The general concept of visually perceiving and misperceiving
movement in scenes is similar to the perception and misperception
of still images. The viewer's eyes take in a limited amount information
(limited by viewpoint, optical abilities, etc) and physiologically/mentally
translates the information into a perception. The human uses
its complex mental template to make the final perception, or
judgment of what is going on. The template was formed by experience,
knowledge, genetic tendencies, physiological abilities (your
visual template is literally blind to the ultraviolet light that
birds see, and the infrared light snakes see), personal bias,
aesthetics, etc. Often the final interpretation, or perception,
is a correct representation of what is being viewed. Sometimes
the perception is off.
Except for more extreme situations (very slow movement, very
small objects), the human eyes/mind is good at detecting the
presence of movement in its field of view. The misperceptions
most commonly happen in the interpretation of the movement. Humans
can correctly detect the presence of movement, but misinterpret
the speed, the direction, even what is moving. A human can think
object A is moving, when it is object B moving (see 'Parking
lot prank' below).
As with still images, viewers are often faced with ambiguous
situations. The viewed information can be perceived in different
ways, often opposing ways. The ambiguity can be because we have
a limited view of a scene. A limited view can be cause by our
point of view (viewing from the left or right, close or far),
because the view is partially obstructed, or the capabilities
of our eyesight is stretched (object is too far away or it's
getting dark). Even with still images, these types of limitations
can cause illusions.
With ambiguous, multiple choice information, the human uses
its template to pick which choice is correct. Often times the
template's pick is correct, while sometimes the pick is wrong.
The following are examples of being presented with ambiguous
information and making the wrong choice about what is going on.
Parking lot prank
A prank you may have heard about is where the two pranksters
park their two cars, one on each side of a parking space. Sometime
later, the unsuspecting victim parks his car in the space. When
the unsuspecting victim is fiddling with his keys or checking
the contents of his wallets or looking in the glove compartment,
the pranksters suddenly drive foreword or backward in unison.
The victim gets the instant sensation that it is his car moving
and panics. He soon figures out what happens, and is embarrassed.
This is an example where a person correctly identifies the presence
of movement, but misinterprets what is moving. Also note that
his misperception was instinctual, and soon corrected when he
figured out what happened ("Damn kids.").
*** In baseball, pitchers use a so called changeup pitch to
fool the batter. A changeup is intended to resemble a fastball
but is slower. The changeup is typically thrown after a fastball,
often consecutive fastballs. Then, seeing the identical fastball
arm and body motion from the pitcher, the batter believes the
ball is again coming fast and swings accordingly. When the changeup
works for the pitcher, the unexpected speed results in the hitter
making feeble or no contact with the ball. A batter can look
the fool swinging with all his might at a slow pitch.
In this case, the batter correctly perceives that the baseball
is moving, but incorrectly guesses its speed. Also note that
batter's perception of speed was not based on ball movement itself,
but the batter's expectations, expectations based on past experience.
famed changeup pitcher, Greg Maddux
* * * *
Duly note that a wrong pick doesn't always mean it was a bad
one. If it looks as if someone's about to throw a brick at you,
it's a prudent move to react as if he will even if he wasn't
going to. When a car is moving towards you, its not a bad idea
to overestimate its speed. These and other perception choices
are more about self preservation than statistical accuracy. Also
not that many instances, for example with the parking lot prank,
a person must react instantly to the ambiguous information. Humans
wouldn't have lasted long on this earth if they weren't able
to react to movement automatically, instinctually, even if sometimes
incorrectly.
Also note that many of the misperceptions were corrected with
time and knowledge. Both the parking lot patsy and the baseball
hitter corrected their misperceptions. Humans aren't omniscient
svengalis. Our perceptions are honed, corrected and filled in
by knowledge, experience, new viewpoints.
* * * *
How we judge movement in motionless images
Judging and perceiving movement in still images illustrates
how we judge movement using things other than movement itself.
We use the overall scene, our experience and expectations about
what is going on. Discussing the movement in motionless images
(which of course is oxymoric) reveals aspects of our template--
what motionless conditions and qualities indicate movement in
our minds.
Even though they are motionless, we see dogs in the act of
running. Blurred ground, field, open mouths and hanging tongues,
ears titled back, all evoke the sense of movement.
Movement and direction is inferred by the blurriness of ball
and background, and even how the image was cropped.
We not only perceive wind, but wind moving left to right
The tree was motionless at the moment the photo was shot,
but have a story about the tree's past and future movement. We
assume lightening knocked down the tree during a storm. We can
also umagine the tree being chopped up and taken away.
