Psychology Chapter 2 PDF

Chapter Two 2. Sensation and Perception 2.1 The Meanings of Sensation and PerceptionPsychologists have traditionally differentiated between sensation and perception. Sensation is the process whereby stimulation of receptor cells in the eyes, ears, nose, tongue, and surface of the skin sends nerve im...

Chapter Two 2. Sensation and Perception 2.1 The Meanings of Sensation and PerceptionPsychologists have traditionally differentiated between sensation and perception. Sensation is the process whereby stimulation of receptor cells in the eyes, ears, nose, tongue, and surface of the skin sends nerve impulses to the brain. Sensations are closely tied to what is happening in the sensory systems themselves. Color, brightness, the pitch of tone or a bitter taste are examples of sensations. The starting point of sensations is a stimulus. A form of energy (such as light waves or sound waves) that can affect sensory organs (such as the eye or the ear). Sensation is therefore the process that detects the stimulus from one’s body or from the environment. How different is sensation from per ception?In real life, you seldom experience simple sensations. Instead of simple sensations, perceptual processes are constantly at work to modify sensory input into what are actually experiences. Perception is the process that organizes sensations into meaningful patterns. It is the process whereby the brain interprets sensations, giving them order and meaning.Thus, hearing sounds and seeing colors is largely a sensory process, but forming a melody and detecting patterns and shapes is largely a perceptual process.Why do we say “largely” in the abo ve expression?We say largely because in everyday life, it is almost impossible to separate sensation from perception. As soon as the brain receives sensations, it automatically interprets or perceives them, and without sensations of some kind perception could not occur. Can you mention examples showing the difference between sensation and perception? Consider, for example, the black marks and letters in this page. Visual Sensation lets you detect the black marks. Visual perception lets you organize the black marks into letters and works. For a real life example of the difference between sensation and perception consider a case study presented by neurologist Oliver Sacks (1985) one of his patients suffered from brain damage that caused him to develop prosopagnosia, the inability to recognize human faces. The patient could recognize people by the sound of their voices, but he could not recognize them by light. His disorder was so severe that he sometimes patted tirehydrants, thinking they were children’s heads. He would even grab his wife's head mistaking it for a hat yet he was not nearsighted; he could easily see a pin on the floor. Thus he had people’s facial features, but he couldn't organize them into recognizable faces (visual perceptions).2.2. The sensory laws: Sensory thresholds and sensory adaptionThere are certain sensory laws that explain how sensation works. Sensory Threshold and sensory adaptation are the two general laws of sensation. Sensory threshold is the minimum point of intensity a sound can be detected. There are two laws of sensory threshold: The law of absolute threshold and the law of difference threshold. The absolute threshold The minimum amount of stimulation a person can detect is called the absolute threshold, or Limen, for example, a cup of coffee would require a certain amount of sugar before you could detect a sweet taste. Because the absolute threshold for a particular sensory experience varies, psychologists operationally define the absolute threshold as the minimum level of stimulation that can be detected 50 percent of the time when a stimulus is presented over and over again. Thus, if you were presented with a low intensity sound 30 times and detected it15 times, that level of intensity would be your absolute threshold for that stimulus. The absolute threshold is also affected by factors other than the intensityof the stimulus; Researchers assume that the detection of a stimulusdepends on both its intensity and the physical and psychological state ofthe individual. One of the most important psychological factors is theresponse bias-how ready the person is to report the presence of aparticular stimulus. Imagine that you are walking down a street at night.Your predisposition to detect a sound depends, in part, on your estimate ofthe probability of being mugged, so you would be more likely to perceivethe sound of footsteps on a neighborhood you believe to be dangerous thanin a neighborhood you believe to be safe. The difference threshold In addition to detecting the presence of a stimulus, you also detectchanges in the intensity of a stimulus. The minimum amount of changethat can be detected is called difference threshold. For example, acup of coffee would require a certain amount of additional sugar before youcould detect an increase in its sweetness. Similarly, you would have toincrease the intensity of the sound from your tape recorder a certainamount before you could detect a change in its volume.Like the absolute