HO_Sensation and Perception

8/17/2019 HO_Sensation and Perception

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S E N S A T I O N A N D P E R C E P T I O N

Introduction

Although the very private processes that connect us with the outside world

extends deep into the brain, this chapter will begin at the surface – at the senseorgans. This is the territory of sensory psychology . Sensation is simply de ned asthe process by which a stimulated receptor (such as the eyes or ears) creates apattern of neural messages that represent the stimulus in the brain, giving rise toour initial experience of the stimulus. An important idea to remember is thatsensation involves converting stimulation (such as a pinpric , a sound, or a !ash of light) into a form the brain can understand (neural signals) – much as a cell phoneconverts an electronic signal into sound waves you can hear.

"sychologists who study sensation do so primarily from a biologicalperspective and they have found out that all our sense organs are, in some verybasic ways, much ali e. All the sense organs transform physical stimulation (such aslight waves or sound waves) into the neural impulses that give us sensations (suchas the experience of light or sound).

#nder most conditions, human sensory experience is highly reliable. $o whenyou catch sight of a friend, the sensation usually registers clearly, immediately, andaccurately. %et, we humans do have our sensory limitations – &ust as other creaturesdo. 'n fact, we lac the accurate senses so remar able in many other species thevision of haw s, the hearing of bats, the sense of smell of rodents, or the sensitivityto magnetic elds found in migratory birds. $o, do we humans excel at anything

%es. The human species has evolved the sensory e*uipment that enables us toprocess a wider range and variety of sensory input that any other species.

+ut sensation is only half the story. +eyond mere sensation is the ama ingrealm of perception. Perceptual psychology will help us understand how weassemble a series of tones into a familiar melody or a collage of shapes andshadings into a familiar face. -e will de ne perception as a mental process thatelaborates and assigns meaning to the incoming sensory patterns. Thus, perceptioncreates an interpretation of sensation .

#ntil *uite recently, the study of perception was primarily the province of psychologists using the cognitive perspective. ow that brain scans have openednew /windows0 on perceptual processes in the brain, neuroscientists have &oinedthem in the *uest to nd biological explanations for perception. 'n the same way,the boundary of sensation blurs into that of perception. "erception is essentially aninterpretation and elaboration of sensation. $een in these terms, sensation refers

&ust to the initial steps in the processing of a stimulus.

I. HOW DOES STIMULATION BECOME SENSATION

The brain never receives stimulation directly from the outside world. 'tsexperience of a tomato is not the same as the tomato itself – although we usuallyassume that the two are identical. either can the brain receive light from a sunset,reach out and touch velvet, or inhale the fragrance of a rose. 't must always rely onsecondhand information from the sensory system which delivers only a codedneural message, out of which the brain must create its own experience.

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Transduction: Changing Stimulation to Sensation't may seem incredible that basic sensations are entirely creations of the

sense organs and brain. +ut remember that all sensory communication with thebrain !ows through neurons in the form of neural signals eurons cannot transmitlight or sound waves or any other external stimulus.

'n all the sense organs, it is the &ob of the sensory receptors, such as the eyesand ears, to convert incoming stimulus information into electrochemical signals 33neural activity – the only language the brain understands. $ensations occur onlywhen the neural signal reaches the cerebral cortex. The whole process seem soimmediate and direct that it fools us into assuming that the sensation of redness ischaracteristic of a tomato or the sensation of cold is a characteristic of ice cream.+ut they are not4

Transduction is the sensory process that converts the information carried bya physical stimulus, such as light or sound waves, into the form of neural messages.

Transduction begins when a sensory neuron detects a physical stimulus (such as thesound wave made by a vibrating guitar string). -hen the appropriate stimulusreaches a sense organ, it activates speciali ed neurons, called receptors , whichrespond by converting their excitation into a nerve signal.

eural impulses carry the codes of sensory events in a form that can befurther processed by the brain. To get to its destination, this information3carryingsignal travels from the receptor cells along a sensory pathway – usually by way of the thalamus and on to speciali ed sensory processing areas in the brain. 5rom thecoded neural impulses arriving from these pathways, the brain then extractsinformation about the basic *ualities of the stimulus, such as its intensity anddirection. 6eep in mind that the stimulus itself terminates in the receptor the onlything that !ows into the nervous system is information carried by the neuralimpulse.

Thresholds: The Boundaries of SensationA!so"ute t#res#o"d for di7erent types of stimulation is the minimum amount of physical energy needed to produce a sensory experience. This threshold will alsovary from one person to another. -hen you point out a faint star to a friend whosays he cannot see it, the star8s light is above your absolute threshold (you can seeit) but below that of your friend (who cannot see it).

