2 Key QuestionsWhat are perceptual constancies, and what is their role in perception?What basic principles do we use to group sensations into meaningful patterns?How is it possible to see depth and judge distance?What effect does learning have on perception?How are perceptions altered by attention, motives, values, and expectations?How reliable are eyewitness reports?Is extrasensory perception possible?
4 MURDERI was in a supermarket when a girl suddenly came running around the corner. She looked back and screamed, “Stop! Stop! You’re killing him! You’re killing my father!” Naturally I was interested! As I quickly retraced her path, I was greeted by a grisly scene. A man was stretched out on the floor with another man on top of him. The guy on top was huge. At 6 feet 6 inches tall and 300 pounds, he looked only half human. He had his victim by the throat an was beating his head against the floor. There was blood everywhere. I decided to do the right thing. I ran.What happened next?
5 Return to the scene of the crime By the time the store manager and I returned to the “scene of the crime”, the police were just arriving. It took quite a while to sort things out, but here is what happened:The “guy on the bottom” had passed out and hit his head. That caused the cut (actually quite minor) which explained the “blood everywhere.” “The guy on top” saw the first man fall and was trying to prevent him from further injuring himself, He was also loosening the man’s collar.
6 What if I never returned… If I had never returned, I would have sworn in court that I had just seen a murder. The girl’s words completely dictated my own perceptions. This perhaps is understandable. But what I will never forget is the shock I felt when I met the “murderer”- the man I had see a few moments before, in broad daylight, as a huge, vicious, horrible-looking creature. The man was not a stranger. He was a neighbor of mine. I had seen him dozens of times before. I know him by name. He is a rather small man.
8 Sensation vs. Perception Sensation involves the actual stimulus (seeing the oval shaper of a bowl held at an angle),Whereas perception involves the brain’s interpretation of that stimulus (seeing a bowl at thousands of angles over the years)
9 Perception: Some Key Terms How we assemble sensations into meaningful patternsSize Constancy:Perceived size of an object remains constant, DESPITE changes in its retinal image sizeNative Perception:A perceptual experience based on innate processesEmpirical Perception:A perception strongly influenced by prior experienceShape Constancy:The perceived shape of an object unaffected by changes in its retinal imageBrightness Constancy:Apparent brightness of an object stays the same under changing lighting conditions
12 Fig. 7.1 Shape constancy. (a) When a door is open its image actually forms a trapezoid. Shape constancy is indicated by the fact that it is still perceived as a rectangle. (b) With great effort you may be able to see this design as a collection of flat shapes. However, if you maintain shape constancy the distorted squares strongly suggest the surface of a sphere. (From Spherescapes-1 by Scott Walter and Kevin McMahon, 1983.)
13 Perceptual GroupingFigure-Ground Organization: Inborn; part of a stimulus stands out as a figure (object) against a plainer background (ground)Reversible Figure: Figure and ground that can be reversed
14 Fig. 7. 2 A reversible figure-ground design Fig. 7.2 A reversible figure-ground design. Do you see two faces in profile, or a wineglass?
15 Perceiving ImagesThe first step in perceiving an image is determining the figure and ground.
17 Gestalt Principles of Organization Nearness:Stimuli that are near each other tend to be grouped togetherSimilarity:Stimuli that are similar in size, shape, color, or form tend to be grouped togetherContinuation, or Continuity:Perceptions tend toward simplicity and continuity
18 Gestalt Principles of Organization (cont.) Closure:Tendency to complete a figure so that it has a consistent overall formContiguity:Nearness in time and space; perception that one thing has caused anotherCommon Region:Stimuli that are found within a common area tend to be seen as a group
24 Depth Perception Depth Perception: Visual Cliff: Depth Cues: Ability to see three-dimensional space and to accurately judge distancesVisual Cliff:Apparatus that looks like the edge of an elevated platform or cliffDepth Cues:Features that supply information about distance and spaceMonocular Depth Cue:Depth cue that can be sensed with one eyeBinocular Depth Cue:Depth cue that can be sensed with two eyes
28 Fig. 7.9 Human infants and newborn animals refuse to go over the edge of the visual cliff
29 Muscular Cues for Depth Perception Accommodation:Bending of the lens of the eye to focus on nearby objectsConvergence:Binocular cue; when you look at something 50 feet or closer, your eyes must turn in (converge) to focus the objectRetinal Disparity:Discrepancy in the images that reach the right and left eyesStereotopic Vision:Three-dimensional sight