threshold, the difference threshold for a particularsensory experience varies from person to person and from occasion tooccasion. Therefore, psychologists formally define the difference thresholdas the minimum change in stimulation that can be detected 50 percent of thetime by a given person. This difference in threshold is called the justnoticeable difference (jnd). The amount of change in intensity ofstimulation needed to produce a jnd is a constant fraction of the originalstimulus. This became known as Weber’s law. For example, because the jndfor weight is about 2% and you held a 50 ounce for weight you would noticea change only if there was at least one ounce change in it. But a personholding a 100 ounce weight would require the addition or subtraction of atleast 2 ounce to notice a change. Research findings indicate the Weber’slaw holds better for stimuli of moderate intensity than stimuli ofextremely low or high intensity. Sensory Adaptation Given that each of your senses is constantly bombarded by stimulation,why do you notice only certain stimuli? One possible reason is that if a stimulus remains constant in intensity,you will gradually stop noticing it. For example, after diving into aswimming pool, you might shiver. Yet a few minutes later you might invitesomeone to join you saying “The water is fine” on entering a friend’sdormitory room, you might be struck by the repugnant stench of month-oldgarbage. Yet a few minutes later you might not notice the odor at all, thistendency of our sensory receptors to have decreasing responsiveness tounchanging stimulus is called sensory adaptation. Sensory adaptation lets you detect potentially important change in yourenvironment while ignoring unchanging aspects of it. For example, when vibrations repeatedlystimulate your skin, you stop noticing them. Thus, if you were having abumpy train ride that made your seat vibrate against your bottom, you wouldinitially notice the vibrations, but it would serve little purpose for youto continue noticing them. Likewise, once you have determined that theswimming pool water is cold or that your friend’s room smells, it wouldserve little purpose to continue noticing those stimuli-especially whenmore important change might be taking place elsewhere in your surroundings.Of course, you will not adapt completely to extremely intense sensations,such as severe pain or freezing cold. This is adaptive, because to ignoresuch stimuli might be harmful or even fatal.2.3 Perception You have seen earlier that perception is a meaning making process. Now youstudy more about this meaning making process of the human intelligent life.It helps you understand the major characteristics of the perceptualprocess: selectivity of perception, form perception, depth perception,perceptual constancy, and perceptual illusion. 2.3.1 Selectivity of perception: Atten tionNote that at any given time, your sense organ is bombarded by many stimuli.Yet you perceive a few of them. Were you aware of, for example, the noisein your room until you read this sentence? You may not. Yet input from theenvironment was coming into your ears all the time. In fact you may beattending to one of such incoming in put ignoring the other noises. Suchselective perception is called attention. Attention is therefore the term given to the perceptual process thatselects certain inputs for inclusion in your conscious experience, orawareness, at any given time, ignoring others. What does this selectivityof perception imply?The selectivity of perception implies, among other things, that our fieldof experience is divided into what is known as “Focus” and “Margin”. Eventsor stimuli that you perceive clearly are the focus of your experience andother items or stimuli that you perceive dimly or vaguely are in the marginof your attention. You may be aware of items in the marginal field but onlyvaguely or partially. To illustrate focus and margin consider that your perceptual field is afootball game. While you are dimly aware of the tangle of players and theactivity of the blockers during the play, it is the ball carrier and hismovement that stands out clearly to you your attention is mainly focused onhim. But at the same time, sensory inputs are coming in from your coldfeet, from your stomach as a result of the last uncomfortable food you ate,and from the fellow behind you who is smoking a cigar. The crowd is alsoshouting. While the play is going on, you are probable not aware of any ofthese sensory inputs. Only when the play is finished or time is called thatyou perceive how cold your feet are, and how noisy the crowd is. The fact that you perceive how cold your feet are, and how noise the crowdis when the play is finished or time is called illustrates anothercharacteristics of attention, that it is constantly shifting. Attentionshifts constantly. What is in the focus of your attention one moment may bein margin; and what is in the margin may become in your focus.Paying attention is in general a function of two factors: factors externalto the perceiver and factors internal to the perceiver. External factorsrefer to factors that are generally found in the objects or stimuli to