A faint stimulus does not abruptly become detectable as its intensityincreases. +ecause of the fu y boundary between detection and non3detection, aperson8s absolute threshold is not absolute4 'n fact, it varies continually with ourmental alertness and physical condition.

Di$erence t#res#o"d (also called the just noticeable di erence or J ! ) is thesmallest amount by which a stimulus can be changed and the di7erence can bedetected half the time. We!er%s La& is the concept that the si e of a J ! isproportional to the intensity of the stimulus9 the J ! is large when the stimulusintensity is high and small when the stimulus intensity is low.

Signal !etection Theory :riginally developed for engineering electronic sensors, signal detection

theory uses the same concepts to explain both the electronic sensing of stimuli bydevices, such as television, and by the human senses, such as vision and hearing.According to si'na" detection t#eor( , sensation depends on the characteristics of the stimulus , the bac"ground stimulation , and the detector .

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$ignal detection theory also helps us understands why thresholds vary.$ensation is not a simple yes3or3no experience but a probability that the signal willbe detected and processed accurately. The theory recogni es that the observer,whose physical and mental status is always in !ux, must compare a sensoryexperience with ever3changing expectations and biological conditions.

Sensory #daptation The great *uantity of incoming sensation would *uic ly overwhelm us if not

for the ability of our sensory systems to adapt. Sensor( adaptation is thediminishing responsiveness of sensory systems to prolonged stimulation, as whenyou adapt to the feel of swimming in cool water. 'n fact, any unchanging stimulationusually shifts into the bac ground of our awareness unless it is *uite intense orpainful. :n the other hand, any change in stimulation will immediately draw yourattention.

II. HOW ARE THE SENSES ALI)E HOW ARE THE* DI++ERENT

<ision, hearing, smell, taste, touch, pain, body position in certain ways allthese senses are the same they transduce stimulus energy into neural impulses.

They are all more sensitive to change than to constant stimulation. And they allprovide us information about the world – information that has survival value. +uthow are they di7erent -ith the exception of pain, each sense taps a di7erent formof stimulus energy, and each sends the information it extracts to a di7erent part of the brain. As a result, di7erent sensations occur because di7erent areas of the brainbecome activated. -hether you hear a bell or see a bell depends ultimately onwhich part of the brain receives stimulation.

Vision: How the Nervous System Processes Light

Animals with good vision have an enormous biological advantage. =oodvision helps us detect desired targets, threats, and changes in our physicalenvironment and to adapt our behavior accordingly.

%ou might thin of the eye as a sort of /video camera0 that the brain uses toma e motion pictures of the world. >i e a camera, the eye gathers light through alens, focuses it, and forms an image in the retina at the bac of the eye. +ut while adigital camera simply forms an electronic image, the eye forms an image that gets

extensive further processing in the brain. The uni*ue characteristic of the eye lies inits ability to extract the information from light waves, which are simply a form of electromagnetic energy. The eye, then, transduces the characteristics of light intoneural signals that the brain can process. This transduction happens in the retina ,the light3sensitive layer of cells at the bac of the eye.

And, as with a camera, things can go wrong. The lenses of those who are/nearsighted0 focus images short of (in front of) the retina9 in those who are/farsighted0 the focal point extends behind the retina. 'n both cases, images are notsharp without corrective lenses.

P#otoreceptors are light3sensitive cells (neurons) in the retina that convert lightenergy to neural impulses. The photoreceptors are as far as light gets into the visualsystem. These photoreceptors consist of two di7erent types of speciali ed neurons –the rods and cones that absorb light energy and respond by creating neuralimpulses.

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+ecause we function sometimes in near dar ness and sometimes in brightlight, we have evolved two types of processors involving two distinct receptor celltypes named for their shapes. The 2;@ million tiny rods /see in the dar 0 – that is,they detect low intensities of light at night, though they cannot ma e the nedistinction that give rise to our sensations of color.

a ing the ne distinctions necessary for color vision is the &ob of the sevenmillion cones that come into play in brighter light. Bach cone is speciali ed todetect the light waves we sense as blue, red, or green. The cones concentrate in thevery center of the retina , in a small region called the ,o-ea , which gives us oursharpest vision. -ith movements of our eyeballs, we use the fovea to scanwhatever interests us visually.

There are other types of cells in the retina that do not respond directly tolight. The bipolar cells handle the &ob of collecting impulses from manyphotoreceptors (rods and cones) and shuttling them on to the ganglion cells. Theretina also contains receptor cells sensitive to edges and boundaries of ob&ects9other cells respond to light and shadow and motion.

+undled together, the axons of the ganglion cells ma e up the optic ner-e ,which transports visual information from the eye to the brain. Again, it is importantto understand that the optic nerve carries no light – only patterns of nerve impulsesconveying information derived from incoming light.

