2 “Sunny Mood”It’s common sense that winter’s gloom can lower your spirits. But until now, little scientific evidence has supported the folklore. This ScienCentral News video has more.
3 4.1 How Do We Sense Our Worlds? Learning ObjectivesDistinguish between sensation and perception.Describe the process of transduction.Distinguish between an absolute threshold and a difference threshold.Discuss sensory detection theory.Define sensory adaptation.
4 How Do We Sense Our Worlds? Sensation: our sense organs’ detection and response to external stimulus energy and the transmission of those responses to the brainPerception: the brain’s processing of detected signals, resulting in internal representations of the stimuli that form a conscious experience of the worldWhat we sense is the result of how we perceive
6 Stimuli Must Be Coded to Be Understood by the Brain Sensory coding: Sensory receptors translate the physical properties of stimuli into patterns of neural impulsesTransduction: a process by which sensory receptors produce neural impulses when they receive physical or chemical stimulationThe brain needs qualitative and quantitative information about a stimulusSensation and perception result from a symphony of sensory receptors and the neurons those receptors communicate with
7 The Stimuli, Receptors, and Pathways for Each Sense
8 Qualitative versus Quantitative Sensory Information
9 Psychophysics Measures the Relationship between Stimuli and Perception Psychologists try to understand the relationship between the world’s physical properties and how we sense and perceive themPsychophysics is a subfield that examines our psychological experiences of physical stimuli
10 “Golf and Perception”Psychologists have found that golfers who’ve played well perceive the hole as bigger than it really is. As this ScienCentral News video explains, the researchers also found those who did poorly see the hole smaller than it really is.
11 Sensory ThresholdsAbsolute threshold: the minimum intensity of stimulation that must occur before you experience a sensationExample: The absolute threshold for hearing is the faintest sound a person can detect 50 percent of the timeDifference threshold: the minimum amount of change required for a person to detect a difference (i.e., the “just noticeable difference”)Weber’s law: states that the just noticeable difference between two stimuli is based on a proportion of the original stimulus rather than on a fixed amount of difference
13 Approximate Absolute Sensory Threshold (Minimum Stimulus) for Each Sense
14 Signal Detection Theory Signal detection theory (SDT): states that detecting a stimulus requires making a judgment about its presence or absence, based on a subjective interpretation of ambiguous informationExample: A radiologist is looking at a CAT scan for the kind of faint shadow that signals an early-stage cancer. Her judgment will likely be affected by her knowledge of the patient, training, experience, motivation, attention and the knowledge of the consequences of being wrongSignal detection research involves a series of trials in which a stimulus is presented in only some trials. In each trial, the participant must state whether he or she sensed the stimulus
15 Payoff Matrices for Signal Detection Theory Note that the percentages in this figure were invented to show representative numbers. Actual percentages vary from question to question.
16 Sensory AdaptationSensory adaptation: a decrease in sensitivity to a constant level of stimulationIf a stimulus is presented continuously, the responses of the sensory systems that detect it tend to diminish over time; when a continuous stimulus stops, the sensory systems usually respond strongly as well
17 FIGURE 4.6 Sensory Adaptation Because of sensory adaptation, people who live near constant noise eventually become less aware of the noise. Pictured here are homes near Heathrow Airport, in London.If jets were flying this close over your home, how long do you think it would take for you to adjust? What other kinds of constant stimulation must people adjust to?
18 4.2 What Are the Basic Sensory Processes? For each of the five major senses — taste, smell, touch, hearing, and vision — identify the type of receptor and trace the neural pathway to the brainDistinguish between the neural processes associated with the experience of immediate pain and the experience of chronic painDiscuss color perception
19 In Taste, Taste Buds Detect Chemicals Gustation: the sense of tasteTaste buds: sensory organs, mostly on the tongue; come in the form of tiny, mushroom-shaped structures (papillae)Stimulated taste buds send signals to the brain, which then produces the experience of tasteDifferent regions of the tongue are not more sensitive to certain tastes (Lindemann, 2001)Every taste experience is composed of a mixture of five basic qualities: sweet, sour, salty, bitter, and the relatively new taste sensation umami (Krulwich, 2007)Mothers can pass their eating preferences on to their offspring
20 “Seeing Hungry”Why does food look more appealing when you are hungry? Scientists are finding that the same chemical in your stomach that causes hunger also changes how your brain perceives food, as this ScienCentral News video explains.
