sensory, attentional and perceptual processes...

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Chapter5• understand the nature of sensory processes,• explain the processes and types of attention,• analyse the problems of form and space perception,• examine the role of socio-cultural factors in perception, and• reflect on sensory, attentional and perceptual processes in everyday life.

After reading this chapter, you would be able to

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IntroductionKnowing the WorldNature and Varieties of StimulusSense Modalities

Visual SensationOther Human Senses (Box 5.1)Auditory Sensation

Attentional ProcessesSelective AttentionDivided Attention (Box 5.2)Sustained AttentionSpan of Attention (Box 5.3)Attention Deficit Hyperactivity Disorder (Box 5.4)

Perceptual ProcessesProcessing Approaches in Perception

The PerceiverPrinciples of Perceptual OrganisationPerception of Space, Depth, and Distance

Monocular Cues and Binocular CuesPerceptual ConstanciesIllusionsSocio-Cultural Influences on Perception

Key TermsSummaryReview QuestionsProject Ideas

Contents

The quality of life is determinedby its activities.

– Aristotle

Chapter 5 • Sensory, Attentional and Perceptual Processes 85

not only from the external world, but also fromour own body. The information collected byour sense organs forms the basis of all ourknowledge. The sense organs register severalkinds of information about various objects.However, in order to be registered, the objectsand their qualities (e.g., size, shape, colour)must be able to draw our attention. Theregistered information must also be sent tothe brain that constructs some meaning outof them. Thus, our knowledge of the worldaround us depends on three basic processes,called sensation, attention, and perception.These processes are highly interrelated; hence,they are often considered as different elementsof the same process, called cognition.

NATURE AND VARIETIES OF STIMULUS

The external environment that surrounds uscontains a wide variety of stimuli. Some ofthem can be seen (e.g., a house), while somecan be heard only (e.g., music). There areseveral others that we can smell (e.g., fragranceof a flower) or taste (e.g., sweets). There arestill others that we can experience by touching(e.g., softness of a cloth). All these stimuli

KNOWING THE WORLD

The world in which we live is full of variety ofobjects, people, and events. Look at the roomyou are sitting in. You will find so many thingsaround. Just to mention a few, you may seeyour table, your chair, your books, your bag,your watch, pictures on the wall and manyother things. Their sizes, shapes, and coloursare also different. If you move to other roomsof your house, you will notice several othernew things (e.g., pots and pans, almirah, TV).If you go beyond your house, you will find stillmany more things that you generally knowabout (trees, animals, buildings). Suchexperiences are very common in our day-to-day life. We hardly have to make any effortsto know them.

If someone asks you, “How can you saythat these various things exist in your room,or house, or in the outside environment?”, youwill most probably answer that you see orexperience them all around you. In doing so,you are trying to tell the person that theknowledge about various objects becomespossible with the help of our sense organs (e.g.,eyes, ears). These organs collect information

In the previous chapters you have already learnt how we respond to various stimulipresent in the external and internal environment with the help of our receptors.While some of these receptors are clearly observable (for example, eyes or ears),others lie inside our body, and are not observable without the help of electrical ormechanical devices. This chapter will introduce you to various receptors that collecta variety of information from the external and internal worlds. The focus will beparticularly on the structure and function of eye and ear, including some interestingprocesses associated with vision and audition. You will also know some importantthings about attention, which helps us to notice and register the information thatour sense organs carry to us. Different types of attention will be described alongwith the factors that influence them. At the end, we will discuss the process ofperception that allows us to understand the world in a meaningful way. You willalso have an opportunity to know how we are sometimes deceived by certain typesof stimuli such as figures and pictures.

Introduction

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provide us with various kinds of information.We have very specialised sense organs to dealwith these different stimuli. As human beingswe are bestowed with a set of seven senseorgans. These sense organs are also knownas sensory receptors or information gatheringsystems, because they receive or gatherinformation from a variety of sources. Five ofthese sense organs collect information fromthe external world. These are eyes, ears, nose,tongue, and skin. While our eyes are primarilyresponsible for vision, ears for hearing, nosefor smell, and tongue for taste, skin isresponsible for the experiences of touch,warmth, cold, and pain. Specialised receptorsof warmth, cold, and pain are found insideour skin. Besides these five external senseorgans, we have also got two deep senses. Theyare called kinesthetic and vestibular systems.They provide us with important informationabout our body position and movement ofbody parts related to each other. With theseseven sense organs, we register ten differentvariety of stimuli. For example, you may noticewhether a light is bright or dim, whether it isyellow, red or green, and so on. With soundyou may notice whether it is loud or faint,whether it is melodious or distracting, and soon. These different qualities of stimuli are alsoregistered by our sense organs.

SENSE MODALITIES

Our sense organs provide us with first-handinformation about our external or internalworld. The initial experience of a stimulus oran object registered by a particular senseorgan is called sensation. It is a processthrough which we detect and encode a varietyof physical stimuli. Sensation also refers toimmediate basic experiences of stimulusattributes, such as “hard”, “warm”, “loud”, and“blue”, which result from appropriatestimulation of a sensory organ. Different senseorgans deal with different forms of stimuli andserve different purposes. Each sense organ ishighly specialised for dealing with a particularkind of information. Hence, each one of themis known as a sense modality.

Functional Limitations of Sense Organs

Before we move on to a discussion of senseorgans, it is important to note that our senseorgans function with certain limitations. Forexample, our eyes cannot see things whichare very dim or very bright. Similarly our earscannot hear very faint or very loud sounds.The same is true for other sense organs also.As human beings, we function within a limitedrange of stimulation. For being noticed by asensory receptor, a stimulus has to be of anoptimal intensity or magnitude. Therelationship between stimuli and thesensations they evoke has been studied in adiscipline, called psychophysics.

In order to be noticed a stimulus has tocarry a minimum value or weight. Theminimum value of a stimulus required toactivate a given sensory system is calledabsolute threshold or absolute limen (AL).For example, if you add a granule of sugar toa glass of water, you may not experience anysweetness in that water. Addition of a secondgranule to water may also not make it tastesweet. But if you go on adding sugar granulesone after another, there will come a point whenyou will say that the water is now sweet. Theminimum number of sugar granules requiredto say that the water is sweet will be the AL ofsweetness.

It may be noted at this point that the AL isnot a fixed point; instead it varies considerablyacross individuals and situations dependingon the people’s organic conditions and theirmotivational states. Hence, we have to assessit on the basis of a number of trials. Thenumber of sugar granules that may producethe experience of “sweetness” in water on50 per cent of occasions will be called the ALof sweetness. If you add more number of sugargranules, the chances are greater that thewater will be reported more often as sweet thanplain.

As it is not possible for us to notice allstimuli, it is also not possible to differentiatebetween all stimuli. In order to notice twostimuli as different from each other, there hasto be some minimum difference between the


value of those stimuli. The smallest differencein the value of two stimuli that is necessary tonotice them as different is called differencethreshold or difference limen (DL). Tounderstand it, we may continue with our“sugar water” experiment. As we have seen,the plain water is experienced as sweet afterthe addition of certain number of sugargranules. Let us remember this sweetness. Thenext question is: how many sugar granuleswill be needed in the water in order toexperience its sweetness as different from theprevious sweetness. Go on adding sugargranules one after another tasting the watereach time. After addition of a few granules,you will notice at a point that the water is nowsweeter than the previous one. The numberof sugar granules added to the water togenerate an experience of sweetness that isdifferent from the previous sweetness on 50per cent of the occasions will be called the DLof sweetness. Thus, difference threshold is theminimum amount of change in a physicalstimulus that is capable of producing asensation difference on 50 per cent of thetrials.

