For more than a century, filmmakers postulated that the mechanism by which we perceive the illusion of motion in motion pictures is persistence of vision. To this day, many students of film, both novice and seasoned, put forth this wacky and wonderful defect of the eye as responsible for our enjoyment of movies. Unfortunately, this theory, while elegant, is absolutely false.
Persistence of vision does indeed exist, although it is now considered an antiquated term (modern psychologists and neurologists prefer to speak of it in terms of positive afterimages). The human eye collects light, which is focused through the lens onto the retina at the back of the eye. The retina is replete with light-sensitive receptor cells. When light strikes these receptors, it triggers a chemical reaction, which is then turned into an electrochemical signal that travels up the optic nerve to the brain. The brain interprets these signals as an image, and, voila, we have vision. This process is hardly foolproof, however. While the receptors in the retina start reacting to light almost instantaneously, they cannot stop reacting quite so quickly. The human eye is capable of perceiving a light event that lasts one millionth of a second, but the receptors may continue reacting to this event for up to one thirtieth of a second, depending on the relative brightness of the event. This lag may cause an image to persist in the eye, and therefore the brain, longer than in reality.
It is really quite easy to observe the effects of the persistence of vision phenomenon. If you smoke, the next time you step out for a butt at night, try waving around your lit cigarette. Even without the help of hallucinogenic substances, persistence of vision will cause you to see spiffy visual trails. If you don't smoke, you can replicate the experiment with a burning stick or twig.
Peter Roget (yes, that Roget) first identified the phenomenon in 1824. Following Roget's discovery, makers of optical toys like the thaumatrope and zoetrope, and later, the inventors of cinema, appropriated the idea, believing that their devices exploited persistence of vision. Allegedly, the phenomenon causes one to perceive a movie as a continuously moving image because each frame persists in the eye just long enough that it appears to seamlessly melt into the next. This theory seemed to be supported by the early finding that at speeds lower than 24 frames per second, the illusion of motion begins to degrade, and below 10 fps, disappears completely. For the time being, persistence of vision as an explanation made sense.
In the late 1970s and early 1980s, a number of researchers presented incontrovertible proof that this explanation is a myth. There are several reasons why persistence of vision can't possibly explain our perception of movies. The most important, and easiest to understand, is that if there is a lower threshold to the film speed necessary to take advantage of persistence of vision, there must also be an upper threshold. If one sped up a film enough, the image of each frame would persist too long, overlapping with the image of the next frame too much, and creating the illusion of visual trails. This is not the case. Video often runs at frame rates of 30-50 fps. Some high speed digital recorders are capable of frame rates exceeding 100 fps. These faster frame rates result in a movie or video that appears more realistic with less flicker, but they do not overtax persistence of vision. No matter how much one increases the film speed, it is impossible to induce the effects one would expect if persistence of vision were really the mechanism underlying the illusion of motion.
In fact, the real reason why movies work is due to a psycho-perceptual illusion called apparent motion.* As the prefix "psycho" implies, this illusion has everything to do with the brain, and little to do with the eye.
During a series of experiments, researchers showed subjects a rapid sequence of images of a visual element, usually a dot. In each image, this element appeared in a different position. When the distance between the dots in succeeding images was very small (less than a quarter of a degree of the visual angle), subjects reported seeing one dot that appeared to move. When the distance between the dots in succeeding images was increased, subjects correctly reported seeing separate dots. Further research seems to indicate that these two different types of displays are processed by different parts of the visual cortex within the brain. The display in which the distance between the dots is small stimulates the same part of the visual cortex that is responsible for our perception of real motion. Similarly, because the differences between frames of film shot at sufficient speed are quite slight, this part of the visual cortex is stimulated, causing the brain to fool itself into seeing motion, even though the individual frames are static. More research is needed to accurately pin down the details of how and why this occurs, but these preliminary findings further debunk the notion that persistence of vision contributes to the illusion of motion.
Persistence of vision is not totally useless in the world of film, as it may contribute to another illusion known as flicker fusion. In addition to the illusion of motion, movies present the illusion of continuity, despite the fact that the frames of film are divided by a visible thin black line. Common sense dictates that these lines should cause a noticeable "flicker effect," yet movies that are shot at 24 fps have minimal flicker, and detectable flickering decreases with increasing film speed. The common explanation for flicker fusion is that the eye (and perhaps, to an extent, the brain) has a perception threshold which, when crossed, renders it unable to distinguish distinct visual stimuli as separate. If a bright light, or in this case, lighted frames of film, flashes at a frequency greater than 20-60 Hz (known as the critical flicker frequency or critical fusion frequency, this threshold is given as a range because it changes based on the individual and the amount of ambient light), a human viewer is unable to distinguish the separate pulses of light. Incidentally, this is also why most computer displays have a refresh rate of greater than 60 Hz. Although some researchers have argued that flicker fusion is solely a cortical phenomenon that occurs within the brain, the most commonly accepted theory at this time is that it is, at least in part, a retinal phenomenon caused by persistence of vision.
The persistence of vision theory as related to the illusion of motion was disproved more than 20 years ago. Why, then, do so many people in the film industry tenaciously cling to this 19th century concept as an explanation for the essentially modern magic of movies? When you try plugging the phrase "persistence of vision" into your search engine of choice, you will find literally thousands of results, put forth by both amateur and professional students of film (but very few legitimate scientists), that tout the phenomenon as responsible for the illusion of motion. Even a few of our noders have fallen prey to this trap. Researchers Joseph and Barbara Anderson have been tirelessly attempting to dispel this myth since 1978 by publishing scholarly articles in the academic journals of the film world (yes, such periodicals exist). The Andersons suggest that the theory persists for psychological reasons, functioning as a myth of creation for film, not unlike the religious myth of Adam and Eve. This myth involves a passive biological process, an idea that is especially attractive to filmmakers, as cinema, by nature, requires a passive viewer. It seems almost poetic. There is also the allure of mystery, the idea that our eyes, unbeknownst to us, act as a sort of "mystic writing pad, a palimpsest, that is like the unconscious." Perhaps most importantly, however, persistence of vision is easy to explain, and requires minimal technical knowledge to understand. It doesn't involve the use of big words or an understanding of the intricacies of neurobiology. It is simple, and on the face of things, it seems to fit. For these varied reasons, many people cling to pseudoscience in the face of truth, and the myth of persistence of vision as related to cinema appears unlikely to lose momentum any time soon.
* There are actually two kinds of apparent motion: short-range apparent motion (also known as fine grain apparent motion), and long-range apparent motion. The intricacies of this psycho-perceptual illusion are really too esoteric to describe here. Suffice it to say that we experience short-range apparent motion when watching movies, and that particular variation of the illusion is what I have described.
Sources:
http://www.uca.edu/org/ccsmi/ccsmi/classicwork/Myth%20Revisited.htm
http://www.grand-illusions.com/percept.htm
http://webeye.ophth.uiowa.edu/dept/RESFELO/ResDay2000/puwat/
http://www.occ.cc.mi.us/moviepage/menustructure/flsinternet/whatsreallyinvolved.htm
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