chapter 8: sensation and perception · 208 chapter 8 / sensation and perception fraser’s spiral...

Chapter OverviewVisit the Understanding PsychologyWeb site at psychology.glencoe.comand click on Chapter 8—ChapterOverviews to preview the chapter.

PSYCHOLOGY

206

Psychology JournalThink about an occasion in

which you were in a crowdedand noisy setting. Did you findyourself listening to or pickingup part of another conversa-tion? Write an explanation forthis occurrence in your journal. ■

http://www.glencoe.com/sec/socialstudies/psychology/psych2001/chapter8/overview.html

From that day forward, Helen “saw” the world in a new way. Shediscovered new ways to experience her world. “Occasionally, if Iam very fortunate, I place my hand gently on a small tree and feel

the happy quiver of a bird in full song.” Helen had entered a world of sensations after she began organizing the stimuli in her world. Try it yourself.

In the next few seconds, something peculiar will start hap pening tothe material youa rereading. Iti soft ennotre alized howcom plext hepro-ces sof rea ding is.

Sensation

■ Main IdeaSensations allow humans to under-stand reality. Sensations occur any-time a stimulus activates a receptor.

■ Vocabulary• sensation• perception• psychophysics• absolute threshold• difference threshold• Weber’s law• signal-detection theory

■ Objectives• Describe the field of study known as

psychophysics.• Define and discuss threshold, Weber’s

law, and signal detection.

Reader’s GuideExploring Psychology

Discovering a New WorldHelen Keller had been blind and deaf

since she was two years old. For the nextfour years, Helen was “wild and unruly.”Then when she was six, Anne Sullivan, ateacher, entered her life. Using the senseof touch as the link between their twoworlds, Anne tried again and again, byspelling words into Helen’s hand, to makeHelen grasp the connection betweenwords and the things they stood for. Thebreakthrough came one day as Annespelled the word water into Helen’s handas water from a spout poured over it. “Istood still, my whole attention fixed uponthe motions of her fingers,” Helen remem-bered. “Suddenly I felt . . . a thrill ofreturning thought; and somehow the mys-tery of language was revealed to me.”

—adapted from ABC’s of the Human Mind,Reader’s Digest, 1990

Chapter 8 / Sensation and Perception 207

sensation: what occurs whena stimulus activates a receptor

perception: the organizationof sensory information intomeaningful experiences

psychophysics: the study ofthe relationships between sen-sory experiences and the physi-cal stimuli that cause them

As you can see, your success in gathering information from your envi-ronment, interpreting this information, and acting on it depends consid-erably on its being organized in ways you expect. In this chapter you willlearn more about sensation and perception, both of which are necessaryto gather and interpret information in our surroundings.

WHAT IS SENSATION?The world is filled with physical changes—an alarm clock sounds; the

flip of a switch fills a room with light; you stumble against a door; steamfrom a hot shower billows out into the bathroom, changing the temper-ature and clouding the mirror. Any aspect of or change in the environ-ment to which an organism responds is called a stimulus. An alarm, anelectric light, and an aching muscle are all stimuli for human beings.

A stimulus can be measured in many physical ways, including its size,duration, intensity, or wavelength. A sensation occurs anytime a stimu-lus activates one of your receptors. The sense organs detect physicalchanges in energy such as heat, light, sound, and physical pressure. Theskin notes changes in heat and pressure, the eyes note changes in light,

and the ears note changes in sound. Other sensorysystems note the location and position of your body.

A sensation may be combined with other sensa-tions and your past experience to yield a perception.A perception is the organization of sensory informa-tion into meaningful experiences (see Figure 8.1).

Psychologists are interested in the relationshipbetween physical stimuli and sensory experiences. Invision, for example, the perception of color corre-sponds to the wavelength of the light, whereas bright-ness corresponds to the intensity of this stimulus.

What is the relationship between color and wave-length? How does changing a light’s intensity affectyour perception of its brightness? The psychologicalstudy of such questions is called psychophysics. Thegoal of psychophysics is to understand how stimulifrom the world (such as frequency and intensity)affect the sensory experiences (such as pitch andloudness) produced by them.

THRESHOLDIn order to establish laws about how people sense

the external world, psychologists first try to determinehow much of a stimulus is necessary for a person tosense it at all. How much energy is required for some-one to hear a sound or to see a light? How much of ascent must be in the room before one can smell it?

208 Chapter 8 / Sensation and Perception

Fraser’s Spiral

Fraser’s spiral illustrates the differencebetween sensation and perception. Ourperception of this figure is that of a spiral,but it is actually an illusion. Trace a circlecarefully. Your finger will always comeback to its starting point. How do we usesensation and perception together tounderstand our world?

Figure 8.1

How much pressure must beapplied to the skin before aperson will feel it?

To answer such questions,a psychologist might set upthe following experiment.First, a person (the partici-pant) is placed in a dark roomand is instructed to look at thewall. He is asked to say “I seeit” when he is able to detect alight. The psychologist thenuses an extremely precisemachine that can project alow-intensity beam of lightagainst the wall.

The experimenter turnson the machine to its low-est light projection. Theparticipant says nothing.The experimenter increasesthe light until finally theparticipant responds, “I seeit.” Then the experimenterbegins another test in theopposite direction. Hestarts with a visible butfaint light and decreases itsintensity until the lightseems to disappear. Manytrials are completed andaveraged. This proceduredetects the absolutethreshold—the weakestamount of a stimulusrequired to produce a sensation. The absolutethreshold is the level ofstimulus that produces apositive response of detec-tion 50 percent of the time.

The absolute thresh-olds for the five senses in humans are the following: in vision—seeing acandle flame 30 miles away on a clear night; for hearing—hearing awatch ticking 20 feet away; for taste—tasting 1 teaspoon of sugar dis-solved in 2 gallons of water; for smell—smelling 1 drop of perfume in a3-room house; for touch—feeling a bee’s wing falling a distance of 1 cen-timeter onto your cheek.

absolute threshold: theweakest amount of a stimulusthat a person can detect halfthe time


G ustav Theodor Fechnerstarted out as a young

professor trying to demon-strate that every person, animal, and plant in the universe is composed ofboth matter and soul. Hefailed. At one point, in themidst of a depression, hepainted his room black andremained in it day and night,seeing no one. When hefinally emerged from his isolation, he walked througha garden, and the flowers looked brighter than he had ever seenthem before.

On the morning of October 22, 1850, as Fechner lay in bed, athought occurred to him. He arrived at the conclusion that a sys-temic relationship between bodily and mental experience could bedemonstrated if a person were asked to report changes in sensationsas a physical stimulus was varied. While testing these ideas, Fechnercreated the area of psychology known as psychophysics. Fechner’smethods of sensory measurement inspired experimental research onthe subject and revolutionized experimental psychology.

Gustav Theodor Fechner1801–1887

“Imagine that you lookat the sky through a tinted glass and pick

out a cloud that is justnoticeably different from

the sky background.Now you use a much

darker glass; the clouddoes not vanish but is

still just barely visible—because although the

absolute levels of inten-sity are much lowerthrough the darker

glass, the ratio of inten-sities between cloud and

sky has not changed.”

Profiles In Psychology

difference threshold: thesmallest change in a physicalstimulus that can be detectedbetween two stimuli

While these thresholds may seem impressive, we respond to very lit-tle of the sensory world. We cannot see X rays or microwaves. Dogs canhear a dog whistle, while we cannot. Humans hear only 20 percent ofwhat a dolphin can hear. Some animals, such as bats and dolphins, havea superior sense of hearing. Other animals, such as hawks, have ex-tremely sharp vision; still others, such as bloodhounds, possess a superiorsense of smell. Humans sense a somewhat limited range of the physicalphenomena in the everyday world.

SENSORY DIFFERENCES AND RATIOS Another type of threshold is the difference threshold. The differ-

ence threshold refers to the minimum amount of difference a person candetect between two stimuli. To return to our example of the person testedin a dark room, a psychologist would test for the difference threshold bygradually increasing the intensity of a visible light beam until the personsays, “Yes, this is brighter than the light I just saw.” With this technique,it is possible to identify the smallest increase in light intensity that isnoticeable to the human eye.