21 “Bitter Taste Buds”The human genome project has scientists diving into what makes us tick—and taste. To find alternatives to salt for masking bitterness in foods, researchers are investigating compounds that block bitter signals. This ScienCentral News video has more.
24 In Smell, the Nasal Cavity Gathers Odorants Olfaction: the sense of smellBasic process:Odorants pass into the nose and nasal cavityContact a thin layer of tissue embedded with smell receptors called the olfactory epitheliumSmell receptors transmit information to the olfactory bulb, the brain center for smellHas the most direct route to the brainSmell’s intensity is processed in brain areas also involved in emotion and memory (Anderson, Christoff et al., 2003)
25 “Super Smellers”Why do nearly half of all people over age 65 lose some of their sense of smell? As this ScienCentral News video reports, some super-smelling mice might hold the answer.
27 In Touch, Sensors In the Skin Detect Pressure, Temperature, and Pain Haptic sense: the sense of touchSense conveys sensations of temperature, pressure, pain, and where our limbs are in spaceThe integration of various signals and higher-level mental processes produces haptic experiencesExamples:Stroking multiple pressure points can produce a tickling sensation, which can be pleasant or unpleasant, depending on the mental state of the person being tickledBrain areas involved in touch sensation respond less to self-produced tactile stimulation than to external tactile stimulation (Blakemore, Wolpert, & Frith, 1998)
28 Figure 4.12 How We Experience Touch: The Haptic Sense
29 Two Types of PainPain is part of a warning system that stops you from continuing activities that may harm youTwo kinds of nerve fibers have been identified for pain:Fast fibers for sharp, immediate pain; activated by strong physical pressure and temperature extremesSlow fibers for chronic, dull, steady pain; activated by chemical changes in tissue when skin is damaged
30 “Brain Pain”The pain of severe burns may be the most excruciating pain a person can experience. But, as this ScienCentral News video reports, the August issue of Scientific American describes how the ultimate in pain may be eased by the ultimate in high-tech distractions.
31 Figure 4.13 How We Experience Touch: The Sense of Pain
32 In Hearing, the Ear Detects Sound Waves Audition: the sense of soundMovements and vibrations of objects cause the displacement of air molecules, which produce a sound wave (change in air pressure that travels through the air)A sound wave’s amplitude determines loudness; its frequency determines pitchThe ears convert sound waves to brain activity, which produces the sensation of sound
34 The Cochlear ImplantCochlear implantation has helped people with severe hearing problems due to the loss of hair cells in the inner earWorks by directly stimulating the auditory nerve; does not not amplify soundWhen devices are implanted in children born deaf, the child’s hearing will be quite functional and he/she will learn to speak reasonably normallyThe problem of audism
35 “Hearing Motion”Imagine if words created a taste in your mouth or music generated bursts of color. These are the effects of a rare condition called synesthesia. Now scientists have found a new type of the condition: people who “hear” motion.
36 FIGURE 4.15 Cochlear Implants Cochlear implants, such as the one fitted on the side of this 10-year-old girl’s head, consist of a microphone around the ear and a transmitter fitted to the scalp, linked to electrodes that directly stimulate the auditory nerve. When implanted at a young age, these devices can enable people with hearing loss to learn to hear and speak.
37 In Vision, the Eye Detects Light Waves Most of the scientific study of sensation and of perception is concerned with visionVery little of what we call seeing takes place in the eyes, but rather as a result of constructive processes that occur throughout much of the brainBasic structures: cornea, lens, pupil, iris, retina
39 Rods and Cones The retina has two types of receptor cells: Rods: respond at extremely low levels of illumination; responsible primarily for night vision; found on outer edges of the retinaCones: less sensitive to low levels of light; responsible primarily for vision under high illumination and for seeing both color and detail; found throughout the retina but concentrated at the foveaContain photopigments that initiate the transduction of light waves into electrical neural impulses
40 Transmission From the Eye to the Brain A variety of retinal cells perform a series of sophisticated computations that help the visual system process the incoming informationCells include: bipolar, amacrine, and horizontal cells; converge on about a million retinal ganglion cellsGanglion cells are the first neurons in the visual pathway with axons, which are gathered into a bundle called the optic nerveAt the optic chiasm, axons in the optic nerves cross to the left and right hemispheres, travel to visual areas of the thalamus and then to the primary visual cortex in the occipital lobe
41 “Artificial Eye”A camera that transmits pictures into the brain is given some visually impaired people the chance at partial eyesight again. This ScienCentral News video has more.