There is a small area of the retina in each eye where everyone is blind,because that part of the retina has no photoreceptors. This !"ind spot is located atthe point where the optic nerve exits each eye, and the result is a gap in the visual

eld. %ou do not experience blindness there because what one eye misses isregistered by the other eye, and the brain / lls in0 the spot with information thatmatches the bac ground.

The visual impairment called blindness can have many causes, which areusually unrelated to the blind spot. +lindness can result, for example, from damageto the retina, cataracts that ma e the lens opa*ue, damage to the optic nerve orfrom damage to the visual processing areas in the brain.

-e loo with our eyes but we see with our brain. A special brain area calledthe $isual corte% creates visual images from the information imported from the eyesthrough the optic nerve. There in the visual cortex, the brain begins wor ing itsmagic by transforming the incoming neural impulses into visual sensations of color,form, boundary, and movement. The visual cortex also manages to ta e the two3dimensional patterns from each eye and assemble them into our three3dimensionalworld of depth. -ith further processing, the cortex ultimately combines these visualsensations with memories, motives, emotions, and sensations of body position andtouch to create a representation of the visual world that ts our current concernsand interests.

Bri'#tness. $ensations of brightness come from the intensity or amplitude of light,determined by how much light reaches the retina. The brain senses brightness bythe volume of neural activity it receives from the eyes.

Creatin' Co"or. Cespite the way the world appears to us, color does not existoutside the brain because color is a sensation that the brain creates based on the

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wavelength of light stri ing our eyes. Eolor exists only in the mind of the viewer – apsychological property of our sensory experience.

The eyes detect the special form of energy that we call visible light. The lightwe see occupies a tiny segment near the middle of the vast e"ectro a'neticspectru which is the entire range of electromagnetic energy. Access to this

electromagnetic spectrum is through a small visual /window0 called the -isi!"espectru . -ithin this narrow visible spectrum, light waves of di7erent wavelengthsgive rise to our sensations of di7erent colors. The eye extracts information from thewavelength of light, and the brain uses that information to construct the sensationswe see as colors.

Sensin' Co"ors. The tric#ro atic t#eor( is the idea that colors are sensed bythree di7erent types of cones sensitive to light in red, blue, and green wavelengths.

The trichromatic theory explains the earliest stage of color sensation. Theopponent/process t#eor( is the idea that cells in the visual system process colorsin complementary pairs. The sensation of a certain color, such as red, inhibits, orinterferes with, the sensation of its compliment, green. This theory explains colorsensation from the bipolar cells onward in the visual system.

Ta en together, the two theories explain two di7erent aspects of color visioninvolving the retina and visual pathways. The trichromatic theory e%plains color

processing in the cones of the retina, while the opponent&process theory e%plainswhat happens in the bipolar cells and beyond'

Co"or B"indness. ot everyone sees colors in the same way, because some peopleare born with a de ciency in distinguishing colors. At the extreme, complete co"or!"indness is the total inability to distinguish colors. 't is typically a genetic disorderthat prevents an individual from discriminating certain colors. The most commonform is red3green color blindness.Hearing: If a Tree Falls in the Forest

'magine how your world would change if your ability to hear were suddenlydiminished.

Fearing provides the ability to locate ob&ects in space. Fearing may be evenmore important than vision in orienting us toward distant events. -e often hearthings before we see the source of the sound. Fearing may also tell of events thatwe cannot see.

T#e P#(sics o, Sound0 Ho& Sound Wa-es are Produced. :n earth, the energyof exploding ob&ects transfers to the surrounding medium – usually air – in the formof sound &a-es . Bssentially the same thing happens with rapidly vibrating ob&ectsas the vibrations push the molecules of air bac and forth. The resulting changes inpressure spread outward in the form of sound waves that can tra-e" 12133 ,eetper second .

The two physical properties of sound waves, fre*uency and amplitude,determine how the sound wave will be sensed by the brain. +re4uenc( refers tothe number of vibrations or cycles the sound wave completes in a given amount of time, which in turn determines the highness or lowness of a sound (the pitch ).5re*uency is usually expressed in cycles per second (cps) or hert* (+*)' A p"itudemeasures the physical strength of the sound wave9 it is de ned in units of soundpressure or energy.

Sensin' Sounds0 Ho& We Hear Sound Wa-es. The psychological sensation of sound re*uires that waves be transduced into neural impulses and sent to the brain.

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Four steps in auditory transduction! Air!orne sound &a-es are re"a(ed to t#e inner ear. <ibrating waves

of air enter the outer ear and move through the air canal to the eardrumor the t( panic e !rane . This tightly stretched sheet of tissue

transmits the vibrations to three tiny bones in the middle ear thehammer , the an$il , and the stirrup . These bones pass the vibrations on tothe primary organ of hearing, the cochlea, located in the inner ear.