A related concept is the just noticeable difference, or JND. This refers tothe smallest increase or decrease in the intensity of a stimulus that a per-son is able to detect. Psychologists have found that a particular sensory


The Human Senses

This chart lists the fundamental features that make up the humansensory system. What is our vestibular sense?

Figure 8.2

Sense Stimulus Sense Receptor SensationOrgan

Sight Light waves Eye Rods and cones Colors, patterns, textures, of retina motion, depth in space

Hearing Sound waves Ear Hair cells located in Noises, tonesinner ear

Skin sensations External contact Skin Nerve endings in skin Touch, pain, warmth, cold

Smell Volatile substances Nose Hair cells of olfactory Odors (musky, flowery,membrane burnt, minty)

Taste Soluble substances Tongue Taste buds of tongue Flavors (sweet, sour,salty, bitter)

Vestibular sense Mechanical and Inner ear Hair cells of semicircular Spatial movement,gravitational forces canals and vestibule gravitational pull

Kinesthesis Body movement Muscles, Nerve fibers in muscles, Movement and positiontendons, tendons, and joints of body partsand joints

experience depends more on changes in the stimu-lus than on the absolute size or amount of thestimulus. For example, if you put a 3-poundpackage of food into an empty backpack, theperceived weight will be greatly increased. Ifyou add the same amount to a backpack witha 100-pound weight in it, however, your per-ception of the weight will hardly increase atall. This is because the perception of theadded weight reflects a proportional change,and 3 pounds does not provide much changein a 100-pound load.

This idea is known as Weber’s law: thelarger or stronger a stimulus, the larger thechange required for a person to notice thatanything has happened to it. By experiment-ing in this way with variations in sounds, tem-peratures, pressures, colors, tastes, and smells,psychologists are learning more about howeach sense responds to stimulation. Some sensesproduce huge increases in sensation in responseto small increases in energy. For instance, the painof an electric shock can be increased more thaneight times by doubling the voltage. On the otherhand, the intensity of a light must be increased manytimes to double its brightness.

Some people are more sensitive to these changesthan others. For example, people who can detect smalldifferences in sensation work as food tasters, wine tasters,smell experts, perfume experts, and so on.

SENSORY ADAPTATIONPsychologists have focused on people’s responses to changes in stim-

uli because they have found that the senses are tuned to change. Sensesare most responsive to increases and decreases, and to new events ratherthan to ongoing, unchanging stimulation. We are able to respond tochanges in our environment because our senses have an ability to adapt,or adjust themselves, to a constant level of stimulation. They get used toa new level and respond only to deviations from it (see Figure 8.3).

A good example of this sensory adaptation is the increase in visualsensitivity that you experience after a short time in a darkened movie the-ater. At first you see only blackness, but after a while your eyes adapt tothe new level, and you can see seats, faces, and so forth. Adaptationoccurs for the other senses as well. Receptors in your skin adapt to thecold water when you go for a swim; disagreeable odors in a lab seem todisappear after a while; street noises cease to bother you after you havelived in a city for a time. Without sensory adaptation, you would feel the

Weber’s law: the principlethat for any change (�s) in astimulus to be detected, aconstant proportion of thatstimulus (s) must be added or subtracted


Can you detect changes in stimuli?What would it take for you to notice a differ-ence in the weight of your backpack?

Procedure1. Fill your backpack with materials so that it

weighs 10 pounds, and put it on your back.

2. Assemble a collection of objects that weighabout 4 ounces (113 g) each, such asapples or oranges.

3. Ask a friend to insert the objects one at atime while you are seated, with the weightof your backpack off your back. Be sureyou cannot see which object is beingplaced in the pack.

Analysis1. After each object is placed in the pack,

stand and report whether or not the back-pack feels heavier.

2. Record the point at which you notice thedifference in the weight of the pack.

3. Use the concept of differencethreshold to explain your results.

Reading CheckAccording to Weber’s

law, how much must a strongstimulus change for a person tonotice the change?

See the SkillsHandbook, page 622,

for an explanation of designingan experiment.

constant pressure of the clothes on your body, and other stimuli wouldseem to be bombarding all your senses at the same time.

Sensory adaptation allows us to notice differences in sensations andreact to the challenges of different or changing stimuli. This principle ishelpful when performing many activities, such as the work of police,security guards, and home inspectors. These people may notice minutechanges and act appropriately.

SIGNAL-DETECTION THEORYThere is no sharp boundary between stimuli that you can perceive and

stimuli you cannot perceive. The signal-detection theory studies therelations between motivation, sensitivity, and decision making in detectingthe presence or absence of a stimulus (Green & Swets, 1966). Detectionthresholds involve recognizing some stimulus against a background ofcompeting stimuli. A radar operator must be able to detect an airplane ona radar screen even when the plane’s blip is faint and difficult to distinguishfrom blips caused by flocks of birds or bad weather, which can produceimages that are like visual “noise.” The radar operator’s judgment will beinfluenced by many factors, and different operators appear to have differ-ent sensitivities to blips. Moreover, a specific individual’s apparent sensitiv-ity seems to fluctuate, depending on the situation. For example, a radaroperator may be able to ignore other stimuli as long as she is motivated tokeep focused, just as you may be motivated to complete your readingassignment no matter what distractions you encounter.

In studying the difficul-ties faced by radar opera-tors, psychologists havereformulated the concept ofabsolute threshold to takeinto account the many fac-tors that affect detection ofminimal stimuli. As a result, signal-detection theoryabandons the idea thatthere is a single trueabsolute threshold for astimulus. Instead, it is basedon the notion that the stim-ulus, here called a signal,must be detected in thepresence of competingstimuli, which can interferewith detection of the signal.

Psychologists haveidentified two differenttypes of processing stimuli,or signals. Preattentive


The Disappearing Circle

Sensation depends on change and contrast in the environment. Holdyour hand over one eye and stare at the dot in the middle of the cir-cle on the right. You should have no trouble maintaining the image ofthe circle. If you do the same with the circle on the left, however, theimage will fade. The gradual change from light to dark does not pro-vide enough contrast to keep the visual receptors in your eye firingat a steady rate. The circle reappears only if you close and reopenyour eye or you shift your gaze to the X. What is the purpose ofsensory adaptation?

Figure 8.3

X

signal-detection theory:the study of people’s tenden-cies to make correct judgmentsin detecting the presence ofstimuli


The Stroop Effect

Try to name the colors of theboxes in a as fast as you can.Then try to read the words inb as fast as you can. Finally,try to name the colors of thewords in b as fast as youcan. You probably proceededmore slowly when naming thecolors in b. Why was it moredifficult to name the colorsin b?

Figure 8.4

1. Review the Vocabulary What is thedifference between sensation and perception?

2. Visualize the Main Idea Complete thechart below by listing the five sensesand their absolute thresholds.

3. Recall Information How does the signal-detection theory explain how you

may be able to study while others arewatching television in the same room?

4. Think Critically Why do you think wedo not respond to all stimuli present inour environment?

Assessment

5. Application Activity Place a watch or clock inan empty room. Move across the room to a pointwhere you do not hear the ticking. Approach theclock one step at a time and mark the spot atwhich you first hear ticking. Repeat this experi-ment 10 times, alternately approaching andbacking away. What are your conclusions? Write a brief analysis.

Sense Absolute Threshold

Sight

a b

Red Blue Yellow Green

Orange Blue Red Yellow

Green Orange Blue Red

Red Green Blue Orange

process is a method for extracting information automatically and simultane-ously when presented with stimuli. Attentive process is a procedure that con-siders only one part of the stimuli presented at a time. For example, whenlooking at Figure 8.4 (b), the Stroop interference effect, preattentive, or auto-matic, processing acts as an interference. The tendency is to read the wordinstead of saying the color of the ink. Apparently people find it almostimpossible not to read color names that appear before their eyes, becausethe name interferes with the response of naming the ink color when the twoare different. In summary, we notice some things automatically in spite ofdistracting information. However, it requires more careful attention tonotice other, less distinct items. This difference, though, is one of degree—alltasks require attention, but some require more attention than others.

The Senses

Reader’s Guide


Why did you see your hand moving even though it was totallydark while doing the experiment above? You have just experi-enced kinesthesis—one of the senses. Although people are

thought to have five senses, there are actually more. In addition to vision,hearing, taste, smell, and touch, there are several skin senses and twointernal senses: vestibular and kinesthetic.