42 The Color of Light is Determined by Its Wavelength An object appears to be a particular color because of the wavelengths of light it reflectsTrichromatic theory: activity in three different types of cones that are sensitive to different wavelengthsOpponent-process theory: Different types of ganglion cells, working in opposing pairs, create the perception that R/G, B/Y are oppositesWe categorize color along three dimensions: hue, saturation, and brightness
43 FIGURE 4.18 The Experience of Color The color of light is determined by the wavelength of the electromagnetic wave that reaches the eye. This graph shows the percentage of light at different wavelengths that is absorbed by each kind of cone.
44 FIGURE 4.20a The Color Spectrum (a) When white light shines through a prism, the spectrum of color that is visible to humans is revealed. As shown here, the visible color spectrum is only a small part of the electromagnetic spectrum: It consists of electromagnetic wavelengths from just under 400 nm (the color violet) to just over 700 nm (the color red). By using nightvision goggles, humans are able to see infrared waves (i.e., waves below red in terms of frequency).
45 FIGURE 4.21 Brightness versus Lightness (a) Which blue is brighter? How do you determine?
46 FIGURE 4.21 Brightness versus Lightness (b) For each pair, which central square is lighter? In fact, the central squares in each pair are identical. Most people see the gray square that is surrounded with red, for example, as lighter than the gray square surrounded with green. Why do they look different?
47 Subtractive Color Mixing Color can be produced through either the subtractive or the additive mixture of wavelengthsSubtractive color mixing: a process of color mixing that occurs within the stimulus itself; a physical, not psychological, processMixing paints is one form of subtractive color mixing because the colors are determined by pigments.Wavelengths that a pigment does not absorb are reflected and enter the eyeRed, yellow, and blue are the subtractive primary colors because together these pigments absorb nearly all the colors of the visible spectrum and when mixed, produce black
48 Additive Color MixingAdditive color mixing: a process of color mixing that occurs when different wavelengths of light interact within the eye’s receptors; a psychological processAdditive primary colors are red, green, and blue because mixing them yields white light
49 “Wearing Red: An Olympic Advantage?” Wondering who’s got an edge in the next game or Olympic event? Here’s a hint: Watch out for the contenders in red. This ScienCentral News video has more.
50 “Red and Lower Test Scores” When final exams loom, here’s a study worth some study: psychology researchers say that seeing even a glimpse of the color red can affect test performance. This ScienCentral News video explains.
51 FIGURE 4.22 Subtractive and Additive Color Mixing (a) Subtractive color mixing is the physical process of color mixing that happens within the stimulus. The subtractive primary colors are red, yellow, and blue. (b) By contrast, additive color mixing happens when lights of different wavelengths are perceived by the eye. The additive primary colors are red, green, and blue.
52 We Have Other Sensory Systems Humans, like other animals, have several internal sensory systems in addition to the five primary sensesKinesthetic sense: perception of the body’s position in space and movements of our bodies and our limbs (some include this with the sense of touch)Vestibular sense: perception of balance; uses information from receptors in the semicircular canals of the inner ear
53 4.3 How Does Perception Emerge from Sensation? Identify the primary sensory areas for touch, hearing, and vision.Discuss the gate control theory of pain.Explain how the brain localizes sound.Distinguish between the “what” and “where” pathways of the visual system.Describe blindsight.
54 How Does Perception Emerge from Sensation? With the exception of olfaction, all sensory information is relayed from the thalamus to cortical and other areas of the brainInformation is projected separately from the thalamus to primary sensory areas of the cerebral cortexIn these areas the perceptual process begins in earnest
55 FIGURE 4.23 Primary Sensory Areas These are the primary brain regions where information about taste, touch, hearing, smell, and vision are projected. Visual information travels in separate “streams” — what you see and where it is — from the occipital lobe (visual cortex) to different parts of the brain for further processing.
