Sensory, Attentional, and Perceptual Processes PDF

Sensory,, AAttentional Sensory ttentional and Perceptual Processes Chapter 5...

Sensory,, AAttentional Sensory ttentional and Perceptual Processes Chapter 5 d After reading this chapter, you would be able to e understand the nature of sensory processes, explain the processes and types of attention, h analyse the problems of form and space perception, T s examine the role of socio-cultural factors in perception, and i reflect on sensory, attentional and perceptual processes in everyday life. R b l E Contents C u Introduction p Knowing the World N re Nature and Varieties of Stimulus Sense Modalities Visual Sensation © e Other Human Senses (Box 5.1) Auditory Sensation Attentional Processes b Selective Attention Divided Attention (Box 5.2) Sustained Attention o Span of Attention (Box 5.3) t Attention Deficit Hyperactivity Disorder (Box 5.4) Perceptual Processes t Processing Approaches in Perception The Perceiver o Principles of Perceptual Organisation Perception of Space, Depth, and Distance n Monocular Cues and Binocular Cues Perceptual Constancies Illusions Socio-Cultural Influences on Perception Key Terms The quality of life is determined Summary by its activities. Review Questions Project Ideas – Aristotle 84 Psychology  Introduction In the previous chapters you have already learnt how we respond to various stimuli present 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 or mechanical devices. This chapter will introduce you to various receptors that collect a variety of information from the external and internal worlds. The focus will be particularly on the structure and function of eye and ear, including some interesting d processes associated with vision and audition. You will also know some important things about attention, which helps us to notice and register the information that e our sense organs carry to us. Different types of attention will be described along with the factors that influence them. At the end, we will discuss the process of h perception that allows us to understand the world in a meaningful way. You will T s also have an opportunity to know how we are sometimes deceived by certain types i of stimuli such as figures and pictures. R b l KNOWING THE WORLD C E u not only from the external world, but also from p our own body. The information collected by our sense organs forms the basis of all our N re The world in which we live is full of variety of objects, people, and events. Look at the room knowledge. The sense organs register several you are sitting in. You will find so many things kinds of information about various objects. However, in order to be registered, the objects © e around. Just to mention a few, you may see your table, your chair, your books, your bag, and their qualities (e.g., size, shape, colour) your watch, pictures on the wall and many must be able to draw our attention. The b other things. Their sizes, shapes, and colours registered information must also be sent to are also different. If you move to other rooms the brain that constructs some meaning out of your house, you will notice several other of them. Thus, our knowledge of the world o new things (e.g., pots and pans, almirah, TV). around us depends on three basic processes, t If you go beyond your house, you will find still called sensation, attention, and perception. many more things that you generally know These processes are highly interrelated; hence, t about (trees, animals, buildings). Such they are often considered as different elements experiences are very common in our day-to- of the same process, called cognition. o day life. We hardly have to make any efforts n to know them. N ATURE AND VARIETIES OF STIMULUS If someone asks you, “How can you say that these various things exist in your room, The external environment that surrounds us or house, or in the outside environment?”, you contains a wide variety of stimuli. Some of will most probably answer that you see or them can be seen (e.g., a house), while some experience them all around you. In doing so, can be heard only (e.g., music). There are you are trying to tell the person that the several others that we can smell (e.g., fragrance knowledge about various objects becomes of a flower) or taste (e.g., sweets). There are possible with the help of our sense organs (e.g., still others that we can experience by touching eyes, ears). These organs collect information (e.g., softness of a cloth). All these stimuli 85 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses provide us with various kinds of information. Functional Limitations of Sense Organs We have very specialised sense organs to deal Before we move on to a discussion of sense with these different stimuli. As human beings organs, it is important to note that our sense we are bestowed with a set of seven sense organs function with certain limitations. For organs. These sense organs are also known as sensory receptors or information gathering example, our eyes cannot see things which systems, because they receive or gather are very dim or very bright. Similarly our ears cannot hear very faint or very loud sounds. information from a variety of sources. Five of these sense organs collect information from The same is true for other sense organs also. the external world. These are eyes, ears, nose, As human beings, we function within a limited tongue, and skin. While our eyes are primarily range of stimulation. For being noticed by a sensory receptor, a stimulus has to be of an d responsible for vision, ears for hearing, nose for smell, and tongue for taste, skin is optimal intensity or magnitude. The e responsible for the experiences of touch, relationship between stimuli and the warmth, cold, and pain. Specialised receptors sensations they evoke has been studied in a h of warmth, cold, and pain are found inside discipline, called psychophysics. T s our skin. Besides these five external sense In order to be noticed a stimulus has to i organs, we have also got two deep senses. They carry a minimum value or weight. The l minimum value of a stimulus required to R are called kinesthetic and vestibular systems. They provide us with important information activate a given sensory system is called b absolute threshold or absolute limen (AL). E about our body position and movement of body parts related to each other. With these For example, if you add a granule of sugar to u seven sense organs, we register ten different a glass of water, you may not experience any C variety of stimuli. For example, you may notice sweetness in that water. Addition of a second p whether a light is bright or dim, whether it is granule to water may also not make it taste N re yellow, red or green, and so on. With sound sweet. But if you go on adding sugar granules you may notice whether it is loud or faint, one after another, there will come a point when whether it is melodious or distracting, and so you will say that the water is now sweet. The © e on. These different qualities of stimuli are also minimum number of sugar granules required registered by our sense organs. to say that the water is sweet will be the AL of sweetness. b It may be noted at this point that the AL is S ENSE MODALITIES not a fixed point; instead it varies considerably o Our sense organs provide us with first-hand across individuals and situations depending t information about our external or internal on the people’s organic conditions and their world. The initial experience of a stimulus or motivational states. Hence, we have to assess t an object registered by a particular sense it on the basis of a number of trials. The organ is called sensation. It is a process number of sugar granules that may produce o through which we detect and encode a variety the experience of “sweetness” in water on of physical stimuli. Sensation also refers to 50 per cent of occasions will