De la primera herramienta formulario A1
III. Componente Estructural - Traducción: Luego de realizar el estudio correspondiente
8. C ONCLUSIONES DE LA pRUEBA pILOTO (Aplicación de la Herramienta -
If you put this book on the floor, it does not suddenly look like part of the floor; if you walk slowly away from it, it does not seem to diminish in size. These are examples of perceptual organization. Perceptual organization integrates sensations into percepts, locates them in space, and preserves their meaning as the perceiver examines them from different vantage points. Here we explore four aspects of perceptual organization: form perception, depth or distance perception, motion perception, and perceptual constancy. FORM PERCEPTION Form perception refers to the organization of sensations into
meaningful shapes and patterns. When you look at this book, you do not perceive it as a patternless collection of molecules. Nor do you perceive it as part of your leg, even though it may be resting in your lap, or think a piece of it has disappeared simply because your hand or pen is blocking your vision of it.
Gestalt Principles The first psychologists to study form perception systematically were the Gestalt psychologists of the early twentieth century. As noted in Chapter 1,
gestalt is a German word that translates loosely to “whole” or “form.” Proponents of
the Gestalt approach argued that in perception the whole (the percept) is greater than the sum of its sensory parts.
Consider the ambiguous picture in Figure 4.27, which some people see as an old woman with a scarf over her head and others see as a young woman with a feather coming out of a stylish hat. Depending on the perceiver’s gestalt, or whole view of the picture, the short black line in the middle could be either the old woman’s mouth or the young woman’s necklace.
Based on experiments conducted in the 1920s and 1930s, the Gestalt psychologists proposed a small number of basic perceptual rules the brain automatically and uncon- sciously follows as it organizes sensory input into meaningful wholes (Figure 4.28).
perceptual organization the process of
integrating sensations into meaningful perceptual units
percepts meaningful perceptual units, such as
images of particular objects
form perception the organization of
sensations into meaningful shapes and patterns
FIGURE 4.27 An ambiguous figure. Whether the perceiver forms a global image of a young or an old woman determines the meaning of each part of the picture; what looks like a young woman’s nose from one perspective looks like a wart on an old woman’s nose from another. The perception of the whole even leads to different inferences about the coat the woman is wearing: In one case, it appears to be a stylish fur, whereas in the other, it is more likely to be interpreted as an old overcoat. (Source: Boring, 1930.)
PERCEPTION 143
Figure-ground perception: People inherently distinguish between figure (the
object they are viewing) and ground (or background), such as words in black ink against a white page.
Similarity: The brain tends to group similar elements together, such as the
circles that form the letter R in Figure 4.28a.
Proximity(nearness): The brain tends to group together objects that are close
to one another. In Figure 4.28b, the first six lines have no particular organiza- tion, whereas the same six lines arranged somewhat differently in the second part of the panel are perceived as three pairs.
Good continuation: If possible, the brain organizes stimuli into continuous
lines or patterns rather than discontinuous elements. In Figure 4.28c, the fig- ure appears to show an X superimposed on a circle, rather than pieces of a pie with lines extending beyond the pie’s perimeter.
Simplicity: People tend to perceive the simplest pattern possible. Most people
perceive Figure 4.28d as a heart with an arrow through it because that is the simplest interpretation.
Closure: Where possible, people tend to perceive incomplete figures as com-
plete. If part of a familiar pattern or shape is missing, perceptual processes complete the pattern, as in the triangle shown in Figure 4.28e The second part of Figure 4.28e demonstrates another type of closure (sometimes called illusory
contour) (Albert, 1993; Kanizsa, 1976). People see two overlapping triangles,
but, in fact, neither one exists; the brain simply fills in the gaps to perceive familiar patterns. Covering the notched yellow circles reveals that the solid white triangle is entirely an illusion. The brain treats illusory contours as if they were real because illusory contours activate the same areas of early visual processing in the visual cortex as real contours (Mendola et al., 1999).