56 In Touch, the Brain Integrates Sensory Information from Different Regions of the Body Touch information from the thalamus is projected to the primary somatosensory cortexIn the 1940s, Wilder Penfield discovered that electrical stimulation of the primary somatosensory cortex could evoke the sensation of touch in different regions of the body (Penfield & Jasper, 1954)The most sensitive regions of the body, such as lips and fingers, have a greater amount of cortex devoted to them
57 Gate Control TheoryPain is a complex experience that depends on biological, psychological, and cultural factorsMelzack’s gate control theory of pain: Pain receptors must be activated and a neural “gate” in the spinal cord must allow the signals through to the brainPain signals transmitted by small-diameter nerve fibers can be blocked at the level of the spinal cord by the firing of larger sensory nerve fibers
58 FIGURE 4.24 Gate Control Theory According to the gate control theory of pain, neural “gates” in the spinal cord allow signals through. Those gates can be closed when information about touch is being transmitted (e.g., by rubbing a sore arm) or by distraction.
59 Controlling PainDrug treatments (ibuprofen, acetaminophen, Novocain, anesthetics)Cognitive states (distraction, positive mood, relaxation) can close the pain gateSome mental processes, such as worrying about or focusing on the painful stimulus, seem to open pain gatesWell-rested research participants rated the same level of a painful stimulus as less painful than did participants who were fearful, anxious, or depressed (Loggia, Mogil, & Bushnell, 2008; Sullivan et al., 2001)
60 “Scared Slow”After a car accident or other scary experience, many people report feeling like the event happened in slow motion. One neuroscientist set out to test whether our brains really perceive things slowed down in response to fear. This ScienCentral News story shows why he had to scare people in order to do the experiment.
61 In Hearing, the Brain Integrates Sensory Information from the Ears Auditory neurons in the thalamus extend their axons to the primary auditory cortexNeurons in the primary auditory cortex code the frequency (or pitch) of auditory stimuliTo locate the origin of a sound (auditory localization), the brain integrates the different sensory information coming from each of our two ears
62 FIGURE 4.25 Auditory Localization (a) Like barn owls, (b) humans draw on the intensity and timing of sounds to locate where the sounds are coming from.
63 In Vision, the Brain Processes Sensory Information from the Eyes The study of perception has focused to a large extent on the primary visual cortex and the multiple areas in which the retinal image is processedAccording to some estimates, up to half of the cerebral cortex may participate in visual perception
64 What Versus WhereVisual areas beyond the primary visual cortex form two parallel processing streams, or pathwaysVentral stream appears to be specialized for the perception and recognition of objectsDorsal stream seems to be specialized for spatial perception (determining where an object is)These two processing streams are therefore known as the “what” stream and the “where” stream
65 BlindsightBlindsight: a condition in which people who are blind have some visual capacities in the absence of any visual awarenessExample: A person might not be able to see anything on his or her left. However, when a stimulus is presented in this blind field, the patient can respond unconsciously to that stimulus
66 4.4 What Factors Influence Visual Perception? Describe the Gestalt principles of perceptual organization.Identify the brain regions associated with facial perception.Identify cues for depth perception.Explain how the visual system perceives motion.Discuss how perceptual constancy is achieved.
67 Object Perception Requires Construction Perceptual psychologists believe that illusions reveal the mechanisms that help our visual systems determine the sizes and distances of objects in the visual environmentResearchers rely on these tricks to reveal automatic perceptual systems that, in most circumstances, result in accurate perception
68 Gestalt Principles of Perceptual Organization The German word Gestalt means “shape” or “form.” As used in psychology, Gestalt means “organized whole.”Gestalt psychology postulated a series of laws to explain how our brains group the perceived features of a visual scene into organized wholes
69 Figure and GroundAmong the most basic organizing principles is distinguishing between figure and groundA classic illustration of this is the reversible figure illusion, in which figure and ground switch back and forth (ambiguous)In identifying what is “figure,” the brain assigns the rest of the scene to the background
71 Proximity and Similarity Two of the most important Gestalt principles concern proximity and similarityPrinciple of proximity: The closer two figures are to each other, the more likely we are to group them and see them as part of the same objectPrinciple of similarity: We tend to group figures according to how closely they resemble each other
72 FIGURE 4.27a Gestalt Principles Gestalt psychology describes how perceived features of a visual scene are grouped into organized wholes.
73 FIGURE 4.27b Gestalt Principles Gestalt psychology describes how perceived features of a visual scene are grouped into organized wholes.