"! T#e coc#"ea ,ocuses t#e -i!rations on t#e !asi"ar e !rane. Ferein the cochlea, the formerly airborne sound waves become /seaborne0because the coiled tube of the cochlea is lled with !uid. As the stirrupvibrates against the o$al window at the base of the cochlea, the vibrationsset the !uid into wave motion. As the !uid wave spreads through thecochlea, it causes vibration in the !asi"ar e !rane , a thin strip of hairytissue running through the cochlea.

#! T#e !asi"ar e !rane con-erts t#e -i!rations into neura"i pu"ses. The swaying of tiny hair cells on the vibrating basilarmembrane stimulates sensory nerve endings connected to the hair cells.

The excited neurons, then, transform the mechanical vibrations of thebasilar membrane into neural activity.

$! +ina""(2 t#e neura" essa'es tra-e" to t#e auditor( corte5 in t#e!rain. eural signals leave the cochlea in a bundle of neurons called theauditory ner$e . The neurons from the two ears meet in the brain stem,which passes the auditory information to both sides of the brain.#ltimately, the signals arrive in the auditory corte% for higher3orderprocessing.

Ps(c#o"o'ica" 6ua"ities o, Sound0 Ho& We Distin'uis# One Sound ,roAnot#er. The brain converts fre*uency and amplitude into three psychologicalsensations pitch, loudness, and timbre.

Sensations of Pitch! A sound wave8s fre uency determines the highness orlowness of a sound – a *uality nown as pitc# . :ur sensitivity to sound spans only alimited range of the sound waves that occur in nature. The range of human auditorysensitivity extends from fre*uencies as low as about ;Gcps to fre*uencies as high as;G,GGGcps. :ther creatures can hear sounds both higher and lower.

Sensations of Loudness! The physical strength or amplitude of a sound wavedetermines "oudness . ore intense sound waves produce louder sounds, while weexperience sound waves with small amplitudes as soft. #mplitude , then, refers tothe physical characteristics of a sound wave, while loudness is a psychologicalsensation. $ound intensity is expressed in units called decibels (d+).

Sensations of Tim%re! Ti !re is the *uality of a sound wave that derives fromthe wave8s complexity.

Hearin' Loss. Aging commonly involves loss of hearing acuity, especially for high

fre*uency sounds. Fearing loss can also come from diseases, such as mumps, thatmay attac the auditory nerves. And it can result from exposure to loud noises suchas gunshots, &et engines, or loud music, that damage the hair cells in the cochlea.

Parts o, t#e Hu an E(e

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common sensory combinations occur in si ling stea s, ing colas, and bowls of cereals.

Position and Mo-e ent To act purposefully and gracefully, we need constantinformation about the position of our limbs and other body parts in relation to each

other and to ob&ects in the environment. -ithout this information, even our simplestactions would be hopelessly uncoordinated. The physical mechanisms that eeptrac of body position, movement, and balance actually consist of two di7erentsystems, the $estibular sense and the "inesthetic sense .

The -esti!u"ar sense is the body position sense that orients us with respectto gravity. 't tells us the posture of our bodies – whether straight, leaning, reclining,or upside down. The vestibular sense also tells us when we are moving or how ourmotion is changing. The receptors for this information are tiny hairs in thesemicircular canals of the inner ear. These hairs respond to our movements bydetecting corresponding movements in the !uid of the semicircular canals.Cisorders of this sense can cause extreme di iness and disorientation.

The 7inest#etic sense eeps trac of body parts relative to each other. %ourinesthetic sense ma es you aware of crossing your legs and tells you which hand is

closer to your cell phone when it rings. 6inesthesis provides constant sensoryfeedbac about what the muscles in your body are doing during motor activities.Jeceptors for inesthesis reside in the &oints, muscles, and tendons.

Jeceptors of the vestibular and inesthetic senses connect to processingregions in the brain8s parietal lobes – which help us ma e sensory /map0 of thespatial relationship among ob&ects and events. This processing usually happensautomatically and e7ortlessly, outside of conscious awareness, except when we aredeliberately learning the movements for a new physical s ill.

S e"" $mell serves a protective function by sensing the odor of possibly dangerousfood, or, for some animals, the scent of a predator. Fumans seem to use the senseof smell primarily in con&unction with taste to locate and identify calorie3densefoods, avoid tainted foods, and it seems, to identify potential mates, too.