You may realise by now that understandingof sensations is not possible withoutunderstanding the AL and DL of different typesof stimuli (for example, visual, auditory), butthat is not enough. Sensory processes do notdepend only on the stimulus characteristics.Sense organs and the neural pathwaysconnecting them to various brain centers alsoplay a vital role in this process. A sense organreceives the stimulus and encodes it as anelectrical impulse. For being noticed thiselectrical impulse must reach the higher braincenters. Any structural or functional defect ordamage in the receptor organ, its neuralpathway, or the concerned brain area may leadto a partial or complete loss of sensation.

Visual Sensation

Among all sense modalities, vision is the mosthighly developed in human beings. Variousestimates indicate that we use it inapproximately 80 per cent of our transactions

with the external world. Audition and othersenses also contribute significantly toinformation gathering from the external world.We shall discuss vision and audition in somedetail. The main features of other senses canbe found in Box 5.1.

Visual sensation starts when light entersthe eyes and stimulates our visual receptors.Our eyes are sensitive to a spectrum of light,the wavelength of which ranges from 380 nmto 780 nm (nm refers to nanometer, which isone billionth of a meter). No sensation isregistered beyond this range of light.

The Human Eye

A diagram of the human eye is shown inFigure 5.1. As you can see, our eye is madeup of three layers. In the outer layer, there isa transparent cornea and a tough sclera thatsurrounds the rest of the eye. It protects theeye and maintains its shape. The middle layeris called choroid, which is richly supplied withblood vessels. The inner layer is known asretina. It contains the photoreceptors (rodsand cones) and an elaborate network ofinterconnecting neurons.

The eye is generally compared with acamera. For example, the eye and camera havea lens. The lens divides the eye into twounequal chambers, namely aqueous chamberand vitreous chamber. The aqueous chamberis located between the cornea and the lens. Itis smaller in size and is filled with a water-like substance, called aqueous humor. Thevitreous chamber is located between the lensand the retina. It is filled with a jelly likeprotein, called vitreous humor. These fluidshelp in holding the lens at its appropriate placeand in proper shape. They also allow enoughflexibility for the occurrence of accommodation— a process through which the lens changesits shape in order to focus the objects atvarying distances. This process is regulatedby ciliary muscles, which are attached to thelens. These muscles flatten the lens to focusthe distant objects and thicken it to focus thenear objects. Like a camera, the eye also hasa mechanism to control the amount of light

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(colour) vision. Each eye contains about 100million rods and about 7 million cones. Thecones are highly concentrated in the centralregion of the retina surrounding the fovea,which is a small circular region of the size of apea. It is also known as the yellow spot. It isthe region of maximum visual acuity. Besidesphotoreceptors, retina also contains a bundleof axons of a cell (called ganglion cell) thatforms the optic nerve, which leads to thebrain.

Working of the Eye

Passing through conjunctiva, cornea, andpupil, the light enters the lens, which focuses

entering into it. The iris serves this purpose.It is a disc-like coloured membrane lyingbetween the cornea and the lens. It controlsthe amount of light entering the eye byregulating pupil dilation. In dim light the pupildilates; in bright light it contracts.

Retina is the inner most layer of an eye. Itis made up of five types of photosensitive cellsamong which rods and cones are mostimportant. Rods are the receptors for scotopicvision (night vision). They operate at lowintensities of light, and lead to achromatic(colourless) vision. Cones are the receptors forphotopic (day light) vision. They operate at highlevels of illumination, and lead to chromatic

Besides vision and audition, there are othersenses that enrich our perceptions. For example,an orange looks attractive not only because of itis colour but also because it has got a specialflavour and taste. These other senses are brieflydescribed here. 1. Smell : The stimulus for smell sensation

consists of molecules of various substancescontained in the air. They enter the nasalpassage where they dissolve in moist nasaltissues. This brings them in contact withreceptor cells contained in olfactoryepithelium. Human beings possess about 50million of these receptors, whereas dogspossess more than 200 million of thesereceptors. Nevertheless, our ability to detectsmell is impressive. It is indicated that humanbeings can recognise and distinguish amongapproximately 10,000 different odours. Thesense of smell also shows sensoryadaptation like other senses.

2. Taste : The sensory receptors for taste arelocated inside small bumps on the tongue,known as papillae. In each papilla there is acluster of taste buds. Human beings possessalmost 10,000 taste buds. While people claimto distinguish a large number of flavours infood, there are only four basic tastes, namelysweet, sour, bitter and salty. How is it thenthat we perceive many more? The answer isthat we are aware not only of the taste ofthe food, but also of its smell, its texture, its

BoxBoxBoxBoxBox 5.1 Other Human SensesOther Human SensesOther Human SensesOther Human SensesOther Human Senses

temperature, its pressure on our tongue, andmany other sensations. When these factors areremoved, we are left with only four basic tastes.Besides, the combination of different flavours invaried proportions results in a different kind offlavour which may be quite unique.

3. Touch and other skin senses : Skin is a sensoryorgan that produces sensations of touch(pressure), warmth, cold, and pain. In our skinthere are specialised receptors for each one ofthese sensations. The receptors of touch are notevenly distributed in our skin. That is why someareas of our body (e.g., face, fingertips) are moresensitive than others (e.g., legs). Pain sensationhas no specific stimulus. Hence, determining itsmechanism has been fairly difficult.

4. The Kinesthetic system : Its receptors are foundprimarily in joints, ligaments, and muscles. Thissystem gives us information about the locationof our body parts in relation to each other, andallows us to perform simple (e.g., touching one’snose) and complex movements (e.g., dancing). Ourvisual system provides a great deal of help inthis respect.

5. The Vestibular system : This system gives usinformation about our body position, movement,and acceleration — the factors that are criticalfor maintaining our sense of balance. The sensoryorgans of this sense are located in the inner ear.While vestibular sacs inform us of our bodypositions, the semicircular canals inform us aboutour movements and acceleration.


it on to the retina. Retina is divided into twoparts: the nasal half and the temporal half.The inner half portion of the eye (towards thenose), taking the center of fovea as mid-point,is called the nasal half. The outer half portionof the eye (towards the temple) from the centerof fovea is called the temporal half. Light fromthe right visual field stimulates the left half ofeach eye (i.e. the nasal half of the right eyeand the temporal half of the left eye), and lightfrom the left visual field stimulates the righthalf of each eye (i.e. the nasal half of the lefteye and the temporal half of the right eye). Aninverted image of the object is formed on theretina. The neural impulse is transmitted tothe visual cortex through the optic nerve wherethe image is re-inverted and processed. Youcan see in Fig.5.1 that the optic nerve leavesthe retina from the area that has nophotoreceptors. In this area visual sensitivityis completely absent. Therefore, it is called theblind spot.

Adaptation

The human eye can function at a very largerange of light intensities. Sometimes we haveto undergo a rapid change in illuminationlevels. For example, when we go to a matineeshow movie, we find it difficult to see thingsin the hall on entering into it. However, afterspending about 15 to 20 minutes there, weare able to see everything. After the show whenwe go out into the open, we find the lightoutside the hall too bright to see things, orsometimes even to keep our eyes open.However, within a minute or so we feelcomfortable, and are able to see thingsproperly. This adjustment is faster than theone made on entering the hall. The process ofgetting adjusted to different intensities of lightis called ‘visual adaptation’.