Each type of sensory receptor takes some sort of external stimulus—light, chemical molecules, sound waves, and pressure—and converts itinto a chemical-electrical message that can be transmitted by the nervoussystem and interpreted by the brain. So far, we know most about theseprocesses in vision and hearing. The other senses have received less atten-tion and are more mysterious in their functioning.

■ Main IdeaThe sense organs—the eyes, ears,tongue, nose, skin, and others—are thereceptors of sensations.

■ Vocabulary• pupil• lens• retina• optic nerve• binocular fusion• retinal disparity• auditory nerve• vestibular system• olfactory nerve• kinesthesis

■ Objectives• Describe the nature and functioning of

the sense organs.• Identify the skin and body senses and

explain how they work.

Exploring Psychology

Seeing in the DarkSit yourself in total darkness, a space

so dark you cannot see your hand beforeyour face. Now hold your hand beforeyour face and move it from side to side.You see your hand in motion.

—from A Second Way of Knowing: The Riddle of Human Perception by Edmund Blair Bolles,1991


pupil: the opening in the iristhat regulates the amount oflight entering the eye

lens: a flexible, elastic, trans-parent structure in the eye thatchanges its shape to focus light on the retina

retina: the innermost coatingof the back of the eye, contain-ing the light-sensitive receptorcells

optic nerve: the nerve thatcarries impulses from the retinato the brain

VISIONVision is the most studied of all the senses, reflecting the high impor-

tance we place on our sense of sight. Vision provides us with a great dealof information about our environment and the objects in it—the sizes,shapes, and locations of things, and their textures, colors, and distances.

How does vision occur? Light enters the eye through the pupil (seeFigure 8.5) and reaches the lens, a flexible structure that focuses light onthe retina. The retina contains two types of light-sensitive receptor cells,or photoreceptors: rods and cones. These cells are responsible for chang-ing light energy into neuronal impulses, which then travel along the opticnerve to the brain, where they are routed to the occipital lobe.

Cones require more light than rods before they begin to respond, andcones work best in daylight. Since rods are sensitive to much lower levels oflight than cones, they are the basis for night vision. There are many morerods (75 to 150 million) than there are cones (6 to 7 million), but only conesare sensitive to color. Rods and cones can be compared to black-and-whiteand color film. Color film takes more light and thus works best in daylight,like our cones. Sensitive black-and-white film works not only in bright lightbut also in shadows, dim light, and other poor lighting conditions, just likeour rods.

The Human EyeFigure 8.5

This cross section of the human eyeshows the passage of light. Note thatthe retina receives an inverted image.What is the main function of the rodsand cones?

Eye muscle

Neuron

Cones

Rods

LIGHT

Optic Nerve

Lens

Iris

Pupil

Cornea

Muscles

Blind spot

Opticnerve

Retina


LightLight is a form of electromagnetic

radiation. Other forms of electromag-netic radiation include radio waves,microwaves, infrared radiation, ultravio-let rays, X rays, and gamma rays. All ofthese are known collectively as the elec-tromagnetic spectrum (see Figure 8.6).

Visible light represents a small por-tion of the electromagnetic spectrum,which is composed of waves of differ-ent length and frequency. You canobserve the wavelengths of visible lightwith a prism. Passing sunlight througha prism breaks the light into a rainbowof colors. Each of these colors is com-prised of light of different wavelengths.While prisms transmit light, otherobjects absorb and reflect light. The ob-ject’s color depends, in part, on the lightthat reaches our eyes. For example, apea looks green because it reflects greenlight and absorbs other colors.

Color DeficiencyWhen some or all of a person’s cones do not function properly, he or

she is said to be color-deficient. There are several kinds of color deficiency,and most color-deficient people do see some colors (see Figure 8.7). Forexample, some people have trouble distinguishing between red and green.Fewer people have no trouble with red and green but cannot distinguishbetween yellow and blue. A very few people are totally color-deficient. Theydepend on their rods, so to them the world looks something like black-and-white television programs—nothing but blacks, whites, and shades of gray.

Color deficiency affects about 8 percent of American men and lessthan 1 percent of American women. It is a hereditary condition. This geneis carried in the genes of women whose vision is usually normal. Thesewomen pass the genes on to their sons, who are born color-deficient.

Binocular FusionBecause we have two eyes, located about 2.5 inches (6.4 cm)

apart, the visual system receives two images. Instead of seeing double,however, we see a single image—a composite of the views of two eyes. The combination of the two images into one is called binocularfusion.

Not only does the visual system receive two images but there is alsoa difference between the images on the retinas. This difference is calledretinal disparity. You can easily observe retinal disparity by bringing an

Your BlindSpot

The point where the optic nerve exits the eye is theblind spot (see Figure 8.5). When light hits that point, theeye registers nothing because this area lacks photorecep-tors—neurons that are sensitive to light. Find your blindspot by holding the diagram below about 3–4 inches (8–10 cm) away from your eyes.

• +Make sure the cross is on the right. Close your right eye

and look directly at the cross with your left eye. Notice thatyou can also see the dot. Focus on the cross, but be awareof the dot as you slowly bring the diagram toward your face.The dot will disappear and then reappear as you bring thediagram toward your face. Now close your left eye and lookdirectly at the dot with your right eye. This time the crosswill disappear and reappear as you bring the diagram slowlytoward your face. What has happened? When you hold thediagram so that the light from the dot falls on the blind spot,you cannot see the dot. This is your blind spot.

retinal disparity: the differ-ences between the imagesstimulating each eye

binocular fusion: theprocess of combining theimages received from the twoeyes into a single, fused image


object, such as an eraser, close to your eyes. Without moving it, look atthe eraser first with one eye, then with the other. You will see a differencein the two images because of the different viewpoint each eye has. Whenyou open both eyes, you will no longer see the difference but will in-stead see the object as solid and three-dimensional, if you have goodbinocular vision.

Retinal disparity is essential to your sense of depth perception. Thebrain interprets a large retinal disparity (a large difference between whatthe right eye and what the left eye are seeing) to mean that an object isnearby. The brain interprets a small retinal disparity (not much differencebetween the images the left and right eyes receive) to mean a distant object.

Gamma rays

10–5 10–3 10–1 101 103 105 107 109 1011 1013 1015 1017

X rays

Wavelength in nanometers (billionths of a meter)

Ultra-violetrays

Infraredrays

Radar

Whitelight

Prism

Visible light

400 500 600 700

Broadcastbands

ACcircuits

Light is the visible portion of the electromagnetic spectrum. When thewavelengths in white light are separated, the visual effect is an array ofcolors because different wavelengths are seen as different colors. Whyare we able to see the wavelengths of the visible spectrum?

The Electromagnetic SpectrumFigure 8.6

PSYCHOLOGY

Student Web ActivityVisit the UnderstandingPsychology Web site at psychology.glencoe.comand click on Chapter 8—Student Web Activities foran activity on sensation andperception.

http://www.glencoe.com/sec/socialstudies/psychology/psych2001/chapter8/webact.html


Nearsightedness and Farsightedness Some of us are born with per-fectly shaped eyeballs. These people have almost perfect vision. If youreyeball is a little too long, however, you are nearsighted. Objects arefocused at a point slightly in front of the retina so that you can see objectsthat are near, but distant objects seem blurry. If your eyeball is too short, you are farsighted. Objects are focused at a point slightly behindthe retina so that you can see distant objects clearly, but near objects appear blurry.

HEARINGHearing depends on vibrations of the air, called sound waves. Sound

waves from the air pass through various bones (see Figure 8.10) until theyreach the inner ear, which contains tiny hairlike cells that move back andforth (much like a field of wheat waving in the wind). These hair cellschange sound vibrations into neuronal signals that travel through theauditory nerve to the brain.

Loudness of sound is determined by the amplitude, or height, of soundwaves. The higher the amplitude, the louder the sound. This strength, orsound-pressure energy, is measured in decibels. The sounds humans hearrange upward from 0 decibels, the softest sound the human ear can detect,to about 140 decibels, which is roughly as loud as a jet plane taking off. Anysound over 110 decibels can damage hearing as can persistent sounds as lowas 80 decibels. Any sound that is painful when you first hear it will damageyour hearing if you hear it often enough. Figure 8.9 lists the decibel levels ofsome common sounds.