Although Gestalt principles are most obvious with visual perception, they apply to other senses as well. For example, the figure–ground principle applies when people at- tend to the voice of a server in a noisy restaurant; her voice becomes figure and all other sounds, ground. In music perception, good continuation allows people to hear a series of notes as a melody; similarity allows them to recognize a melody played on a violin while other instruments are playing; and proximity groups notes played together as a chord.
From an evolutionary perspective, the Gestalt principles exemplify the way the brain organizes perceptual experience to reflect the regularities of nature. In nature, the parts of objects tend to be near one another and attached. Thus, the principles of proximity and good continuation are useful perceptual rules of thumb. Similarly, ob- jects often partially block, or occlude, other objects, as when a squirrel crawls up the bark of a tree. The principle of closure leads humans and other animals to assume the existence of the part of the tree that is covered by the squirrel’s body.
Combining Features More recent research has focused on the question of how the brain combines the simple features detected in primary areas of the cortex (particularly
figure–ground perception a fundamental rule
of perception described by Gestalt psychology which states that people inherently differentiate between figure (the object they are viewing, sound to which they are listening, etc.) and ground (background)
similarity a Gestalt rule of perception which
states that the brain tends to group similar elements within a perceptual field
proximity a Gestalt rule of perception which
states that, other things being equal, the brain groups objects together that are close to each other
good continuation a Gestalt rule of
perception which states that, if possible, the brain organizes stimuli into continuous lines or patterns rather than discontinuous elements
simplicity a Gestalt rule of perception which
states that people tend to perceive the simplest pattern possible
closure a Gestalt rule of perception which
states that people tend to perceive incomplete figures as complete
FIGURE 4.28 Gestalt principles of form perception. The Gestalt psychologists discovered a set of laws of perceptual organization, including (a) similarity, (b) proximity, (c) good continuation, (d) simplicity, and (e) closure. (Source: Part (e) adapted from Kanizsa, 1976.)
(a) (b) (c) (d) (e)
the primary visual cortex) into larger units that can be used to identify objects. Object identification requires matching the current stimulus array against past percepts stored in memory to determine the identity of the object (such as a ball, a chair, or a particular person’s face). Imaging studies and research on patients and animals with temporal lobe lesions suggest that this process occurs along the “what” visual pathway.
One prominent theory of how the brain forms and recognizes images was developed by Irving Biederman (1987, 1990; Bar & Biederman, 1998). Consider the fol- lowing common scenario. It is late at night, and you are channel surfing—rapidly pressing the television remote control in search of something to watch. From less than a second’s glance, you can readily perceive what most shows are about and whether they might be interesting. How does the brain, in less than a second, recognize a complex visual array on a television screen in order to make such a rapid decision?
Biederman and his colleagues have shown that we do not need even a half a second to recognize most scenes; 100 milliseconds—a tenth of a second—will typically do. Biederman’s theory, called recognition-by-components, asserts that we perceive and categorize objects in our environment by breaking them down into component parts and then matching the components and the way they are arranged against similar “sketches” stored in memory. According to this theory, the brain combines the simple features extracted by the primary cortex (such as lines of particular orientations) into a small number of elementary geometrical forms (called geons, for “geometric ions”). From this geometrical “alphabet” of 20 to 30 geons, the outlines of virtually any object can be constructed, just as millions of words can be constructed from an alphabet of
26 letters. Figure 4.29 presents examples of some of these geons. Biederman argues that combining primitive visual sensations into geons not only allows rapid identification of objects but also explains why we can recognize objects even when parts of them are blocked or missing. The reason is that the Gestalt principles, such as good continuation, apply to perception of geons. In other words, the brain fills in gaps in a segment of a geon, such as a blocked piece of a circle. The theory predicts, and research sup- ports the prediction, that failures in identifying objects should occur if the lines where separate geons connect are missing or ambiguous, so that the brain can no longer tell where one compo- nent ends and another begins (Figure 4.30).
