Persistent Fallacy Gives Good Rhetoric

By James Leahy

“Persistence of vision”, according to the lead article in the last issue of Vertigo, “describes the optical illusion that makes cinema possible”. Nonsense. Movies move, and depend upon an illusion of movement. Julian Hochberg gives a succinct account: “Motion pictures are possible because we perceive continuous movement in response to a rapid succession of static views. The phenomenon is often called apparent movement (or stroboscopic movement). Writers about the cinema routinely appeal to ‘visual persistence’ to explain this fundamental phenomenon. Such persistence, however, though real enough (see after-images) simply cannot explain apparent movement. At best, it names the fact that we do not detect the brief periods ... during which the motion picture screen is dark. At worst, persistence would result in the superposition of successive views” (“The Perception of Motion Pictures”, p. 604 of The Oxford Companion to the Mind).

Persistence of vision, then, may underpin fusion, but there’s no intrinsic link between fusion and illusions of movement. Fusion is demonstrated by a single light source every time we fail to perceive that our light bulbs, powered by a 240-cycle-per-second alternating current, go off each time the current reverses.

Scientists have been quite clear about these phenomena for more than ninety years, following the identification and description of several illusions of movement by Max Wertheimer in a paper published in 1912. This gave an account of a two-year programme of research, one, legend has it, provoked by a train journey. During a delay in Frankfurt station, Wertheimer had an experience that has been shared by many of us. The movement of the train at the neighbouring platform made it appear for a moment that his own train was moving. Before continuing his journey, he rushed out of the station to purchase a child’s stroboscope. Fellow psychologists Kurt Koffka and Wolfgang Köhler assisted him in the research, which led directly to the birth of the Gestalt School of experimental psychology.

Wertheimer referred to the illusion that is the basis of the cinema as “the phi phenomenon”. It is easily demonstrated using two bright lights with a dark background. If the first is turned off shortly before the second is turned on, the viewer perceives a single light moving from the first position to the second. An ideal demonstration apparatus (both “hands on” and cheap) would allow the viewer/experimenter to vary the intensities of the two lights, their distance apart, and the time lapse between one going off and the other on. This would reveal that the illusion is only effective within certain limits of distance, time and brightness. For example, when the lights are too far apart they are seen as two. The same happens when the gap between switching off and on is too long. Modern research indicates that phi movement is an illusion generated at the level of the central cortex, whereas persistence of vision is a retinal phenomenon.

Why the persistent potency of persistence of vision? Clearly the notion encourages a certain kind of rhetorical flourish, one which proposes a continuity where science suggests the more likely product to be a blur. Then one must never underestimate the “Yah Boo School” of academic debate: “My theory is as good as yours!” However, the latter assertion confuses a phenomenon with a theory. If an apple falls on a scientist’s head, that’s a phenomenon; if the scientist picks himself up, and links the fall of the apple to the motion of the moon round the earth, and the earth round the sun, he’s generated a theory, one which can be tested by, for example, seeing if it predicts or can explain other observable phenomena. After-images, phi movement and their much-loved off-spring the movies are all phenomena, not theories.

The desire to compartmentalize knowledge (undertaken, perhaps, because it may be easier to measure its acquisition by cutting it up into discrete chunks) is another factor in keeping error alive. Thus, in the Micropædia of the 2002 printing of The Encyclopædia Britannica, in the article in Volume 8 headed “motion picture”, one reads that “an optical phenomenon known as persistence of vision gives the illusion of actual, smooth and continuous motion.” 19 pages later, on p. 381, there’s a more accurate account, in the discussion of “movement perception”. Here persistence of vision is linked to “the so-called phi function”; these are described as the two phenomena which “are exploited by motion pictures, which consist of rapid successions of still frames”. Ironically, Max Wertheimer’s son Michael, an emeritus professor of psychology in his own right, is listed as an Adviser on the Authority of entries in the Micropædia. Guess he wasn’t asked to read the entry entitled “motion picture”!

To explore this topic further, go to where you’ll find a selection of references and quotations compiled and introduced by Stephen Herbert.

James Leahy first saw a laboratory demonstration of the “phi phenomenon” in 1959, during his second year at Cambridge. He was then studying Psychology, a Part 2 of the Natural Sciences Tripos. In the same term he had his first piece of film criticism published. This ultimately led to his becoming an editor of Vertigo. Richard Gregory, the lecturer who introduced and explained the demonstration, went on to write Eye and Brain: the Psychology of Seeing, become a Fellow of the Royal Society, and edit The Oxford Companion to the Mind.