beperceived. Some of the external characteristics of objects that determinewhether you are going to attend them or not are size and intensity,repetition, novelty (or newness), and movement. Other things beingconstant, bigger and brighter stimuli are more likely to capture yourattention than smaller and dimmer objects. That is why announcements andnotice are written in big and block letters. In the same way, people whodress bright colored clothes tend to capture your attention. Repetition is the second factor. You are more likely to attend to stimulithat repeatedly or frequently occur in your perceptual field. A misspelledword is more likely to be detected if it occurs many times in a paragraphthan when it occurs only once or twice. You are going to notice a person ifhe continuously follows you as compared to a person you meet only once ortwice. That is, by the way, why slogans, advertisings, and announcement arerepeated continuously to audiences and spectators. In a word, repetition isattention getting. However, no matter how big or bright a stimulus is, orelse no matter how frequently it may occur, you may not give it attentionas if it occurs in the same way all the time. This is basically because youare likely to adapt to it and then stop responding to it. This is calledsensory adaptation or habituation. It is the tendency to ignore a stimulusthat occurs continuously in the same way. Hence, the third factor ofattention is novelty-the extent to which a stimulus creates a contrast withthe rest objects in the environment. Novel or new objects create a sharpcontrast with the environment and hence tend to capture your attention.Remember here why you are given a special attention as a guest, why first-born children get more attention from parents etc.The last but not the least external factor in attention getting ismovement. Moving objects tend to get your attention more than non-moving orstagnant objects. Your eyes are involuntarily attracted to movement the waybutterflies are attracted to light. This is because moving objects are instinctually felt dangerous or threatening and you arereflexively responding to them to defend yourself. Moreover, moving objectsbring with them changes in stimulation or newness in their presentation.In general, stimuli in the environment that are bigger and brighter, ormore frequently occurring or newer or moving are likely to get yourattention. Paying attention is not, however, determined only by thesecharacteristics of objects. Even when a stimulus is bigger, brighter, new, frequent, or moving, you may not give it attention if you arenot psychologically ready to attend to it. Hence, attention giving alsodepends on your psychological states as an observer. What are some of the internal psychological states of the observer thataffect as to which stimulus on pa ys attention to or ignore?Psychologists have identified two important psychological factors: Set orexpectancy and motives or needs. Set refers to the idea that you may be“ready” and “primed for” certain kinds of sensory input. Set, orexpectancy, therefore, varies from person to person. It is important notonly in the selection of sensory input for inclusion in the focus of yourattention. It is also important in organizing the selected sensory input.To illustrate the role of set in attention, consider the husband who isexpecting an important phone call. He will hear the telephone ring in thenight while his wife does not. The wife, on the other hand, may more likelyto hear the baby crying than the telephone ringing. Of course, if the wifeis expecting an important cell, the reverse may be true.What other examples, do you think, illustrate the role of set or expectancy in perception?Motives and needs are the second psychological factors influencing you asan observer. There are differences between you and your friend in what youselect to perceive as a result of differences in your motives and needs.You and your friend attend to and organize the sensory input in ways thatmatch your respective needs. People who are hungry, thirst, or sexuallyaroused are likely to pay attention to events in the environment, whichwill satisfy these needs. 2.3.2 From perception Visual sensations, as discussed under sensation, provide the raw materialsthat are to be organized into meaningful patterns, shapes, forms, andconcepts or ideas or form perception. The meaningful shapes or patterns orideas that are made perhaps out of meaningless and discrete or pieces andbites of sensations refer to form perception. To perceive forms (meaningfulshapes or patterns), you need to distinguish a figure (an object) from itsground (or its surrounding). Let us look at this idea further.Figure-Ground Perception Figure-ground perception is the perception of objects and forms of everydayexperience as standing out from a background. Pictures (figure) hang on awall (ground), words (figure) are seen on a page (ground), and melody(figure) stands out from the repetitive chords in the musical background(ground), the pictures, words, and the melody are perceived as the figure,while the wall, the page, and the chords are the ground. The ability to distinguish an object from