The )iology of &lfaction +iologically, the sense of smell, begins with chemical

events in the nose. There, odors in the form of airborne chemical molecules interactwith receptor proteins associated with speciali ed nerve cells. These cells,incidentally, are the body8s only nerve cells that come in direct contact with theoutside environment.

:dor molecules can be complex and varied. $cientists have catalogued atleast 2,@GG di7erent odor3producing molecules. Bxactly how the nose ma es senseof this cacophony of odors is not completely understood, but we do now that nasalreceptors sense the shape of odor molecules.

The nose8s receptor cells transduce information about the stimulus andconvey it to the brain8s olfactory bulbs , located on the underside of the brain &ustbelow the frontal lobes. There, sensations of smell are initially processed and thenpassed on to many other parts of the brain. #nli e all the other senses, smellsignals are not relayed through the thalamus, suggesting that smells has veryancient evolutionary roots.

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The Psychology of Smell :lfaction has an intimate connection with both emotionand memory. This may explain why the olfactory bulbs lie very close to, andcommunicate directly with, structures in the limbic system and temporal lobes thatare associated with emotion and memory. Therefore, it is not surprising that bothpsychologists and writers have noticed that certain smells can evo e emotion3laden

memories, sometimes of otherwise3forgotten events. 'f you thin about it for amoment, you can probably recall a vivid memory /image0 of the aroma associatedwith a favorite food from your childhood.

Taste >i e smell, taste is a sense based on chemistry. The senses of taste and smellhave a close and cooperative wor ing relationship – so many of the subtledistinctions you may thin as !avors really come from odors. ( uch of the /taste0 of an onion is odor, not !avor. And when you have a cold, you8ll notice that food seemstasteless because your nasal passages are bloc ed.)

The sense of taste, or 'ustation , involves four primary *ualities ordimensions sweet, sour, bitter, and salty. >ess well nown is the fth taste calledumami which is the savory !avor found in protein3rich foods such as meat, seafood,and cheese. 't is also associated with monosodium glutamate ( $=), often used inAsian cuisine.

The taste receptor cells, located in the taste buds on the top and side of thetongue, sample !avors from food and drin as they pass by on the way to thestomach. These taste receptors cluster in small mucous3membrane pro&ectionscalled papillae. Bach is especially sensitive to a molecule of a particular shape. Aspeciali ed nerve /hotline0 carries nothing but taste messages to speciali edregions of the cortex. There, tastes are reali ed in the parietal lobe8s somatosensoryarea.

*evelopmental +hanges in Taste 'nfants have heightened taste sensitivity butthis supersensitivity decreases with age. any elderly people complain that foodhas lost its taste. Taste receptors can be easily damaged by alcohol, smo e, acids,or hot foods but, fortunately, we fre*uently replace our gustatory receptors.+ecause of this constant renewal, the taste system boasts the most resistance topermanent damage of all our senses, and a total loss of taste is extremely rare.

T#e S7in Senses Eonsider the s in8s remar able versatility 't protects us againstsurface in&ury, holds in body !uids, and helps regulate body temperature. The s inalso contains nerve endings that, when stimulated, produce sensations of touch,pain, warmth, and cold. >i e several other sense, these s7in senses are connectedto the somatosensory cortex located in the brain8s parietal lobes.

The s in8s sensitivity to stimulation varies tremendously over the body,depending in part on the number of receptors in each area. -e are ten times moreaccurate in sensing stimulation on our ngertips than stimulation on our bac s. :ursensitivity is greatest where we need it most – on our face, tongue, and hands."recise sensory feedbac from these parts of the body permits e7ective eating,spea ing, and grasping.

S(nest#esia0 Sensation Across T#e Senses A small minority of people have acondition called s(nest#esia which allows them to sense their worlds acrosssensory domains. $ome actually taste shapes, others associate days of the weewith colors. Their de ning characteristic involves sensory experience that lin s onesense with another. Eross3sensory sensations reported in synesthesia are real, not

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&ust metaphors. Jesearch also shows that this ability runs in families, so it probablyhas a genetic component.

$ynesthesia apparently involves communication between di7erent brainareas that process di7erent sensations – often regions that lie close to each other in

the cortex. +rain imaging studies implicate a cortical area called the TP- , lying incon&unction of the temporal, parietal, and occipital lobes. This region simultaneouslyprocess information coming from many pathways.

III. WHAT IS THE RELATIONSHIP BETWEEN SENSATION AND PERCEPTION

:ur brain attaches meaning to incoming sensory information. Coes a bittertaste mean poison Coes a red !ag mean danger Coes a smile signify a friendlyoverture "erception brings meaning to sensation, so perception produces aninterpretation of the world, not a perfect representation of it.

The tas of perception is to organi e sensation into stable, meaningful percepts . A percept , then, is not &ust a sensation but the associated meaning aswell.