be called the AL n immediate basic experiences of stimulus of sweetness. If you add more number of sugar attributes, such as “hard”, “warm”, “loud”, and granules, the chances are greater that the “blue”, which result from appropriate water will be reported more often as sweet than stimulation of a sensory organ. Different sense plain. organs deal with different forms of stimuli and As it is not possible for us to notice all serve different purposes. Each sense organ is stimuli, it is also not possible to differentiate highly specialised for dealing with a particular between all stimuli. In order to notice two kind of information. Hence, each one of them stimuli as different from each other, there has is known as a sense modality. to be some minimum difference between the 86 Psychology value of those stimuli. The smallest difference with the external world. Audition and other in the value of two stimuli that is necessary to senses also contribute significantly to notice them as different is called difference information gathering from the external world. threshold or difference limen (DL). To We shall discuss vision and audition in some understand it, we may continue with our detail. The main features of other senses can “sugar water” experiment. As we have seen, be found in Box 5.1. the plain water is experienced as sweet after Visual sensation starts when light enters the addition of certain number of sugar the eyes and stimulates our visual receptors. granules. Let us remember this sweetness. The Our eyes are sensitive to a spectrum of light, next question is: how many sugar granules the wavelength of which ranges from 380 nm will be needed in the water in order to to 780 nm (nm refers to nanometer, which is d experience its sweetness as different from the one billionth of a meter). No sensation is previous sweetness. Go on adding sugar registered beyond this range of light. e granules one after another tasting the water h each time. After addition of a few granules, The Human Eye you will notice at a point that the water is now s A diagram of the human eye is shown in T sweeter than the previous one. The number i Figure 5.1. As you can see, our eye is made of sugar granules added to the water to l up of three layers. In the outer layer, there is R generate an experience of sweetness that is a transparent cornea and a tough sclera that b different from the previous sweetness on 50 surrounds the rest of the eye. It protects the E per cent of the occasions will be called the DL eye and maintains its shape. The middle layer u of sweetness. Thus, difference threshold is the is called choroid, which is richly supplied with C minimum amount of change in a physical blood vessels. The inner layer is known as p stimulus that is capable of producing a retina. It contains the photoreceptors (rods N re sensation difference on 50 per cent of the and cones) and an elaborate network of trials. interconnecting neurons. You may realise by now that understanding The eye is generally compared with a © e of sensations is not possible without camera. For example, the eye and camera have understanding the AL and DL of different types a lens. The lens divides the eye into two of stimuli (for example, visual, auditory), but unequal chambers, namely aqueous chamber b that is not enough. Sensory processes do not and vitreous chamber. The aqueous chamber depend only on the stimulus characteristics. is located between the cornea and the lens. It Sense organs and the neural pathways o is smaller in size and is filled with a water- connecting them to various brain centers also t like substance, called aqueous humor. The play a vital role in this process. A sense organ vitreous chamber is located between the lens receives the stimulus and encodes it as an t and the retina. It is filled with a jelly like electrical impulse. For being noticed this protein, called vitreous humor. These fluids o electrical impulse must reach the higher brain help in holding the lens at its appropriate place centers. Any structural or functional defect or and in proper shape. They also allow enough n damage in the receptor organ, its neural flexibility for the occurrence of accommodation pathway, or the concerned brain area may lead — a process through which the lens changes to a partial or complete loss of sensation. its shape in order to focus the objects at varying distances. This process is regulated Visual Sensation by ciliary muscles, which are attached to the Among all sense modalities, vision is the most lens. These muscles flatten the lens to focus highly developed in human beings. Various the distant objects and thicken it to focus the estimates indicate that we use it in near objects. Like a camera, the eye also has approximately 80 per cent of our transactions a mechanism to control the amount of light 87 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses  Box 5.1 Other Human Senses Besides vision and audition, there are other temperature, its pressure on our tongue, and senses that enrich our perceptions. For example, many other sensations. When these factors are an orange looks attractive not only because of it removed, we are left with only four basic tastes. is colour but also because it has got a special Besides, the combination of different flavours in flavour and taste. These other senses are briefly varied proportions r esults in a different kind of described here. flavour which may be quite unique. 1. Smell : The stimulus for smell sensation 3. Touch and other skin senses : Skin is a sensory consists of molecules of various substances or gan that pr oduces sensations of touch d contained in the air. They enter the nasal (pressure), warmth, cold, and pain. In our skin e passage where they dissolve in moist nasal there ar e specialised receptors for each one of tissues. This brings them in contact with these sensations. The receptors of touch are not receptor cells contained in olfactory evenly distributed in our skin. That is why some h epithelium. Human beings possess about 50 areas of our body (e.g., face, fingertips) are more s million of these receptors, whereas dogs sensitive than others (e.g., legs). Pain sensation T i possess mor e than 200 million of these has no specific stimulus. Hence, determining its receptors. Nevertheless, our ability to detect mechanism has been fairly difficult. R l smell is impressive. It is indicated that human 4. The Kinesthetic system : Its receptors are found beings can recognise and distinguish among primarily in joints, ligaments, and muscles. This E b approximately 10,000 different odours. The system gives us information about the location sense of smell also shows sensory of our body parts in relation to each other, and u adaptation like other senses. allows us to perform simple (e.g., touching one’s C 2. Taste : The sensory receptors for taste are nose) and complex movements (e.g., dancing). Our p located inside small bumps on the tongue, visual system provides a great deal of help in N re known as papillae. In each papilla there is a this respect. cluster of taste buds. Human beings possess 5. The Vestibular system : This system gives us almost 10,000 taste buds. While people claim information about our body position, movement, to distinguish a large number of flavours in and acceleration — the factors that are critical © e food, there are only four basic tastes, namely for maintaining our sense of balance. The sensory sweet, sour, bitter and salty. How is it then organs of this sense are located in the inner ear. that we perceive many more? The answer is While vestibular sacs inform us of our body b that we are aware not only of the taste of positions, the semicircular canals inform us about the food, but also of its smell, its texture, its our movements and acceleration. o entering into it. The iris serves this purpose. (colour) vision. Each eye contains about 100 