PERCEIVING AN ORGANIZED WORLD

Perceptual Organization:

Perceptual organization occurs when one groups the basic elements of the sensory world into the coherent objects that one perceives. The ability to organize the perceptual world makes one's perception more efficient; that is, perceptual judgements can be made accurately in a short period of time. It is the basis for many special perceptual abilities, such as reading a road map, identifying the melody in an unfamiliar piano piece, recognizing the face of a friend, and seeing various animal shapes in the clouds.

There are two basic principles of perceptual organization. One involves perceiving the world in terms of a figure embedded in some perceptual background. The other asserts that organization tends toward simplicity.

Organizing the perceptual world generally means identifying a part of the world as the target of perception and viewing that target image in relation to its surrounding stimuli. The target stimulus is called the figure, while the surrounding stimuli make up the ground, or background. This differentiation is called figure-ground perception. In perception, we give most of our attention to the figure. Studies of the way people view pictures have shown that most attention is given to the figure and little is reserved for the ground. This difference was recorded by recording the eye movement patterns of individuals while they look at pictures. Some important factors that influence the way people determine the figure from the ground include size: smaller areas are more likely to be seen as figure than larger areas. Familiarity is also important, meaning that familiar shapes and forms are more often perceived as figures. Also, objects that are symmetrical are more often seen as figures.

The Gestalt psychologists proposed the law of Pragnanz, also called the law of simplicity, which stated that we have a tendency to see things in the simplest form possible. Some perceptual elements are seen as belonging together because they provide better continuity of the stimulus.

Another principle involved in organization by minimum tendency is called closure. Closure refers to the tendency to fill information that is missing from the perceptual array by closing in gaps. For example, if one was given a picture of a half completed triangle, and they were asked to describe what the figure was, the normal tendency would be to say it was a triangle, even though most of the triangle is missing. This tendency is referred to as closure.

A third organizing tendency in perception refers tot he ways in which perceptual elements are grouped, or laws of grouping. Two such laws are similarity and proximity. As the names of these laws convey, similarity refers to the tendency to group things on the basis of how similar they are to one another. Proximity refers to the closeness, in terms of physical location, objects are to one another.

Although the above senses are all visual sense, these organizing principles exist for all senses. For example, figure-ground principles can be seen in the way people perceive music; they listen to the melody as distinct from its harmonies. At times, the instrument or voice dominates the others in a group, thus becoming the figure, while the other voices and instruments become the ground. In taste, some strong flavors can be distinguished from more subtle flavors, which serve as the ground.

The ability to perceive depth is a function of the arrangement of objects in the perceptual environment, the capacities of the eyes, and the interpretive processes of the brain, which include the use of memory stores. There are two types of depth perception cues: those requiring one eye, and others that require two eyes. Depth perception cues that require only one eye are called monocular cues, while those that require two eyes are called binocular cues.

There are two depth cues that require two eyes: binocular disparity and convergence. Because your eyes are separated by a space, each retina receives a slightly different view of the world. There is a simple way to demonstrate this to yourself. Close your right eye and hold your left thumb about 6 inches in front of your left eye. Then position your right thumb behind your left thumb so that it is hidden from view. If you now look at your fingers with your other eye, you should be able to see both thumbs. This difference in the images received by the two eyes is known as binocular disparity. In a sense, the brain compares the information from the two eyes by overlayng the retinal images. The greater the disagreement between the two retinal patterns, the close the object.

Convergence depends on the muscle tension that results from the external eye muscles that control eye movement. When you look at objects up close, your eyes converge and the tension in the eye muscles is noticeable. By monitoring the tension of the eye muscles as an object is moved closer, the brain is provided with a measure of convergence that can be used to make judgments about the distance of objects.

When your head is moved, the images produced by objects in the environment move across the retina. The relative rate of this movement will vary as a function of distance, providing a cue for depth known as motion parallax. This cue is easy to see in a moving vehicle. For example, when you are traveling on a train and looking out the window, nearby objects will appear to move by rapidly while objects that are farther away will appear to move more slowly.

The elevation of objects above the horizon in our visual field is an important cue to their depth. Objects located higher in the field are farther away. This cue is very important to artists in simulating depth in their paintings.

Objects in our environment often overlap, with nearer objects obscuring parts of distant objects. This depth cue is known as interposition or superposition.