Perceptua" Processin'0 +indin' Meanin' in Sensation

The ,hat and ,here Pathways in the )rain The primary visual cortex, at thebac of the brain, splits visual information into two interconnected streams. :nestream, which !ows mainly to the temporal lobe, extracts information about anob&ect8s color and shape. This &#at pat#&a( allows us to determine what ob&ectsare. The other stream, the &#ere pat#&a( , pro&ects to the parietal lobe, whichdetermines an ob&ect8s location. Bvidence suggests that other senses also havewhat and where streams that interact with those in the visual system.

-e are conscious of information in the what pathway but not necessarily inthe where pathway. This fact explains a curious phenomenon nown as !"indsi'#t .'t is the ability to locate ob&ects despite damage to the visual system ma ing itimpossible for a person consciously to see and identify ob&ects. +lindsight is thoughtto involve unconscious visual processing in the where pathway.

Feature *etectors $peciali ed groups of cells in the visual pathways extract veryspeci c stimulus features, such as an ob&ect8s length, slant, color, boundary,location, and movement. "erceptual psychologists call these cells ,eaturedetectors .

-e now about feature detectors from animal and human experiments inwhich brain in&ury or disease selectively robs an individual of the ability to detectcertain features, such as colors or shapes. There is even a part of the temporal lobe– near the occipital cortex – with feature detectors that are especially sensitive tofeatures of the human face.

Top-*own and )ottom-.p Processing 5orming a precept involves twocomplimentary processes that psychologists call top&down processing and bottom&up processing . 'n top/do&n processin' , our goals, past experience, nowledge,expectations, memory, motivations, or cultural bac ground guide our perceptions of ob&ects – or events. 'n !otto /up processin' , the characteristics of the stimulus

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exert a strong in!uence on our perceptions. +ottom3up processing relies heavily onour brain8s feature detectors to sense these stimulus characteristics.

Thus, bottom3up processing involves sending sensory data into the systemthrough receptors and sending it /upward0 to the cortex, where a basic analysis,

involving the feature detectors, is performed to determine the characteristics of thestimulus. "sychologists also refer to this as stimulus&dri$en processing because theresulting percept is determined, or /driven0, by the stimulus features. +y contrast,top3down processing !ows in the opposite direction, with the percept being drivenby some concept in the cortex – at the /top0 of the brain. +ecause this sort of thin ing relies heavily on concepts in the perceiver8s own mind, it is also nown asconceptually dri$en processing .

Perceptual +onstancies The ability to see an ob&ect as being the same shapefrom di7erent angles or distance is &ust one example of a perceptua" constanc( .'n fact, there are many inds of perceptual constancies. These include color constancy , which allows us to see a !ower as being the same color in the reddishlight of sunset as in the white glare of midday. Si*e constancy allows us to perceivea person as the same si e at di7erent distances and also serves as a strong cue fordepth perception. Together, these constancies help us identify and trac ob&ects in achanging world.

Inattentional )lindness and +hange )lindness $ometimes we don8t noticethings that occur right in front of our noses – particularly if they are unexpected andwe haven8t focused our attention on them. "sychologists call this inattentiona"!"indness . agicians rely on it for many of their tric s. They also rely on c#an'e!"indness , a related phenomenon in which we fail to notice that something isdi7erent now than it was before.

-e do notice changes that we anticipate, such as a red light turning to green.+ut laboratory studies show that many people don8t notice when, in a series of photographs of the same scene, a red light is replaced by a stop sign. :ne way thismay cause trouble in the world outside the laboratory is that people underestimatethe extent to which they can be a7ected by change blindness. This probably occursbecause our perceptual systems and our attention have limits on the amount of information they can process, so our expectations coming from the /top down0

cause us to overloo the unexpected.

Perceptua" A !i'uit( and Distortion A primary goal of perception is to get anaccurate / x0 on the world – to recogni e friends, foes, opportunities, and dangers.$urvival sometimes depends on accurately perceiving the environment, but theenvironment is not always easy to /read0. 5igures or ob&ects are sometimes hard to

nd because they blend easily with the bac ground. $ometimes our perceptions canbe wildly inaccurate because we misinterpret an image – as what happens insensory and perceptual illusions .

,hat Illusions Tell .s a%out Sensation and Perception -hen your minddeceives you by interpreting a stimulus pattern incorrectly, you are experiencing ani""usion . 'n the blac 3and3white Fermann grid, note how dar , fu y spots appear atthe intersections of the white bars. +ut when you focus on an intersection, the spotvanishes. -hy The answer lies in the way receptor cells in the visual pathwayinteract with each other. The ring of certain cells that are sensitive to light3darboundaries inhibits the activity of ad&acent cells that would otherwise detect thewhite grid lines. This inhibiting process ma es us sense dar er regions – the grayish

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areas – at the white intersections &ust outside our focus. Bven though we now (top3down) that the s*uares in the Fermann grid are blac and the lines are white, this

nowledge cannot overcome the illusion, which operates at a more basic, sensorylevel.