Pitch depends on sound-wave frequency, or the rate of the vibrationof the medium through which the sound wave is transmitted. Low fre-quencies produce deep bass sounds, and high frequencies produce shrillsqueaks. If you hear a sound composed of a combination of different fre-quencies, you can hear the separate pitches even though they occursimultaneously. For example, if you strike two keys of a piano at the sametime, your ear can detect two distinct pitches.

auditory nerve: the nervethat carries impulses from theinner ear to the brain, resultingin the perception of sound

A Changing Flag

Stare steadily at the lowest right-handstar for about 45 seconds. Then stareat a blank piece of paper. You shouldsee a negative afterimage of this fig-ure. This occurs because the recep-tors for green, black, and yellowbecome fatigued or neuronal firingrates shift, allowing the complemen-tary colors of each to predominatewhen you stare at the white paper.What happens when you shift your glance to a blank wall somedistance away? Why?

Figure 8.8

� � � � � ��

� � � � � ��

� � � � � ��

� � � � � �

� � � � � ��

Testing for ColorDeficiency

Can you see numeralsin the dot patterns thatmake up this figure?Those with normalvision will see a num-ber, while those withred-green deficiencywill see only randompatches of color. Whatis the cause of colordeficiency?

Figure 8.7


Sources of sounds can be located whenyour ears work together. When a noiseoccurs on your right, for example, thesound wave comes to both ears, but itreaches your right ear a fraction of a secondbefore it reaches the left. It is also slightlylouder in the right ear. These differences tellyou from which direction it is coming.

The Pathway of SoundThe ear is designed to capture soundwaves (see Figure 8.10). The outer earreceives sound waves, and the earflapdirects the sounds down a short tubecalled the auditory canal. The vibration ofair (the sound wave) causes air in the audi-tory canal to vibrate, which in turn causesthe eardrum to vibrate.

The middle ear is an air-filled cavity.Its main structures are three tiny bones—the hammer, anvil, and stirrup. Thesebones are linked to the eardrum at oneend and to the cochlea at the other end.When sound waves cause the eardrum tovibrate, these bones vibrate and pushagainst the cochlea.

The cochlea makes up the inner ear.The cochlea is a bony tube that containsfluids and neurons. The pressure againstthe cochlea makes the liquid inside thecochlea move. Tiny hairs inside the cochlea pick up the motion. Thesehairs are attached to sensory cells. The sensory cells turn the sound vibra-tions into neuronal impulses. The auditory nerve carries these impulses tothe brain. This neuronal input goes to the hearing areas of the cerebralcortex of the brain.

DeafnessThere are two types of deafness. Conduction deafness occurs when any-

thing hinders physical motion through the outer or middle ear or when thebones of the middle ear become rigid and cannot carry sounds inward.People with conduction deafness can usually be helped with a convention-al hearing aid. A hearing aid picks up sound waves, changes them into mag-nified vibrations, and sends them to the inner ear. Sensorineural deafnessoccurs from damage to the cochlea, the hair cells, or the auditory neurons.People with complete sensorineural deafness cannot be helped with a con-ventional hearing aid, but may be helped with a special hearing aid called acochlear implant. A cochlear implant is a miniature electronic device that issurgically implanted into the cochlea. The device changes sound waves intoelectrical signals. These signals are fed into the auditory nerve, which carriesthem to the brain. The brain then processes the sensory input.

Decibel Levels

The loudness of a sound (its amplitude) is measuredin decibels. Each increase of 10 decibels makes asound 10 times louder. A normal conversation at 3 feet measures about 60 decibels, which is 10,000times louder than a whisper of 20 decibels. Soundbecomes painful at 130 decibels. What is the mea-surement in decibels of a subway train?

Figure 8.9

Rock band at 15 feet

Jet takeoff at 200 feet

Riveting machine

Subway train at 15 feet

Water at foot of Niagara Falls

Automobile interior at 55 mph

Freeway traffic at 50 feet

Normal conversation at 3 feet

Quiet restaurant

Quiet office

Library

Whisper at 3 feet

Normal breathing

Threshold of severe pain 140

120

100

80

60

40

20

0

PsychologicalResponse

DecibelScale Example

Painfully loud

Prolonged exposureproduces damage

to hearing

Very loud

Quiet

Very quiet

Just audible

Threshold of hearing

Reading CheckHow does sound occur?


BALANCEThe body’s sense of balance is regulated by the vestibular system inside

the inner ear. Its prominent feature is the three semicircular canals. Hair cellsproject into the fluid within each of the canals. When you turn your head,these canals also move. Inertia causes the fluid in the canals to resist changesin motion, which bends receptor hair cells projecting into the fluid.

The stimuli for vestibular responses include movements such as spin-ning, falling, and tilting the body or head. Overstimulation of the vestibu-lar sense by such movements can result in dizziness and motion sickness,as you probably have experienced by going on amusement-park rides.Although you are seldom directly aware of your sense of balance, in its ab-sence you would be unable to stand or walk without stumbling or falling.

SMELL AND TASTESmell and taste are known as the chemical senses because their recep-

tors are sensitive to chemical molecules rather than to light energy or soundwaves. For you to smell something, the appropriate gaseous moleculesmust come into contact with the smell receptors in your nose. These mol-ecules enter your nose in vapors that reach a special membrane in theupper part of the nasal passages on which the smell receptors are locat-ed. These receptors send messages about smells through the olfactorynerve to the brain. For you to taste something, appropriate liquid chem-icals must stimulate receptors in the taste buds on your tongue. Taste

information is relayedto the brain alongwith data about thetexture and tempera-ture of the substancein your mouth (seeFigure 8.11).

Studies show thatfour primary sensoryexperiences—sour,salty, bitter, andsweet—make up taste(Beebe-Center, 1949).The combining oftaste, smell, and tac-tile sensations isknown as flavor.Research suggeststhat a person candetect flavors any-where on the tongue,using the taste budspictured in Figure8.11. There are people

The Human EarFigure 8.10

Ear flap

Hammer

Anvil Nerves

Auditorynerve

Ear canal

Eardrum

Soundwaves

Stirrup

Cochlea

The earflap funnels sound waves down the ear canal to theeardrum. The bones of the middle ear pick up the vibrationsand transmit them to the inner ear. What is the function of thecochlea?

olfactory nerve: the nervethat carries smell impulses fromthe nose to the brain

vestibular system: threesemicircular canals that providethe sense of balance, located inthe inner ear and connected tothe brain by a nerve


who have greater taste sensitivities than others. So-called supertastershave two or more times the taste buds than nontasters, resulting inincreased sensitivity to sweet, bitter, sour, and salty.

Much of what is referred to as taste is actually produced by the senseof smell. As people age, their sense of taste does not seem to decline.When older people complain that food does not taste as good as it oncedid, the reason usually is a loss of smell rather than a failing sense of taste(Bartoshuk, 1989). You have undoubtedly noticed that when your nose isblocked by a cold, foods usually taste bland.

Sensations of warmth, cold, and pressure also affect taste. Try to imag-ine eating cold chicken soup or drinking a hot soda. Now imagine the tex-tural differences between a spoonful of pudding and a crunchy chocolate bar,and you will see how the texture and temperature of food influence taste.

The chemical senses seem to play a relatively unimportant role inhuman life when compared to their functions in lower animals. Insects,for example, often depend on smell to communicate with one another,especially in mating. In humans, smell and taste have become more amatter of pleasure rather than of survival.

THE SKIN SENSESReceptors in the skin are responsible for providing the brain with at

least four kinds of information about the environment: pressure, warmth,cold, and pain. Sensitivity to pressure varies from place to place in the skin.Some spots, such as your fingertips, are densely populated with receptorsand are, therefore, highly sensitive. Other spots, such as the middle of yourback or the back of your calf, contain relatively few receptors. Pressuresensations can serve as protection. For example, feeling the light pressureof an insect landing on your arm warns you of the danger of being stung.

Some skin receptors are particularly sensitive to hot or cold stimuli. Tocreate a hot or cold sensation, a stimulus must have a temperature greateror less than the temperature of the skin in the sensing area. If you plungeyour arm into a sink of warm water on a hot day, you will experience lit-tle or no sensation of its heat. If you put your arm in the same water on acold day, however, the water will feel quite warm.

