The brain determines how you see cinema: Film and the persistence of vision - part 1

Nearly any source on film will tell you that the visual phenomenon "persistence of vision" is why you see a series of still images as moving pictures. The curious thing is that the phenomenon "persistence of vision" apparently is not real. Good thing early filmmakers did not know this, for according to most film histories, this principle is credited as the basis for motion pictures and it is this phenomenon that led early filmmakers to launch the art form called cinema. Before we discuss what is real and what is illusion in the perception of motion in film, let us look at the idea behind "persistence of vision" and early film development. According to film history, this principle was first used in flipbooks, where a series of slightly different images appear on successive pages and give the illusion of movement when the pages are fanned. Photographic experiments in recording movement that used trip wires to take a series of photographs were found to work the same way as a flipbook. Early efforts that projected a series of similar photos in a continuous roll of film were seen only as blurred images. However, when experimenters projected discrete, slightly different images that were separated by a brief moment of darkness, audiences could see the flickering images as moving pictures rather than as a series of short-duration images, if the projection rate was fast enough. Early film pioneers discovered that images at less than 10 frames per second were seen as separate images but at greater frame per second ratios, the audience saw motion, although they still noticed the flickering. Hence, early films were called "flickers" because of the flickering image on the screen. As frames per second increased, flickering became less noticeable, but more film was required. In the silent film era, projector speeds were 16 to 18 frames per second and could even vary within a film. The development of synchronous sound meant projector speed had to be standardized at 24 frames per second, which is about the standard now for film or video. Film historians and film scholars have long cited "persistence of vision" as the phenomenon at work that allows us to perceive the series of still pictures as something moving on screen. However, it appears that this concept is based on some early ideas about human visual perception that science has long since left behind. For some reason, neither the scientists nor the film scholars noted the discrepancy. The original idea of "persistence of vision" was based on early observations by English-Swiss physician Peter Mark Roget. In 1824, he noted that moving wheel spokes observed between vertical slots blurred into a static image. Another source of the "persistence of vision" idea may have been the 1894 (motion pictures were invented in 1895) observations by Belgian physicist Joseph Plateau, who was studying the illusions of apparent motion in which static images were perceived as moving. Plateau concluded that if several images that are slightly but progressively different were shown for short intervals and in rapid succession, that the impressions they made on the retina would be joined together. The eye saw a single object changing form or position, using an idea called "retinal fusion." Other ideas of the time about retinal afterimages, "fusion" to describe light and color mixing and "flicker fusion" to describe how sequence of blinking lights, give the illusion of movement. All these are fairly close to the usual description of the phenomenon in film texts. However, after 1900, researchers studying perception abandoned the idea of retinal effects in fusion in favor of brain interpretation. Writings on cinema sometimes took note of new research up to the 1930s but afterward reverted to the "perception of vision" explanation for how we see motion in movies, sometimes crediting the eye as the source of the effect and other times referring to the brain. In either case, the idea of fusion of images remained. But science moved on. Can current research answer why we see a series of still pictures as continuous moving image? The question has two parts: Why do we see it as continuous, and why does it move? Research is ongoing. First, why do separate still images look continuous rather than like flashes of light? Flicker fusion is the term used in perception studies for the fusion for that flickering light. Flicker fusion threshold is defined as the frequency at which all flicker of an intermittent light stimulus disappears. Like other neuropsychological thresholds, this threshold is not an absolute but is set as the point at which flicker sometimes will be seen and sometimes will not be seen in 50 percent of trials. The flicker fusion threshold is higher for brighter lights and since the rods in the retina have a faster response than the cones, flicker can be seen in peripheral vision at higher frequencies. Flicker fusion is important in all technologies that present "moving images," since nearly all depend, like film, on presenting a rapid succession of static images. If the frame rate falls below the flicker fusion threshold for the given viewing conditions, flicker will be apparent and movements will appear jerky. Without being able to describe the physiological reasons why, early filmmakers were able to determine this threshold. The 24- frames-per-second rate is well over threshold, minimizing any perception of flicker. Second, why do still pictures seem to move? This has to do with the phenomenon called apparent motion. Here the research is more complex. To give it sufficient space, this discussion will be continued in the next column next week.