Sensation and Perception PDF

CHAPTER 5 CHAPTER OUTLINE Sensation and Perception Perception Is Influenced by Expectations: Perceptual Sets Stimuli Are Recognizable under Changing Conditions: Perceptual Constancies SENSORY PROCESSES Stimulus Detection: The Absolute Threshold Signal Detection Theory The Difference Threshold Fo...

CHAPTER 5 CHAPTER OUTLINE Sensation and Perception Perception Is Influenced by Expectations: Perceptual Sets Stimuli Are Recognizable under Changing Conditions: Perceptual Constancies SENSORY PROCESSES Stimulus Detection: The Absolute Threshold Signal Detection Theory The Difference Threshold Focus on Neuroscience: The Neuroscience of Subliminal Perception and Prosopagnosia PERCEPTION OF DEPTH, DISTANCE, AND MOVEMENT Sensory Adaptation Depth and Distance Perception Perception of Movement THE SENSORY SYSTEMS ILLUSIONS: FALSE PERCEPTUAL HYPOTHESES Vision Audition Taste and Smell: The Chemical Senses The Skin and Body Senses Applications: Mona Lisa’s Smile EXPERIENCE, CRITICAL PERIODS, AND PERCEPTUAL DEVELOPMENT Frontiers: Sensory Prosthetics: Restoring Lost Function PERCEPTION: THE CREATION OF EXPERIENCE Perception Is Selective: The Role of Attention Perceptions Have Organization and Structure Perception Involves Hypothesis Testing Cross-Cultural Research on Perception Research Foundations: Critical Periods: The Role of Early Experience Restored Sensory Capacity All our knowledge has its origins in our perceptions. —Leonardo da Vinci What are the issues here? What do we need to know? Where can we find the information to answer these questions? In August 1933, three reporters for the Saint John Telegraph travelled to Moncton to investigate reports of a mysterious hill where cars ran uphill on their own. This was not the first time such stories had emerged. As early as 1880, area farmers noted that horses seemed to be straining with a loaded cart even though they appeared to be going downhill. If the carts were unhitched at the bottom of the hill, they would roll uphill on their own, as would barrels or bales! It was as if some mysterious magnetic force were pulling these items uphill. The three reporters were skeptical and spent the morning looking for the hill with strange magnetic powers. Indeed, they stopped at the bottom of every hill in and around Moncton waiting to see their 1931 Ford Roadster roll uphill. After hours of frustrating searching they stopped at the base of Lutes Mountain and got out of the car to stretch. To their surprise, the roadster calmly rolled uphill away from them. There are at least six magnetic or gravity hills in Canada and hundreds around the world. Not a single site has any unusual magnetic field. N ature gives us a marvellous set of sensory contacts with our world. If our sense organs are not defective, we experience light waves as brightnesses and colours, air vibrations as sounds, chemical substances as odours or tastes, and so on. However, such is not the case for people with a rare and mysterious condition called synesthesia, which means, quite literally, “mixing of the senses” (Cytowic, 2002; Harrison & Baron-Cohen, 1997). They may experience sounds as colours or tastes as touch sensations that have different shapes. Women are more likely to be synaesthetes than men (1 in 1150 versus 1 in 7150, respectively; Rice et al., 2005). Interestingly, Maurer and Mondloch (2006) have suggested that we are all born synaesthetic: The neural pathways of infants are fairly undifferentiated and lead to cross-modal perceptions. The Russian psychologist A.R. Luria (1968) studied a highly successful writer and musician whose life was a perpetual stream of mixed-up sensations. On one occasion, Luria asked him to report on his experiences while listening to electronically generated musical tones. To a medium-pitch tone, the man experienced a brown strip with red edges, together with a sweet-and-sour flavour. A very highpitched tone evoked the following sensation: “It looks something like a fireworks tinged with a pinkred hue. The strip of colour feels rough and unpleasant, and it has an ugly taste—rather like that of a briny pickle. . . . You could hurt your hand on this.” Sensory-impaired people such as those who experience synesthesia provide glimpses into different aspects of how we “sense” and “understand” our world. These processes, previewed in Figure 5.1, begin when specific types of stimuli activate specialized sensory receptors. Whether the stimulus is light, sound waves, a chemical molecule, or pressure, your sensory receptors must translate this information into the only language your nervous system understands: the language of nerve impulses. This process is called transduction. Once this translation occurs, specialized neurons called feature detectors break down and analyze the specific features of the