To study illusions at the level of perception, psychologists often employa !i'uous 8'ures – stimulus patterns that can be interpreted (top3down) in twoor more distinct ways, as in the .ubin $ase and the ec"er cube . These gures aredesigned to confound your interpretations, not &ust your sensations. Bach suggeststwo con!icting meanings once you have seen both, your perception will cycle bacand forth between them as you loo at the gure. $tudies suggest that thesealternating interpretations may involve the shifting of perceptual control betweenthe left and right hemispheres of the brain.

There is also color and brightness constancy , our ability to see an ob&ect asessentially unchanged under di7erent lighting conditions. #nder normal condition,this helps us from being misled by shadows.

$tudies have shown that context and or culture contribute to how weperceive sensory illusions and that our interpretations are learned .

'pplying the Lessons of Illusions $everal prominent modern artist, fascinatedwith the visual experiences created by ambiguity, have used perceptual illusion as acentral artistic feature of their wor . 'n S"y and /ater by .E. Bscher, you can seebirds and shes only through the process of gure3ground reversal, much li e theJubin vase illusion. The e7ect of these paintings on us underscores the function of human perception to ma e sense of the world and to x on the best interpretationwe can ma e.

To interpret illusions, we draw on our personal experiences, learning, andmotivation. 6nowing this, those who understand the principles of perception oftencan control illusions to achieve desired e7ects far beyond the world of painting.Architects and interior designers create illusions that ma e spaces seem larger orsmaller than they really are. They may ma e a small apartment appear morespacious when it is painted in light colors and sparsely furnished. $imilarly, set andlighting designers in movies and theatrical productions purposely create visual

illusions on lm and on stage. -e also ma e everyday use of illusion on our choicesof cosmetics and clothing. 'n such ways, we use illusions to distort /reality0 andma e our loves more pleasant.

T#eoretica" E5p"anations ,or Perception The fact that most people perceivemost illusions and ambiguous gures in essentially the same way suggests thatfundamental perceptual principles are at wor . Two in!uential theories, 0estalt theory and learning&based inference , explain how we form our perceptions. The=estalt theory emphasi es how we organi e incoming stimulation into meaningfulperceptual patterns – because of the way our brains are innately /wired0. :n theother hand, learning3based inference emphasi es learned in!uences on perception,including the power of expectations, context, and culture. 'n other words, =estalttheory emphasi es nature , and learning3based inference emphasi es nurture . Perceptual &rgani/ation: The 0estalt Theory About 2GG years ago, perceptualtric s captured the interest of a group of =erman psychologists, who argued thatthe brain in innately wired to perceive not &ust stimuli but also patterns instimulation. They called such a pattern a 0estalt , the =erman word for /perceptual

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pattern0 or /con guration0. Thus, from the raw material of stimulation, the brainforms a perceptual whole that is more than the mere sum of its sensory parts. Thisperspective became nown as 9esta"t ps(c#o"o'( .

The =estaltists li ed to point out that we perceive a s*uare as a single gure

rather than merely as four individual lines. $imilarly, when you hear a familiar song,you don not focus on the individual notes. Jather, your brain extracts the melody,which is your perception of the overall pattern of notes. $uch examples show thatwe always attempt to organi e sensory information into meaningful patterns, themost basic elements of which are already present in our brains at birth.

'! Figure and 0round :ne of the most basic of perceptual processes identi edby =estalt psychology divides our perceptual experience into 1gure andground . A 8'ure is simply a pattern or image that grabs our attention."sychologists sometimes call this a 0estalt . Bverything else becomes'round , the bac drop against which we perceive the gure. <isually, a gurecould be a bright !ashing sign or a word on the bac ground of a page.

)! +losure: Filling in the )lan(s :ur minds seem to abhor a gap and wheredivision occurs, we perceive subjecti$e contours or boundaries that exist notin the stimulus but only in the sub&ective experience of our minds. C"osurema es us see incomplete gures as wholes by supplying the missingsegments, lling in gaps, and ma ing inferences about potentially hiddenob&ects. 'n general, humans have a natural tendency to perceive stimuli ascomplete and balanced even when pieces are missing. Elosure is alsoresponsible for lling in your /blind spot0.

+! The 0estalt Laws of Perceptual 0rouping2. >aw of similarity – we tend to group things together that are similar. Anysuch tendency to perceive things as belonging together because theyshare common features re!ects the law of similarity.