t It is a disc-like coloured membrane lying million rods and about 7 million cones. The between the cornea and the lens. It controls cones are highly concentrated in the central t the amount of light entering the eye by region of the retina surrounding the fovea, o regulating pupil dilation. In dim light the pupil which is a small circular region of the size of a dilates; in bright light it contracts. pea. It is also known as the yellow spot. It is n Retina is the inner most layer of an eye. It the region of maximum visual acuity. Besides is made up of five types of photosensitive cells photoreceptors, retina also contains a bundle among which rods and cones are most of axons of a cell (called ganglion cell) that important. Rods are the receptors for scotopic forms the optic nerve, which leads to the vision (night vision). They operate at low brain. intensities of light, and lead to achromatic (colourless) vision. Cones are the receptors for Working of the Eye photopic (day light) vision. They operate at high Passing through conjunctiva, cornea, and levels of illumination, and lead to chromatic pupil, the light enters the lens, which focuses 88 Psychology  Vitreous humor Fovea Cornea Proximal Iris stimulus Aqueous humor d Optic nerve h e T i s Pupil R l Distal stimulus b Lens E u Blind spot Ciliary muscle Retina C N re p Fig.5.1 : Structure of the Human Eye it on to the retina. Retina is divided into two Adaptation © e parts: the nasal half and the temporal half. The human eye can function at a very large The inner half portion of the eye (towards the range of light intensities. Sometimes we have nose), taking the center of fovea as mid-point, b to undergo a rapid change in illumination is called the nasal half. The outer half portion levels. For example, when we go to a matinee of the eye (towards the temple) from the center show movie, we find it difficult to see things o of fovea is called the temporal half. Light from in the hall on entering into it. However, after t the right visual field stimulates the left half of spending about 15 to 20 minutes there, we each eye (i.e. the nasal half of the right eye are able to see everything. After the show when t and the temporal half of the left eye), and light we go out into the open, we find the light from the left visual field stimulates the right outside the hall too bright to see things, or o half of each eye (i.e. the nasal half of the left sometimes even to keep our eyes open. eye and the temporal half of the right eye). An However, within a minute or so we feel n inverted image of the object is formed on the comfortable, and are able to see things retina. The neural impulse is transmitted to properly. This adjustment is faster than the the visual cortex through the optic nerve where one made on entering the hall. The process of the image is re-inverted and processed. You getting adjusted to different intensities of light can see in Fig.5.1 that the optic nerve leaves is called ‘visual adaptation’. the retina from the area that has no Light adaptation refers to the process of photoreceptors. In this area visual sensitivity adjusting to bright light after exposure to dim is completely absent. Therefore, it is called the light. This process takes nearly a minute or blind spot. two. On the other hand, dark adaptation 89 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses refers to the process of adjusting to a dimly Colour Vision illuminated environment after exposure to In our interaction with the environment we bright light. This may take half an hour or not only experience a variety of objects, but even longer depending on the previous level also their colours. It may be noted that colour of exposure of the eye to light. There are certain is a psychological property of our sensory ways in which these processes can be experience. It is created when our brain facilitated. An interesting activity is given interprets the information received from the below to demonstrate this process to you. external world. It may be remembered that light is described physically in terms of Activity 5.1 wavelength, not in terms of colour. As we have read earlier, the visible spectrum is a range d Move from a lighted area to a dark room and note of energy (380-780 nm) that our how much time you take to see things clearly in e that room. photoreceptors can detect. The energy lower Next time put on red goggles while you stay or higher than the visible spectrum is harmful h in the lighted place. Then move into the dark room to the eyes. The sun light is a perfect mixture and note how much time you take to see things of seven colours just like a rainbow. The T s clearly in that room. colours observed are violet, indigo, blue, i You will notice that the use of red goggles has l green, yellow, orange, and red, abbreviated R gr eatly reduced the time r equired for dark adaptation. as ‘VIBGYOR’. b Do you know why has this happened? E Discuss with your friends and the teacher. The Dimensions of Colour u A person with normal colour vision can C Photochemical Basis of Light and Dark distinguish more than seven million different p Adaptation : You may wonder why the light shades of colour. Our experiences of colour N re and dark adaptations take place. According can be described in terms of three basic to the classical view, light and dark dimensions, called hue, saturation, and adaptations occur due to certain brightness. Hue is a property of chromatic © e photochemical processes. The rods have a colours. In simple words, it refers to the name photo-sensitive chemical substance, called of the colour, e.g., red, blue, and green. Hue varies with wavelength, and each colour is b rhodopsin or visual purple. By the action of light the molecules of this chemical substance identified with a specific wavelength. For get bleached or broken down. Under such example, blue has a wavelength of about 465 o conditions the light adaptation takes place in nm, and green of about 500 nm. Achromatic t the eyes. On the other hand, the dark colours like black, white or grey are not adaptation is achieved by the removal of light, characterised by hues. Saturation is a t and thereby allowing for restorative processes psychological attribute that refers to the to regenerate the pigment in the rods with the relative amount of hue of a surface or object. o help of vitamin A. The regeneration of The light of single wavelength (monochromatic) rhodopsin in rods is a time consuming appears to be highly saturated. As we mix n process. That is why dark adaptation is a different wavelengths, the saturation slower process than light adaptation. It has decreases. The colour grey is completely been found that people who suffer from unsaturated. Brightness is the perceived vitamin A deficiency do not achieve dark intensity of light. It varies across both adaptation at all, and find it really difficult to chromatic and achromatic colours. White and move in the dark. This condition is generally black represent the top and bottom of the known as night blindness. A parallel chemical brightness dimension. White has the highest believed to be found in cones is known as degree of brightness, whereas black has the iodopsin. lowest degree. 