The Sense of Viewing Motion: Apparent Motion

These two shots of Erik Sobbe rehearsing for The Nutcracker at American Dance Center studio in December 2010 make a good example of "Short-range (near) apparent motion."

Although the animation (below) created from the two shots makes it appear as if Erik is moving his legs from underneath to outside and back while remaining in the air, in truth these show two succesive leaps with a different action at the top of each leap.

By putting these two shots together in a rapid enough rate we can take advantage of the brain's sampling rate of the environment around it and get the sense that we can "see" the legs move through intermediate positions..

1 2 3 4 5
 Plie  Plie  Plie
On the ground Jump up - legs underneath Back on the ground, set to jump Jump up - legs out Back on the ground


Even today we don't fully know why our vision can give us a sense of motion. But we have a number of answers toward that goal, thanks to wondering why we experience a sense of seeing motion when we see a set of still images, each slightly different from the last. For years we called these still images in sequence movies, or "motion pictures" even though we know that all the images are still images.

A large part of the answers about vision and a sense of motion we worked out early in the last century by an Austrian scientist, Max Wertheimer. At the time a number of theories were being floated, among them the concept called "persistence of vision." This concept assumed that the eye itself held on to each still picture for a while as the next still picture was flashed on a screen.

What "persistence of vision" failed to explain were two things:
1) that if this were the case then our view of "movies" would be one with a series of double images, one fading after another and
2) it failed to even ask the question of how we experience a visual sense of motion at all other times.

Wertheimer showed in 1912 that no image "persisted" on the eye and instead that the brain did what today we might call sampling, at close intervals, checking to see what the eyes were showing against what it had just checked on.

Unfortunately film writers got the wrong information a long time ago and stayed with it, teaching "persistence" for decades, even up to and including today. So a lot of people studying film and video were taught the wrong information because they were not up on the latest scientific experiments - - - from 1912.

Short-range Apparent Motion Phi Motion
Here the objects in the "movement" are close together physically. Note how there seems be a sense of leg movement between tucked under and straight out even though there are no intermediate images. Phi motion is a different category
of visual motion sensing.
Beta motion. This is the kind of animation you can see on lighted theater marquees where a series of items become visible, each in turn, usually around the circumference of the marquee.

Above you can see how the phi-motion animation was constructed from two objects drawn on opposite sides of a frame. Note the timing for on and for off durations.

Each image is on for 2-tenths of a second and off for 5-hundreths of a second (1/20th second)


Persistence of Wrong Information

For years we were told in film classes and film books that "Persistence of Vision" explains why we see a sequence of images on film as motion. You can still run into this teaching in books and classes. However, persistence of vision was disproved way back in 1912 by Max Wertheimer. The original question was the wrong question and they were looking at the wrong structures. As obvious as the eye seems they needed to look at the brain, not the retina. Instead of asking why a series of still images looks like motion they should have asked how we see motion, period. Then you could learn how movies and flip books and other image sequences interact with motion detection mechanisms in the brain.

What we might see if vision did persist as an artifact on the retina.
Imagine the brain trying to make sense of these overlapping images instead of
processing differences between 50 to 60 discrete visual samplings per second.

This quoted section from a paper by Joseph and Barbara Anderson (follow up article from their 1978 article)
It is worth reading in whole:

There is no motion on the screen, just a succession of still images. If there were persistence of these images in the eye of the viewer, figures on the screen would pile up, one on top of the other, resulting in a kind of chronophotographic display.

... ...

It is only with hindsight that the problem seems to divide into such clearly separable categories -- the fusing of the flickering light, called flicker fusion in the literature of perceptual psychology, and the appearance of motion which is referred to as apparent motion.  Early writers, without the benifit of hindsight, continually confused the two issues.

... ...

In 1912 Max Wertheimer published his "Experimental Studies on the Seeing of Motion," the classic work on apparent motion that is cited as the founding work of Gestalt Psychology.  Through a series of experiments utilizing variations of the two-element display, Wertheimer isolated what he considered three primary stages of apparent motion: 
(1) beta movement (the object at A seen as moving across the intervening space to position B),
(2) partial movement (each object seen moving a short distance), and
(3) phi movement (objectless or pure motion).

In a classic experiment he [Wertheimer (1912)] presented two separated lines in rapid succession.  When the time interval between the lines was just right, the observer reported seeing movement between them -- a disembodied movement in which the line did not move from one place to another.  Though the observer still saw stationary lines flashing on and off, movement was experienced between them.  This is now known as the phi phenomenon. (Kaufman 368)

In another series of experiments Wertheimer convincingly refuted the "trace" or after-image theory. (Wertheimer)  His conclusions were clear:  "It is not sufficient to draw upon pure peripheral processes in relation to a single eye: we must have recourse to processes which lie behind the retina." (Wertheimer 1084).

Terms to note when reading this paper: real motion, short-range apparent motion (what we should be calling this phenomenon), long-range apparent motion, fine grain illusion, akinetopsia (inability to see motion, still objects are visible, in-motion objects disappear), persistence of vision (not this) or phi movement (not this) (not always right, seems to change from time to time depending on who is adding info)