Chapter 7 Perceiving the World

Key Questions What 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?

Terms Perception Accommodation Selective attention Native Convergence Divided attention Empirical Retinal disparity Habituate Shape constancy Stereoscopic vision Orientation response Brightness constancy Random dot stereograms Bottom-up processing Figure-ground organization Top-down processing Pictorial depth cues Perceptual expectancy Reversible figures Moon illusion Tachistoscope Illusory figures Apparent distance hypothesis Perceptual categories Engineering psychologist Perceptual reconstruction Perceptual habits Display Ames room Weapons focus Control Perceptual features Reality testing Natural design Active movement Dishabituation Feedback Context Extrasensory perception Camouflage Frames of reference Perceptual hypothesis Adaptation level Parapsychology Ambiguous stimuli Illusion Zener cards Impossible figure Hallucination Run of luck Depth perception Stroboscopic movement Replicated Visual cliff Ganzfeld Depth cues Muller-Lyer illusion Stage ESP Monocular cues Size-distance invariance Binocular cues

MURDER I 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?

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.

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.

Bowl

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)

Perception: Some Key Terms How we assemble sensations into meaningful patterns Size Constancy: Perceived size of an object remains constant, DESPITE changes in its retinal image size Native Perception: A perceptual experience based on innate processes Empirical Perception: A perception strongly influenced by prior experience Shape Constancy: The perceived shape of an object unaffected by changes in its retinal image Brightness Constancy: Apparent brightness of an object stays the same under changing lighting conditions

Color Changing Card Trick

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.)

Perceptual Grouping Figure-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

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?

Perceiving Images The first step in perceiving an image is determining the figure and ground.

Do you see the arrow?

Gestalt Principles of Organization Nearness: Stimuli that are near each other tend to be grouped together Similarity: Stimuli that are similar in size, shape, color, or form tend to be grouped together Continuation, or Continuity: Perceptions tend toward simplicity and continuity

Gestalt Principles of Organization (cont.) Closure: Tendency to complete a figure so that it has a consistent overall form Contiguity: Nearness in time and space; perception that one thing has caused another Common Region: Stimuli that are found within a common area tend to be seen as a group

Gestalt Principles: Common Region Similarity

Gestalt Principles: Closure

Gestalt Principles: Continuity

Fig. 7.3 How we organize perceptions

© E.R. Degginger/Animals Animals Fig. 7.5 A challenging example of perceptual organization. Once the camouflaged insect (known as a giant walkingstick) becomes visible, it is almost impossible to view the picture again without seeing the insect.

Depth Perception Depth Perception: Visual Cliff: Depth Cues: Ability to see three-dimensional space and to accurately judge distances Visual Cliff: Apparatus that looks like the edge of an elevated platform or cliff Depth Cues: Features that supply information about distance and space Monocular Depth Cue: Depth cue that can be sensed with one eye Binocular Depth Cue: Depth cue that can be sensed with two eyes

Binocular Depth

Size-distance relationship Size-distance relationship When other monocular cues tell us an image is further away, it actually appears larger.

Let’s have a catch

Fig. 7.9 Human infants and newborn animals refuse to go over the edge of the visual cliff

Muscular Cues for Depth Perception Accommodation: Bending of the lens of the eye to focus on nearby objects Convergence: Binocular cue; when you look at something 50 feet or closer, your eyes must turn in (converge) to focus the object Retinal Disparity: Discrepancy in the images that reach the right and left eyes Stereotopic Vision: Three-dimensional sight

Fig. 7.10 The eyes must converge, or turn in toward the nose, to focus close objects.

Figure 7-11 & 7-12 Page 211

Pictorial Cues for Depth Features found in paintings, drawings, and photographs that supply information about space, depth, and distance Linear Perspective: (7-14a) Based on apparent convergence of parallel lines in environment Relative size: (7-14b) Depict 2 objects of the same size at different distances, the more distant objects smaller Height 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

Pictorial Cues for Depth Light and shadow (7-14c) (7-16) Most objects lighted in ways that create clear patterns of light and shadow Overlap (Interposition): (7-14d) When one object partially blocks another Texture Gradients: (7-14e) Texture changes can contribute to depth perception; coarse texture implies closeness, fine texture implies distance Aerial perspective: Smog, fog, dust, and haze add to the apparent distance of an object Relative Motion (Motion Parallax): (7-17) Nearby objects move a lot as your head moves; distant objects move slightly

Fig. 7-14 (a) Linear perspective. (b) Relative size Fig. 7-14 (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.