its general backgroundis basic to all form perception. And gestalt psychologists stress thatform perception in an active, rather than a passive, process likeselectivity of perception. Hence, there can be a shift in your perceptionof figure and ground such that the figure may become the ground and viceversa. Factors that determine your attention equally determine what shouldbecome the figure and what should become the ground. By the way, what helps you in general to separate the figure from thege neral around in your visual perception?This will take you to the second feature of form perception calledcontours.Contours in Form Perception You are able to separate forms from the general ground only because you canperceive contours. Contours are formed whenever a marked difference occursin the brightness or color of the background. If you, for instance, look ata piece of paper that varies continuously in brightness from white at oneborder to black at the opposite border, you will perceive no contour. Thepaper will appear uniform, and if you are asked to say where the sheetstops being light and starts to become dark, you can only guess. On theother hand, if the change is marked rather than gradual-suppose severalshades are skipped-you will see the paper as divided in to two parts. Inperceiving the division at the place where the brightness gradient changesabruptly, you have perceived a contour. In general, contours give shape tothe objects in our visual world because they mark one object off fromanother or they mark an object off from the general ground. When contoursare disrupted visually, as in camouflage, objects are difficult todistinguish from the background. Organization in form Perception When several objects are present in the visual field, we tend to perceivethem as organized into patterns or groupings. The Gestalt psychologistsstudied such organization intensively in the early part of this century.They emphasized that organized perceptual experience has properties, whichcannot be predicated from a simple analysis of the components. In otherwords, Gestalt psychologist said “the whole is more than the sum of itsparts”. This simply means that what is perceived has its own newproperties, properties that emerge from the organization, which takesplace.Organization in perception partially explains our perception of complexpatterns as unitary forms, or objects. We see objects as objects onlybecause grouping processes operate in perception. Without them, the variousobjects and patterns we perceive-a face on a television screen, a car atree, a book-would not “ hang together” as objects or patterns, they wouldmerely be so many disconnected sensations-dots, lines or blotches, forexample. What are some of the laws of per ceptual organization?One organizing principle is proximity, or nearness. The laws ofproximity says that items which are close together in space or time tend tobe perceived as belonging together or forming an organized group. Another organizing principle of perception is similarity. Most people seeone triangle formed by the dots with its apex at the top and anothertriangle formed by the rings with its apex at the bottom. They perceivetriangle because similar items such as, the rings and the dots, tend to beorganized together. Otherwise, they would see a hexagon or a six-pointedstar, where all the dots are the same.Grouping according to similarity, however, does not always occur. A figureis more easily seen as a six-pointed star than as one figure composed ofdots and another figure made up of rings. In this case, similarity iscompeting with the organizing principle of symmetry, or good figure.Neither the circle nor the dots by themselves from a symmetrical pattern.The law of good figure says that there is a tendency to organizethings to make a balanced or symmetrical figure that includes all theparts. In this case, such a balanced figure can be achieved only by using all the dots and rings to perceive a six pointedstar the law of good figure wins out over the law of similarity because therings by themselves or the dots by themselves do not form symmetrical goodsfigures. Still another principle or organization is continuation, the tendency toperceive a line that starts in one way as continuing in the same way. Forexample, a line that starts out as a curve is seen as continuing onsmoothly curved course. A straight line is seen as continuing on a straightcourse or, if it does change direction as forming an angle rather than acurve. We see the dots as several curved and straight lines. Even thoughthe curved and straight lines cross and have dots in common, it is onlywith an effort that we can perceive a straight line suddenly becoming acurved line at one of these functions.Finally, the law of closure makes our perceived world or form more completethan the sensory stimulation that is presented. The law of closure refersto perceptual processes that organize the perceived world by filling ingaps in stimulation 2.3.3 Depth perception If we live in a two-dimensional world, form perception would be sufficient.But because we live in a three-dimensional world, we have evolved depthperception-the ability