stimuli. At the next stage, these numerous stimulus “pieces” are reconstructed into a neural representation that is then compared with previously stored information, such as our knowledge of what particular objects look, smell, or feel like. This matching of a new stimulus with our internal storehouse of knowledge allows us to recognize the stimulus and give it meaning. We then consciously experience a perception. Sensation Stimulus is received by sensory receptors Receptors translate stimulus properties into nerve impulses (transduction) Feature detectors analyze stimulus features Stimulus features are reconstructed into neural representation Neural representation is compared with previously stored information in brain Matching process results in recognition and interpretation of stimuli Perception FIGURE 5.1 Sensory and perceptual processes proceed from the reception and translation of physical energies into nerve impulses to the active process by which the brain receives the nerve impulses, organizes and confers meaning on them, and constructs a perceptual experience. 1. Describe the six stages that constitute the process of sensory processing and perception of information. 138 CHAPTER FIVE 2. How do psychologists differentiate between sensation and perception? How does this process help us understand the mysterious mixing of the senses in synesthesia? We know that specific parts of the brain are specialized for different sensory functions. In people with synesthesia, there is some sort of cross-wiring, so that activity in one part of the brain evokes responses in another part of the brain dedicated to another sensory modality (Ward, 2008). Functional MRI studies have shown that for people with synesthesia with word-colour linkages, hearing certain words is associated with neural activity in parts of the visual cortex. This activity does not occur in people without synesthesia, even if they are asked to imagine colours in association with certain words (Nunn et al., 2002). Several explanations have been offered for the sensory mixing (Cytowic & Eagleman, 2009; Hubbard & Ramachandran, 2005). One theory is that the pruning of neural connections that occurs in infancy has not occurred in people with synesthesia, so that brain regions retain connections that are absent in most people. In support of this theory, diffusion tensor imaging, which lights up white matter pathways in the brain, has revealed increased connectivity in patients with synesthesia (Rouw & Scholte, 2007). Another theory is that with synesthesia, there is a deficit in neural inhibitory processes in the brain that ordinarily keep input from one sensory modality from “overflowing” into other sensory areas and stimulating them. Whatever the processes involved, both normal perceptual processes and synesthesia relate to one of the big mysteries in cognitive neuroscience called the binding problem. How do we bind all our perceptions into one complete whole while keeping its sensory elements separate? When you hold a rose in your hand, see its coloured petals, feel the petals’ velvety quality, and smell its aroma, these disparate sensory experiences are somehow fused into your total experience of the rose. People with synesthesia may create additional perceptions of that rose that are inconsistent with its physical properties. In some ways, sensation and perception blend together so completely that they are difficult to separate, for the stimulation we receive through our sense organs is instantaneously organized and transformed into the experiences that we refer to as perceptions. Nevertheless, psychologists do distinguish between them. Sensation is the stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain. Perception—making “sense” of what our senses tell us—is the active process of organizing this stimulus input and giving it meaning (Mather, 2006; May, 2007). Because perception is an active and creative process, the same sensory input may be perceived in FIGURE 5.2 Quickly read these two lines of symbols out loud. Did your perception of the middle symbol in each line depend on the symbols that surrounded it? different ways at different times. For example, read the two sets of symbols in Figure 5.2. The middle symbols in both sets of curved lines are exactly the same and they send identical input to your brain, but you probably perceive them differently. Your interpretation, or perception, of the characters is influenced by their context—that is, by the characters that preceded and followed them, and by your learned expectation of what normally follows the letter A and the number 12. This simple illustration shows how perception takes us a step beyond sensation. SENSORY PROCESSES Locked within the silent, dark recesses of your skull, your brain cannot “understand” light waves, sound