"Persistence of vision" and the perception of motion

[mmcirvin]

This fascinating article fulminates over the persistence of "persistence of vision" as an explanation for the perception of motion in film and video—one that is at best an extreme oversimplification, and completely wrong when it is claimed to reside entirely in the retina. In the process, the article states something I did not know, that there seem to be two different mechanisms of motion perception in the brain, as revealed by an experiment in which subjects look at flashes in different positions presented in rapid succession:

[Kolers and Pomerantz, 1971] found that when two elements appeared on the screen, good illusory motion was seen with proper timing.  (This is the usual binary or two-element display, the limiting case for apparent motion.)  When four, eight, or sixteen elements appeared, smooth continuous motion was never attained.  However, with thirty-two or sixty-four or more elements, smooth continuous motion was again perceived.  Thus, if smooth continuity of motion was rated as a function of number of elements presented, the result would be a U-shaped curve.  Kolers concluded:  "It seems there is no necessary continuity of processing between spatially separated and spatially contiguous flashes; the ways the visual system constructs the two perceptions of motion seems to be quite different." (Kolers 39)

The article goes on to confidently identify motion perception in movies with the "short-range" mechanism associated with closely spaced flashes. But it seems to me that that's not the whole story: isn't the long-range mechanism frequently exploited in cheap-ass limited animation, when only the extremes of a repeated motion are shown in alternation?

It's interesting to speculate, too, about whether and how that long-range mechanism might have evolved adaptively in a world without Hanna, Barbera, or experimental psychologists. Perhaps it allows us to infer motion from the visible extreme positions of an object moving along a curve through space, where it appears to freeze momentarily as it changes direction. There wouldn't be anything in nature corresponding to the four- or eight-element case.

Comments

[pentomino.livejournal.com]

How about an object moving behind a series of obstructions?

That sounds pretty important for a species of intelligent hunters.

[mmcirvin.livejournal.com]

Actually, that could be an adaptive explanation for the related "phi phenomenon (http://www.psych.purdue.edu/Magniphi/PhiIsNotBeta/phi1.html)" (that link has cool demonstrations). In cases like the four or eight flashing balls, you don't see the balls themselves moving, but you often will see a disembodied sensation of motion in the area around them. Which might just be your brain telling you "the gaps between the trees aren't themselves moving, but there's something moving around back there".