90 Psychology Colour Mixtures The Human Ear There is an interesting relationship among Ear is the primary receptor of auditory stimuli. colours. They form complementary pairs. While its well-known function is hearing, it When mixed in correct proportions the also helps us in maintaining our body balance. complementary colours yield an achromatic The structure of an ear is divided into three grey or white. Examples of complementary segments, called the external ear, the middle colours are red-green and yellow-blue. Red, ear, and the inner ear (Fig.5.2). green and blue are called primary colours, External Ear : It contains two main structures, because on mixing, the light of these three namely pinna and auditory meatus. Pinna is colours can produce almost any colour. The a cartilaginous funnel-shaped structure that most common example is the television screen. d collects sound waves from the surroundings. It contains spots of blue, red and green Auditory meatus is a canal protected by hair e colours. The combinations of these three and wax that carries sound waves from pinna produce different colours and shades that we to the tympanum or ear drum. h see on the TV screen. Middle Ear : The middle ear starts with T i s After Images tympanum, a thin membrane highly sensitive l to sound vibrations. This is followed by the R This is quite an interesting phenomenon tympanic cavity. It is connected to the related to visual sensations. The effect of a b pharynx with the help of Eustachian tube, E visual stimulus persists for some time even which maintains the air pressure in tympanic u after the removal of that stimulus from the cavity. From the cavity the vibrations pass to C visual field. This effect is called after image. three ossicles known as malleus (hammer), p After images are positive and negative. incus (anvil), and stapes (stirrup). They N re Positive after images resemble the original increase the intensity of sound vibrations stimulus in terms of hue, saturation, and about 10 times, and send them to the inner brightness. They usually occur after a brief ear. © e intense stimulation of dark adapted eyes. On the other hand, negative after images appear Inner Ear : The inner ear has a complicated in complementary colours. These images structure known as membranous labyrinth, b appear when a person stares at the patch of a which is encapsulated in a bony shell called particular colour for at least 30 seconds, and bony labyrinth. A lymph-like fluid is found in then transfers the gaze to a neutral the space between bony labyrinth and o background (e.g., a white or grey surface). If membranous labyrinth. This is called t the person looks at the blue colour, the perilymph. negative after image will appear in yellow. The bony labyrinth has three semi- t Similarly, a red stimulus will yield a negative circular canals at right angle to each other, a cavity, called vestibule, and a coiled structure, o after image of green colour. called cochlea. The semicircular canals have fine hair cells, which are highly sensitive to n Auditory Sensation postural changes as well as changes in the Audition or hearing is also an important sense body orientation. Inside the bony cochlea, modality that carries great value for us. It there is a membranous cochlea, which is also provides us with reliable spatial information. known as scala media. It is filled with Besides orienting us to certain objects or endolymph, and has a spirally coiled individuals, it plays a vital role in spoken membrane, called basilar membrane. It has communication also. Auditory sensation got fine hair cells arranged in a series to form begins when sound enters our ear and the organ of corti. This is the main organ for stimulates the chief organs of hearing. hearing. 91 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses  Hammer Saccule Semicircular Utricle canals Anvil Facial nerve Auditory nerve d External canal h e T i s Pinna Cochlea R b l E Eustachian tube C p u Tympanic m embrane Stirrup Round window N re Fig.5.2 : Structure of the Human Ear Working of the Ear © e disturbs the surrounding medium (i.e. air), and pushes the air molecules back and forth. b Pinna collects the sound vibrations and serves This results in changes in pressure that spread them to the tympanum through the auditory outward in the form of sound waves, travelling meatus. From the tympanic cavity the o at a rate of about 1,100 ft/sec. These changes vibrations are transferred to the three ossicles, t travel in waves much like the ripples set up which increase their strength and transmit by a stone thrown into a pond. When these them to the inner ear. In the inner ear the t sound waves strike our ears, they initiate a cochlea receives the sound waves. Through set of mechanical pressure changes that o vibrations the endolymph is set in motion, ultimately trigger the auditory receptors. which also vibrates the organ of corti. Finally, The simplest kind of sound wave is one n the impulses are sent to the auditory nerve, that causes successive pressure changes over which emerges at the base of cochlea and time in the form of a single repeating sine wave reaches the auditory cortex where the impulse (Fig.5.3). Sound waves vary in amplitude as is interpreted. well as in wavelength. Amplitude is a general measure of stimulus magnitude. It is the Sound as a Stimulus amount of change in pressure, i.e. the extent We all know that sound is the stimulus for of displacement of the molecules from the ears. It results from pressure variations in the position of rest. In Fig.5.3 the amplitude of external environment. Any physical movement sound wave is represented as the distance of 92 Psychology the crest or trough from its mean position. sound reflects the complexity of its sound Wavelength is the distance between the two waves. Most of the sounds found in natural crests. Sound waves are basically formed due environments are complex. to alternate compression and decompression (rarefaction) of air molecules. A complete Activity 5.2 change in pressure from compression to rarefaction and again to compression makes Vision and hearing are generally believed to be a cycle of the wave. the two most highly prized senses. What would your life be if you lost any one of your senses? Which sense would you find more traumatic to lose? Why? Think and write down. What if you could magically improve the d performance of one of your senses, which sense would you choose to improve? Why? Could you e Amplitude improve the performance of this one sense without magic? Think and write down. h Discuss with your teacher. RT one cycle l i ATTENTIONAL PROCESSES s E b In the previous section we have discussed Fig.5.3 : Sound Waves some sensory modalities that help us in u collecting information from the external world C Sound waves are described in terms of and also from our internal system. A large p their frequency, which is measured in terms number of stimuli impinge upon our sense N re of cycles per second. Its unit is called Hertz organs simultaneously, but we do not notice (Hz). Frequency and wavelength have an all of them at the same time. Only a selected inverse relationship. When the wavelength few of them are noticed. For example, when © e increases, the frequency decreases, and when you enter your classroom you encounter the wavelength decreases, the frequency several things in it, such as doors, walls, increases. Amplitude and frequency both are windows, paintings on walls, tables, chairs, b physical dimensions. Besides these, there are students, schoolbags, water bottles, and so three psychological dimensions of sound, on, but you selectively focus only on one or o namely loudness, pitch and timbre. two of them at one time. The process through t Loudness of the sound is determined by which certain stimuli are selected from a group its amplitude. Sound waves with large of others is generally referred to as attention. t amplitude are perceived as loud; those with At this point it may be noted that besides small amplitude are perceived as soft. selection, attention also refers to several other o Loudness is measured in decibels (db). Pitch properties like alertness, concentration, and refers to highness or lowness of a sound. The search. Alertness refers to an individual’s n seven notes used in Indian classical music readiness to deal with stimuli that appear represent a gradual increase in their pitch. before her/him. While participating in a race Frequency determines the pitch of a sound in your school, you might have seen the wave. The higher the