factor.