74 The “Best” FormsGood continuation: the tendency to interpret intersecting lines as continuous rather than as changing direction radicallyClosure: the tendency to complete figures that have gapsIllusory contours: We sometimes perceive contours and cues to depth even though they do not exist
75 FIGURE 4.27c Gestalt Principles Gestalt psychology describes how perceived features of a visual scene are grouped into organized wholes.
76 FIGURE 4.27d Gestalt Principles Gestalt psychology describes how perceived features of a visual scene are grouped into organized wholes.
77 FIGURE 4.27e Gestalt Principles Gestalt psychology describes how perceived features of a visual scene are grouped into organized wholes.
78 Bottom-Up and Top-Down Information Processing How do we assemble the information about parts into a perception of a whole object?Bottom-up processing: Data are relayed in the brain from lower to higher levels of processingTop-down processing: Information at higher levels of mental processing can influence lower, “earlier” levels in the processing hierarchyThe flight crew of New Zealand Flight 901 failed to notice the 12,000-foot volcano looming in front of them because the pilots saw what they expected to see
79 FIGURE 4.28 ContextContext plays an important role in object recognition.How does context aid your interpretation of the shapes shown here?
80 FIGURE 4.29 Mount ErebusBecause the pilots on Air New Zealand Flight 901 did not expect to see this Antarctic volcano ahead of them, they failed to see it.
81 Face Perception The visual system is sensitive to faces: We can more readily discern information about a person’s mood, attentiveness, sex, race, and age by looking at a person’s face than by listening to them talk, watching them walk, or studying their clothing (Bruce & Young, 1986)Whites are much better at recognizing white faces than at recognizing black faces (Brigham & Malpass, 1985)Prosopagnosia: deficits in the ability to recognize facesCortical regions, even specific neurons, seem to be specialized to perceive faces and are sensitive to facial expression and gaze direction
82 “Vote for You!”Researchers have shown that voters can be swayed toward choosing a candidate by making the candidate look more like a voter. These visual manipulations were unnoticed, but were enough to swing the 2004 presidential election. The implications of the research go beyond digital manipulation and give a deeper insight into this year’s heated race.
83 “Election Faces”New research shows that when it comes to voting, at least part of your decision is made in the blink of an eye. As this ScienCentral video explains, voting involves parts of the brain that make quick decisions about the people you meet or see, including candidates for election.
84 FIGURE 4.30 Perceiving Faces Brain imaging shows increased activity in the right hemisphere when faces are viewed.
85 Depth Perception is Important for Locating Objects How are we able to construct a three-dimensional mental representation of the visual world from two-dimensional retinal input?Binocular depth cues: available from both eyes together and contribute to bottom-up processingMonocular depth cues: available from each eye alone and provide organizational information for top-down processing
86 Binocular Depth Perception Binocular disparity (or retinal disparity): This cue is caused by the distance between humans’ two eyesThe brain uses the disparity between these two retinal images to compute distances to nearby objectsStereoscopic vision: the ability to determine an object’s depth based on that object’s projections to each eyeConvergence: When eye muscles turn the eyes inward, the brain knows how much the eyes are converging and uses this information to perceive distance
87 FIGURE 4.32 Binocular Disparity To demonstrate your own binocular disparity, hold one of your index fingers out in front of your face and close first one eye and then the other. Your finger appears to move because each eye, due to its position relative to the finger, has a unique retinal image.
88 FIGURE 4.33 ConvergenceHold one of your index fingers out in front of your face, about a foot away. Slowly bring your finger toward your eyes. Are you able to perceive your eyes converging?
89 Monocular Depth Perception We can perceive depth with one eye because of monocular depth cuesPictorial depth cues:OcclusionRelative sizeFamiliar sizeLinear perspectiveTexture gradientPosition relative to horizon
90 FIGURE 4.34 Pictorial Depth Cues Using the bulleted list below as a reference, try to identify the six depth cues in Edvard Munch’s painting Evening on Karl Johan Street (circa 1892).
91 Motion Cues for Depth Perception Motion parallax: The brain uses cues from the relative movements of objects that are at various distances from the observerWhen you watch the scenery from a moving car, near objects such as mailboxes seem to pass quickly, far objects more slowly, whereas objects farther away appear to match your speedObjects at an intermediate distance (a house) move opposite the direction of closer ones (a mailbox), whereas distant objects (a mountain) move in the same direction relative to the intermediate-distance object
92 FIGURE 4.35 Motion Parallax Near objects seem to pass us more quickly in the opposite direction of our movement. Objects farther away seem to move more slowly.