;. >aw of proximity (nearness) – we tend to group things together that arenear each other.

?. >aw of continuity – we prefer smoothly connected and continuous guresD. >aw of common fate – when visual elements are moving together, we tend

to perceive them as one =estalt@. >aw of "rMgnan (meaningfulness) – we perceive the simplest pattern

possible, or the percept re*uiring the least mental e7ort. This is alsocalled the minimum principle of perception

Learning-)ased Inference: The Nurture of Perception Fermann vonFelmholt 8s theory of "earnin'/!ased in,erence emphasi ed how people use priorlearning to interpret new sensory information. +ased on experience, the observerma es inferences – guesses or predictions – about what the sensations mean. Thistheory explains why we assume a birthday party is in progress when we see lightedcandles on a ca e. -e have learned to associate ca es, candles, and birthdays.:rdinarily, perceptual inferences are fairly accurate. :n the other hand, confusingsensations and ambiguous arrangements can create perceptual illusions anderroneous conclusions. :ur perceptual interpretations are only hypotheses aboutour sensations.

According to this theory, the most important factors in determining anaccurate percept include the conte%t , our e%pectations , and our perceptual set .

'! +onte1t and 21pectations – once you identify a context, you formexpectations about what persons, ob&ects and events you are li ely toexperience. "erceptual identi cation depends on context and expectations aswell as ob&ect8s physical properties.

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)! Perceptual Set – readiness to detect a particular stimulus in a givencontext. This involves focused alertness. :ften, a perceptual set leads you totransform an ambiguous stimulus into the one you were expecting to see

*epth Perception: Nature or Nurture3 Although depth perception appears earlyin human development, the idea of being cautious when there is danger of fallingseems to develop later in infancy. Cevelopmental psychologists believe thatcrawling and depth perception are lin ed in that crawling helps infants develop theirunderstanding of the three3dimensional world.

:ur sense of depth or distance relies on multiple cues. Cepth cues aregrouped in to categories binocular cues and monocular cues .

A. Binocu"ar cues – are information ta en in by both eyes that aid in depthperception.2. Binocular con$ergence suggests how the lines of vision from each eye

converge at di7erent angles on ob&ects at di7erent distances.;. .etinal disparity arises from the di7erence in perspectives of the two

eyes.+. Monocu"ar cues – are information about depth that relies on the input of

&ust one eye.2. .elati$e si*e . 'f two ob&ects that are assumed to be the same si e cast

di7erent3si ed images on the retina, observers usually &udge them to lie atdi7erent distances.

;. 2inear perspecti$e . The apparent convergence of parallel lines.?. 2ight and Shadow . >ighter3colored ob&ects seem closer9 dar er ob&ects

seem farther away. :b&ects that re!ect the most light appear to be nearerthan more dimly lit ob&ects in the distance.

D. 3nterposition . Eloser ob&ects will cut o7 our vision of more distant ob&ectsbehind them. "artially hidden ob&ects are more distant than the ob&ectsthat hide them.

@. .elati$e motion . :b&ects at di7erent distances appear to move throughyour eld of vision at a di7erent rate. earer ob&ects appear to move by atgreat speed, while more distant ob&ects stay in your eld of view longer,appearing to move by more slowly.

H. #tmospheric perspecti$e . Fa e or fog ma es ob&ects in the distance loofu y, less distinct, or invisible.

Seein' and Be"ie-in' -e all sense the world in roughly the same way butbecause we attach di7erent meaning to our sensations, it is clear that people

percei$e the world in many di7erent ways.

+unda enta" +eatures o, t#e Hu an SensesSense Sti u"us Sense Or'an Receptor Sensation

<ision >ight waves Bye Jods andcones of theretina

Eolors,brightness,patterns,motion,textures

Fearing $ound waves Bar Fair cells ofthe basilarmembrane

"itch,loudness,timber

$ in senses Bxternalcontact $ in erve endingsin s in Touch, warmth,cold$mell <olatile

substancesose Fair cells of

olfactoryepithelium

:dors

Taste $olublesubstances

Tongue Taste buds oftongue

5lavors

"ain any intense et of pain $peciali ed Acute pain,

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or extremestimulitemperature,chemicals,mechanicalstimuli, etc.

bers all overthe body

pain receptors,overactive orabnormalneurons

chronic pain

6inestheticand vestibularsenses

+ody position,movement,and balance

$emicircularcanals,s eletalmuscles,

&oints, tendons

Fair cells insemicircularcanals9neuronsconnected tos eletalmuscles,

&oints, andtendons

"osition of thebody parts inspace

T#e Her ann 9rid T#e Ru!in :ase

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HO_Sensation and Perception

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