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Chapter 6


Perception involves the selection, organization, and interpretation of sensory information. It quickly became one of psychology's primary concerns as early researchers attempted to explain illusions.

In organizing sensory data into whole perceptions, our first task is to discriminate figure from ground. We then organize the figure into meaningful form by following certain rules for grouping stimuli. We transform two-dimensional retinal images into three-dimensional perceptions by using binocular 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 can also create an illusion of movement.

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

Studies of sensory restriction reveal that, for many species, infancy is a critical period during which experience must activate the brain's innate visual mechanisms. For example, when cataracts are removed from adults who have been blind from birth, they can distinguish figure and ground and they 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 world upside down, people manage to adapt and move about with ease. Clear evidence that perception is influenced by our experience comes from the many demonstrations of perceptual set and context effects.

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

1. Describe how the process of perception is directed and limited by selective attention.

Selective attention means that at any moment our awareness focuses on only a limited aspect of all that we are capable of experiencing. For example, even if a stimulus figure can evoke more than one perception, we consciously experience only one at a time. The cocktail party phenomenon provides another example of selective attention. The ability to attend to one voice among many enables us to converse coherently in the midst of auditory chaos. Selective attention also limits our perception, as many stimuli will pass by unnoticed.

Perceptual Illusions

2. Explain how illusions help us to understand perception.

Illusions mislead us by playing on the ways we typically organize and interpret our sensations, and thus understanding illusions provides valuable clues to the ordinary mechanisms of perception. For example, several well-known illusions are based on the perceived relationship between size and distance, which is generally valid. As visual illusions indicate, among our senses vision is dominant. When there is a conflict between vision and other sensations, vision usually dominates, a phenomenon called visual capture.

3. Discuss 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 human perceiver organizes them into a gestalt, a German word meaning a "form" or a "whole." They further demonstrated that the whole may differ from the sum of its parts. Clearly, our brains do more than merely register information about the world. We are always filtering sensory information and inferring perceptions in ways that make sense to us.

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. Gestalt principles for grouping that describe this process include proximity (we group nearby figures together), similarity (we group similar figures together), continuity (we perceive smooth, continuous patterns rather than discontinuous ones), closure (we fill in gaps to create a whole object), and connectedness (we perceive spots, lines, or areas as a single unit when uniform and linked).

5. Discuss research on depth perception involving the use of the visual cliff and describe the binocular and monocular cues in depth perception.

Research on the visual cliff (a miniature cliff with a drop-off covered by sturdy glass) reveals that many species have depth perception at, or very soon after, birth. We transform two-dimensional retinal images into three-dimensional perceptions by using binocular and monocular cues.

The binocular cues include retinal disparity (the difference between the two images the retinas receive of an object) and convergence (the extent to which the eyes converge inward when looking at an object).

The monocular cues 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 texture suggests increasing distance), relative height (higher objects are farther away), relative motion (as we move, objects at different distances change their relative positions in our visual image, with those closest moving most), linear perspective (the converging of parallel lines indicates greater distance), and relative brightness (dimmer objects seem more distant).

6. Describe stroboscopic movement and the phi phenomenon.

Our brain computes motion as objects move across or toward the retina. The brain will also interpret a rapid series of slightly varying images as movement, a phenomenon called stroboscopic movement. By flashing 24 still pictures a second, a motion 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. Theatre and restaurant marquees exploit the effect with a succession of lights that create the impression of, say, a moving arrow.

7. Describe the perceptual constancies and show how they operate in visual illusions.

Perceptual constancy refers to the principle that we perceive objects as unchanging (having consistent lightness, shape, and size) even as illumination and retinal images change.

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 circumstances 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 the horizon seems larger. Similarly, the lines in the Muller-Lyer illusion may be interpreted as varying in distance from us and thus are perceived to be of different lengths. Finally, in the distorted room illusion we perceive both corners as being the same distance away. Thus anything in the near corner appears disproportionately large compared to anything in the far corner


8. Describe the debate over the role of nature and nurture in perception and discuss what research findings on sensory restriction and restored vision have contributed to this debate.

In the classic version of the nature-nurture debate, the German philosopher Immanuel Kant maintained that knowledge comes from our innate ways of organizing sensory experiences. On the other side, the British philosopher John Locke argued that we learn to perceive the world through our experiences of it. It's now clear that different aspects of perception depend more or less on nature'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's innate visual mechanisms. When cataracts are removed from adults who have been blind from birth, these people remain unable to perceive the world normally. Generally, they can distinguish figure from ground and perceive colors, but they are unable to distinguish shapes and forms. In controlled experiments, infant kittens and monkeys have been reared with severely restricted visual input. When their visual exposure is returned to normal, they, too, suffer enduring visual handicaps.

9. Explain what the use of distorting goggles indicates regarding the adaptability of perception.

Human perception is remarkably adaptable. Given glasses that shift the world slightly to the left or right, or even turn it upside down, people manage to adapt their movements and, with practice, to move about with ease.

10. Discuss the effects of assumptions, expectations, schemas, and contexts on our perceptions.

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

Through experience we also form schemas, or concepts, which organize and interpret unfamiliar information, a fact that helps explain why some of us "see" monsters, faces, and UFOs that others do not.

A given stimulus may trigger radically different perceptions, partly because of our different schemas, but also because of the immediate context. For example, we discern whether a speaker said "cults and sects" or "cults and sex" from the surrounding words.

Is There Perception Without Sensation?

11. State the claims of ESP and explain why most research psychologists remain skeptical.

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 typically turned out to be nothing more than the illusions of stage magicians, because checks of psychic visions have been no more accurate than guesses made by others, and because sheer chance guarantees that some stunning coincidences are sure to occur. Perhaps the most important reason for their skepticism, however, is the absence of a reproducible ESP result. New studies using the "ganzfeld procedure" have recently raised hopes of a possible telepathy phenomenon. Follow-up studies should soon indicate the phenomenon's reliability.





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