30 Fig. 7.10 The eyes must converge, or turn in toward the nose, to focus close objects.
32 Pictorial Cues for Depth Features found in paintings, drawings, and photographs that supply information about space, depth, and distanceLinear Perspective: (7-14a)Based on apparent convergence of parallel lines in environmentRelative size: (7-14b)Depict 2 objects of the same size at different distances, the more distant objects smallerHeight in the picture plane (7-14b)Objects are placed higher (closer to the horizon line) in a drawing tend to be perceived as more distant
33 Pictorial Cues for Depth Light and shadow (7-14c) (7-16)Most objects lighted in ways that create clear patterns of light and shadowOverlap (Interposition): (7-14d)When one object partially blocks anotherTexture Gradients: (7-14e)Texture changes can contribute to depth perception; coarse texture implies closeness, fine texture implies distanceAerial perspective:Smog, fog, dust, and haze add to the apparent distance of an objectRelative Motion (Motion Parallax): (7-17)Nearby objects move a lot as your head moves; distant objects move slightly
34 Fig. 7-14 (a) Linear perspective. (b) Relative size Fig (a) Linear perspective. (b) Relative size. (c) Light and shadow. (d) Overlap. (e) Texture gradients. Drawings in the top row show fairly “pure” examples of each of the pictorial depth cues. In the bottom row, the pictorial depth cues are used to assemble a more realistic scene.
35 Prrrrrrrroject!Sensation and Perception: Visual Illusions
39 Fig The apparent motion of objects viewed during travel depends on their distance from the observer. Apparent motion can also be influenced by an observer’s point of fixation. At middle distances, objects closer than the point of fixation appear to move backward; those beyond the point of fixation appear to move forward. Objects at great distances, such as the sun or moon, always appear to move forward.
40 Some Illusions Moon Illusion: Apparent-Distance Hypothesis: Apparent change in size that occurs as the moon moves from the horizon (large moon) to overhead (small moon)Apparent-Distance Hypothesis:Horizon seems more distant than the night sky
41 An impossible figure—the “three-pronged widget.”
42 Fig The Ponzo illusion may help you understand the moon illusion. Picture the two white bars as resting on the railroad tracks. In the drawing, the upper bar is the same length as the lower bar. However, because the upper bar appears to be farther away than the lower bar, we perceive it as longer. The same logic applies to the moon illusion.
47 Perceptual Learning Other-Race Effect: Active Movement: Context: Change in the brain that alters how we process sensory informationPerceptual Habits:Ingrained patterns of organization and attentionOther-Race Effect:Tendency to be better at recognizing faces from one’s own racial group than faces from other racial or ethnic groupsActive Movement:Self-generated action; accelerates perceptual adaptationContext:Information surrounding a stimulus; affects perception (Fig. 7-25)Frames of Reference:Internal standards for judging stimuli
49 Fig. 7. 21 The effects of prior experience on perception Fig The effects of prior experience on perception. The doctored face looks far worse when viewed right side up because it can be related to past experience.
51 Ames Room Video An Ames room (named for the man who designed it) Is a lopsided space that appears square when viewed from a certain point (fig 7-23)
52 Fig The Ames room. From the front, the room looks normal; actually, the right-hand corner is very short, and the left-hand corner is very tall. In addition, the left side of the room slants away from viewers. The diagram shows the shape of the room and reveals why people appear to get bigger as they cross the room toward the nearer, shorter right corner.
53 Illusions: Is What You See What You Get? Length, position, motion, curvature, or direction is constantly misjudgedHallucination:When people perceive objects or events that have no external basis in realityStroboscopic Movement:Illusory motion perceived when objects are shown in rapidly changing positionsMuller-Lyer Illusion: (Fig. 27a)Two equal-length lines topped with inward or outward pointing V’s appear to be of different length; based on experience with edges and corners of rooms and buildings
56 Fig. 7-27 Some interesting perceptual illusions Fig Some interesting perceptual illusions. Illusions are a normal part of perception.