frequency, the higher will participants on the starting line in an alert be the pitch. The range of hearing is generally state waiting for the whistle to blow in order 20 Hz-20,000 Hz. Timbre refers to the nature to run. Concentration refers to focusing of or quality of a sound. For example, the sound awareness on certain specific objects while of a car engine and a person talking differ with excluding others for the moment. For example, respect to quality or timbre. The timbre of a in the classroom, a student concentrates on 93 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses the teacher’s lecture and ignores all sorts of have already indicated that our perceptual noises coming from different corners of the system has a limited capacity to receive and school. In search an observer looks for some process information. This means that it can specified subset of objects among a set of deal only with a few stimuli at a given moment objects. For example, when you go to fetch of time. The question is, which of those stimuli your younger sister and brother from the will get selected and processed? Psychologists school, you just look for them among have identified a number of factors that innumerable boys and girls. All these activities determine the selection of stimuli. require some kind of effort on the part of people. Attention in this sense refers to “effort Factors Affecting Selective Attention allocation”. Several factors influence selective attention. d Attention has a focus as well as a fringe. These generally relate to the characteristics When the field of awareness is centered on a e of stimuli and the characteristics of particular object or event, it is called focus or individuals. They are generally classified as the focal point of attention. On the contrary, h “external” and “internal” factors. when the objects or events are away from the External factors are related to the features T s center of awareness and one is only vaguely of stimuli. Other things held constant, the size, i aware of them, they are said to be at the fringe l intensity, and motion of stimuli appear to be R of attention. important determinants of attention. Large, Attention has been classified in a number b bright, and moving stimuli easily catch our E of ways. A process-oriented view divides it into attention. Stimuli, which are novel and two types, namely selective and sustained. u moderately complex, also easily get into our We will briefly discuss the main features of C focus. Studies indicate that human these types of attention. Sometimes we can p photographs are more likely to be attended to also attend to two different things at the same N re than the photographs of inanimate objects. time. When this happens, it is called divided Similarly, rhythmic auditory stimuli are more attention. Box 5.2 describes when and how readily attended to than verbal narrations. the division of attention is possible. © e Sudden and intense stimuli have a wonderful capacity to draw attention. Selective Attention Internal factors lie within the individual. b Selective attention is concerned mainly with These may be divided into two main categories, the selection of a limited number of stimuli or viz. motivational factors and cognitive factors. objects from a large number of stimuli. We Motivational factors relate to our biological to t Box 5.2 Divided Attention no In day-to-day life we attend to several things at the same time. You must have seen people driving a car and talking to a friend, or attending to phone calls on a mobile set, or putting on sunglasses, or listening to music. If we watch them closely, we will notice that they are still allocating more effort time. However, this becomes possible only with highly practiced activities, because they become almost automatic and require less attention to perform than new or slightly practiced activities. Automatic pr ocessing has three main characteristics; (i) It occurs without intention, (ii) It takes to driving than to other activities, even though place unconsciously, and (iii) It involves very little (or some attention is given to other activities. It no) thought processes (e.g., we can read wor ds or tie indicates that on certain occasions attention can our shoelaces without giving any thought to these be allocated to more than one thing at the same activities). 94 Psychology or social needs. When we are hungry, we notice given moment of time are not completely even a faint smell of food. A student taking an blocked. The filter only attenuates (weakens) examination is likely to focus on a teacher’s their strength. Thus some stimuli manage to instructions more than other students. escape through the selective filter to reach Cognitive factors include factors like interest, higher levels of processing. It is indicated that attitude, and preparatory set. Objects or personally relevant stimuli (e.g., one’s name events, which appear interesting, are readily in a collective dinner) can be noticed even at a attended by individuals. Similarly we pay very low level of sound. Such stimuli, even quick attention to certain objects or events to though fairly weak, may also generate which we are favourably disposed. Preparatory response occasionally by slipping through the set generates a mental state to act in a certain selective filter. d way and readiness of the individual to respond Multimode theory was developed by to one kind of stimuli and not to others. Johnston and Heinz (1978). This theory e believes that attention is a flexible system that Theories of Selective Attention allows selection of a stimulus over others at h three stages. At stage one the sensory A number of theories have been developed to T s explain the process of selective attention. We representations (e.g., visual images) of stimuli i are constructed; at stage two the semantic l will briefly discuss three of these theories. R representations (e.g., names of objects) are Filter theory was developed by Broadbent constructed; and at stage three the sensory b (1956). According to this theory, many stimuli E and semantic representations enter the simultaneously enter our receptors creating consciousness. It is also suggested that more u a kind of “bottleneck” situation. Moving processing requires more mental effort. When C through the short-term memory system, they the messages are selected on the basis of stage p enter the selective filter, which allows only one one processing (early selection), less mental N re stimulus to pass through for higher levels of effort is required than when the selection is processing. Other stimuli are screened out at that moment of time. Thus, we become aware based on stage three pr ocessing (late selection). © e of only that stimulus, which gets access through the selective filter. Sustained Attention Filter-attenuation theory was developed b by Triesman (1962) by modifying Broadbent’s While selective attention is mainly concerned theory. This theory proposes that the stimuli with the selection of stimuli, sustained not getting access to the selective filter at a attention is concerned with concentration. It to t Box 5.3 Span of Attention no Our attention has a limited capacity to receive stimuli. The number of objects one can attend to at a brief exposure (i.e. a fraction of a second) is called “span of attention” or “perceptual span”. More specifically, the span of attention refers to the span of attention varies within the limit of seven plus or minus two. This is popularly known as the “magic number”. It means that at a time, people can attend to a set of five to seven numbers, which can be extended to nine or more under exceptional conditions. That is amount of information an observer can grasp from perhaps the reason why motorbikes or cars are given a complex array of stimuli at a single momentary a number plate that contains only four digit numbers exposure. This can be deter mined by the use of an with some alphabets. In case of violation of driving rules instrument, called “tachistoscope”. On the basis of a traffic police can easily read and note these numbers several experiments, Miller has reported that our along with the alphabets. 