Light adaptation refers to the process ofadjusting to bright light after exposure to dimlight. This process takes nearly a minute ortwo. On the other hand, dark adaptation

Fig.5.1 : Structure of the Human Eye

Cornea

Iris

Aqueoushumor

Pupil

Lens

Ciliary muscle RetinaBlind spot

Optic nerve

Vitreous humor

Fovea

Distalstimulus

Proximalstimulus

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refers to the process of adjusting to a dimlyilluminated environment after exposure tobright light. This may take half an hour oreven longer depending on the previous levelof exposure of the eye to light. There are certainways in which these processes can befacilitated. An interesting activity is givenbelow to demonstrate this process to you.

Colour Vision

In our interaction with the environment wenot only experience a variety of objects, butalso their colours. It may be noted that colouris a psychological property of our sensoryexperience. It is created when our braininterprets the information received from theexternal world. It may be remembered thatlight is described physically in terms ofwavelength, not in terms of colour. As we haveread earlier, the visible spectrum is a rangeof energy (380-780 nm) that ourphotoreceptors can detect. The energy loweror higher than the visible spectrum is harmfulto the eyes. The sun light is a perfect mixtureof seven colours just like a rainbow. Thecolours observed are violet, indigo, blue,green, yellow, orange, and red, abbreviatedas ‘VIBGYOR’.

The Dimensions of Colour

A person with normal colour vision candistinguish more than seven million differentshades of colour. Our experiences of colourcan be described in terms of three basicdimensions, called hue, saturation, andbrightness. Hue is a property of chromaticcolours. In simple words, it refers to the nameof the colour, e.g., red, blue, and green. Huevaries with wavelength, and each colour isidentified with a specific wavelength. Forexample, blue has a wavelength of about 465nm, and green of about 500 nm. Achromaticcolours like black, white or grey are notcharacterised by hues. Saturation is apsychological attribute that refers to therelative amount of hue of a surface or object.The light of single wavelength (monochromatic)appears to be highly saturated. As we mixdifferent wavelengths, the saturationdecreases. The colour grey is completelyunsaturated. Brightness is the perceivedintensity of light. It varies across bothchromatic and achromatic colours. White andblack represent the top and bottom of thebrightness dimension. White has the highestdegree of brightness, whereas black has thelowest degree.

Move from a lighted area to a dark room and notehow much time you take to see things clearly inthat room.

Next time put on red goggles while you stayin the lighted place. Then move into the dark roomand note how much time you take to see thingsclearly in that room.

You will notice that the use of red goggles hasgreatly reduced the time required for darkadaptation.

Do you know why has this happened?Discuss with your friends and the teacher.

ActivityActivityActivityActivityActivity 5.1

Photochemical Basis of Light and DarkAdaptation : You may wonder why the lightand dark adaptations take place. Accordingto the classical view, light and darkadaptations occur due to certainphotochemical processes. The rods have aphoto-sensitive chemical substance, calledrhodopsin or visual purple. By the action oflight the molecules of this chemical substanceget bleached or broken down. Under suchconditions the light adaptation takes place inthe eyes. On the other hand, the darkadaptation is achieved by the removal of light,and thereby allowing for restorative processesto regenerate the pigment in the rods with thehelp of vitamin A. The regeneration ofrhodopsin in rods is a time consumingprocess. That is why dark adaptation is aslower process than light adaptation. It hasbeen found that people who suffer fromvitamin A deficiency do not achieve darkadaptation at all, and find it really difficult tomove in the dark. This condition is generallyknown as night blindness. A parallel chemicalbelieved to be found in cones is known asiodopsin.


Colour Mixtures

There is an interesting relationship amongcolours. They form complementary pairs.When mixed in correct proportions thecomplementary colours yield an achromaticgrey or white. Examples of complementarycolours are red-green and yellow-blue. Red,green and blue are called primary colours,because on mixing, the light of these threecolours can produce almost any colour. Themost common example is the television screen.It contains spots of blue, red and greencolours. The combinations of these threeproduce different colours and shades that wesee on the TV screen.

After Images

This is quite an interesting phenomenonrelated to visual sensations. The effect of avisual stimulus persists for some time evenafter the removal of that stimulus from thevisual field. This effect is called after image.After images are positive and negative.Positive after images resemble the originalstimulus in terms of hue, saturation, andbrightness. They usually occur after a briefintense stimulation of dark adapted eyes. Onthe other hand, negative after images appearin complementary colours. These imagesappear when a person stares at the patch of aparticular colour for at least 30 seconds, andthen transfers the gaze to a neutralbackground (e.g., a white or grey surface). Ifthe person looks at the blue colour, thenegative after image will appear in yellow.Similarly, a red stimulus will yield a negativeafter image of green colour.

Auditory Sensation

Audition or hearing is also an important sensemodality that carries great value for us. Itprovides us with reliable spatial information.Besides orienting us to certain objects orindividuals, it plays a vital role in spokencommunication also. Auditory sensationbegins when sound enters our ear andstimulates the chief organs of hearing.

The Human Ear

Ear is the primary receptor of auditory stimuli.While its well-known function is hearing, italso helps us in maintaining our body balance.The structure of an ear is divided into threesegments, called the external ear, the middleear, and the inner ear (Fig.5.2).

External Ear : It contains two main structures,namely pinna and auditory meatus. Pinna isa cartilaginous funnel-shaped structure thatcollects sound waves from the surroundings.Auditory meatus is a canal protected by hairand wax that carries sound waves from pinnato the tympanum or ear drum.

Middle Ear : The middle ear starts withtympanum, a thin membrane highly sensitiveto sound vibrations. This is followed by thetympanic cavity. It is connected to thepharynx with the help of Eustachian tube,which maintains the air pressure in tympaniccavity. From the cavity the vibrations pass tothree ossicles known as malleus (hammer),incus (anvil), and stapes (stirrup). Theyincrease the intensity of sound vibrationsabout 10 times, and send them to the innerear.

Inner Ear : The inner ear has a complicatedstructure known as membranous labyrinth,which is encapsulated in a bony shell calledbony labyrinth. A lymph-like fluid is found inthe space between bony labyrinth andmembranous labyrinth. This is calledperilymph.

The bony labyrinth has three semi-circular canals at right angle to each other, acavity, called vestibule, and a coiled structure,called cochlea. The semicircular canals havefine hair cells, which are highly sensitive topostural changes as well as changes in thebody orientation. Inside the bony cochlea,there is a membranous cochlea, which is alsoknown as scala media. It is filled withendolymph, and has a spirally coiledmembrane, called basilar membrane. It hasgot fine hair cells arranged in a series to formthe organ of corti. This is the main organ forhearing.

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Working of the Ear

Pinna collects the sound vibrations and servesthem to the tympanum through the auditorymeatus. From the tympanic cavity thevibrations are transferred to the three ossicles,which increase their strength and transmitthem to the inner ear. In the inner ear thecochlea receives the sound waves. Throughvibrations the endolymph is set in motion,which also vibrates the organ of corti. Finally,the impulses are sent to the auditory nerve,which emerges at the base of cochlea andreaches the auditory cortex where the impulseis interpreted.

Sound as a Stimulus

We all know that sound is the stimulus forears. It results from pressure variations in theexternal environment. Any physical movement

disturbs the surrounding medium (i.e. air),and pushes the air molecules back and forth.This results in changes in pressure that spreadoutward in the form of sound waves, travellingat a rate of about 1,100 ft/sec. These changestravel in waves much like the ripples set upby a stone thrown into a pond. When thesesound waves strike our ears, they initiate aset of mechanical pressure changes thatultimately trigger the auditory receptors.