57 Fig Why does line (b) in the Müller-Lyer illusion look longer than line (a)? Probably because it looks more like a distant corner than a nearer one. Because the vertical lines form images of the same length, the more “distant” line must be perceived as larger. As you can see in the drawing on the right, additional depth cues accentuate the Müller-Lyer illusion. (After Enns & Coren, 1995.)
58 Size-Distance Invariance If two objects make images of the same size, the more distant object must be larger, this is known as Size-distance invarianceSame concept explains the Muller-Lyer illusion. If the V-tipped line looks father away than the arrowhead-tipped line, then you must compensate by seeing the V-tipped line as longer
59 Attention and Perception Selective Attention:Refers to the fact that we give some messages priorityDivided Attention:Divide your mental effort among tasks, each of which requires more or less attentionOrientation Response:Bodily changes that prepare an organism to receive information from a particular stimulusEx: Pupils enlarge, breathing stops briefly
62 Perceptual Expectancies Bottom-Up Processing:Analyzing information starting at the bottom (small units) and going upward to form a complete perceptionTop-Down Processing:Pre-existing knowledge that is used to rapidly organize features into a meaningful wholePerceptual Set:Past experiences, motives, contexts, or suggestions that prepare us to perceive in a certain way
64 Factors Affecting the Accuracy of Eyewitness Perceptions Stress:High levels impair accuracyWeapon Focus:Presence of a weapon impairs eyewitness’ accuracyExposure Time:Less time an eyewitness has to observe an event, the less s/he will perceive and remember itAccuracy-Confidence:Confidence is not a good predictor of his/her accuracyCross-Racial Perceptions:Eyewitnesses are better at identifying members of their own race than of other races
65 More Factors Affecting the Accuracy of Eyewitness Perceptions Post-Event Information:Testimony reflects not only what was actually seen but also information obtained later onColor Perception:Judgments of color made under monochromatic light are very unreliableUnconscious Transference:A culprit who is identified may have been seen in another situation or contextAlcohol Intoxication:Impairs later ability to recall eventsAttitudes and Expectations:May affect eyewitness’ perception of events
66 Implications of Eyewitness Testimony Reality Testing:Obtaining additional information to check your perceptionsHabituate:Tend to respond less to predictable and unchanging stimuliDishabituation:Reversal of habituation
69 Extrasensory Perception (ESP): Fact or Fallacy? Parapsychology: Study of ESP and other psi phenomena (events that seem to defy accepted scientific laws)Clairvoyance:Purported ability to perceive events unaffected by distance or physical barriersTelepathy:Purported ability to read mindsPrecognition:Purported ability to accurately predict the future (“Minority Report” and the “Pre-Cogs,” like Agatha)Psychokinesis (Mind Over Matter):Purported ability to influence physical objects by willpower
70 More ESP IssuesZener Cards: Deck of 25 cards, each having one of five symbolsRun of Luck:Statistically unusual outcome that could occur by chance alone (e.g., getting five heads in a row, two jackpots within six pulls of a slot machine)Stage ESP:Simulation of ESP for entertainment purposesConclusion:Existence of ESP has NOT been scientifically demonstrated; positive results are usually inconclusive and easily criticizedIn sum:Be skeptical! If it seems too good to be true, it probably is!
71 Fig. 7-35 ESP cards used by J. B Fig ESP cards used by J. B. Rhine, an early experimenter in parapsychology.
73 (a) (b c)Fig Fake psychokinesis. (a) The performer shows an observer several straight keys. While doing so, he bends one of the keys by placing its tip in the slot of another key. Normally, this is done out of sight, behind the “psychic’s” hand. It is clearly shown here so you can see how the deception occurs. (b) Next, the “psychic” places the two keys in the observer’s hand and closes it. By skillful manipulation, the observer has been kept from seeing the bent key. The performer then “concentrates” on the keys to “bend them with psychic energy.” (c) The bent key is revealed to the observer. “Miracle” accomplished! (Adapted from Randi, 1983.)
74 Extrasensory Perception Telepathy – mind readingClairvoyance – perceiving remote eventsPrecognition – Knowing things before they happenTelekinesis (psychokinesis) – moving objects with one’s mind (not technically ESP)