95 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses refers to our ability to maintain attention on factor. Intense and long lasting stimuli an object or event for longer durations. It is facilitate sustained attention and result in also known as “vigilance”. Sometimes people better performance. Temporal uncertainty is have to concentrate on a particular task for a third factor. When stimuli appear at regular many hours. Air traffic controllers and radar intervals of time they are attended better than readers provide us with good examples of this when they appear at irregular intervals. phenomenon. They have to constantly watch Spatial uncertainty is a fourth factor. Stimuli and monitor signals on screens. The that appear at a fixed place are readily occurrence of signals in such situations is attended, whereas those that appear at usually unpredictable, and errors in detecting random locations are difficult to attend. signals may be fatal. Hence, a great deal of Attention has several practical d vigilance is required in those situations. implications. The number of objects one can e readily attend to in a single glance is used to Factors Influencing Sustained Attention design the number plates of motorbikes and h Several factors can facilitate or inhibit an cars so that the traffic police can easily notice T s individual’s performance on tasks of sustained them in the case of traffic rule violations i attention. Sensory modality is one of them. (Box 5.3). A number of children fail to perform R l Performance is found to be superior when the well in school simply due to the problem of stimuli (called signals) are auditory than when attention. Box 5.4 presents some interesting E b they are visual. Clarity of stimuli is another information about a disorder of attention. C p u N re Box 5.4 Attention Deficit H Hyyperactivity Disorder (ADHD) DHD) © e This is a very common behavioural disorder found found to account for ADHD more reliably than other among children of the primary school age. It is factors. Currently ADHD is considered to have multiple characterised by impulsivity, excessive motor causes and effects. b activity, and an inability to attend. The disorder is Disagreement remains over the most effective more prevalent among boys than among girls. If method of treatment of ADHD. A drug, called Ritalin, is not managed properly, the attention difficulties may widely used, which decreases children’s over-activity o persist into adolescence or adult years. Difficulty and distractibility, and at the same time increases their in sustaining attention is the central feature of this attention and ability to concentrate. However, it does t disorder, which gets reflected in several other not “cure” the problem, and often also results in such domains of the child. For example, such children negative side-effects as the suppr ession in normal t are highly distractible; they do not follow growth of height and weight. On the other hand, instructions, have difficulty in getting along with behavioural management programmes, featuring o parents, and are negatively viewed by their peers. positive reinforcement and structuring lear ning They do poorly in school, and show difficulties in materials and tasks in such a way that minimises errors n reading or lear ning basic subjects in schools in and maximises immediate feedback and success, have spite of the fact that there is no deficit in their been found quite useful. Successful modification of intelligence. ADHD is claimed with cognitive behavioural training Studies generally do not provide evidence for programme in which rewards for desired behaviours a biological basis of the disorder, whereas some are combined with training in the use of verbal self- relationship of the disorder with dietary factors, instructions (stop, think, and then do). With this particularly food colouring, has been documented. procedure, the ADHD children lear n to shift their On the other hand, social-psychological factors (e.g., attention less frequently and to behave reflectively — home environment, family pathology) have been a learning that is relatively stable over time. 96 Psychology  the whole is known as bottom-up processing. P ERCEPTUAL P ROCESSES The notion that recognition process begins In the previous section we have examined that from the whole, which leads to identification the stimulation of sensory organs leads us to of its various components is known as top- experience something such as, a flash of light down processing. The bottom-up approach or a sound, or a smell. This elementary lays emphasis on the features of stimuli in experience, called sensation, does not provide perception, and considers perception as a us with any understanding of the stimulus process of mental construction. The top-down that stimulated the sense organ. For example, approach lays emphasis on the perceiver, and it does not inform us about the source of the considers perception as a process of light, sound or fragrance. In order to make recognition or identification of stimuli. Studies d sense out of the raw material provided by the show that in perception both the processes sensory system, we process it further. In doing interact with each other to provide us with an e so, we give meaning to stimuli by using our understanding of the world. h learning, memory, motivation, emotions, and other psychological processes. The process by THE P ERCEIVER T s which we recognise, interpret or give meaning i to the information provided by sense organs is Human beings are not just mechanical and R l called perception. In interpreting stimuli or passive recipients of stimuli from the external events, individuals often construct them in b world. They are creative beings, and try to E their own ways. Thus perception is not merely understand the external world in their own u an interpretation of objects or events of the ways. In this process their motivations and C external or internal world as they exist, instead expectations, cultural knowledge, past p it is also a construction of those objects and experiences, and memories as well as values, events from one’s own point of view. N re beliefs, and attitudes play an important role The process of meaning-making involves in giving meaning to the external world. Some certain sub-processes. These are shown in of these factors are described here. Fig.5.4. © e Motivation Processing Approaches in Perception The needs and desires of a perceiver strongly b How do we identify an object? Do we identify influence her/his perception. People want to a dog because we have first recognised its furry fulfil their needs and desires through various o coat, its four legs, its eyes, ears, and so on, or means. One way to do this is to perceive do we recognise these different parts because t objects in a picture as something that will we have first identified a dog? The idea that satisfy their need. Experiments were recognition process begins from the parts, conducted to examine the influence of hunger t which serve as the basis for the recognition of on perception. When hungry persons were no Fig.5.4 : Sub-processes of Perception 97 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses shown ambiguous pictures, they were found to perceive them as pictures of food objects more often than satiated (non-hungry) persons. Expectations or Perceptual Sets The expectations about what we might perceive in a given situation also influence our perception. This phenomenon of perceptual familiarisation or perceptual generalisation reflects a strong tendency to see what we d expect to see even when the