Recognition-by-components is not a complete theory of form perception. It was intended to explain how people make relatively rapid initial determinations about what they are seeing and what might be worth closer inspection. More subtle discriminations re- quire additional analysis of qualities such as color, texture, and movement, as well as the integration of these different mental “maps” (Ullman, 1995). For example, participants asked to find a triangle in a large array of geometric shapes can do so very quick- ly, whether the triangle is one of 10 or 50 other shapes (Tries- man, 1986). If they are asked to find the red triangle, not only does their response time increase, but the length of time required is directly proportional to the number of other geometric shapes in view. Apparently, making judgments about the conjunction of two
recognition-by-components the theory
whichasserts that we perceive and categorize objects in our environment by breaking them down into component parts and then matching the components and the way they are arranged against similar “sketches” stored in memory
FIGURE 4.29 Recognition by components. The simple geons in (a) can be used to create thousands of different objects (b) simply by altering the relations among them, such as their relative size and placement. (Source: Biederman, 1990, p. 49.)
(a) (b) 1 1 2 2 2 3 3 3 3 3 3 3 4 4 5 5 5 5 5
FIGURE 4.30 Identifiable and unidentifiable images. People can rapidly identify objects (a) even if many parts of them are missing, as long as the relations among their components, or geons, remain clear (b). When they can no longer tell where one geon ends and another begins (c), the ability to identify the objects will disappear. (Source: Biederman, 1987, p. 135.)
PERCEPTION 145
attributes—in this case, shape and color—requires not only consulting two maps (one of shape and the other of color) but also superimposing one on the other. That we can carry out such complex computations as quickly as we can is remarkable.
Perceptual Illusions Sometimes the brain’s efforts to organize sensations into co- herent and accurate percepts fail. This is the case with perceptual illusions, in which normal perceptual processes produce perceptual misinterpretations. Impos- sible figures are one such type of illusion; they provide conflicting cues for three- dimensional organization, as illustrated in Figure 4.31. Recognizing the impossibility of these figures takes time because the brain attempts to impose order by using principles such as simplicity on data that allow no simple solution. Each portion of an impossible figure is credible, but as soon as the brain organizes sensations in one way, another part of the figure invalidates it. Other illusions, although not impossible figures, still play tricks on us. Roger Shepherd’s turning tables illusion in Figure 4.32 represent one such illusion. Although the tables are, in fact, the same size, our brain does not process the information that way.
I N T E R I M S U M M A R Y
Perception involves the organization and interpretation of sensory experience. Form
perception refers to the organization of sensations into meaningful shapes and patterns
(percepts). The Gestalt psychologists described several principles of form perception. More recently, a theory called recognition-by-components has argued that people perceive and categorize objects by first breaking them down into elementary units. The brain’s efforts to organize percepts can sometimes produce perceptual illusions.
DEPTh PERCEPTION A second aspect of perceptual organization is depth, or dis-
tance, perception. You perceive this book as having height, width, and breadth and
being at a particular distance; a skilled athlete can throw a ball 15 yards into a small hoop not much bigger than the ball. We make three-dimensional judgments such as these based on a two- dimensional retinal image—and do so with such rapidity that we have no awareness of the computations our nervous system is making. Julian Beever has mastered this phenomenon in his sidewalk art (Figure 4.33).
perceptual illusions perceptual
misinterpretations produced in the course of normal perceptual processes
depth perception the organization of
perception in three dimensions; also called distance perception
(a) (b)
FIGURE 4.31 Impossible figures. The brain cannot form a stable percept because each time it does, another segment of the figure renders the percept impossible. Escher, who painted the impossible figure in (b), made use of perceptual research.
FIGURE 4.32 Roger Shepherd’s turning tables illusion. The tables are the same shape and size in spite of the fact that our brain processes them as different shapes and sizes.
Although we focus again on the visual system, other sensory systems provide cues for depth perception as well, such as auditory cues and kinesthetic sensa- tions about the extension of the body. Two kinds of visual information provide particularly important information about depth and distance: binocular cues and monocular cues.
Binocular Cues Because the eyes are in slightly different locations, all but the most distant objects produce a different image on each retina, or a retinal disparity. To see this in action, hold your finger about 6 inches from your nose and alternately close your left and right eye. You will note that each eye sees your finger in a slightly differ- ent position. Now, do the same for a distant object; you will note only minimal differ- ences between the views. Retinal disparity is greatest for close objects and diminishes with distance.