Many kinds of stimuli—scratches, punctures, pressure, heat, andcold—can produce pain. What they have in common is real or potentialinjury to bodily tissues. Pain makes it possible for you to prevent damageto your body; it is an emergency system that through your prior experi-ence with pain demands immediate action.

Perceptions of PainWhereas other senses rely primarily on a single stimulus, pain results

from many different stimuli. For example, pain can be caused by intensepressure, bright lights, loud noises, intense heat, and so on. There are twotypes of pain sensations—the sharp, localized pain you may feel immedi-ately after an injury and the dull, generalized pain you may feel later.

Have you ever stubbed your toe and then rubbed it to reduce the pain?According to the gate control theory of pain, we can lessen some pains byshifting our attention away from the pain impulses or by sending other

Surface oftongue

Trench containsburied taste buds

Taste bud

When you chew,chemicals of the foodmix with saliva and rundown into trenches inyour tongue. Oncethere, taste buds reactto chemicals dissolvedin saliva. How do thesenses of taste andsmell work together?

The HumanTongueFigure 8.11


signals to compete with the pain sig-nals. This creates a sort of competitionbetween nonpain and pain impulses.This bottleneck, or gate, limits thenumber of impulses that can be trans-mitted. Thus, by increasing nonpainfulimpulses (rubbing your toe), youdecrease the pain impulses, and thesensation of pain is dulled.

The gate control theory of paincould explain how athletes are able tocomplete a game even though theyhave injured themselves. Although asoccer player may know that she hasbruised her side, she may not feel thepain fully until the game is over and shehas calmed down.

THE BODY SENSESThe sense of movement and body position is kinesthesis. It cooper-

ates with the vestibular and visual senses to maintain posture and balance.The sensation of kinesthesis comes from receptors in and near the mus-cles, tendons, and joints. When any movement occurs, these receptorsimmediately send messages to the brain. Without kinesthetic sensations,your movements would be jerky and uncoordinated. You could not walkwithout looking at your feet and complex physical activities, such as con-ducting surgery, piano playing, and acrobatics, would be impossible.

Orange Juiceand Toothpaste

Have you ever brushed your teeth and then had orangejuice for breakfast? How did the orange juice taste? Usuallythe orange juice will taste bitter. Our taste buds have mem-branes that contain fatlike phospholipids, while toothpastescontain a detergent that breaks down fat and grease. So thetoothpaste first assaults the membranes with its detergents,leaving them raw. Then chemicals in the toothpaste, suchas formaldehyde, chalk, and saccharin, cause a sour tastewhen they mix with the citric and ascorbic acids of orangejuice. Try eating artichokes, then drinking water. Whatdoes the water taste like?

1. Review the Vocabulary What are thefive basic senses? Describe two addi-tional senses that humans have.

2. Visualize the Main Idea Use a flow-chart similar to the one below todescribe the pathway of sound.

3. Recall Information What is the elec-tromagnetic spectrum and why do wesee only a portion of it?

4. Think Critically Why can we seesteadily and read street signs eventhough we may be walking or running?

The Pathway of Sound

Assessment

5. Application Activity Take a friend to a brightly lit area and then to a dimly lit area andnotice how the size of his or her pupils change.How can you explain this?

kinesthesis: the sense ofmovement and body position


The purpose of the excerpt above is to demonstrate to you how useful your powers of perception are. Perception goes beyondreflexive behavior and allows us to confront changes in our

environment. Perceptual thinking is essential for us to adapt to change.People do not usually experience a mass of colors, noises, tempera-

tures, and pressures. Rather, we see cars and buildings, hear voices andmusic, and feel pencils, desks, and physical contact. We do not merelyhave sensory experiences; we perceive objects. The brain receives infor-mation from the senses and organizes and interprets it into meaningfulexperiences—unconsciously. This process is called perception.

Perception

■ Main IdeaThe way we interpret sensations andorganize them into meaningful experi-ences is called perception.

■ Vocabulary• Gestalt• subliminal messages• motion parallax• constancy• illusions• extrasensory perception (ESP)

■ Objectives• Outline the principles involved in

perception.• Describe how we learn to perceive

and what illusions are.

Reader’s GuideExploring Psychology

Trying to Catch a FlyThe frog’s bug detector shows the

rigidity of reflexive behavior. If you severthe frog’s optic nerve, it will grow backtogether, and the bug detector will stillwork fine. If you sever the optic nerve andthen rotate the frog’s eye 180 degrees, thenerve will still heal and reestablish all theold connections; however, this time theresults will not be so good. The bugdetector does not know that everythinghas been rotated, so it miscomputes abug’s location. If the bug is high, the frogshoots its tongue low. If the bug is to theright, the tongue goes to the left. The frog never learns to compensate for thechanged situation.

—from A Second Way of Knowing: The Riddle of Human Perception by Edmund Blair Bolles,1991


PRINCIPLES OF PERCEPTUAL ORGANIZATIONThrough the process of perception, the brain is always trying to

comprehend the confusion of stimuli that bombard the senses. The brainmakes sense of the world by creating whole structures out of bits andpieces of information in the environment. Each whole that is organizedby the brain is called a Gestalt. Here, the whole is more than the sum ofthe parts. (Gestalt is a German word meaning “pattern” or “configuration.”)

Gestalt psychologists have tried to identify the principles the brain usesin constructing perceptions (Koffka, 1963). Some of the principles theyhave discovered are demonstrated in Figure 8.12. For example, people tend to see dots in patterns and groups. Principles that people usein organizing such patterns are proximity, continuity, similarity, simplicity, andclosure. If the elements of the pattern are close to one another or are simi-lar in appearance, they tend to be perceived as belonging to one another.

The Gestalt principles of organization help explain how we group oursensations and fill in gaps to make sense of our world. In music, forinstance, you tend to group notes on the basis of their closeness, or prox-imity, to one another in time—you hear melodies, not single notes.Similarity and continuity are also important. They allow you to follow the

Gestalt Principles

Humans see patterns and groupings in their environment ratherthan disorganized arrays of bits and pieces. Why do we use theprinciples of organization illustrated here?

Figure 8.12

SimilarityWhen similar and dissimilarobjects are mingled, we seethe similar objects as groups.

SimplicityWe see the simplestshapes possible.

ClosureWhen we see a familiarpattern or shape withsome missing parts, wefill in the gaps.

We see a star instead of five Vs.

ProximityWhen we see a number of similarobjects, we tend to perceive themas groups or sets of those thatare close to each other.

[ab cd ef gh ij kl mn]

ContinuityWe tend to see continuouspatterns, not disrupted ones.

Two curves or two pointed shapes?

A

B

D

C

Gestalt: the experience thatcomes from organizing bits andpieces of information intomeaningful wholes


sound of a particular voice or instrument evenwhen many other sounds are occurring. Closureaids us in perceiving an object even though theremay be gaps in what our senses pick up. The rule ofsimplicity states that we tend to perceive complexfigures as divided into several simpler figures.

FIGURE-GROUND PERCEPTIONOne form of perceptual organization is the divi-

sion of experience into figure and ground (see Figure8.13). Figure-ground perception is the ability to dis-criminate properly between a figure and its back-ground. When you look at a three-dimensionalobject against the sky, you have no trouble distin-guishing between the object and its background.Objects become the figure and stand out from thebackground. It is when something is two-dimensional,as in Figure 8.13, that you may have trouble telling the figure from theground. Nevertheless, such figure-ground perceptions give clues as to thenature of perception. That we can perceive a single pattern in more thanone way demonstrates that we are not passive receivers of stimuli.

Figure and ground are important in hearing as well as in vision. Whenyou follow one person’s voice at a noisy meeting, that voice is a figure andall other sounds become ground. Similarly, when you listen to a piece ofmusic, a familiar theme may leap out at you: the melody becomes thefigure, and the rest of the music merely background.

PERCEPTUAL INFERENCEOften we have perceptions that are not

based entirely on current sensory informa-tion. When you hear barking as youapproach your house, you assume it is yourdog—not a cat or a rhinoceros or evenanother dog. When you take a seat in a darktheater, you assume it is solid and will holdyour weight even though you cannot seewhat supports the seat. When you are driv-ing in a car and see in the distance that theroad climbs up a steep hill then disappearsover the top, you assume the road will con-tinue over and down the hill, not come to anabrupt end just out of sight.