waves, or the other forms of energy that make up the language of the environment. Contact with the outer world is possible only because certain neurons have developed into specialized sensory receptors that can transform these energy forms into the code language of nerve impulses. The particular stimuli to which different animals are sensitive vary considerably. The sensory equipment of any species is an adaptation to the environment in which it lives. Many species have senses that humans lack altogether. Carrier pigeons, for example, use Earth’s magnetic field to find their destination on cloudy nights when they can’t navigate by the stars. Sharks sense electric currents leaking through the skins of fish hiding in undersea crevices, and rattlesnakes find their prey by detecting infrared radiation given off by small rodents. Whatever the source of stimulation, its energy must be converted into nerve impulses, the only language the nervous system understands (Liedtke, 2006). Transduction is the process whereby the characteristics of a stimulus are converted into nerve impulses. We now consider the range of stimuli to Sensation and Perception which humans and other mammals are attuned and the manner in which the various sense organs carry out the transduction process. As a starting point, we might ask the following: How many senses do humans have? Certainly there appear to be more than the five classical senses with which we are familiar: vision, audition (hearing), touch, gustation (taste), and olfaction (smell). For example, there are senses that provide information about balance and body position. Also, the sense of touch can be subdivided into separate senses of pressure, pain, and temperature. Receptors deep within the brain monitor the chemical composition of our blood. The immune system also has sensory functions that allow it to detect foreign invaders and to receive stimulation from the brain (Nossal & Hall, 1995). Like those of other organisms, human sensory systems are designed to extract from the environment the information that we need to function and survive. Although our survival does not depend on having eyes like eagles or owls, noses like bloodhounds, or ears as sensitive as those of the worm-hunting robin, we do have specialized sensors that can detect many different kinds of stimuli with considerable sensitivity. The scientific area of psychophysics, which studies relations between the physical characteristics of stimuli and sensory capabilities, is concerned with two kinds of sensitivity. The first concerns the absolute limits of sensitivity. For example, what is the softest sound or the weakest salt solution that humans can detect? The second kind of sensitivity has to do with differences between stimuli. What is the smallest difference in brightness that we can detect? How much difference must there be in two tones before we can tell that they are not identical? Stimulus Detection: The Absolute Threshold How intense must a stimulus be before we can detect its presence? Researchers answer this question by systematically presenting stimuli of varying intensities and asking people whether they can detect them. Because we are often unsure of whether we TABLE 5.1 have actually sensed very faint stimuli, researchers designate the absolute threshold as the lowest intensity at which a stimulus can be detected correctly 50 percent of the time. Thus, the lower the absolute threshold, the greater the sensitivity. From studies of absolute thresholds, the general limits of human sensitivity for the five major senses can be estimated. Some examples are presented in Table 5.1. As you can see, many of our senses are surprisingly sensitive. Yet some other species have absolute thresholds that seem incredible by comparison. For example, a female silkworm moth that is ready to mate needs to release 2.8 billionths of a gram of an attractant chemical molecule per second to attract every male silkworm moth within a radius of 1.6 kilometres. Signal Detection Theory I (M.W.P.) can remember lying in bed as a child after seeing a horror movie, straining my ears to detect any unusual sound that might signal the presence of a monster in the house. My vigilance caused me to detect faint and ominous sounds that probably would have gone unnoticed had I seen a comedy or a western earlier in the evening. Perhaps you have had a similar experience. At one time it was assumed that each person had a more or less fixed level of sensitivity for each sense. But psychologists who study stimulus detection found that people’s apparent sensitivity can fluctuate quite a bit. They concluded that the concept of a fixed absolute threshold is inaccurate because there is no single point on the intensity scale that separates nondetection from detection of a stimulus. There is instead a range