Ambiguous movement: The Barber pole illusion
There are instances where, due to limited including obscured
viewpoint, one cannot determine the direction of movement. A
standard example is the candy striped barber pole.
A barber pole has diagonal stripes and is rotated to the left
or right. Looking from a particular angle it can look as if the
stripes are moving straight up or down (depending on direction
of rotation). Faced with different plausible choices (moving
up or rotating), humans unconsciously pick one. The pick may
or may not be correct.
.....
Strobescopic Effect and Ambiguous movement
While humans perceive realistic movement in the quick flashing
of still images that is a movie, there is a strange, unrealistic
movement they sometimes notice. In old time Westerns, the wheels
of a moving wagon sometimes appear to be still, rotating slower
than they should or even rotating backwards. This happens when
the rotation speed of the spokes is not in synchronicity with
speed of the film.
The below three still images of a wagon wheel look to show
the wheel in the same position, but they show the wheel at different
rotations. The middle picture is rotated 90 degrees from the
left image, and the right is rotated an additional 90 degrees.
That each spoke is shaped and colored identical to the others
is essential to the illusion. If these were the stills in a movie
the rotating wheel would appear to be motionless. If they were
the stills in a movie but the rotation was 80 degrees instead
of 90, the wheels would appear to be going backwards.
The wagon wheel illusion in a movie is an example of the stroboscopic
effect. In the dark, a strobe gives off intermittent flashes
of light. The viewer views a moving object though short intermittent
snapshots rather than a continuous view. This can lead to misperception
of movement, such as with the movie wagon wheel.
Say you are watching a swinging pendulum under stroboscopic
lighting. If the strobe flashes a quick burst of light once every
second and it takes the pendulum exactly one second to swing
back and forth, the pendulum will appear to be motionless. Each
flash catches the pendulum in the same position, the pendulum
having done quite a bit of moving in the dark. If the flashes
catch the pendulum at its extreme right position, the pendulum
will appear to being pulled or blown right.
This is an example of the human making an Occham's razor perception
from the limited information. The information is ambiguous, there
are different possible explanations for what you see, and the
viewer chooses the simplest explanation, the one that seems to
make the most sense. If you and others saw no movement in an
object, it would be considered bizarre for you to proclaim that
the object was moving. However, this bizarre proclemation would
be correct with the pendulum. This should offer some insight
into the limitations of Ockham's
razor. Sometimes the simplest explanation is wrong because
it formed from limited information.
The human's perception of flashing still images, including
with movies and strobes, bring up interesting questions about
how human perceive and about our understanding of very fast objects.
Do the above snapshots show a moving or still pendulum? One
can't tell.
Animated advertising signs that use a grid of flashing lights
give an example of how humans interpret changing information.
When you look at the above, it soon appears that there is a single
yellow dot moving around and around. However, it is many dots
changing color (the equivalent of lights turning on and off).
Bank signs and sport stadium scoreboards use this technique to
create apparent animated words and characters. When one doesn't
know what is happening (of and on lights), the information can
be interpreted different ways, but is usually perceived as one
light moving. Even when one knows what is happening, it is still
perceived as a single moving light. The phenomenon is commonly
referred to as the Phi phenomenon.
There is a different but related phenomenon of the Phi phenomenum,
called the Beta phenomenon. An example of this is when two different,
but same looking pictures of balls or dots or whatever are flashed
one after the other on a screen. Even though they are two distinct
pictures, humans commonly perceive it as one thing moving. Whether
the two images are flashed in quick succession or with longer
delay, the perception of a single object moving is the same.
Movies as quasi strobe images
When watching a movie, the people or horses on the screen
appear to move realistically, but are stroboscopic-like flashes
of still images. To the mind, this quick succession of still
images of a horse most closely matches real movement. The mind
is faced with information that can be interpreted different ways.
To the mind, real movement may not be the perfect choice (the
eyes and detect the shuttering of the film), but it seems better
than other choices.
"Close enough counts"
As the movie motion shows, visual information that is close enough
to what the brain's template detects as real can be perceived
as real. When judging ambiguous information, the brain chooses
what the information most closely resembles. The motion picture
running horse information closely, if not totally, matches up
with brain's template for a running horse, so is perceived as
a running horse. Close enough is good enough. The mind often
interprets the images from movies and illusory strobe flashed
objects to be moving differently than they are, because the illusory
movement seems more plausible.
Neurons in our unconscious brains are used to identify and
interpret movement. They help identify not only the presence
of movement, but judge the direction and speed. There are still
images with patterns that the neurons interpret, if only briefly,
as motion. The below are examples.