The simplest kind of sound wave is onethat causes successive pressure changes overtime in the form of a single repeating sine wave(Fig.5.3). Sound waves vary in amplitude aswell as in wavelength. Amplitude is a generalmeasure of stimulus magnitude. It is theamount of change in pressure, i.e. the extentof displacement of the molecules from theposition of rest. In Fig.5.3 the amplitude ofsound wave is represented as the distance of

Fig.5.2 : Structure of the Human Ear

Hammer

Externalcanal

Anvil

Semicircularcanals

Saccule

Utricle

Facial nerve

Auditory nerve

Eustachian tube

RoundwindowStirrup

Tympanicmembrane

Pinna Cochlea


the crest or trough from its mean position.Wavelength is the distance between the twocrests. Sound waves are basically formed dueto alternate compression and decompression(rarefaction) of air molecules. A completechange in pressure from compression torarefaction and again to compression makesa cycle of the wave.

sound reflects the complexity of its soundwaves. Most of the sounds found in naturalenvironments are complex.

Vision and hearing are generally believed to bethe two most highly prized senses. What wouldyour life be if you lost any one of your senses?Which sense would you find more traumatic tolose? Why? Think and write down.

What if you could magically improve theperformance of one of your senses, which sensewould you choose to improve? Why? Could youimprove the performance of this one sense withoutmagic? Think and write down.

Discuss with your teacher.


Fig.5.3 : Sound Waves

Am

plitu

de

one cycle

Sound waves are described in terms oftheir frequency, which is measured in termsof cycles per second. Its unit is called Hertz(Hz). Frequency and wavelength have aninverse relationship. When the wavelengthincreases, the frequency decreases, and whenthe wavelength decreases, the frequencyincreases. Amplitude and frequency both arephysical dimensions. Besides these, there arethree psychological dimensions of sound,namely loudness, pitch and timbre.

Loudness of the sound is determined byits amplitude. Sound waves with largeamplitude are perceived as loud; those withsmall amplitude are perceived as soft.Loudness is measured in decibels (db). Pitchrefers to highness or lowness of a sound. Theseven notes used in Indian classical musicrepresent a gradual increase in their pitch.Frequency determines the pitch of a soundwave. The higher the frequency, the higher willbe the pitch. The range of hearing is generally20 Hz-20,000 Hz. Timbre refers to the natureor quality of a sound. For example, the soundof a car engine and a person talking differ withrespect to quality or timbre. The timbre of a

ATTENTIONAL PROCESSES

In the previous section we have discussedsome sensory modalities that help us incollecting information from the external worldand also from our internal system. A largenumber of stimuli impinge upon our senseorgans simultaneously, but we do not noticeall of them at the same time. Only a selectedfew of them are noticed. For example, whenyou enter your classroom you encounterseveral things in it, such as doors, walls,windows, paintings on walls, tables, chairs,students, schoolbags, water bottles, and soon, but you selectively focus only on one ortwo of them at one time. The process throughwhich certain stimuli are selected from a groupof others is generally referred to as attention.

At this point it may be noted that besidesselection, attention also refers to several otherproperties like alertness, concentration, andsearch. Alertness refers to an individual’sreadiness to deal with stimuli that appearbefore her/him. While participating in a racein your school, you might have seen theparticipants on the starting line in an alertstate waiting for the whistle to blow in orderto run. Concentration refers to focusing ofawareness on certain specific objects whileexcluding others for the moment. For example,in the classroom, a student concentrates on

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the teacher’s lecture and ignores all sorts ofnoises coming from different corners of theschool. In search an observer looks for somespecified subset of objects among a set ofobjects. For example, when you go to fetchyour younger sister and brother from theschool, you just look for them amonginnumerable boys and girls. All these activitiesrequire some kind of effort on the part ofpeople. Attention in this sense refers to “effortallocation”.

Attention has a focus as well as a fringe.When the field of awareness is centered on aparticular object or event, it is called focus orthe focal point of attention. On the contrary,when the objects or events are away from thecenter of awareness and one is only vaguelyaware of them, they are said to be at the fringeof attention.

Attention has been classified in a numberof ways. A process-oriented view divides it intotwo types, namely selective and sustained.We will briefly discuss the main features ofthese types of attention. Sometimes we canalso attend to two different things at the sametime. When this happens, it is called dividedattention. Box 5.2 describes when and howthe division of attention is possible.

Selective Attention

Selective attention is concerned mainly withthe selection of a limited number of stimuli orobjects from a large number of stimuli. We

have already indicated that our perceptualsystem has a limited capacity to receive andprocess information. This means that it candeal only with a few stimuli at a given momentof time. The question is, which of those stimuliwill get selected and processed? Psychologistshave identified a number of factors thatdetermine the selection of stimuli.

Factors Affecting Selective Attention

Several factors influence selective attention.These generally relate to the characteristicsof stimuli and the characteristics ofindividuals. They are generally classified as“external” and “internal” factors.

External factors are related to the featuresof stimuli. Other things held constant, the size,intensity, and motion of stimuli appear to beimportant determinants of attention. Large,bright, and moving stimuli easily catch ourattention. Stimuli, which are novel andmoderately complex, also easily get into ourfocus. Studies indicate that humanphotographs are more likely to be attended tothan the photographs of inanimate objects.Similarly, rhythmic auditory stimuli are morereadily attended to than verbal narrations.Sudden and intense stimuli have a wonderfulcapacity to draw attention.

Internal factors lie within the individual.These may be divided into two main categories,viz. motivational factors and cognitive factors.Motivational factors relate to our biological

In day-to-day life we attend to several things atthe same time. You must have seen people drivinga car and talking to a friend, or attending to phonecalls on a mobile set, or putting on sunglasses, orlistening to music. If we watch them closely, wewill notice that they are still allocating more effortto driving than to other activities, even thoughsome attention is given to other activities. Itindicates that on certain occasions attention canbe allocated to more than one thing at the same

BoxBoxBoxBoxBox 5.2 Divided AttentionDivided AttentionDivided AttentionDivided AttentionDivided Attention

time. However, this becomes possible only with highlypracticed activities, because they become almostautomatic and require less attention to perform thannew or slightly practiced activities.

Automatic processing has three maincharacteristics; (i) It occurs without intention, (ii) It takesplace unconsciously, and (iii) It involves very little (orno) thought processes (e.g., we can read words or tieour shoelaces without giving any thought to theseactivities).


or social needs. When we are hungry, we noticeeven a faint smell of food. A student taking anexamination is likely to focus on a teacher’sinstructions more than other students.Cognitive factors include factors like interest,attitude, and preparatory set. Objects orevents, which appear interesting, are readilyattended by individuals. Similarly we payquick attention to certain objects or events towhich we are favourably disposed. Preparatoryset generates a mental state to act in a certainway and readiness of the individual to respondto one kind of stimuli and not to others.

Theories of Selective Attention

A number of theories have been developed toexplain the process of selective attention. Wewill briefly discuss three of these theories.

Filter theory was developed by Broadbent(1956). According to this theory, many stimulisimultaneously enter our receptors creatinga kind of “bottleneck” situation. Movingthrough the short-term memory system, theyenter the selective filter, which allows only onestimulus to pass through for higher levels ofprocessing. Other stimuli are screened out atthat moment of time. Thus, we become awareof only that stimulus, which gets accessthrough the selective filter.