to judge the distance of objects. Given that imageson the retina are two dimensional, how can we perceive depth? That is, howcan we determine the distance of objects (the distal stimulus) from thepattern of stimulation on our retinas (the proximal stimulus)? Depth perception depends on the use binocular cues and monocular cues.There are two kinds of binocular cues: retinal disparity and convergence.The two kinds of binocular cues require the interaction of both eyes.Retinal disparity is, the degree of difference between the image of anobject that are focused on the two retinas. The closer the object, thegreater is the retinal disparity. To demonstrate retinal disparity foryourself, point a forefinger vertically between your eyes. Look at thefinger with one eye closed. Then look at it with the other closed. You willnotice that the background shifts as you view the scene with differentviews of the same stimulus. The “view master” device you might have used asa child creates the impression of visual depth by presenting slightlydifferent image to the eyes at the same time mimicking retinal disparity.Retinal disparity is greater when an object is near you than when it isfarther away from you. Certain cells in visual cortex detect the degree ofretinal disparity, which the brain uses to estimate the distance of anobject focused on the retinas.The second binocular cue to depth is convergence, the degree to which theeyes turn inward to focus on an object. As you can confirm for yourself,the closer the objects are the greater the convergence of the eyes. Hold aforefinger vertically in front of your face and move it toward your nose.You should notice an increase in ocular muscle tension as your fingerapproaches your nose. Neurons in the cerebral cortex translate the amountof muscle tension into an estimate of the distance of your finger. Not thatconvergence is associated with important everyday activities. For example,drinking alcohol impairs depth perception by disrupting the normalconvergence of the eyes and using a computer terminal for hours induce eyefatigue caused by continues convergence.Binocular cues require two eyes, whereas monocular cues require only one.This means that even people who have lost sight in one eye may still havegood depth perception. One monocular is accommodation, which is the changein the shape of the lens that lets you focus the image of an object on theretina. Neuron in the rectum assume that the greater the accommodation ofthe lens, the closer the object. But prolonged accommodation can alter yourdepth perception. For example, if you stare at a near object for a longtime and then look at a more distant object, the more distant object willlook farther away than it is. This is attributable to the brain’sovercompensation for the continuous accommodation of the lens while it wasfocused on the near object. A second monocular cue is motion parallax, the tendency to perceiveourselves as passing objects faster when they are closer to us than whenthey are farther away. You will notice this when you drive on a rural road.You perceive yourself passing nearby telephone poles faster than you arepassing a farmhouse.The remaining monocular cues are called pictorial cues because artists usethem to create depth in their drawings and paintings. Leonardo da Vinciformalized pictorial cues 500 year ago in teaching his art students how touse them to make their paintings look more realistic. He noted that anobject that overlaps another object will appear closer, a cue calledinterposition. Because your psychology professor overlaps the blackboard,you know that she or he is closer to you than the blackboard is. Comparingthe relative size of objects also provides a cue to their distance. If twopeople are about the same height and one casts a smaller image on yourretina. You will perceive that person as farther away. You probably have noticed that parallel objects, such as railroad tracks,seem to get closer as the further away (and farther apart as they getcloser). The pictorial cue, linear perspective, may even have practicalapplication. During World War II, naval aviation cadets flying at nightsometimes crashed into airplanes ahead of them, apparently because offailure to judge the distance of those plans. Taking advantage of linearperspective solved this problem. Two taillights set a standard distanceapart replaced the traditional single taillight. As a result, when pilotsnoticed that the taillights of an airplane appeared to move farther apart,they realized that they were getting closer to it.An object’s elevation provides another cue to its distance. Objects thatare higher in your visual field seem to be farther away. If you paint apicture, you create depth by placing more distant objects higher on theCanvas. Shading patterns provide cues to distance because areas that are inshadow tend to recede, while areas that are in light tend to stand out.Painters use shading to make balls, balloons, and organs appear round.Aerial perspective depends on the clarity of objects. Closer objects seemclearer than more distant ones. A distant mountain will look hazier than anear one. The final monocular cue, the