This phenomenon of filling in the gaps inwhat our senses tell us is known as perceptualinference (Gregory, 1970). Perceptual inference

?Did You Know?Did You Know?

Hearing Colors All of her life, a woman,finally diagnosed with synesthesia, hadseen colors when she heard words or let-ters. She always saw yellow with hints ofgreen when she heard the word king, forinstance. Synesthesia is the mingling orswapping of sensory information in whichstimulating one sense triggers consciousexperience in another sense. People withsynesthesia may experience spoken wordsas tastes or shapes, colors as smells, andtouches as sounds. The condition occursin about 1 in 25,000 people and may resultfrom “crossed wires” in the sensory areasof the brain.

What Is It?

What did you see thefirst time you looked atthis illustration—a vaseor two profiles? Peopleinvariably organize theirexperience into figureand ground. Whatdoes the fact that weperceive a single pat-tern more than oneway demonstrate?

Figure 8.13


subliminal messages: briefauditory or visual messagesthat are presented below theabsolute threshold

is largely automatic and unconscious. We need only a few cues to inform usthat a noise is our dog barking or that a seat is solid. Why? Because we haveencountered these stimuli and objects in the past and know what to expect

from them in the present. Perceptual inference, thus, often dependson experience. On the other hand, we are probably born with

some of our ability to make perceptual inferences.

LEARNING TO PERCEIVEIn large part, perceiving is something that people learn to do. For

example, infants under one month will smile at a nodding object the sizeof a human face, whether or not it has eyes, a nose, or other human fea-tures. At about 20 weeks, however, a blank oval will not make mostinfants smile, but a drawing of a face or a mask will. The infant haslearned to distinguish something that looks like a person from otherobjects. Infants 28 weeks and older are more likely to smile at a femalethan a male face. By 30 weeks, most infants smile more readily when theysee a familiar face than when they see someone they do not know. Ittakes, however, 7 or 8 months for infants to learn to recognize differentpeople (Ahrens, 1954).

Experiments with human beings have also shown that active involve-ment in one’s environment is important for accurate perception. Peoplewho have been blind from birth and who have had their sight restored byan operation have visual sensations, but initially they cannot tell the dif-ference between a square and a circle or see that a red cube is like a bluecube (Valvo, 1971). In fact, some had difficulty making such simple dis-tinctions six months after their vision was restored.

Learning to perceive is influenced by our needs, beliefs, and expecta-tions. When we want something, we are more likely to see it.Psychologists (Wispe & Drambarean, 1953) have demonstrated that hun-gry people are faster at identifying food-related words when words are flashed quickly in front of them. What we identify as truth may betwisted and reconstructed to fit our own belief systems. Previous experi-ences influence what we see (Lachman, 1996). For example, if you havealways perceived all elderly women as honest, you might not even ques-tion the elderly woman at the next table when your wallet disappears ina restaurant, even if she is the most obvious suspect. This is called a per-ceptual set. This set prepares you to see what you want to see.

Subliminal PerceptionIn his 1957 book The Hidden Persuaders, Vance Packard divulged that

advertisers were using a revolutionary breakthrough in marketing tech-niques—subliminal advertising. The word subliminal comes from theLatin: sub (“below”) and limen (threshold). This concept used subliminal messages, which are brief auditory or visual messages presented below the absolute threshold so that there is less than a 50 per-cent chance that they will be perceived.

This drawing seems todefy basic geometriclaws.

Pop-OutFeatures

Reality is a jumble ofsensations and details.The letter P probablypops out to you. TheQs may also pop out,but not as much as theP. You may not havenoticed the O, though.Why does the P popout?

Figure 8.14

QQQQQQQQQQQQQPQQQQOQQQQQQQQQQQQQ


One advertiser, James Vicary, falsely claimed that the words “EatPopcorn” and “Drink Coke” had been flashed on a movie screen in a New Jersey theater on alternate nights for six weeks. Although the flashes were so brief (1/3000 of a second, once every five seconds) thatnone of the moviegoers even seemed to notice them, Vicary claimed that the sales of popcorn had risen 58 percent and Coke sales had risen18 percent.

The public response to this announcement was long, loud, and hys-terical. Congressional representatives called for FCC regulations, whileseveral state legislatures passed laws banning subliminal ads. Eventually,Vicary admitted that the data from the movie theater experiment werefalse, and many people believe the experiment never really took place.However, public furor over the potential for abuse of subliminal advertis-ing remained. in 1974, the FCC condemned subliminal advertising,regardless of its efficacy.

The idea for subliminal ads was a natural outgrowth of a long seriesof controversial studies on subliminal perception—the ability to noticestimuli that affect only the unconscious mind. Most of these earlier stud-ies involved presenting verbal or visual material at intensities that wereconsidered too low for people to perceive. A more critical look at thestudies, however, revealed several flaws in the way they were designedand carried out. For example, no attempt was made to assess or controlfactors other than the subliminal message that might have influenced thepurchase of Coke or popcorn. The temperature in the theater or thelength of the movie might have contributed to the increase in sales.Unfortunately, the study was not presented in enough detail to be evalu-ated by scientists.

Even if it is possible for people to perceive information at very lowlevels of intensity, there is no clear evidence that these weak, often lim-ited messages would be more powerful in influencing people than wouldconscious messages. Nevertheless, many people believe that subliminaladvertising is powerful.

DEPTH PERCEPTIONDepth perception—the ability to recognize distances and three-

dimensionality—develops in infancy. Psychologists have placed infantson large tables and found that they most likely will not crawl over the edge. From this observation, it is possible to infer that infants dohave depth perception.

Monocular Depth CuesPeople use many monocular depth cues to perceive distance and

depth. Monocular depth cues are cues that can be used with a single eye.There are at least a half-dozen monocular cues external to us that we use.In the absence of any other cues, the size of an object—bigger is nearer—will be used. We use relative height—objects that appear farther away

The NeckerCube

The Necker cube is anambiguous figure. Youcan will yourself to seeit as if you were lookingdown on it, with cornerX closest to you, or asif you were looking upat it, with corner Yclosest to you. Howmight you make thiscube less ambiguousby adding details?

Figure 8.15

X

Y


Reading CheckHow do monocular and

binocular depth cues differ?

from another object arehigher on your plane ofview. Interposition, or theoverlapping of images,causes us to view the objectwe can see in its entirety tobe closer than one whoseoutline is interrupted byanother object. Light andshadows yield informationabout an object’s shape andsize. Brightly lit objectsappear closer, while objectsin shadows appear fartheraway. Texture-density gradi-ent means that the fartherremoved an object is, theless detail we can identify(see Figure 8.16).

Another cue is motionparallax—the apparent

movement of objects that occurs when you move your head from sideto side or when you walk around. You can demonstrate motion paral-lax by looking toward two objects in the same line of vision, one nearyou and the other some distance away. If you move your head back andforth, the near object will seem to move more than the far object. In this way, motion parallax gives you clues as to which objects are closerthan others.

Another distance cue, linear perspective, is based on the fact that par-allel lines converge when stretched into the distance (see Figure 8.16).For instance, when you look at a long stretch of road or railroad tracks,it appears that the sides of the road or the tracks converge on the hori-zon. A final related cue is relative motion. When you are riding in a car, for example, and look at distant mountains, the objects in a nearbyfield seem to be moving in the opposite direction to your move-ment. Yet, when you look at an animal in a nearby field, the mountainsor land beyond the animal seem to be moving in the same direction you are.

Binocular Depth CuesBinocular depth cues depend upon the movement of both eyes. For

example, convergence is the process by which your eyes turn inward tolook at nearby objects. Another cue is the information provided by reti-nal disparity, as discussed earlier in the chapter. Because each of your eyesoccupies a different position, each eye receives a slightly different image.That difference is retinal disparity. The brain interprets a large retinal dis-parity to mean a close object and a small retinal disparity to mean a distant object.

Monocular Cues

As you look down a long stretch of highway, the parallel sides of theroad seem to converge. The expansive desert appears furrowed upclose and smooth in the distance. How would you use the relativeheight cue to perceive the distance of the desert?