results do not e accurately reflect external reality. For example, if your milkman delivers you milk daily at h about 5.30 A.M., any knocking at the door around that time is likely to be perceived as T s Fig.5.5 : An Item to test the ‘Field Dependent’ and the presence of the milkman even if it is i ‘Field Independent’ Cognitive Style l someone else. R the triangle. Those who can do it quickly will b Activity 5.3 be called “field independent”; those who take E long time will be called “field dependent”. u To demonstrate expectancy tell your friend to close C her eyes. Write 12, 13, 14, 15 on the board. Ask Cultural Background and Experiences p her to open her eyes for 5 seconds, look at the Different experiences and learning N re board, and note down what she saw. Repeat replacing only the 12, 14, 15 with A, C, D viz. ‘A opportunities available to people in different 13 C D’. Ask her again to note down what she cultural settings also influence their perception. People coming from a pictureless © e saw. Most people write down B in place of 13. environment fail to recognise objects in pictures. Hudson studied the perception of b pictures by African subjects, and noted several Cognitive Styles difficulties. Many of them were unable to Cognitive style refers to a consistent way of identify objects depicted in pictures (e.g., o dealing with our environment. It significantly antelope, spear). They also failed to perceive t affects the way we perceive the environment. distance in pictures, and interpreted There are several cognitive styles that people pictures incorrectly. Eskimos have been found t use in perceiving their environment. One most to make fine distinction among a variety of extensively used in studies is the “field snow that we may be unable to notice. Some o dependent and field independent” cognitive aboriginal groups of Siberian region have been style. Field dependent people perceive the found to differentiate among dozens of skin n external world in its totality, i.e. in a global or colours of reindeers, which we would not be holistic manner. On the other hand, field able to do. independent people perceive the external These studies indicate that the perceiver world by breaking it into smaller units, i.e. in plays a key role in the process of perception. an analytic or differentiated manner. People process and interpret stimuli in their Look at Fig.5.5. Can you find out the own ways depending on their personal, social triangle hidden in the picture? How much time and cultural conditions. Due to these factors do you take to locate it? Try to find out the our perceptions are not only finely tuned, but time other students of your class take to locate also modified. 98 Psychology  and sky stay behind the figure and are P RINCIPLES OF PERCEPTUAL ORGANISATION perceived as background. Our visual field is a collection of different To test this experience, look at the Fig.5.6 elements, such as points, lines, and colours. given below. You will see either the white part However, we perceive these elements as of the figure, which looks like a vase (flower organised wholes or complete objects. For pot), or the black part of the figure, which looks example, we see a bicycle as a complete object, like two faces. not as a collection of different parts (e.g., saddle, wheel, handle). The process of organising visual field into meaningful wholes is known as form perception. d You may wonder how different parts of an objects are organised into a meaningful whole. e You may also ask if there are certain factors h that facilitate or inhibit this process of organisation. T s Several scholars have tried to answer such i questions, but the most widely accepted R l answer has been given by a group of researchers, called Gestalt psychologists. E b Prominent among them are Köhler, Koffka, u and Wertheimer. Gestalt means a regular C figure or a form. According to Gestalt p psychologists, we perceive different stimuli not as discrete elements, but as an organised N re Fig.5.6 : Rubin’s Vase “whole” that carries a definite form. They believe that the form of an object lies in its We distinguish figure from the ground on whole, which is different from the sum of their the basis of the following characteristics: © e parts. For example, a flower pot with a bunch 1. Figure has a definite form, while the of flowers is a whole. If the flowers are removed, background is relatively formless. b the flower pot still remains a whole. It is the 2. Figure is more organised as compared to configuration of the flower pot that has its background. changed. Flower pot with flowers is one 3. Figure has a clear contour (outline), while o configuration; without flowers it is another the background is contourless. t configuration. 4. Figure stands out from the background, The Gestalt psychologists also indicate that while the background stays behind the t our cerebral processes are always oriented figure. towards the perception of a good figure or 5. Figure appears more clear, limited, and o pragnanz. That is the reason why we perceive relatively nearer, while the background everything in an organised form. The most appears relatively unclear, unlimited, and n primitive organisation takes place in the form away from us. of figure-ground segregation. When we look The discussion presented above indicates at a surface, certain aspects of the surface that human beings perceive the world in clearly stand out as separate entities, whereas organised wholes rather than in discrete parts. others do not. For example, when we see words The Gestalt psychologists have given us several on a page, or a painting on a wall, or birds laws to explain how and why different stimuli flying in the sky, the words, the painting, and in our visual field are organised into the birds stand out from the background, and meaningful whole objects. Let us look at some are perceived as figures, while the page, wall, of these principles. 99 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses The Principle of Proximity The Principle of Smallness Objects that are close together in space or time According to this principle, smaller areas tend are perceived as belonging together or as a to be seen as figures against a larger group. For example, Fig.5.7 does not look like background. In Fig.5.10 we are more likely to a square pattern of dots, but as a series of see a black cross rather than a white cross columns of dots. Similarly, Fig.5.7 also looks within the circle because of this principle. like a group of dots together in rows. Fig.5.7 : Proximity e d h The Principle of Similarity Fig.5.10 : Smallness T i s Objects that are similar to one another and The Principle of Symmetry l have similar characteristics are perceived as R a group. In Fig.5.8 the little circles and squares This principle suggests that symmetrical areas b are evenly spaced both horizontally and tend to be seen as figures against E vertically so that the proximity does not come asymmetrical backgrounds. For example, in u into play. Instead, we tend to see alternating Fig.5.11 the black areas are seen as figures C columns of circles and squares. (as they have symmetrical properties) against p their white asymmetrical background. N re © e b Fig.5.8 : Similarity o The Principle of Continuity Fig.5.11 : Symmetry t This principle states that we tend to perceive The Principle of Surroundedness objects as belonging together if they appear t to form a continuous pattern. For instance, According to this principle, the areas we are more likely to identify two lines a-b surrounded by others tend to be perceived as o and c-d crossing than to identify four lines figures. For example, the image in Fig.5.12 looks like five figures against the white n meeting at the center p. background rather than the word ‘LIFT’. Fig.5.9 : Continuity LIFT Fig.5.12 : Surroundedness 100 Psychology The Principle of Closure