texture gradient, affects depth perceptionbecause the nearer an object, the more details we can make out and thefarther an object, the fewer details we can make out. When you look acrossa field, you can see every blade of grass near you, but only an expanse ofgreen far away from you. Even 7 month old infants respond to the texturegradient cue. When presented with drawings that use the texture gradient tomake some objects appear to be in the foreground and others in thebackground, infants will reach for an object in the foreground.2.3.4 Perceptual Constancies The image of a given object focused on your retina may vary in size, shape,and brightness. Yet you will continue to perceive the object as stable insize, shape, and brightness because of perceptual constancy. This isadaptive, because it provides you with a more visually stable world, makingit easier for you to function in it, as an object gets farther away fromyou; it produces a smaller image on your retina. If you know the actualsize of an object, size constancy makes you interpret a change in itsretinal size as a change in its distance rather than a change in its size.When you see a car a block away, it does not seem smaller than one that ishalf a block away, even though the more distant car produces a smallerimage on your retina. Size constancy can be disrupted by alcohol. In onestudy, young adults drank alcohol and were then asked to estimate the sizeof an object. They tended to underestimate its size. Disruption of sizeconstancy might be one way that alcohol intoxication promotes automobileaccidents.Shape constancy assures that an object of known shape will appear tomaintain its normal shape regardless of the angle from which you view it.Close this book and hold it at various orientations relative to your lineof sight. Unless you look directly at the cover when it is on a planeperpendicular to your line of vision, it will never cast a rectangularimage on your retinas, yet you will continue to perceive it as rectangular.Shape constancy occurs because your brain compensates for the slant of anobject relative to your line of sight. Though the amount of light reflected from a given object can vary, weperceive the object as having a constant brightness, this is calledbrightness constancy. A white shirt appears equally bright in dim light orbright light. But brightness constancy is relative to other objects. If youlook at a white shirt in dim light in the presence on nonwhite objects inthe same light in the presence on nonwhite objects in the samelight, it will maintain its brightness. But if you look at the whiteshirt by itself, perhaps by viewing a large area of it though a hollowtube, it will appear dully in dim light and brighter in sunlight. 2.3.5. Perceptual Illusion In Edgar Allen Poe’s “The sphinx” a man looks out of his window and ishorrified by what he perceives to be a monstrous animal on a distantmountain. He learns only later that the“monster” was actually an insect on his window. Because he perceived theanimal as far away, he assumed it was relatively large. And because henever had seen such a creature, he assumed that it was a monster. Thisshows how the misapplication of a visual cue, in this case perceived sizeconstancy, can produce a visual illusion. Visual illusions provide clues to the processes involved in normal visualperception. For example, from ancient times to modern times, people havebeen mystified by the moon illusion illustrated in Figure in which the moonappears larger when it is at the horizon than when it is overhead. This isan illusion because the moon is the same distance from us at the horizon aswhen it is overhead. Thus, the retinal image it produces is the same sizewhen it is at the horizon as when it is overhead. Perhaps Franz Muller-Lyer, developed the most widely studied illusion. Note in Fig B that thevertical line at the bottom appears longer than the one at the top. But ifyou take a ruler and measure the lines, you will find that they are equalin length. In the Muller-Lyer illusion, left (Fig A), in which lines ofequal length appear unequal; the line with the wings pointing outward lookslonger than the line with the wings pointing inward. Fig A Fig B Though no explanation has achieved universal acceptance, a favoredone relies on size constancy and the resemblance of the figure on theright to the inside corner of a room and the resemblance of the figure onthe left to the outside corner of a building. Given that the lines projectimages of equal length on the retina, the lines that appear farther awaywill be perceived as longer. Because an inside corner of a room appearsfarther away than an outside corner of a building, the line on the rightappears farther and, therefore, longer than the line on the left. In general, perception is the act of knowing through sensation. But, somepeople appear to have an ability to know other people, objects, and eventswithout any sensory contact an experience called extra sensory perception(ESP) or paranormal ability. Have you ever heard or experienced suchphenomena? What specific type? Do you believe it is true? Do you thinkpsychologists and scientists believe in ESP? Why?