Figure 8.16

motion parallax: the appar-ent movement of stationaryobjects relative to one anotherthat occurs when the observerchanges position


CONSTANCYWhen we have learned to perceive certain objects in our environ-

ment, we tend to see them in the same way, regardless of changing con-ditions. You probably judge the whiteness of the various portions of thesepages to be fairly constant, even though you may have read the bookunder a wide range of lighting conditions. The light, angle of vision, distance, and, therefore, the image on the retina all change, but your per-ception of the object does not. Thus, despite changing physical condi-tions, people are able to perceive objects as the same by the processes ofsize, shape, brightness and color constancy (see Figure 8.17).

An example of size constancy will illustrate how we have an auto-matic system for perceiving an object as being the same size whether it isfar or near. A friend walking toward you does not seem to change into agiant even though the images inside your eyes become larger and largeras she approaches. To you, her appearance stays the same size becauseeven though the size of your visual image is increasing, you are per-ceiving an additional piece of information: distance is decreasing. Theenlarging eye image and the distance information combine to produce aperception of an approaching object that stays the same size.

Distance information compensates for the enlarging eye image toproduce size constancy. If information about distance is eliminated, yourperception of the size of the object begins to correspond to the actual sizeof the eye image. For example, it is difficult for most people to estimatethe size of an airplane in the sky because they have little experience judg-ing such huge sizes and distances. Pilots, however, can determinewhether a flying plane is large and far away or small and close becausethey are experienced in estimating the sizes and distances of planes.

An example of brightness and color constancy is when you look outa window at night and see that the trees, grass, or parked cars do notappear to be the same color or brightness as they are during the daytime.You already know, though, that thegrass is green, for instance. You per-ceive this quality (of color andbrightness) even under conditionsof different illumination.

ILLUSIONSIllusions are incorrect percep-

tions. Illusions can be useful in teach-ing us about how our sensation andperceptual systems work. Illusionsare created when perceptual cues aredistorted so that our brains cannotcorrectly interpret space, size, anddepth cues. For example, look at thelines in Figure 8.18. Which lines in

Shape Constancy

We perceive the opening door as being rectangular in shape,although our view of the shape of it changes as it opens.Why are perceptual constancies important to our under-standing of the world?

Figure 8.17

constancy: the tendency toperceive certain objects in thesame way regardless of chang-ing angle, distance, or lighting

illusions: perceptions thatmisrepresent physical stimuli


extrasensory perception(ESP): an ability to gain infor-mation by some means otherthan the ordinary senses

each set are longer?Measure the lengths of thepairs of lines with a ruler,then look again. Do thelines look as long now thatyou know they are thesame? For most people, theanswer is no.

A possible explanationof this type of illusion is thateven though the patternsare two-dimensional, yourbrain treats them as three-dimensional. These illusionshave features that usuallyindicate distance in three-dimensional space. The topline in Figure 8.18a, forexample, can be thought ofas the far corner of a room;the bottom line is like thenear corner of the building.In Figure 8.18b and Figure8.18c, the converging linescreate the illusion of dis-tance so that the lower barlooks nearer and shorter

than the upper bar. This perceptual compensation seems to be uncon-scious and automatic.

Figure 8.19 shows two individuals in a room. Their sizes look dra-matically different because you perceive the room as rectangular. In fact,the ceiling and walls are slanted so that the back wall is both shorter andcloser on the right than on the left. Yet even when you know how thisillusion was achieved, you still accept the peculiar difference in the sizesof the two people because the windows, walls, and ceiling appear rectan-gular. Your experience with rectangular rooms overrides your knowledgeof how this trick is done.

EXTRASENSORY PERCEPTIONWe are fascinated by things that cannot be seen, easily explained, or

sometimes even verified, such as flying saucers, atoms, genes, andextrasensory perception. Extrasensory perception (ESP)—receivinginformation about the world through channels other than the normalsenses—is a hotly debated topic. There are four types of ESP: (1) clairvoy-ance is perceiving objects or information without sensory input; (2) telepa-thy involves reading someone else’s mind or transferring one’s thoughts; 3) psychokinesis involves moving objects through purely mental effort;

Lines of Different Lengths?

The Müller-Lyer illusion (a) and the Ponzo illusion (b) are depicted here.The lines between the arrowheads in (a) are exactly the same length,as are the heavy black lines in (b). Some psychologists believe thatthe reason the lines in (a) seem of different lengths is because theyare interpreted as offering different cues to their distance from theviewer. The lines in (b) may appear to be different in length becausethe brain interprets this diagram as though it is from a scene such asthat in (c). Why does the brain interpret scenes in certain ways?

Figure 8.18

a cb


and (4) precognition is the ability to foretell events. Since the1960s, James Randi, known as “The Amazing Randi,” hascampaigned against people who claim they have ESP. Hehas exposed many of these people as frauds.

Many people are convinced that ESP exists because of anintense personal experience that can never be scientifically val-idated. For instance, we all have some fears before traveling, andwe imagine the worst: our plane will crash, our train will bederailed, or we will have an automobile accident. These eventsalmost never happen, and we easily forget about our frighten-ing premonitions. If the improbable should actually take place,however, our premonitions turn into compelling evidence forthe existence of precognition. Such coincidences sometimesbecome widely publicized evidence supporting paranormalphenomena, and we may quickly forget all the occasions whenour premonitions were completely wrong. If we are truly inter-ested in validating the existence of ESP, though, we must keeptrack of the frequency of its failures as well as its successes.

Scientists have been investigating ESP since the 1900s. (Probably themost famous parapsychologist is J.B. Rhine.) Many scientists do not acceptthe results of experiments supporting ESP because the findings are highlyunstable. One of the basic principles of scientific research is that one sci-entist should be able to replicate another scientist’s results. Not only dodifferent ESP experiments yield contradictory findings, but the sameindividual seems to show ESP on one day but not on the next. Perhapsthe most telling argument against ESP is that when strict controls are usedin an ESP experiment, there is little likelihood of demonstrating ESP. Thisis contrary to what one normally expects when trying to demonstrate aphenomenon using the scientific methods described in Chapter 2.

A Strange Room

The room, constructed by AdelbertAmes, changes depth cues to dis-tort our perception. What makesthis illusion work?

Figure 8.19

1. Review the Vocabulary Describe theGestalt principles of organization. Howdo these principles help us organizereality?

2. Visualize the Main Idea Use a graphicorganizer similar to the one below tolist and briefly describe monoculardepth cues.

3. Recall Information What are thebinocular depth cues? How do theyhelp us judge reality?

4. Think Critically How do illusionsdemonstrate the difference betweensensations and perceptions?

Assessment

5. Application Activity Consider the followingquestion: You have suddenly lost all perceptualconstancies—what specific problems do youencounter? Create a scene—in the form of a play,narrative, or newspaper article—that illustrates aproblem you might encounter.

Monocular Depth Cues


the mountains were hills or clouds. Turnbullexplained that they were hills but much largerthan any Kenge had seen before. Kenge agreedto ride with Turnbull to the mountains for furtherinspection. A passing thunderstorm obstructedthe travelers’ view and did not clear until the tworeached the mountains. When Kenge peered up atthe enormous mountain range, he was amazed.

As Turnbull and Kenge turned to leave,Kenge noticed a wide open plain on whichstood a herd of buffalo. Kenge wanted to knowwhat type of “insects” the buffalo were. Turnbull

explained that the animalswere not insects but buf-falo. When the two arrivedclose to the herd of buffalo,Kenge now knew that the“insects” were buffalo allalong, but he still could notunderstand why they hadappeared to be so small.

Resu l ts : Turnbul l ’saccounts support the ideathat human perception de-velops (at least in the case

of size constancy) as we use the environmentaround us, or by nurture. However, someresearch with infants supports the nature side ofperception. For example, individuals who wereblind at birth and later gained their sight wereable to perceive figure-ground relationships. Arewe born with certain perceptual abilities and notothers, or is perception something we learn? InKenge’s case, perceptual ability was a learnedphenomenon. Certain perceptual skills may benecessary for our survival. In Kenge’s case, hedid not need a wide range of size constancy tosurvive in the dense jungle.