How does the brain translate retinal disparity into depth perception? Most cells in the primary visual cortex are binocular cells. Some of these cells respond most
binocular cues visual input integrated from
two eyes that provides perception of depth
monocular cues visual input from a single eye
alone that contributes to depth perception
binocular cells neurons that receive
information from both eyes
FIGURE 4.33 Depth perception. The sidewalk drawing by Julian Beever on the left is constructed on a flat surface but appears to be three-dimensional—until, that is, a side angle is viewed.
(a) (b)
FIGURE 4.34 Monocular depth cues. The photo of the Taj Mahal in India illustrates all of the monocular cues to depth perception: interposition (the trees blocking the sidewalk and the front of the building), elevation (the most distant object seems to be the highest), texture gradient (the rela- tive clarity of the breaks in the walkways closer to the camera), linear perspective (the convergence of the lines of the walkways surrounding the water), shading (the indentation of the arches toward the top of the building), aerial perspective (the lack of the detail of the bird in the distance), familiar size (the person standing on the walkway who seems tiny), and relative size (the diminishing size of the trees as they are farther away).
PERCEPTION 147
vigorously when the same input arrives from each eye, whether the input is a vertical line, a horizontal line, or a line moving in one direction. Other binocular cells respond to disparities between the eyes.
Like many cells receptive to particular orientations, binocular cells require en- vironmental input early in life to assume their normal functions. Researchers have learned about binocular cells by allowing kittens to see with only one eye at a time, covering one eye or the other on alternate days. As adults, these cats are unable to use binocular cues for depth (Blake & Hirsch, 1975; Crair et al., 1998; Packwood & Gordon, 1975).
Another binocular cue, convergence, is actually more kinesthetic than visual. When looking at a close object (such as your finger 6 inches in front of your face), the eyes converge, whereas distant objects require ocular divergence. Convergence of the eyes toward each other thus creates a distance cue produced by muscle move- ments in the eyes.
Monocular Cues Although binocular cues are extremely important for depth per- ception, people do not crash their cars whenever an eyelash momentarily gets into one eye because they can still rely on monocular cues. The photograph of the Taj Mahal in Figure 4.34 illustrates the main monocular depth cues involved even when we look at a nonmoving scene:
Interposition: When one object blocks part of another, the obstructed object is
perceived as more distant.
Elevation: Objects farther away are higher on a person’s plane of view and
thus appear higher up toward the horizon.
Texture gradient: Textured surfaces, such as cobblestones or grained wood,
appear coarser at close range, and finer and more densely packed at greater distances.
Linear perspective: Parallel lines appear to converge in the distance.
Shading: The brain assumes that light comes from above and hence interprets
shading differently toward the top or the bottom of an object.
Aerial perspective: Since light scatters as it passes through space, and espe-
cially through moist or polluted air, objects at greater distances appear fuzzier than those nearby.
Familiar size: People tend to assume an object is its usual size and therefore
perceive familiar objects that appear small as distant.
Relative size: When looking at two objects known to be of similar size, people
perceive the smaller object as farther away.
Artists working in two-dimensional media rely on monocular depth cues to rep- resent a three-dimensional world. Thus, people have used interposition and eleva- tion to convey depth for thousands of years. Other cues, however, such as linear perspective, were not discovered until as late as the fifteenth century; as a result, art before that time appears flat to the modern eye. Although some monocular cues ap- pear to be innate, cross-cultural research suggests that perceiving three dimensions in two-dimensional drawings is partially learned. For example, people in technologi- cally less developed cultures who have never seen photography often initially have difficulty recognizing even their own images in two-dimensional form (Berry et al., 1992).
A final monocular depth cue arises from movement. When people move, images of nearby objects sweep across their field of vision faster than objects farther away. This disparity in apparent velocity produces a depth cue called motion parallax. The relative motion of nearby versus distant objects is particularly striking when we look out the window of a moving car or train. Nearby trees appear to speed by, whereas