Filter-attenuation theory was developedby Triesman (1962) by modifying Broadbent’stheory. This theory proposes that the stimulinot getting access to the selective filter at a

given moment of time are not completelyblocked. The filter only attenuates (weakens)their strength. Thus some stimuli manage toescape through the selective filter to reachhigher levels of processing. It is indicated thatpersonally relevant stimuli (e.g., one’s namein a collective dinner) can be noticed even at avery low level of sound. Such stimuli, eventhough fairly weak, may also generateresponse occasionally by slipping through theselective filter.

Multimode theory was developed byJohnston and Heinz (1978). This theorybelieves that attention is a flexible system thatallows selection of a stimulus over others atthree stages. At stage one the sensoryrepresentations (e.g., visual images) of stimuliare constructed; at stage two the semanticrepresentations (e.g., names of objects) areconstructed; and at stage three the sensoryand semantic representations enter theconsciousness. It is also suggested that moreprocessing requires more mental effort. Whenthe messages are selected on the basis of stageone processing (early selection), less mentaleffort is required than when the selection isbased on stage three processing (lateselection).

Sustained Attention

While selective attention is mainly concernedwith the selection of stimuli, sustainedattention is concerned with concentration. It

Our attention has a limited capacity to receivestimuli. The number of objects one can attend to ata brief exposure (i.e. a fraction of a second) is called“span of attention” or “perceptual span”. Morespecifically, the span of attention refers to theamount of information an observer can grasp froma complex array of stimuli at a single momentaryexposure. This can be determined by the use of aninstrument, called “tachistoscope”. On the basis ofseveral experiments, Miller has reported that our

BoxBoxBoxBoxBox 5.3 Span of AttentionSpan of AttentionSpan of AttentionSpan of AttentionSpan of Attention

span of attention varies within the limit of seven plusor minus two. This is popularly known as the “magicnumber”. It means that at a time, people can attend toa set of five to seven numbers, which can be extendedto nine or more under exceptional conditions. That isperhaps the reason why motorbikes or cars are givena number plate that contains only four digit numberswith some alphabets. In case of violation of driving rulesa traffic police can easily read and note these numbersalong with the alphabets.

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refers to our ability to maintain attention onan object or event for longer durations. It isalso known as “vigilance”. Sometimes peoplehave to concentrate on a particular task formany hours. Air traffic controllers and radarreaders provide us with good examples of thisphenomenon. They have to constantly watchand monitor signals on screens. Theoccurrence of signals in such situations isusually unpredictable, and errors in detectingsignals may be fatal. Hence, a great deal ofvigilance is required in those situations.

Factors Influencing Sustained Attention

Several factors can facilitate or inhibit anindividual’s performance on tasks of sustainedattention. Sensory modality is one of them.Performance is found to be superior when thestimuli (called signals) are auditory than whenthey are visual. Clarity of stimuli is another

factor. Intense and long lasting stimulifacilitate sustained attention and result inbetter performance. Temporal uncertainty isa third factor. When stimuli appear at regularintervals of time they are attended better thanwhen they appear at irregular intervals.Spatial uncertainty is a fourth factor. Stimulithat appear at a fixed place are readilyattended, whereas those that appear atrandom locations are difficult to attend.

Attention has several practicalimplications. The number of objects one canreadily attend to in a single glance is used todesign the number plates of motorbikes andcars so that the traffic police can easily noticethem in the case of traffic rule violations(Box 5.3). A number of children fail to performwell in school simply due to the problem ofattention. Box 5.4 presents some interestinginformation about a disorder of attention.

This is a very common behavioural disorder foundamong children of the primary school age. It ischaracterised by impulsivity, excessive motoractivity, and an inability to attend. The disorder ismore prevalent among boys than among girls. Ifnot managed properly, the attention difficulties maypersist into adolescence or adult years. Difficultyin sustaining attention is the central feature of thisdisorder, which gets reflected in several otherdomains of the child. For example, such childrenare highly distractible; they do not followinstructions, have difficulty in getting along withparents, and are negatively viewed by their peers.They do poorly in school, and show difficulties inreading or learning basic subjects in schools inspite of the fact that there is no deficit in theirintelligence.

Studies generally do not provide evidence fora biological basis of the disorder, whereas somerelationship of the disorder with dietary factors,particularly food colouring, has been documented.On the other hand, social-psychological factors (e.g.,home environment, family pathology) have been

BoxBoxBoxBoxBox 5.4 Attention Deficit HAttention Deficit HAttention Deficit HAttention Deficit HAttention Deficit Hyperactivity Disorder (ADyperactivity Disorder (ADyperactivity Disorder (ADyperactivity Disorder (ADyperactivity Disorder (ADHD)HD)HD)HD)HD)

found to account for ADHD more reliably than otherfactors. Currently ADHD is considered to have multiplecauses and effects.

Disagreement remains over the most effectivemethod of treatment of ADHD. A drug, called Ritalin, iswidely used, which decreases children’s over-activityand distractibility, and at the same time increases theirattention and ability to concentrate. However, it doesnot “cure” the problem, and often also results in suchnegative side-effects as the suppression in normalgrowth of height and weight. On the other hand,behavioural management programmes, featuringpositive reinforcement and structuring learningmaterials and tasks in such a way that minimises errorsand maximises immediate feedback and success, havebeen found quite useful. Successful modification ofADHD is claimed with cognitive behavioural trainingprogramme in which rewards for desired behavioursare combined with training in the use of verbal self-instructions (stop, think, and then do). With thisprocedure, the ADHD children learn to shift theirattention less frequently and to behave reflectively —a learning that is relatively stable over time.


PERCEPTUAL PROCESSES

In the previous section we have examined thatthe stimulation of sensory organs leads us toexperience something such as, a flash of lightor a sound, or a smell. This elementaryexperience, called sensation, does not provideus with any understanding of the stimulusthat stimulated the sense organ. For example,it does not inform us about the source of thelight, sound or fragrance. In order to makesense out of the raw material provided by thesensory system, we process it further. In doingso, we give meaning to stimuli by using ourlearning, memory, motivation, emotions, andother psychological processes. The process bywhich we recognise, interpret or give meaningto the information provided by sense organs iscalled perception. In interpreting stimuli orevents, individuals often construct them intheir own ways. Thus perception is not merelyan interpretation of objects or events of theexternal or internal world as they exist, insteadit is also a construction of those objects andevents from one’s own point of view.

The process of meaning-making involvescertain sub-processes. These are shown inFig.5.4.

Processing Approaches in Perception

How do we identify an object? Do we identifya dog because we have first recognised its furrycoat, its four legs, its eyes, ears, and so on, ordo we recognise these different parts becausewe have first identified a dog? The idea thatrecognition process begins from the parts,which serve as the basis for the recognition of

the whole is known as bottom-up processing.The notion that recognition process beginsfrom the whole, which leads to identificationof its various components is known as top-down processing. The bottom-up approachlays emphasis on the features of stimuli inperception, and considers perception as aprocess of mental construction. The top-downapproach lays emphasis on the perceiver, andconsiders perception as a process ofrecognition or identification of stimuli. Studiesshow that in perception both the processesinteract with each other to provide us with anunderstanding of the world.

THE PERCEIVER

Human beings are not just mechanical andpassive recipients of stimuli from the externalworld. They are creative beings, and try tounderstand the external world in their ownways. In this process their motivations andexpectations, cultural knowledge, pastexperiences, and memories as well as values,beliefs, and attitudes play an important rolein giving meaning to the external world. Someof these factors are described here.

Motivation

The needs and desires of a perceiver stronglyinfluence her/his perception. People want tofulfil their needs and desires through variousmeans. One way to do this is to perceiveobjects in a picture as something that willsatisfy their need. Experiments wereconducted to examine the influence of hungeron perception. When hungry persons were

Fig.5.4 : Sub-processes of Perception

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shown ambiguous pictures, they were foundto perceive them as pictures of food objectsmore often than satiated (non-hungry)persons.