Seeing Is BelievingPeriod of Study: Late 1950s and early 1960s

Introduction: In the late 1950s and early1960s, an anthropologist, C.M. Turnbull, trav-eled to the Ituri Forest in the present-day coun-try of the Democratic Republic of the Congo(formerly Zaire) to study the life and culture ofthe BaMbuti Pygmies. Turnbull traveled fromone village to another. A 22-year-old mannamed Kenge from a localPygmy village accompa-nied and assisted him.Kenge had spent all of hislife living in the denseforests surrounding his vil-lage. He was accustomedto seeing only the imagescontained within the forests.

Hypothesis: BecauseKenge had been isolated bythe forests all of his life, thesight of new images wouldappear complex and confusing. The thick forestsblocked the local villagers’ view of distant objectssuch as animals, mountains, and the sun andmoon on the horizon. Because Kenge had neverseen these things, his perceptual development (inthis case, size constancy, or the ability to per-ceive a familiar object as being the same sizeregardless of its distance from you) was limited.He understood only what he could see directly infront of him. The information collected fromTurnbull’s experience with Kenge raised thequestion of whether perceptual understanding isa learned ability or biological mechanism: a ques-tion of nature versus nurture.

Method: The discovery of Kenge’sperceptual limitations took place whenTurnbull and Kenge came to a clearing on the eastern edge of the Ituri Forest. At that point Kenge and Turnbull en-joyed a clear view of the RuwenzoriMountains. Confused by the sight of themountains, Kenge asked Turnbull if

Analyzing the Case Study 1. Why did some images seem confusing to Kenge?

2. According to Turnbull, how do we learn size constancy?

3. Critical Thinking Do you think that Kenge couldadjust to life in your city or town? Explain the difficultieshe might encounter.

Summary and Vocabulary

Chapter Vocabularysensation (p. 208)perception (p. 208)psychophysics (p. 208)absolute threshold (p. 209)difference threshold (p. 210)Weber’s law (p. 211)signal-detection theory (p. 212)pupil (p. 215)lens (p. 215)retina (p. 215)optic nerve (p. 215)binocular fusion (p. 216)retinal disparity (p. 216)auditory nerve (p. 218)vestibular system (p. 220)olfactory nerve (p. 220)kinesthesis (p. 222)Gestalt (p. 224)subliminal messages (p. 226)motion parallax (p. 228)constancy (p. 229)illusions (p. 229)extrasensory perception (ESP)(p. 230)

Sensation■ The absolute threshold is the weakest amount of

a stimulus required to produce a sensation; the dif-ference threshold is the minimum amount of dis-tinction a person can detect between two stimuli.

■ Senses are most responsive to increases anddecreases, rather than unchanging stimulation.

■ Sensory adaptation allows people to notice differ-ences in sensations and react to the challenges ofdifferent or changing stimuli.

People take in information through their senses. Through theirsenses they perceive the world around them.

Main Idea: Sensationsallow humans to under-stand reality. Sensationsoccur anytime a stimu-lus activates a receptor.

The Senses■ Vision provides people with a great deal of

information about the environment and theobjects in it.

■ Hearing depends on vibrations of the air, calledsound waves.

■ The body’s sense of balance is regulated by thevestibular system inside the inner ear.

■ Smell and taste are known as the chemical senses because their receptors are sensitive tochemical molecules.

■ Receptors in the skin are responsible for providingthe brain with information about pressure,warmth, cold, and pain.

■ Kinesthesis cooperates with the vestibular andvisual senses to maintain our sense of movementand body position.

Main Idea: The senseorgans—the eyes, ears,tongue, nose, skin, andothers—are the recep-tors of sensations.

Perception■ The Gestalt principles of organization help

explain how we group our sensations and fill in gaps to make sense of our world.

■ Figure-ground perception is the ability to discrimi-nate properly between figure and ground.

■ Perceptual inference is the phenomenon of fillingin the gaps in what our senses tell us.

■ Learning to perceive is influenced by our needs,beliefs, and expectations.

■ People use monocular depth cues and binoculardepth cues to perceive distance and depth.

■ Incorrect perceptions, created when perceptualcues are distorted, are called illusions.

Main Idea: The waywe interpret sensationsand organize them intomeaningful experiencesis called perception.


Assessment

Reviewing VocabularyChoose the letter of the correct term or conceptbelow to complete the sentence.

1. The organization of sensory information intomeaningful experiences is __________ .

2. The psychological study of questions such asthe relationship between color and wavelengthis called __________.

3. The minimum amount of difference a personcan detect between two stimuli is the__________.

4. __________ are incorrect perceptions.5. __________ is the sense of movement and body

position.6. The combination of two images into one

is __________ .7. A(n)__________ is created when a stimulus

activates a receptor.8. Advertisers sometimes use __________, which

are brief auditory or visual stimuli presentedbelow the absolute threshold so that there is lessthan a 50 percent chance that they will be perceived.

9. The weakest amount of stimuli required to pro-duce a sensation is called the __________.

10. A body’s sense of balance is regulated by the__________ inside the inner ear.

Self-Check QuizVisit the Understanding Psychology Web site at psychology.glencoe.com and click on Chapter 8—Self-Check Quizzes to prepare for the Chapter Test.

a. sensationb. psychophysicsc. absolute thresholdd. difference thresholde. binocular fusion

f. perceptiong. vestibular systemh. kinesthesisi. subliminal messagesj. illusions

Recalling Facts1. Using a graphic organizer similar to the one

below, define stimulus and give at least fourexamples of stimuli.

2. What is the psychological principle thatexplains why you are more likely to notice whena single lightbulb burns out in a room with threelamps than when a single lightbulb burns out ina sports arena?

3. What is our vestibular system? Why do we need it?4. List four kinds of information we receive from

our skin.5. Define figure-ground perception and provide an

example of it.

Critical Thinking1. Synthesizing Information Which of the senses

do you consider most important to you? Whydo you think so?

2. Making Inferences How might being color-deficient affect your daily life? Your choice ofcareer?

3. Analyzing Concepts What is the differencebetween vestibular sense and kinesthesis? Inwhat situations are both senses needed?

4. Analyzing Concepts What sensation do youexperience when you close your eyes and gentlypress on one of your eyeballs at the outer edge?How can you explain the visual experience inthe absence of light rays?

5. Evaluating Information One of the objectionsto the use of subliminal advertising techniques isthat they could be used to manipulate or influ-ence large numbers of people without theirknowing it. Do you think this is an importantobjection? Why or why not?

PSYCHOLOGY


A stimulus is

http://www.glencoe.com/sec/socialstudies/psychology/psych2001/quiz.shtml?BOOK=006&CHAPTER=8&TITLE=Sensation+and+Perception

Psychology Projects1. The Senses Peel a fresh potato and an apple.

Have a friend close his eyes and smell a freshonion while he takes a bite of each one. Can hetell which food is which without his sense ofsmell? Try this experiment with various people,using different foods that have similar textures.Report your findings in a chart. Explain the rela-tionship between the senses of taste and smell.

2. Perception Watch a movie with your parentsor grandparents. As soon as the film is over, askeach person to write a paragraph describing thelast scene in the movie. Read each paragraphaloud. What was everyone’s perception of thescene? How could you explain any differences?

3. Sensation Sensory adaptation refers to theability of the senses to adjust themselves to aconstant level of stimulation. Create a simpleexperiment to test for sensory adaptation. Ask aclassmate to demonstrate the experiment.

Technology ActivitySearch the Internet for informationand examples of optical illusions.

Present examples to your class and explain how theillusions were created.

Psychology Journal1. Reread your journal entry about hear-

ing another conversation in a crowded setting. Is your explanation still valid? If neces-sary, how would you change it?

2. Think about what it would be like if our sensesdid not have the limits they do. What visualproblems might we have? What would we hearif our sense of hearing had a different range?Write answers to these questions in your journal.

Assessment


Practice and assess key socialstudies skills with GlencoeSkillbuilder Interactive WorkbookCD-ROM, Level 2.

Building SkillsInterpreting a Graph Review the graph, andthen answer the questions that follow.1. What three body parts are the least sensi-

tive to touch?2. Why are certain parts of the body more

sensitive to touch than others?3. How does the information in the graph

help explain why people reading in Brailleuse their fingertips?

See the Skills Handbook, page628, for an explanation of inter-

preting graphs.

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