These two processes are commonly confused and much misunderstood, but lead to a fascinating, almost metaphysical, theory of what film really is. In effect, Persistance of Vision and the Phi Phenomenon can be distinguished by the fact that the former takes place in the retina whereas the latter happens in the visual cortex, much later in the process of visual recognition. Persistance of Vision: Persistance of Vision is one of two types of temporal retinal response; a slow response, having to do with summing and integration effects, by which the rods are replenished after having been saturated by light. The most well-known effect is that by which the photoreceptors' activity is prolonged some time after the end of the stimulus. It works best when the eye is adapted to darkness, when a bright flash can sometimes last several seconds. On oft-cited example is the 'circle of fire' we see when we turn a flashlight in circles in the dark: though this is not entirely due to retinal persistance, it is obvious that this effect is in no way related to film perception, which is not the perception of a continuous movement but of a discontinuous projection. Persistance of Vision does account for the thaumatropes (is that the right name?), those early disks with for example a bird cage on one side and a bird on the other. When the disk is flipped rapidly, the two images are superimposed. Again, this is not an illusion of movement, and as we will see, it is the phi phenomenon which is responsable for the perception of movement, whereas persistance of vision gives an effect of immobility. For completeness' sake I will mention the other type of temporal retinal response, a fast response, which is the flicker effect. a function of brightness and speed which define a critical frequency at which we do not perceive flicker. Visual masking occurs when the two effects, persistance of vision and flicker effect, are combined. In effect, if two images follow each other quickly, persistance of vision causes interactions changing the perception of both images. Inserting a black flicker between the images reduces the persistance of vision. In cinema this elimination of retinal persistance helps the perception of movement. Phi Phenomenon: The phi phenomenon was discovered by Wertheimer in 1912. The original experiment was as follows: a subject is presented with two different points of light slightly separated in space which flash once each at differing intervals. If the two flashes are close in time, the two lights are perceived as simultaneous. If the two flashes are far apart in time, the two lights are perceived as distinct. However, between 30 and 200 milliseconds of difference between the two flashes, the subject perceives one single light that travels from the first point to the second. This perception of apparent movement explains the flashing lightbulbs on Broadway marquis that seem to define a continuous movement, or the world headlines snaking their way around a building in Times Square. The phi phenomenon, which occurs not in the retina but in the occiptal lobe of the brain, is a complex visual process by which movement is detected. It seems that this mechanism is the same for both apparent and real movement, one reason that filmic movement is psychologically indistinguishable from real movement (as Christian Metz once wrote, "movement can not be recreated but only created"). Film and reality are thus indistinguishable, film residing on a completely innate biological process of motion detection. Thus the film viewer is presented with a discontinuous, luminous stimulus, which gives an impression of continuity and apparent movement within the image, due to the phi phenomenon. The phi phenomenon is not completely understood and a series of recent experiments have only served to raise more questions. For example, in the case of two points of light that are of different colors, say red and green, the subject perceives a single point of light that travels across the field of vision and which CHANGES COLOR IN THE MIDDLE. This presupposes that the second light has been perceived, but that the subject is not privy to the existence of a second, distinct light, nor to its location until after the phi phenomenon, several milliseconds later. Some have posited the theory of a Marxist brain that holds back information, or else of a Stalinist brain that lies to us. For film perception, it is clear that the apparent movement that we perceive in the cinema occurs only between the frames, during the darkness (don't forget that for two hours spent at the movies, we spend forty minutes of it in complete darkness). So just as we create our dreams at night between the days, in the darkness behind our eyelids, at the movies we create the movement, during the darkness between the static images. The individual frames must be similar, but not too similar, as well as different, but not too different, for us to perceive a movement in the image. Between the frames, the brain bridges the gap, compensating for the differences, creating the movement. "Film happens between the frames" - Jonas Mekas The wheel-turning-backwards effect, which can just as easily be perceived under a neon light, due to its flickering, is simply a result of the phi phenomenon causing the perception of apparent movement between discontinuous flashes, as always. The spokes of the wheel are filmed at a certain frequency. The camera records the spokes as they turn, capturing distinct instants of the rotation. If in the second of two film frames a given spoke is closer to the spoke behind it rather than the spoke ahead of it at the moment the shutter clicked, than the apparent movement will create a backwards turning wheel. These perceptual effects are discussed in most textbooks on biopsychology and sensation and perception. A good discussion of these effects as they bear on film perception can be found in "L'Image" by Jacques Aumont, Nathan University Press, Paris. -Pip Chodorov <[log in to unmask]> ---- To signoff SCREEN-L, e-mail [log in to unmask] and put SIGNOFF SCREEN-L in the message. Problems? Contact [log in to unmask]