of uncertainty, and people set their own decision criterion, a standard of how certain they must be that a stimulus is present before they will say they detect it. The decision criterion can also change from time to time, depending on such factors as fatigue, expectation, and the potential significance of the stimulus. Signal detection theory is concerned with the factors that influence sensory judgments. In a typical signal detection experiment, participants are told that after a warning light appears, Some Approximate Absolute Thresholds for Various Senses Sense Modality Absolute Threshold Vision Hearing Taste Smell Touch Candle flame seen at approximately 50 kilometres on a clear, dark night Tick of a watch under quiet conditions at approximately 6 metres Single teaspoon of sugar in approximately 7.5 litres of water One drop of perfume diffused into the entire volume of a large apartment Wing of a fly or bee falling on your cheek from a distance of 1 centimetre Source: Based on Galanter, 1962. 139 3. What two kinds of sensory capabilities are studied by psychophysics researchers? 4. What is the absolute threshold, and how is it technically defined and measured? 5. Why do signal detection theorists view stimulus detection as a decision? 6. What kinds of personal and situational factors influence signal detection decision criteria? 140 CHAPTER FIVE The Difference Threshold Participant’s response Stimulus 7. What is the technical definition of a difference threshold? How does Weber’s law help us compare just noticeable difference (jnd) sensitivities in the various senses? Present Absent “Yes” Hit False alarm “No” Miss Correct rejection FIGURE 5.3 This matrix shows the four possible outcomes in a signal detection experiment in which participants decide whether a stimulus has been presented or not presented. The percentages of responses that fall within each category can be affected both by characteristics of the participants and by the nature of the situation. a barely perceptible tone may or may not be presented. Their task is to tell the experimenter whether they heard the tone. Under these conditions, there are four possible outcomes, as shown in Figure 5.3. When the tone is in fact presented, the participant may say “Yes” (a hit) or “No” (a miss). When no tone is presented, the participant may also say “Yes” (a false alarm) or “No” (a correct rejection). At low stimulus intensities, both the participant’s and the situation’s characteristics influence the decision criterion (Colonius & Dzhafarov, 2006; Pitz & Sachs, 1984, Verghese, 2001). Bold participants who frequently say “Yes” have more hits, but they also have more false alarms than do conservative participants. Participants also can be influenced to become bolder or more conservative by manipulating the rewards and costs for giving correct or incorrect responses. Increasing the rewards for hits or the costs for misses results in lower detection thresholds (i.e., more “Yes” responses at low intensities). Thus, a Navy radar operator may be more likely to notice a faint blip on her screen during a wartime mission, when a miss might have disastrous consequences, than during a peacetime voyage. Conversely, like physicians who will not perform a risky medical procedure without strong evidence to support their diagnosis, participants become more conservative in their “Yes” responses as costs for false alarms are increased, resulting in higher detection thresholds (Irwin & McCarthy, 1998). Experience also plays a role in signal detection—experienced drivers respond more quickly to signs of danger partly because they have a lower threshold for detecting and identifying hazardous situations than do novice drivers (Wallis & Horswill, 2007). Signal detection research shows us that perception is, in part, a decision. Distinguishing between stimuli can sometimes be as important as detecting stimuli in the first place. When we try to match the colours of paints or clothing, very subtle differences can be quite important. Likewise, a slight variation in taste might signal that food is tainted or spoiled. Professional wine tasters and piano tuners make their livings by being able to make very slight discriminations between stimuli. The difference threshold is defined as the smallest difference between two stimuli that people can perceive 50 percent of the time. The difference threshold is sometimes called the just noticeable difference (jnd). Fortunately, as the German physiologist Ernst Weber (pronounced Veh-ber) discovered in the 1830s, there is some degree of lawfulness in the range of sensitivities within our sensory systems. Weber’s law states that the difference threshold, or jnd, is directly proportional to the magnitude of the stimulus with which the comparison is being made, and can be expressed as a