Perception and Misperception of Movement

grangeThe general concept of visually perceiving and misperceiving movement in scenes is similar to the perception and misperception of still images (see previous post). 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.”).

Change of speed pitches in baseball.  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

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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.

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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.

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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 (see early blog post).  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.

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