CHAPTER PREVIEW

Perception involves the selection, organization, and interpretation of sensory information. It quick-ly became one of psychology’s primary concerns as early researchers attempted to explain illu-sions.

In organizing sensory data into whole perceptions, our first task is to discriminate figure fromground. We then organize the figure into meaningful form by following certain rules for groupingstimuli. We transform two-dimensional retinal images into three-dimensional perceptions by usingbinocular cues, such as retinal disparity, and monocular cues, such as the relative sizes of objects.Our brain computes motion as objects move across the retina. A quick succession of images canalso create an illusion of movement.

The perceptual constancies enable us to perceive objects as enduring in shape, size, and light-ness, regardless of viewing angle, distance, and illumination. The constancies explain several well-known illusions.

Studies of sensory deprivation reveal that, for many species, infancy is a critical period duringwhich experience must activate the brain’s innate visual mechanisms. For example, when cataractsare removed from adults who have been blind from birth, they can distinguish figure and groundand can perceive color, but they are unable to distinguish shapes and forms.

At the same time, human vision is remarkably adaptable. Given glasses that turn the worldupside down, people manage to adapt and move about with ease. Clear evidence that perception isinfluenced by our experience comes from the many demonstrations of perceptual set and contexteffects. Because perceptions vary, they may not be what the designer of a machine assumes.Human factors psychologists study how people perceive and use machines and how machines andphysical environments can be better suited to that use.

Although parapsychologists have tried to document ESP, most research psychologists remainskeptical, particularly because the results of experiments have not been reproducible.

CHAPTER GUIDE

➤ Exercise: Fact or Falsehood?➤ Videos: Moving Images: Experiencing Psychology Through Film, Program 10: Sensation Without Perception: Visual

Prosopagnosia; Discovering Psychology, Updated Edition: Sensation and Perception; Sensation and Perception;Module 8 of Psychology: The Human Experience: Sensation and Perception

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Perception 6:

Selective Attention

➤ Lecture: Inattentional Blindness; Change Blindness; Mindsight—A Sixth Sense➤ Exercises: Field Dependence–Independence; Human Earphones➤ Transparency: 80 Selective Attention➤ Video: Video Clip 23 of Digital Media Archive: Psychology: Neisser’s Selective Attention Test

1. Describe the interplay between attention and perception.

Selective attention means that at any moment, awareness focuses on only a limited aspect of allthat we are capable of experiencing. For example, even if a stimulus figure can evoke more thanone perception, we consciously experience only one at a time. The cocktail party effect providesanother example of selective attention. The ability to attend to one voice among many enables usto converse coherently in the midst of auditory chaos. Selective attention also limits our percep-tion, as many stimuli will pass by unnoticed. This lack of awareness is evident in studies of inattentional blindness. Forms of this include change blindness, change deafness, and choiceblindness.

Perceptual Illusions

➤ Exercises: The Wundt-Jastrow Illusion; Perceptual Illusions and Principles; A Kinetic Depth Illusion; MusicalIllusions and Paradoxes on CD

➤ Projects: Playing Cards and Illusions; Instant Object Recognition; Kinesthetic Capture➤ PsychSim: Visual Illusions➤ Film/Video: An Introduction to Visual Illusions; Segment 10 of the Scientific American Frontiers Series, 2nd ed.:

Lights, Camera, Magic!➤ Transparency: 81 Perceptual Illusions

2. Explain how illusions help us to understand some of the ways we organize stimuli into meaningfulperceptions.

Illusions mislead us by playing on the ways we typically organize and interpret our sensations, andthus understanding illusions provides valuable clues to the ordinary mechanisms of perception.For example, several well-known illusions are based in the perceived relationship between sizeand distance, which is generally valid. Others reflect group principles and assumptions made aboutthe relationship between light and shadow. As visual illusions indicate, among our senses, vision isdominant. When there is a conflict between vision and other sensations, vision usually dominates,a phenomenon called visual capture. Hearing captures touch.