Expectations or Perceptual Sets

The expectations about what we mightperceive in a given situation also influence ourperception. This phenomenon of perceptualfamiliarisation or perceptual generalisationreflects a strong tendency to see what weexpect to see even when the results do notaccurately reflect external reality. For example,if your milkman delivers you milk daily atabout 5.30 A.M., any knocking at the dooraround that time is likely to be perceived asthe presence of the milkman even if it issomeone else.

To demonstrate expectancy tell your friend to closeher eyes. Write 12, 13, 14, 15 on the board. Askher to open her eyes for 5 seconds, look at theboard, and look down recording what she saw.Repeat replacing only the 12, 14, 15 with A, C, Dviz. ‘A 13 C D’. Ask her again to note downwhat she saw. Most people write down B in placeof 13.

ActivityActivityActivityActivityActivity 5.3the triangle. Those who can do it quickly willbe called “field independent”; those who takelong time will be called “field dependent”.

Cultural Background and Experiences

Different experiences and learningopportunities available to people in differentcultural settings also influence theirperception. People coming from a picturelessenvironment fail to recognise objects inpictures. Hudson studied the perception ofpictures by African subjects, and noted severaldifficulties. Many of them were unable toidentify objects depicted in pictures (e.g.,antelope, spear). They also failed to perceivedistance in pictures, and interpretedpictures incorrectly. Eskimos have been foundto make fine distinction among a variety ofsnow that we may be unable to notice. Someaboriginal groups of Siberian region have beenfound to differentiate among dozens of skincolours of reindeers, which we would not beable to do.

These studies indicate that the perceiverplays a key role in the process of perception.People process and interpret stimuli in theirown ways depending on their personal, socialand cultural conditions. Due to these factorsour perceptions are not only finely tuned, butalso modified.

Cognitive Styles

Cognitive style refers to a consistent way ofdealing with our environment. It significantlyaffects the way we perceive the environment.There are several cognitive styles that peopleuse in perceiving their environment. One mostextensively used in studies is the “fielddependent and field independent” cognitivestyle. Field dependent people perceive theexternal world in its totality, i.e. in a global orholistic manner. On the other hand, fieldindependent people perceive the externalworld by breaking it into smaller units, i.e. inan analytic or differentiated manner.

Look at Fig.5.5. Can you find out thetriangle hidden in the picture? How much timedo you take to locate it? Try to find out thetime other students of your class take to locate

Fig.5.5 : CEFT


PRINCIPLES OF PERCEPTUAL ORGANISATION

Our visual field is a collection of differentelements, such as points, lines, and colours.However, we perceive these elements asorganised wholes or complete objects. Forexample, we see a bicycle as a complete object,not as a collection of different parts (e.g.,saddle, wheel, handle). The process oforganising visual field into meaningful wholesis known as form perception.

You may wonder how different parts of anobjects are organised into a meaningful whole.You may also ask if there are certain factorsthat facilitate or inhibit this process oforganisation.

Several scholars have tried to answer suchquestions, but the most widely acceptedanswer has been given by a group ofresearchers, called Gestalt psychologists.Prominent among them are Köhler, Koffka,and Wertheimer. Gestalt means a regularfigure or a form. According to Gestaltpsychologists, we perceive different stimuli notas discrete elements, but as an organised“whole” that carries a definite form. Theybelieve that the form of an object lies in itswhole, which is different from the sum of theirparts. For example, a flower pot with a bunchof flowers is a whole. If the flowers are removed,the flower pot still remains a whole. It is theconfiguration of the flower pot that haschanged. Flower pot with flowers is oneconfiguration; without flowers it is anotherconfiguration.

The Gestalt psychologists also indicate thatour cerebral processes are always orientedtowards the perception of a good figure orpragnanz. That is the reason why we perceiveeverything in an organised form. The mostprimitive organisation takes place in the formof figure-ground segregation. When we lookat a surface, certain aspects of the surfaceclearly stand out as separate entities, whereasothers do not. For example, when we see wordson a page, or a painting on a wall, or birdsflying in the sky, the words, the painting, andthe birds stand out from the background, andare perceived as figures, while the page, wall,

and sky stay behind the figure and areperceived as background.

To test this experience, look at the Fig.5.6given below. You will see either the white partof the figure, which looks like a vase (flowerpot), or the black part of the figure, which lookslike two faces.

Fig.5.6 : Rubin’s Vase

We distinguish figure from the ground onthe basis of the following characteristics:1. Figure has a definite form, while the

background is relatively formless.2. Figure is more organised as compared to

its background.3. Figure has a clear contour (outline), while

the background is contourless.4. Figure stands out from the background,

while the background stays behind thefigure.

5. Figure appears more clear, limited, andrelatively nearer, while the backgroundappears relatively unclear, unlimited, andaway from us.The discussion presented above indicates

that human beings perceive the world inorganised wholes rather than in discrete parts.The Gestalt psychologists have given us severallaws to explain how and why different stimuliin our visual field are organised intomeaningful whole objects. Let us look at someof these principles.

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The Principle of Proximity

Objects that are close together in space or timeare perceived as belonging together or as agroup. For example, Fig.5.7 does not look likea square pattern of dots, but as a series ofcolumns of dots. Similarly, Fig.5.7 also lookslike a group of dots together in rows.

Fig.5.10 : Smallness

Fig.5.9 : Continuity

Fig.5.8 : Similarity

The Principle of Similarity

Objects that are similar to one another andhave similar characteristics are perceived asa group. In Fig.5.8 the little circles and squaresare evenly spaced both horizontally andvertically so that the proximity does not comeinto play. Instead, we tend to see alternatingcolumns of circles and squares.

The Principle of Continuity

This principle states that we tend to perceiveobjects as belonging together if they appearto form a continuous pattern. For instance,we are more likely to identify two lines a-band c-d crossing than to identify four linesmeeting at the center p.

The Principle of Smallness

According to this principle, smaller areas tendto be seen as figures against a largerbackground. In Fig.5.10 we are more likely tosee a black cross rather than a white crosswithin the circle because of this principle.

The Principle of Symmetry

This principle suggests that symmetrical areastend to be seen as figures againstasymmetrical backgrounds. For example, inFig.5.11 the black areas are seen as figures(as they have symmetrical properties) againsttheir white asymmetrical background.

Fig.5.11 : Symmetry

Fig.5.12 : Surroundedness

LIFT

The Principle of Surroundedness

According to this principle, the areassurrounded by others tend to be perceived asfigures. For example, the image in Fig.5.12looks like five figures against the whitebackground rather than the word ‘LIFT’.

Fig.5.7 : Proximity


Monocular Cues (Psychological Cues)

Monocular cues of depth perception areeffective when the objects are viewed with onlyone eye. These cues are often used by artiststo induce depth in two dimensional paintings.Hence, they are also known as pictorial cues.Some important monocular cues that help usin judging the distance and depth in twodimensional surfaces are described below. Youwill find some of them applied in Fig.5.14.

The Principle of Closure

We tend to fill the gaps in stimulation andperceive the objects as whole rather than theirseparate parts. For example, in Fig.5.13 thesmall angles are seen as a triangle due to ourtendency to fill the gaps in the object providedby our sensory input.