Weber fraction. For example, the jnd value for weights is a Weber fraction of approximately 1/50 (Teghtsoonian, 1971). This number means that if you lift a weight of 50 grams, a comparison weight must weigh at least 51 grams in order for you to be able to judge it as heavier. If the weight were 500 grams, a second weight would have to weigh at least 510 grams (i.e., 1/50 5 10 grams/500 grams) for you to discriminate between them. Although Weber’s law breaks down at extremely high and low intensities of stimulation,1 it holds up reasonably well within the most frequently encountered range, therefore providing a reasonable barometer of our abilities to discern differences in the various sensory modalities. Table 5.2 lists Weber fractions for the various senses. The TABLE 5.2 Weber Fractions for Various Sensory Modalities Sensory Modality Weber Fraction Audition (tonal pitch) Vision (brightness, white light) Kinesthesis (lifted weights) Pain (heat produced) Audition (loudness) Touch (pressure applied to skin) Smell (India rubber) Taste (salt concentration) 1/333 1/60 1/50 1/30 1/20 1/7 1/4 1/3 Sources: Geldard, 1962; Teghtsoonian, 1971. 1This breakdown led Gustav Fechner to develop his own, more general, law in 1851. Fechner’s Law states that perceived sensation is proportional to the logarithm of physical stimulus intensity. Sensation and Perception 141 Focus on Neuroscience THE NEUROSCIENCE OF SUBLIMINAL PERCEPTION AND PROSOPAGNOSIA A subliminal stimulus is one that is so weak or brief that, although it is received by the senses, it cannot be perceived consciously—the stimulus is well below the absolute threshold. There is little question that subliminal stimuli can register in the nervous system (Kihlstrom, 2008; Matthen, 2007; Merikle & Daneman, 1998). But can such stimuli affect attitudes and behaviour without our knowing it? The answer appears to be yes—to a limited extent. In the late 1950s, James Vicary, a public-relations executive, arranged to have subliminal messages flashed on a theatre screen during a movie. The messages urged the audience to “drink Coca-Cola” and “eat popcorn.” Vicary’s claim that the subliminal messages increased popcorn sales by 50 percent and soft-drink sales by 18 percent aroused a public furor. Consumers and scientists feared possible abuse of subliminal messages to covertly influence the buying habits of consumers, and even to achieve mind control and brainwashing. The National Association of Broadcasters reacted by outlawing subliminal messages on American TV. The outcries were, in large part, false alarms. Several attempts to reproduce Vicary’s results under controlled conditions failed, and many other studies conducted in laboratory settings, on TV and radio, and in movie theatres indicated that there is little reason to be seriously concerned about significant or widespread control of consumer behaviour through subliminal stimulation (Dixon, 1981; Drukin, 1998). Ironically, Vicary admitted years later that his study was a hoax, designed to revive his floundering advertising agency. Nonetheless, his false report stimulated a great deal of useful research on the power of subliminal stimuli to influence behaviour. As far as consumer behaviour is concerned, the conclusion is that persuasive stimuli above the absolute threshold are far more influential than subliminal attempts to sneak into our subconscious mind, perhaps because we are more certain to “get the message.” Though consumer behaviour cannot be controlled subliminally, can such stimuli affect more subtle phenomena, such as attitudes? Here the effects are stronger (Arendt et al., 1997; Greenwald & Banaji, 1995). In one study, Jon Krosnick (1992) showed participants nine slides of a particular person and then measured their attitudes toward the target person. For half of the participants, each photograph was immediately preceded by an unpleasant picture (e.g., a face on fire) that was presented subliminally. The remaining participants were shown pleasant subliminal stimuli, such as smiling babies. Participants shown the associated unpleasant subliminal stimuli expressed somewhat negative attitudes toward the person, indicating a process of subconscious attitude conditioning, whereas those who saw the positive subliminal stimuli did not. Evidence consistent with subliminal perception can be seen when examining patients who have very specific types of brain damage. For example, individuals with prosopagnosia are unable to recognize familiar faces. In essence, they have a type of visual agnosia that is specific for faces. Such individuals typically have cortical damage in areas involved with object perception. In some cases, they may be aware that they are looking at a face, but they cannot tell you who the individual is. Nonetheless, they may be able to categorize the