As parallel lines recede in the distance, the lines appear to come closer and closer together. Standing on a railroad track and looking down the rails, or driving down a straight stretch of highway and observing the road ahead are good ways to demonstrate this cue known as linear perspective. Many children use this perspective in demonstrating depth in their artwork.

Another depth cue comes from the changes in coloration of distant objects. Objects that are farther away appear to be more bluish. This color change is primarily a function of atmospheric haze, but it also depends on the level of illumination. This cue, known as aerial perspective, is only operative outdoors and at considerable distances.

Brightness is also another cue we use to judge depth. If two objects are the same size and exist at the same distance from us but differ in brightness, we will perceive the dimmer object as being farther away. This judgement is due to relative brightness, a cue we have learned to depend on because brighter objects are typically closer to us.

James J. Gibson proposed the higher-order information, produced by the relative position and movement of objects may allow you to perceive depth and size directly and do so without performing and complex calculations or computations. Texture gradient, one of Gibson's depth cues, states that objects that make up the texture gradient tend to be about the same size. Consequently, their change in retinal size provides you with a cue for depth. According to Gibson, this allows you to infer the depth of most objects directly in the natural environment by simply noting their location on the texture gradient.

A debate that has been argued on for a long time is whether or not depth cues are innate or learned. Research has demonstrated that it is both. At birth, the binocular cues are of little use, because the infant cannot coordinate its eye movements and will not be able to do so reliably until it is approximately 3 months of age. And even then, infants are not capable of interpreting the disparate images from the eyes in terms of depth, because the ability to use the cue of binocular disparity does not emerge until some time between the ages of 3.5 and 6 months. This acquisition of binocular skills may be the result of learning or simply due to maturation, but some capability, probably monocular in nature, exists at birth.

Researchers have also studied whether depth cues are innate or learned in animals. One way to test this is to place animals on the visual cliff, where the floor of a table drops away a few feet. When a young kid or lamb that has just learned to walk is placed on the shallow side, it avoids venturing over the cliff, even though it faces no danger in doing so. Similarly, chicks less than a day old avoid the area where the floor is recessed. Other studies have shown that even animals raised in impoverished environments that lack normal depth cues avoid the deep side of the visual cliff.

Testing human infants on the visual cliff raises some ethical questions. Infants must be able to crawl before they can be tested, and this is when they are typically 6 months old or older. During that time, they have had many experiences that could contribute to the learning of depth perception. Studies of human infants show that these babies also prefer the shallow side to the deep side, but since human babies could have learned some depth cues, these studies could not establish the real answer to the question. However, another method to test this ability has been established. Using newborn infants, researchers suspended them in a sling above either the shallow or deep sides of the cliff and found a difference in the infants' hear rate when the infants viewed the deep versus the shallow side. This difference did not necessarily indicate that the infants were afraid of the deep side; it may have meant simply that the infants were showing a greater interest in one side than the other. But it did show that infants could perceive some elements of depth.

A somewhat similar process to depth perception in the visual environment exists for the sense of hearing and is usually labeled auditory localization, the ability to locate the direction and distance of sounds. Audition depends largely on two ears (binaural) for sound localization, although sounds can be localized using only one ear (monaural), but that is difficult and often inaccurate. Sound localization is important for many animals and especially critical for some. Owls and bats use localization to find food, and frogs use it to locate a mate. And humans also use this: teachers use it to locate the person that has just spoken in class.

Sounds are localized in binaural listening by two important cues: time differences and intensity differences. A sound located to your left will reach your left ear slightly ahead of its arrival at your right ear. Similarly, it will reach your left ear with slightly greater sound intensity. Although these differences are very small, it is sufficient for the brain to use in judging the distance and direction of the sound source. Time differences are especially important for localizing sounds below 1500Hz (low sounds), while intensity differences are most useful for higher frequencies, especially those above 6000Hz. Sounds that are in the median plane, or directly in front or behind you, will reach the ears with equal intensity and at the same time. The solution to this problem would be to move the head, which will then create the interaural differences needed to localize the sound successfully.

The appearance of objects in our perceptual world is always changing. However, it is important to understand that these changes are in appearance and do not represent changes in the object itself. Many times, in young children, the concept of size constancy does not exist. This holds that objects stay the same size, even though the size of the retinal image changes as the distance of the object varies. Children, often seeing images far away, think that these objects are actually very small, when in fact, when coming closer, the objects appear bigger.