Fig.5.13 : Closure

PERCEPTION OF SPACE, DEPTH, AND DISTANCE

The visual field or surface in which thingsexist, move or can be placed is called space.The space in which we live is organised in threedimensions. We perceive not only the spatialattributes (e.g., size, shape, direction) ofvarious objects, but also the distance betweenthe objects found in this space. While theimages of objects projected on to our retinaare flat and two dimensional (left, right, up,down), we still perceive three dimensions inthe space. Why does it happen so? It occursdue to our ability to transfer a two dimensionalretinal vision into a three dimensionalperception. The process of viewing the worldin three dimensions is called distance or depthperception.

Depth perception is important in our dailylife. For example, when we drive, we use depthto assess the distance of an approachingautomobile, or when we decide to call a personwalking down the street, we determine theloudness with which to call.

In perceiving depth, we depend on twomain sources of information, called cues. Oneis called binocular cues because they requireboth eyes. Another is called monocular cues,because they allow us to perceive depth withjust one eye. A number of such cues are usedto change a two dimensional image into a threedimensional perception.

Relative Size : The size of retinal image allowsus to judge distance based on our past andpresent experience with similar objects. Asthe objects get away, the retinal imagebecomes smaller and smaller. We tend toperceive an object farther away when itappears small, and closer when it appearsbigger.

Interposition or Overlapping : These cues occurwhen some portion of the object is covered byanother object. The overlapped object isconsidered farther away, whereas the objectthat covers it appears nearer.

Linear Perspective : This reflects aphenomenon by which distant objects appearto be closer together than the nearer objects.For example, parallel lines, such as rail tracks,

Fig.5.14 : Monocular Cues

The above picture will help you to understand somemonocular cues: Interposition and relative size (see thetrees). Which other cues can you locate in the picture?

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appear to converge with increasing distancewith a vanishing point at the horizon. The morethe lines converge, the farther away theyappear.

Aerial Perspective : The air containsmicroscopic particles of dust and moisturethat make distant objects look hazy or blurry.This effect is called aerial perspective. Forexample, distant mountains appear blue dueto the scattering of blue light in theatmosphere, whereas the same mountains areperceived to be closer when the atmosphereis clear.

Light and Shade : In the light some parts ofthe object get highlighted, whereas some partsbecome darker. Highlights and shadowsprovide us with information about an object’sdistance.

Relative Height : Larger objects are perceivedas being closer to the viewer and smallerobjects as being farther away. When we expecttwo objects to be the same size and they arenot, the larger of the two will appear closerand the smaller will appear farther away.

Texture Gradient : It represents a phenomenonby which the visual field having more densityof elements is seen farther away. In theFig.5.15 the density of stones increases as welook farther away.

that are close. The rate of an object’smovement provides a cue to its distance. Forexample, when we travel in a bus, closerobjects move “against” the direction of the bus,whereas the farther objects move “with” thedirection of the bus.

Binocular Cues (Physiological Cues)

Some important cues to depth perception inthree dimensional space are provided by boththe eyes. Three of them have particularly beenfound to be interesting.

Retinal or Binocular Disparity : Retinaldisparity occurs because the two eyes havedifferent locations in our head. They areseparated from each other horizontally by adistance of about 6.5 centimeters. Because ofthis distance, the image formed on the retinaof each eye of the same object is slightlydifferent. This difference between the twoimages is called retinal disparity. The braininterprets a large retinal disparity to mean aclose object and a small retinal disparity tomean a distant object, as the disparity is lessfor distant objects and more for the nearobjects.

Convergence : When we see a nearby objectour eyes converge inward in order to bring theimage on the fovea of each eye. A group ofmuscles send messages to the brain regardingthe degree to which eyes are turning inward,and these messages are interpreted as cuesto the perception of depth. The degree ofconvergence decreases as the object movesfurther away from the observer. You canexperience convergence by holding a finger infront of your nose and slowly bringing it closer.The more your eyes turn inward or converge,the nearer the object appears in space.

Accommodation : Accommodation refers to aprocess by which we focus the image on theretina with the help of ciliary muscle. Thesemuscles change the thickness of the lens ofthe eye. If the object gets away (more than 2meters), the muscle is relaxed. As the objectmoves nearer, the muscle contracts and thethickness of the lens increases. The signal

Fig.5.15 : Texture Gradient

Motion Parallax : It is a kinetic monocular cue,and hence not considered as a pictorial cue.It occurs when objects at different distancesmove at a different relative speed. The distantobjects appear to move slowly than the objects


about the degree of contraction of the muscleis sent to the brain, which provides the cuefor distance.

the image that it casts on the retina is a circle,or an ellipse, or roughly a short line (if theplate is viewed from the edge). It is also calledform constancy.

Brightness Constancy

Visual objects not only appear constant intheir shape and size, they also appear constantin their degree of whiteness, greyness, orblackness even though the amount of physicalenergy reflected from them changesconsiderably. In other words, our experienceof brightness does not change in spite of thechanges in the amount of reflected lightreaching our eyes. The tendency to maintainapparent brightness constant under differentamount of illumination is called brightnessconstancy. For example, surface of a paperwhich appears white in the sun light, is stillperceived as white in the room light. Similarly,coal that looks black in the sun also looksblack in room light.

ILLUSIONS

Our perceptions are not always veridical.Sometime we fail to interpret the sensoryinformation correctly. This results in amismatch between the physical stimuli andits perception. These misperceptions resultingfrom misinterpretation of information receivedby our sensory organs are generally known asillusions. These are experienced more or lessby all of us. They result from an externalstimulus situation and generate the same kindof experience in each individual. That is whyillusions are also called “primitiveorganisations”. Although illusions can beexperienced by the stimulation of any of oursenses, psychologists have studied them morecommonly in the visual than in other sensemodalities.

Some perceptual illusions are universaland found in all individuals. For example, therail tracks appear to be converging to all ofus. These illusions are called universalillusions or permanent illusions as they donot change with experience or practice. Someother illusions seem to vary from individual

Hold a pencil in front of you. Close your right eyeand focus on the pencil. Now open the right eyeand close the left eye. Keep doing it simultaneouslywith both the eyes. The pencil will appear to movefrom side to side in front of your face.


PERCEPTUAL CONSTANCIES

The sensory information that we receive fromour environment constantly changes as wemove around. Yet we form a stable perceptionof an object seen from any position and inany intensity of light. Perception of the objectsas relatively stable in spite of changes in thestimulation of sensory receptors is calledperceptual constancy. Here we will examinethree types of perceptual constancies that wecommonly experience in our visual domain.

Size Constancy

The size of an image on our retina changeswith the change in the distance of the objectfrom the eye. The further away it is, the smalleris the image. On the other hand, ourexperience shows that within limits the objectappears to be about the same size irrespectiveof its distance. For example, when youapproach your friend from a distance, yourperception of the friend’s size does not changemuch despite the fact that the retinal image(image on retina) becomes larger. Thistendency for the perceived size of objects toremain relatively unchanged with changes intheir distance from the observer and the sizeof the retinal image is called size constancy.

Shape Constancy

In our perceptions the shapes of familiarobjects remain unchanged despite changes inthe pattern of retinal image resulting fromdifferences in their orientation. For example,a dinner plate looks the same shape whether

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to individual; these are called personalillusions. In this section, we will describe someimportant visual illusions.

Geometrical Illusions

In Fig.5.16 the Muller-Lyer illusion has beenshown. All of us perceive line A as shorter thanline B, although both the lines are equal. Thisillusion is experienced even by children. Thereare some studies that suggest that even

animals experience this illusion more or lesslike us. Besides Muller-Lyer illusion, severalother visual illusions are experienced byhuman beings (also birds and animals). InFig.5.17 you can see the illusion of verticaland horizontal lines. Although both the linesare equal, we perceive the vertical line as longerthan the horizontal line.