93 Size Perception Depends on Distance Perception The size of an object’s retinal image depends on that object’s distance from the observerTo determine an object’s size the visual system needs to know how far away it isDepth cues can fool us into seeing depth when it is not there; a lack of depth cues can fool us into not seeing depth when it is there
94 FIGURE 4.36 Distance Perception This picture, by Rebecca Robinson, captures what appears to be a tiny Sarah Heatherton standing on James Heatherton’s head. This illusion occurs because the photo fails to present depth information: It does not convey the hill on which Sarah is standing.
95 Ames BoxesAmes boxes: first crafted in the 1940s by Adelbert Ames, a painter turned scientistAmes boxes’ rooms play with linear perspective and other distance cues to create size illusions
96 FIGURE 4.37 The Ames BoxAmes played with depth cues to create size illusions. For example, as illustrated here, he made a diagonally cut room appear rectangular by using crooked windows and floor tiles. When one child stands in a near corner and another (of similar height) stands in a far corner, the room creates the illusion that they are equidistant from the viewer. Therefore, the closer child looks like a giant compared with the child farther away.
97 The Ponzo Illusion Classic example of a size/distance illusion Explained: Monocular depth cues make the two-dimensional figure seem three-dimensional (Rock, 1984)This illusion shows how much we rely on depth perception to gauge size; the brain uses depth cues even when depth is absent
98 FIGURE 4.38 The Ponzo Illusion The two horizontal lines appear to be different sizes but are actually the same length.
99 Motion Perception Has Internal and External Cues How does the brain know what is moving?After receiving damage to secondary visual areas of her brain — areas critical for motion perception—M.P., a German woman, saw the world as a series of snapshots rather than as a moving image (Zihl, von Cramon, & Mai, 1983)Neurons specialized for detecting movement fire when movement occurs
100 Motion AftereffectsWaterfall effect: If you stare at a waterfall and then turn away, the scenery you are now looking at will seem to move upwardExplained:The visual cortex has neurons that respond to movement in a given directionWhen you stare at a moving stimulus long enough, these direction-specific neurons adapt to the motion and become fatiguedWhen the stimulus is removed, other motion detectors that respond to all other directions are more active than the fatigued motion detectors
101 Compensation for Head and Eye Movement When you see what appears to be a moving object, how do you know whether the object is moving, you are moving, or your eyes are moving?Explained:The brain calculates an object’s perceived movements by monitoring the movement of the eyes, and perhaps also of the head, as they track a moving objectMotion detectors track an image’s motion across the retina
102 FIGURE 4.39 Perceiving Movement These diagrams illustrate the two ways that the visual system detects movement.
103 Stroboscopic Motion Perception Stroboscopic movement: a perceptual illusion that occurs when two or more slightly different images are presented in rapid successionMax Wertheimer conducted experiments in 1912 by flashing, at different intervals, two vertical lines placed close togetherWhen the interval was about 60 milliseconds, the line appeared to jump from one place to anotherAt slightly longer intervals, the line appeared to move continuously — a phenomenon called phi movement
104 FIGURE 4.40 How Moving Pictures Work This static series would appear transformed if you spun the wheel. When the slightly different images were presented in rapid succession, the stroboscopic movement would tell your brain that you were watching a moving horse.
105 Perceptual Constancies Are Based on Ratio Relationships How does the brain know that a person is 6 feet tall when the retinal image of that person changes size?Perceptual constancy: The brain correctly perceives objects as constant despite sensory data that could lead it to think otherwiseThe brain computes a ratio based on relative magnitude rather than on sensations’ absolute magnitude; perceptual systems are tuned to detect changes from baseline conditions, not just to respond to sensory inputsSizeColorShapeLightness
106 FIGURE 4.41 Perceptual Constancy When you look at each of these photos, your retinal image of the bearded man is the same. Why, then, does he appear larger in (a) than in (b)?
107 FIGURE 4.42 The Tabletop Illusion Created by the psychologist Roger Shepard, this illusion demonstrates the brain’s automatic perceptual processes. Even when we know the two tabletops are the same size and shape — even if we have traced one image and placed it on top of the other — perspective cues make us see them as different.