There are many changes in the perceptual world, and we need only respond to the ones that are meaningful. For example, the visual system is interested in properties of objects and not in the accidental properties of light reflected from those objects. Thus, we want to see the same color of an object even if the light source changes from the sun to a light bulb. We want to see a square as a square even if it is view from an oblique angle. And we want to detect the motion of the object, not the motion of our eyes. Perceptual constancies provide us with a way of creating some stability in a world of constant flux.

Objects that exist in the real world and as images on our retina represent two kinds of stimuli. The stimulus of the object as it actually exists is called the distal stimulus, whereas the retinal image of that object is called the proximal stimulus. When we move away from objects, or when they move away from us, the distal stimulus remains the same, but the proximal stimulus changes in size. Once size constancy has been learned, the perceiver correctly learns to interpret changes in the retinal image size as a cue to changes in distance. Research has shown that infants, even in their first year, can demonstrate some minimal size constancy, but their performance does not equal adult's performance until the age of 6.

An important variable in size constancy is distance. Imagine viewing an airplane against a cloudless blue sky. You might perceive it to be an actual airplane flying at an altitude of 30,000 ft, yet it could be a model airplane whose altitude was only several hundred feet. Against a uniform sky, there are almost no cues for distance, so it becomes difficult to judge the size of the plane accurately, based solely on its retinal image size. This relationship between size and distance was first described by Helmholtz and has become known as the size-distance invariance hypothesis. The hypothesis holds that perceivers make judgments of the actual size of objects by comparing the size of the retinal image with their perception of the object's distance.

Another important factor is relative size. We often make judgments about the size of objects in comparison to other objects located nearby. Suppose that you have set the table for dinner. When you stand near the table, the retinal images of the plates, knives, and spoons are much larger than they will be when you look at the table from a distance of 10 ft. But all of those retinal images are changing in size together, which provides an added cue to the task of judging the actual size of any one object.

The fact that the shape of an object, such as a door, does not change, even though the retinal image changes, is an example of perceptual constancy known as shape constancy. If you look at an ordinary door, closed in front of you, the image produced on your retina is that of a rectangle. Yet if the door is partially opened, the retinal image changes to that of a trapezoid. However, if you ask the perceiver if the door is seen as trapezoidal, the answer will be no.

Also, the size-distance invariance hypothesis has been changed to the shape-slant invariance hypothesis. This hypothesis proposes that shape constancy is the result of a comparison of the shape of the retinal image and the perceived slant of the object. Another example when shape constancy is used is when you view a screen at a slant. The images on the screen are considerably distorted in terms of retinal image shapes, but shape constancy is reasonably good. One explanation for why constancy is maintained in this situation is what Irvin Rock has called the taking-into account theory. The perceptual system picks up information regarding the slant of the screen; that is, this unusual viewing condition is taken into account. These adjustments in the perceptual system occur rapidly and without awareness. As a result, perceptual order is maintained instead of the perceptual chaos that might otherwise result.

When sunlight shines through the window onto a floor, the amount of light reflected from that portion of the floor changes. Yet, that part of the floor is not seen as lighter. The fact that object lightness tends to be perceived as unchanging, despite changes in the amount of light striking the surface of an object is called lightness constancy. This constancy appears to exist from birth.

Lightness is determined essentially by the percentage of light reflected from an object's surface, a quality that is called albedo, defined as the proportion of light reaching a surface that is reflected back to the eye. The albedo is a constant property of any object. Although the light illuminating the object may increase, the light reflected will be a constant proportion of that increase, since the object is usually illuminated by the same light source.

We often perceive movement where no movement occurs. Those illusions of movement are collectively referred to as apparent movement. One example of this kind of movement, the autokinetic effect, is produced by shining a point of light in an otherwise black room. After watching the light for a brief interval, subjects will begin to see it move. Another kind of apparent movement in which nothing moves is called stroboscopic movement, familiar to people as motion pictures. Although the reality of movement in motion pictures seems undeniable, the stimulus for that perceived movement is nothing more than the successive presentation of a series of still pictures, projected on the screen at a speed of 24 frames per second. This projection speed represents the optimal rate, given what is known about processing in the visual system, for movement to be perceived. Psychologists have studied this form of movement for more than 75 years since Max Wertheimer did his pioneering experiments on a form of stroboscopic motion that he called the phi phenomenon. Wertheimer found that two points of light could be successively illuminated so that subjects saw only one light, moving back and forth, rather than two discrete lights blinking on and off in succession. This phenomenon is used today in some advertising signs designed to create the illusion of movement.