Perceptual Organization

➤ Lectures: Object Recognition; Visual Agnosia; Autostereograms; Auditory Organization➤ Exercises: Perceptual Organization; An Auditory Analogue of the Visual Reversible Figure; The Ganzfeld; Binocular

Vision; Binocular Vision Versus Monocular Vision; Variation in the Size of the Retinal Image; Perceived Distanceand Perceived Size; Brightness Contrast

➤ Project: Perceived Lunar Size➤ Videos: Video Clips 4 and 5 of Digital Media Archive: Psychology: Depth Cues and Müller-Lyer Illusion➤ Transparencies: 82 Reversible Figure and Ground; 83 Organizing Stimuli into Groups; 84 Monocular Cues; 85 The

Interplay Between Perceived Size and Distance

3. Describe Gestalt psychology’s contribution to our understanding of perception.

Gestalt psychologists described principles by which we organize our sensations into perceptions.They provided many compelling demonstrations of how, given a cluster of sensations, the humanperceiver organizes them into a gestalt, a German word meaning a “form” or a “whole.” They fur-ther demonstrated that the whole may differ from the sum of its parts. Clearly, our brains do morethan merely register information about the world. We are always filtering sensory information andinferring perceptions in ways that make sense to us.

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4. Explain the figure-ground relationship, and identify principles of perceptual grouping in form perception.

Our first task in perception is to perceive any object, called the figure, as distinct from its surroundings, called the ground. We must also organize the figure into a meaningful form. Gestaltprinciples for grouping that describe this process include proximity (we group nearby figurestogether), similarity (we group similar figures together), continuity (we perceive smooth, continu-ous patterns rather than discontinuous ones), connectedness (we perceive spots, lines, or areas as asingle unit when uniform and linked), and closure (we fill in gaps to create a whole object).

5. Explain the importance of depth perception, and discuss the contribution of visual cliff research toour understanding of this ability.

Depth perception is the ability to see objects in three dimensions although the images that strikethe eye are two dimensional. It enables us to judge distance. Research on the visual cliff (a minia-ture cliff with a drop-off covered by sturdy glass) reveals that that depth perception is in partinnate. Many species perceive the world in three dimensions at, or very soon after, birth.

6. Describe two binocular cues for perceiving depth, and explain how they help the brain to computedistance.

Binocular cues require information from both eyes. In the retinal disparity cue, the brain com-putes the relative distance of an object by comparing the slightly different images an object castson our two retinas. The greater the difference, the greater the distance. In the convergence cue, thebrain calculates the degree of neuromuscular strain when our two eyes turn inward to look at anearby object. The greater the strain, the closer the object.

7. Explain how monocular cues differ from binocular cues, and describe several monocular cues forperceiving depth.

Monocular cues enable us to judge depth using information from only one eye. The monocularcues include relative size (the smaller image of two objects of the same size appears more distant),interposition (nearby objects partially obstruct our view of more distant objects), relative clarity(hazier objects appear more distant), texture gradient (a gradual change to a less distinct texturesuggests increasing distance), relative height (higher objects are farther away), relative motion ormotion parallax (as we move, objects at different distances change their relative positions in ourvisual image, with those closest moving most), linear perspective (the converging of parallel linesindicates greater distance), and light and shadow (dimmer objects seem more distant). Artists usemonocular cues to portray depth on a flat canvas.

8. State the basic assumptions we make in our perception of motion, and explain how these perceptions can be deceiving.

Our basic assumption is that shrinking objects are retreating and enlarging objects are approach-ing. The brain will also interpret a rapid series of slightly varying images as continuous move-ment, a phenomenon called stroboscopic movement. By flashing 24 still pictures a second, amotion picture creates perceived movement. The phi phenomenon, another illusion of movement,is created when two or more adjacent lights blink on and off in succession. Lighted signs exploitthe effect with a succession of lights that create the impression of, say, a moving arrow.

9. Explain the importance of perceptual constancy.

Perceptual constancy is necessary to recognize an object. It enables us to see an object asunchanging (having consistent lightness, color, shape, and size) even as illumination and retinalimages change.

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10. Describe the shape and size constancies, and explain how our expectations about perceived sizeand distance contribute to some visual illusions.

Shape constancy is our ability to perceive familiar objects (for example, an opening door) asunchanging in shape, and size constancy is perceiving objects as unchanging in size, despite thechanging images they cast on our retinas.

Given the perceived distance of an object, we instantly and unconsciously infer the object’s size.The perceived relationship between distance and size is generally valid but under special circum-stances can lead us astray. For example, one reason for the Moon illusion is that cues to objects’distances at the horizon make the Moon behind them seem farther away. Thus, the Moon on thehorizon seems larger. Similarly, the lines in the Müller-Lyer illusion may be interpreted as varyingin distance from us and thus are perceived to be of different lengths. Finally, in the distorted roomillusion, we perceive both corners as being the same distance away. Thus anything in the near cor-ner appears disproportionately large compared to anything in the far corner.