Monocular Cues (Psychological Cues) We tend to fill the gaps in stimulation and Monocular cues of depth perception are perceive the objects as whole rather than their effective when the objects are viewed with only separate parts. For example, in Fig.5.13 the one eye. These cues are often used by artists small angles are seen as a triangle due to our to induce depth in two dimensional paintings. tendency to fill the gaps in the object provided Hence, they are also known as pictorial cues. by our sensory input. Some important monocular cues that help us in judging the distance and depth in two dimensional surfaces are described below. You will find some of them applied in Fig.5.14. e d h Fig.5.13 : Closure T i s PERCEPTION OF SPACE, DEPTH, AND DISTANCE R l The visual field or surface in which things E b exist, move or can be placed is called space. The space in which we live is organised in three u dimensions. We perceive not only the spatial C attributes (e.g., size, shape, direction) of p various objects, but also the distance between N re the objects found in this space. While the images of objects projected on to our retina Fig.5.14 : Monocular Cues are flat and two dimensional (left, right, up, © e The above picture will help you to understand some down), we still perceive three dimensions in monocular cues: Interposition and relative size (see the the space. Why does it happen so? It occurs trees). Which other cues can you locate in the picture? due to our ability to transfer a two dimensional b retinal vision into a three dimensional Relative Size : The size of retinal image allows perception. The process of viewing the world us to judge distance based on our past and o in three dimensions is called distance or depth present experience with similar objects. As t perception. the objects get away, the retinal image Depth perception is important in our daily becomes smaller and smaller. We tend to t life. For example, when we drive, we use depth perceive an object farther away when it to assess the distance of an approaching appears small, and closer when it appears o automobile, or when we decide to call a person bigger. walking down the street, we determine the n Interposition or Overlapping : These cues occur loudness with which to call. when some portion of the object is covered by In perceiving depth, we depend on two another object. The overlapped object is main sources of information, called cues. One considered farther away, whereas the object is called binocular cues because they require that covers it appears nearer. both eyes. Another is called monocular cues, because they allow us to perceive depth with Linear Perspective : This reflects a just one eye. A number of such cues are used phenomenon by which distant objects appear to change a two dimensional image into a three to be closer together than the nearer objects. dimensional perception. For example, parallel lines, such as rail tracks, 101 Chapter 5 Sensory, Attentional and Perceptual Pr ocesses appear to converge with increasing distance that are close. The rate of an object’s with a vanishing point at the horizon. The more movement provides a cue to its distance. For the lines converge, the farther away they example, when we travel in a bus, closer appear. objects move “against” the direction of the bus, whereas the farther objects move “with” the Aerial Perspective : The air contains direction of the bus. microscopic particles of dust and moisture that make distant objects look hazy or blurry. Binocular Cues (Physiological Cues) This effect is called aerial perspective. For example, distant mountains appear blue due Some important cues to depth perception in to the scattering of blue light in the three dimensional space are provided by both atmosphere, whereas the same mountains are the eyes. Three of them have particularly been d perceived to be closer when the atmosphere found to be interesting. e is clear. Retinal or Binocular Disparity : Retinal Light and Shade : In the light some parts of disparity occurs because the two eyes have h the object get highlighted, whereas some parts different locations in our head. They are T s become darker. Highlights and shadows separated from each other horizontally by a i provide us with information about an object’s distance of about 6.5 centimeters. Because of R l distance. this distance, the image formed on the retina of each eye of the same object is slightly b Relative Height : Larger objects are perceived E different. This difference between the two as being closer to the viewer and smaller images is called retinal disparity. The brain u objects as being farther away. When we expect interprets a large retinal disparity to mean a C two objects to be the same size and they are close object and a small retinal disparity to p not, the larger of the two will appear closer mean a distant object, as the disparity is less N re and the smaller will appear farther away. for distant objects and more for the near Texture Gradient : It represents a phenomenon objects. by which the visual field having more density © e Convergence : When we see a nearby object of elements is seen farther away. In the our eyes converge inward in order to bring the Fig.5.15 the density of stones increases as we image on the fovea of each eye. A group of look farther away. b muscles send messages to the brain regarding the degree to which eyes are turning inward, and these messages are interpreted as cues o to the perception of depth. The degree of t convergence decreases as the object moves further away from the observer. You can t experience convergence by holding a finger in front of your nose and slowly bringing it closer. o The more your eyes turn inward or converge, n the nearer the object appears in space. Accommodation : Accommodation refers to a process by which we focus the image on the Fig.5.15 : Texture Gradient retina with the help of ciliary muscle. These Motion Parallax : It is a kinetic monocular cue, muscles change the thickness of the lens of and hence not considered as a pictorial cue. the eye. If the object gets away (more than 2 It occurs when objects at different distances meters), the muscle is relaxed. As the object move at a different relative speed. The distant moves nearer, the muscle contracts and the objects appear to move slowly than the objects thickness of the lens increases. The signal 102 Psychology about the degree of contraction of the muscle the image that it casts on the retina is a circle, is sent to the brain, which provides the cue or an ellipse, or roughly a short line (if the for distance. plate is viewed from the edge). It is also called form constancy. Activity 5.4 Brightness Constancy Hold a pencil in front of you. Close your right eye Visual objects not only appear constant in their and focus on the pencil. Now open the right eye and close the left eye. Keep doing it simultaneously shape and size, they also appear constant in with both the eyes. The pencil will appear to move their degree of whiteness, greyness, or from side to side in front of your face. blackness even though the amount of physical energy reflected from them changes d considerably. In other words, our experience e P ERCEPTUAL C ONSTANCIES of brightness does not change in spite of the changes in the amount of reflected light The sensory information that we receive from h reaching our eyes. The tendency to maintain our environment constantly changes as we apparent brightness constant under different T s move around. Yet we form a stable perception amount of illumination is called brightness i of an object seen from any position and in constancy. For example, surface of a paper R l any intensity of light. Perception of the objects which appears white in the sunlight, is still as relatively stable in spite of changes in the perceived as white in the room light. Similarly, E b stimulation of senso