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Perception z We have previously examined the sensory processes by which stimuli are encoded. z Now we will examine the ultimate purpose of sensory information z PERCEPTION - the conscious representation of the external environment. z Perception involves the selection, organization, and interpretation of sensory information. z Perception quickly became one of psychology’s primary concerns as early researchers attempted to explain illusions. Fact or Falsehood z Understanding illusions provides important clues to the ordinary mechanisms of perception. z Infants just learning to crawl do not perceive depth. z Persons who have sight in only one eye are totally unable to gauge distances. z The moon appears larger at the horizon than in the sky because distance cues make it appear closer. Fact or Falsehood z A person who is born blind but gains sight as an adult cannot recognize common shapes and forms. z If required to look through a pair of glasses that turns the world upside down, we soon adapt and coordinate our movements without difficulty. z If people are told that an infant is “David,” they are likely to see “him” as bigger and stronger than if the same infant is called “Diana.” Fact or Falsehood z Psychics have frequently been successful in helping the police solve crimes. z Laboratory evidence clearly indicates that some people do have ESP. Selective Attention z Selective attention means that at any moment we focus our awareness on only a limited aspect of all that we experience. z Until reading this sentence, you have been unaware that your shoes are pressing against your feet or that your nose is in your line of vision. Now, suddenly, your attentional spotlight shifts. Your feet feel encased, your nose stubbornly intrudes on the page before you. Selective Attention z The cocktail party effect is our ability to attend selectively to only one voice among many. z Change blindness experiment. Page 233 in book Perceptual Illusions z Illusions mislead us by playing on the ways we typically organize and interpret our sensations. z Understanding illusions provides valuable clues to the ordinary mechanisms of perception. Perceptual Illusions z Is AB or BC z See pages 234 longer? and 235 in your book for several examples of perceptual illusions. Perceptual Organization z Some of the best examples that perception involves organization of sensory input was provided by the Gestalt Psychologists. z Gestalt psychologists hypothesized that “the whole is greater than the sum of the parts.” z They were interested in showing the global nature of our perceptions Perceptual Organization z Our yen for assembling visual features into complete forms involves bottom-up processing, starting with entry-level sensory analysis, as well as top-down processing that uses our experiences and expectations to interpret those sensations. z Sensation and perception blend into one continuous process, progressing upward from specialized detector cells and downward from our assumptions. Figure and Ground z Gestalt Psychologists also thought that an important part of our perception was the organization of a scene in to its: z Figure - the object of interest z Ground - the background Figure and Ground Gestalt Grouping Principles z Gestalt theorists argued that our perceptual systems automatically organized sensory input based on certain rules. y Proximity: we group nearby figures together y Similarity: we group figures that are similar to each other together. y Closure: we fill in gaps to create a complete, whole object y Good Continuation: we organize stimuli into smooth, continuous patterns y Connectedness: When they are uniform and linked, we perceive spots, lines, or areas as a single unit. Organizing Stimuli Depth Perception z One of our more important perceptual abilities involves seeing in three- dimensions z Depth perception is difficult because we only have access to two- dimensional images z How do we see a 3-D world using only the 2-D retinal images? Depth Perception Cues z Cue - stimulus characteristics that influence our perceptions z We are able to see in 3-D because the visual system can utilize depth cues that appear in the retinal images. Types of Depth Cues z Depth cues are usually divided into categories, we will consider two types of depth cues: z Monocular - depth cues that appear in the image in either the left or right eye z Binocular - depth cues that involve comparing the left and right eye images Binocular Cues z Convergence y When you look at a distant object, the lines of vision from your eyes are parallel. However, when you look at something 50 feet or less in distance, your eyes must converge (turn in) to focus the object. z Retinal Disparity y A difference in the images that reach the right and left eyes. RD is based on the fact that the eyes are about 2.5 inches apart. Because of this, each eye receives a slightly different view of the world. Binocular Depth Cues z Monocular depth cues allow us to see in 3-D with the view of only one eye, but our best depth perception occurs if we look through both eyes. z This is because our right and left eyes see a slightly different view of the world. z This difference between the image in the two eyes is know as Retinal Disparity. Autostereogram z Another way to create the illusion of depth through binocular stereopsis is with an Autostereogram. z An autostereogram is formed by superimposing two repeating patterns z The two patterns are slightly offset, and when viewed properly, this offset is seen as a binocular disparity. Autostereogram Monocular Depth Cues z Relative Size z Interposition z Relative Clarity z Texture gradient z Relative Height z Relative Motion z Linear Perspective z Light and Shadow z Position relative to Horizon Relative Size Interposition Relative Clarity Texture Gradient Relative Height Relative Height Linear Perspective Ponzo Illusion z Converging lines indicate that top line is farther away than bottom line Light and Shadow Motion Perception z Your brain computes motion based partly on its assumption that shrinking objects are retreating (not getting smaller) and enlarging objects are approaching. z Large objects, such as a train, appear to move more slowly than smaller objects, such as a car moving at the same speed. Motion Perception z Marquees and holiday lights sometimes create another illusion of movement using the phi phenomenon. z When two adjacent stationary lights blink on and off in quick succession, we perceive a single light moving back and forth between them. Ames Room z The Ames room is designed so that the monocular depth cues give the illusion that the two people are equally far away Other Size-Distance Illusions z In each of these examples, the top and bottom lines are actually the same length. (a) Müller-Lyer illusion z In each case the top line looks longer. z Why? (b) Ponzo illusion Muller-Lyer Illusion z Perceptual psychologists have hypothesized that the top horizontal line looks longer because it also looks farther away. z Specifically, the inward pointing arrows signify that the horizontal line is closest to you, and the outward pointing arrows signify the opposite case. Perceptual Constancy z When viewing conditions change, the retinal image changes even if the objects being viewed remain constant. z Example: as a person walks away from you their retinal image decreases in size. z important function of the perceptual system is to represent constancy in our environment even when the retinal image varies. Size Constancy z Cylinders at positions A and B are the same size even though their image sizes differ Point B z The depth cues such as linear perspective and texture help the visual system judge the size accurately Point A Shape Constancy z It is hard to tell if the figure on the upper right is a trapezoid or a square slanted backward. z If we add texture, the texture gradient helps us see that it is actually a square. Lightness Constancy z We perceive an object as having a constant lightness even while its illumination varies. z Perceived lightness depends on relative luminance – the amount of light an object reflects relative to its surroundings. Perceptual Interpretation z Sensory Deprivation and Restored Vision y In both humans and animals, a period of sensory restriction does no permanent harm if it occurs later in life. z Perceptual Adaptation y Humans adapt to a distorted environment rather quickly. Perceptual Set z Our experiences, assumptions, and expectations may give us a perceptual set, or mental predisposition, that greatly influences what we perceive. y Isn’t it weird when you see your teacher at the grocery store? z Schemas determine our perceptual set. y Children’s drawings reflect their schemas of reality, as well as their abilities to represent what they see. Claims of ESP z Telepathy, or mind-to-mind communication— one person sending thoughts to another or perceiving another’s thoughts. z Clairvoyance, or perceiving remote events, such as sensing that a friend’s house is on fire. z Precognition, or perceiving future events, such as a political leader’s death or a sporting event’s outcome. z Psychokinesis, or “mind over matter,” such as levitating a table or influencing the roll of a die.
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