11. Discuss lightness constancy and its similarity to color constancy.

Lightness constancy enables us to perceive an object as having a constant lightness even when thelight that falls on it changes. Perceived lightness depends on relative luminance. Color constancyis our ability to perceive the color of an object as unchanging even when its illumination changes.For both constancies, the brain perceives the quality of lightness or color relative to surroundingobjects.

Perceptual Interpretation

➤ Lectures: Functional Blindness; Cases of Restored Vision; “Thin-Slicing”; Context and Perception; Bad HumanDesigns; Banner Blindness and Web Design

➤ Exercises: Displacement Glasses; Discovering Personal Bias; Perceptual Set; Perceptual Set and Gender Stereotypes;Social Transmission of a Narrative

➤ Videos: Segment 11 of the Scientific American Frontiers Series, 2nd ed.: Cockpit Confusion; Module 10 of The BrainSeries, 2nd ed.: Perception: Inverted Vision

➤ Feature Film: At First Sight➤ Transparencies: 86 Perceptual Set; 87 Perception Is a Biopsychosocial Phenomenon

12. Describe the contribution of restored-vision and sensory deprivation research in our understand-ing of the nature-nurture interplay in our perceptions.

In the classic version of the nature-nurture debate, the German philosopher Immanuel Kant main-tained that knowledge comes from our innate ways of organizing sensory experiences. On theother side, the British philosopher John Locke argued that we learn to perceive the world throughour experiences of it. It’s now clear that different aspects of perception depend more or less onnature’s endowments and on the experiences that influence what we make of our sensations.

For many species, infancy is a critical period during which experience must activate the brain’sinnate visual mechanisms. When cataracts are removed from adults who have been blind frombirth, these people remain unable to perceive the world normally. Generally, they can distinguishfigure from ground and perceive colors, but they are unable to recognize shapes, forms, and com-plete faces. In controlled experiments, infant kittens and monkeys have been reared with severelyrestricted visual input. When their visual exposure is returned to normal, they, too, suffer enduringvisual handicaps.

13. Explain how the research on distorting goggles increases our understanding of the adaptability ofperception.

Human perception is remarkably adaptable. Given glasses that shift the world slightly to the leftor right, or even turn it upside down, people manage to adapt their movements and, with practice,to move about with ease. Although kittens and monkeys also can adapt, chicks cannot.

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14. Define perceptual set, and explain how it influences what we do and do not perceive.

Clear evidence that perception is influenced by our experiences—our learned assumptions andbeliefs—as well as by sensory input comes from the many demonstrations of perceptual set, amental predisposition to perceive one thing and not another.

Through experience, we also form concepts, or schemas, which organize and interpret unfamiliarinformation, a fact that helps explain why some of us “see” monsters, faces, and UFOs that othersdo not.

15. Explain why the same stimulus can evoke different perceptions in different contexts.

A given stimulus may trigger radically different perceptions, partly because of our differentschemas, but also because of the immediate context. For example, we discern whether a speakersaid “morning” or “mourning” or “dye” or “die” from the surrounding words.

16. Describe the role human factors psychologists play in creating user-friendly machines and work settings.

Human factors psychologists explore how people and machines interact and how physical envi-ronments can be adapted to human behaviors. They help to design appliances, machines, and worksettings that fit our natural perceptions. Sometimes, they use “natural mapping” to make simpledesign changes that reduce our frustration in using common appliances. They are mindful of the“curse of knowledge,” whereby technology developers assume that others share their expertise. Aspsychologists, their most powerful tool is research. By testing users’ responses to several alterna-tives, they seek to increase both human safety and productivity.

Is There Extrasensory Perception?

➤ Lecture: Belief in ESP➤ Exercises: Belief in ESP Scale; ESP Tricks➤ Projects: Testing for ESP; The Psychic Challenge➤ Video: Segment 2 of the Scientific American Frontiers Series, 2nd ed.: Water, Water Everywhere

17. Identify the three most testable forms of ESP, and explain why most research psychologists remainskeptical of ESP claims.

Claims are made by parapsychologists for three varieties of extrasensory perception (ESP):telepathy (mind-to-mind communication), clairvoyance (perceiving remote events), and precognition (perceiving future events). Closely linked with these are claims of psychokinesis, or “mind over matter.”

Research psychologists remain skeptical because the acts of so-called psychics have typicallyturned out to be nothing more than the illusions of stage magicians, because checks of psychicvisions have been no more accurate than guesses made by others, and because sheer chance guar-antees that some stunning coincidences are sure to occur. An important reason for their skepticism,however, is the absence of a reproducible ESP result. In addition, to believe in ESP, one mustbelieve that the brain is capable of perceiving without sensory input.

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