Prrrrrrrroject! Sensation and Perception: Visual Illusions

Relative Motion Video

Fig. 7.15 On a dry lake bed, relative size is just about the only depth cue available for judging the camera’s distance from this vintage aircraft. What do you estimate the distance to be?

OR IS IT... Page 216 Figure 7-20

Fig. 7.17 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.

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

An impossible figure—the “three-pronged widget.”

Fig. 7.19 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.

Optical Illusions Handout (Ambiguous figures) Homework Personal experience Perceptual abilities Personal needs Homework

Perceptual Learning Other-Race Effect: Active Movement: Context: Change in the brain that alters how we process sensory information Perceptual Habits: Ingrained patterns of organization and attention Other-Race Effect: Tendency to be better at recognizing faces from one’s own racial group than faces from other racial or ethnic groups Active Movement: Self-generated action; accelerates perceptual adaptation Context: Information surrounding a stimulus; affects perception (Fig. 7-25) Frames of Reference: Internal standards for judging stimuli

Context Effects

Fig. 7. 21 The effects of prior experience on perception Fig. 7.21 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.

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)

Fig. 7-23 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.

Illusions: Is What You See What You Get? Length, position, motion, curvature, or direction is constantly misjudged Hallucination: When people perceive objects or events that have no external basis in reality Stroboscopic Movement: Illusory motion perceived when objects are shown in rapidly changing positions Muller-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

Muller-Lyer Illusion Handout AB-BC

Stroboscopic Movement Airplane

Fig. 7-27 Some interesting perceptual illusions Fig. 7-27 Some interesting perceptual illusions. Illusions are a normal part of perception.

Fig. 7.28 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.)

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 invariance Same 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

Attention and Perception Selective Attention: Refers to the fact that we give some messages priority Divided Attention: Divide your mental effort among tasks, each of which requires more or less attention Orientation Response: Bodily changes that prepare an organism to receive information from a particular stimulus Ex: Pupils enlarge, breathing stops briefly

Selective Attention How many passes

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Perceptual Expectancies Bottom-Up Processing: Analyzing information starting at the bottom (small units) and going upward to form a complete perception Top-Down Processing: Pre-existing knowledge that is used to rapidly organize features into a meaningful whole Perceptual Set: Past experiences, motives, contexts, or suggestions that prepare us to perceive in a certain way

Tachisoscope Flash-X

Factors Affecting the Accuracy of Eyewitness Perceptions Stress: High levels impair accuracy Weapon Focus: Presence of a weapon impairs eyewitness’ accuracy Exposure Time: Less time an eyewitness has to observe an event, the less s/he will perceive and remember it Accuracy-Confidence: Confidence is not a good predictor of his/her accuracy Cross-Racial Perceptions: Eyewitnesses are better at identifying members of their own race than of other races

More Factors Affecting the Accuracy of Eyewitness Perceptions Post-Event Information: Testimony reflects not only what was actually seen but also information obtained later on Color Perception: Judgments of color made under monochromatic light are very unreliable Unconscious Transference: A culprit who is identified may have been seen in another situation or context Alcohol Intoxication: Impairs later ability to recall events Attitudes and Expectations: May affect eyewitness’ perception of events

Implications of Eyewitness Testimony Reality Testing: Obtaining additional information to check your perceptions Habituate: Tend to respond less to predictable and unchanging stimuli Dishabituation: Reversal of habituation

Eye Witness Lineup Bomber on roof

Eye Witness Testimony Video

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 barriers Telepathy: Purported ability to read minds Precognition: 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

More ESP Issues Zener Cards: Deck of 25 cards, each having one of five symbols Run 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 purposes Conclusion: Existence of ESP has NOT been scientifically demonstrated; positive results are usually inconclusive and easily criticized In sum: Be skeptical! If it seems too good to be true, it probably is!

Fig. 7-35 ESP cards used by J. B Fig. 7-35 ESP cards used by J. B. Rhine, an early experimenter in parapsychology.

Do you have ESP ability? Test

(a) (b c) Fig. 6.37 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.)

Extrasensory Perception Telepathy – mind reading Clairvoyance – perceiving remote events Precognition – Knowing things before they happen Telekinesis (psychokinesis) – moving objects with one’s mind (not technically ESP)