Fig.5.17 : Vertical-Horizontal Illusion

to be important. In the absence of these, thelight points will not appear as moving. Theywill appear either as one point, or as differentpoints appearing one after another, withoutany experience of motion.

Experience of illusions indicates thatpeople do not always perceive the world as itis; instead they engage in its construction,sometimes based on the features of stimuliand sometimes based on their experiences ina given environment. This point will be furthercleared in the section that follows now.

SOCIO-CULTURAL INFLUENCES ON

PERCEPTION

Several psychologists have studied theprocesses of perception in different socio-cultural settings. The questions they try toanswer through these studies are: Doesperceptual organisation of people living indifferent cultural settings take place in anuniform manner? Are the perceptual processesuniversal, or they vary across different culturalsettings? Because we know, people living indifferent parts of the world look different, manypsychologists hold the view that their ways ofperceiving the world must be different in somerespects. Let us examine some studies relatingto perception of illusion figures and otherpictorial materials.

You are already familiar with Muller-Lyerand Vertical-Horizontal illusion figures.Psychologists have used these figures withseveral groups of people living in Europe,Africa, and many other places. Segall,Campbell, and Herskovits carried out the mostextensive study of illusion susceptibility bycomparing samples from remote Africanvillages and Western urban settings. It wasfound that African subjects showed greatersusceptibility to horizontal-vertical illusion,whereas Western subjects showed greatersusceptibility to Muller-Lyer illusion. Similarfindings have been reported in other studiesalso. Living in dense forests the Africansubjects regularly experienced verticality (e.g.,long trees) and developed a tendency tooverestimate it. The Westerners, who lived in

Fig.5.16 : Muller-Lyer Illusion

A B

Apparent Movement Illusion

This illusion is experienced when somemotionless pictures are projected one afteranother at an appropriate rate. This illusionis referred to as “phi-phenomenon”. When wesee moving pictures in a cinema show, we areinfluenced by this kind of illusion. Thesuccession of flickering electrical lights alsogenerate this illusion. This phenomenon canbe experimentally studied with the help of aninstrument by presenting two or more lightsin a succession. For the experience of thisillusion, Wertheimer had reported the presenceof appropriate level of brightness, size, spatialgap, and temporal contiguity of different lights


Key TermsKey Terms

Absolute threshold, After images, Binocularcues, Bottom-up processing, Cochlea, Cones,Dark adaptation, Depth perception, Differencethreshold, Divided attention, Eustachian tube,Figure-ground segregation, Filter theory,Filter-attenuation theory, Gestalt, Lightadaptation, Loudness, Monocular cues, Organof corti, Perceptual constancies, Phi-phenomenon, Pitch, Primary colours, Retina,Rhodopsin, Rods, Selective attention,Sustained attention, T imbre, Top-downprocessing, Visual illusions, Wavelength

an environment characterised by right angles,developed a tendency to underestimate thelength of lines characterised by enclosure (e.g.,arrowhead). This conclusion has beenconfirmed in several studies. It suggests thatthe habits of perception are learnt differentlyin different cultural settings.

In some studies people living in differentcultural settings have been given pictures foridentification of objects and interpretation ofdepth or other events represented in them.Hudson did a seminal study in Africa, andfound that people, who had never seenpictures, had great difficulty in recognisingobjects depicted in them and in interpretingdepth cues (e.g., superimposition). It wasindicated that informal instruction in homeand habitual exposure to pictures werenecessary to sustain the skill of pictorial depthperception. Sinha and Mishra have carried outseveral studies on pictorial perception usinga variety of pictures with people from diversecultural settings, such as hunters andgatherers living in forests, agriculturists living

in villages, and people employed and living incities. Their studies indicate thatinterpretation of pictures is strongly relatedto cultural experiences of people. While peoplein general can recognise familiar objects inpictures, those less exposed to pictures havedifficulty in the interpretation of actions orevents depicted in them.

Summary• Knowledge of our internal and external world becomes possible with the help of senses. Five

of them are external senses, and two are internal senses. The sense organs receive variousstimuli and send them in the form of neural impulses to specialised areas of brain forinterpretation.

• Vision and audition are the two most widely used senses. Rods and cones are the receptorsfor vision. Rods function in low intensities of light, whereas cones function at high intensitiesof light. They are responsible for achromatic and chromatic vision, respectively.

• Light and dark adaptations are two interesting phenomena of the visual system. Hue, saturationand brightness are the basic dimensions of colour.

• Sound serves as stimulus for auditory sensations. Loudness, pitch, and timbre are the propertiesof sound. Organ of corti located in the basilar membrane is the chief organ of hearing.

• Attention is a process through which we select certain information by filtering out many othersthat appear to be irrelevant at a given moment of time. Activation, concentration, and searchare important properties of attention.

• Selective and sustained attention are two major types of attention. Divided attention is evidentin the case of highly practiced tasks in which there is much automaticity of informationprocessing.

• The span of attention is the magical number of seven plus and minus two.• Perception refers to the processes of interpretation and informed construction of the information

received from sensory organs. Human beings perceive their world in terms of their motivations,expectations, cognitive styles, and cultural background.

• Form perception refers to the perception of a visual field set off from rest of the field by visiblecontours. The most primitive form of organisation takes place in the form of figure-groundsegregation.

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1. Explain the functional limitations of sense organs.

2. What is meant by light and dark adaptation? How do they take place?

3. What is colour vision and what are the dimensions of colour?

4. How does auditory sensation take place?

5. Define attention. Explain its properties.

6. State the determinants of selective attention. How does selective attention differ fromsustained attention?

7. What is the main proposition of Gestalt psychologists with respect to perception of thevisual field?

8. How does perception of space take place?

9. What are the monocular cues of depth perception? Explain the role of binocular cues inthe perception of depth?

10. Why do illusions occur?

11. How do socio-cultural factors influence our perceptions?

Review QuestionsReview QuestionsReview QuestionsReview QuestionsReview Questions

• Gestalt psychologists have identified several principles that determine our perceptualorganisations.

• The image of an object projected on to the retina is two dimensional. Three dimensionalperception is a psychological process that depends on correct utilisation of certain monocularand binocular cues.

• Perceptual constancies refer to invariance of our perceptions of an object seen from anyposition and in any intensity of light. There is good evidence for size, shape, and brightnessconstancies.

• Illusions are the examples of nonveridical perceptions. They refer to misperceptions resultingfrom misinterpretation of information received by our sensory organs. Some illusions areuniversal, while others are more personal and culture-specific.

• Socio-cultural factors play an important role in our perceptions by generating differentialfamiliarity with and salience of stimuli as well as certain habits of perceptual inferenceamong people.

1. Collect ten advertisements from magazines. Analyse the content and message beingconveyed in each advertisement. Comment on the use of various attentional and perceptualfactors to promote the given product.

2. Give a toy model of a horse/elephant to visually challenged and sighted children. Let thevisually challenged children feel by touching the toy model for some time. Ask the childrento describe the model. Show the same toy model to sighted children. Compare theirdescriptions and find out their similarities and differences.

Take another toy model (e.g., a parrot) and give it to a few visually challenged childrento have a feel of it by touching. Then give a sheet of paper and a pencil and ask them todraw the parrot on the sheet. Show the same parrot to sighted children for some time,remove the parrot from their sight, and ask to draw the parrot on a sheet of paper.

Compare the drawings of the visually challenged and sighted children and examinetheir similarities and differences.

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sensory, attentional and perceptual processes...

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