visual stimulus as a face, and some patients can correctly “guess” who the face belongs to. How can this happen if the stimuli cannot be perceived? Consider the following study by J.K. Steeves and colleagues (2006). Steeves et al. studied patient D.F., a 47-year-old woman who suffered brain damage at age 34 from accidental carbon monoxide poisoning. D.F. has a great deal of difficulty recognizing the size, shape, and orientation of objects, but she is able to perceive colour. Thus, she is often able to recognize objects (e.g., an orange versus a tomato) based on colour and texture information alone. Similarly, people may be identified by nonfacial cues, such as clothing choice and voice pitch. Earlier studies using fMRI imaging (Culham, 2004; James et al., 2003) had identified specific lesions in D.F.’s cortex. In particular, damage was observed in the lateral occipital area (LOA) in both hemispheres (Figure 5.4). The LOA has been associated with object perception in the intact cortex. D.F. and three control participants with no brain damage were shown a series of face and object stimuli while imaging with fMRI. Activation was examined in the LOA and in a second area associated with facial processing: the fusiform gyrus. Here we find the fusiform facial area (FFA), a brain region specifically associated with facial perception (Barton, Press, Keenan, & O’Connor, 2002). For all participants, including D.F., there was greater brain activation in the FFA when viewing faces than when viewing scenes. However, the control participants showed greater activation in the LOA as well when viewing faces. This area was damaged in D.F. Despite this damage, D.F. was able to accurately categorize the stimuli as faces versus objects 95 percent of the time. In a second test, D.F. was shown a series of 30 images (5 faces and 25 objects) and was asked to describe what they were. All five faces were accurately identified as a face, but not one of the objects was correctly described. In a third test, all participants were shown a series of 60 famous individuals (e.g., John F. Kennedy, Princess Diana) and were asked to name them or provide information about the individual if they could not come up with the name. The controls continued 142 CHAPTER FIVE FFA (on underside) LOA FIGURE 5.4 Approximate locations of the lateral occipital area (LOA) and the fusiform facial area (FFA). The FFA is actually on the underside or ventral surface of the cortex. correctly identified 93 percent of the images; D.F. could not identify a single one. There are three points we should take away from this study. First, it would appear that higher-order facial recognition is a complex process involving several brain regions, including the LOA and the FFA, in addition to the primary visual cortex. Nonetheless, an individual such as D.F. can glean a certain amount of information about visual stimuli even when one of these areas is severely damaged. It is likely that D.F. uses certain heuristic rules to “identify” faces (e.g., elongated oval targets with skin tone are likely to be faces) even though she is not really aware that the stimulus is, in fact, a face. Second, this research emphasizes the importance of the case study to investigate psychological phenomena. D.F. is a unique individual who provides an extraordinary opportunity to examine the role of brain regions in visual processing. In addition, the combination of behavioural testing and fMRI imaging allows the researchers to precisely identify the regions and deficits involved with this disorder. Finally, the study highlights the subtle manner in which subliminal stimuli may have an effect. Philip Merikle and his colleagues (e.g., Merikle & Skanes, 1992; Merikle et al., 2001) have argued that the effect is one of biasing perception— subliminal cues can bias what we perceive at a conscious level and may alter our conscious experience of those stimuli. Todorov & Bargh (2002) demonstrated that subliminal presentations of aggressively toned words cause people to judge the ambiguous behaviours of others as more aggressive and to increase their own tendency to behave more aggressively. More recently, Radel et al. (2013) have shown that exposure to sublimninal motivational conversations results in greater perservance on difficult tasks. We may not be consciously aware of stimuli, but perhaps, like D.F., aspects of the stimuli are processed at a different level and are available for us to use in subsequent decisions. smaller the fraction, the greater the sensitivity to differences. As highly visual creatures, humans show greater sensitivity in their visual sense than they do in, for example, their sense of smell. Undoubtedly, many creatures who depend on their sense of smell to track their prey would show quite a different order of sensitivity. Weber fractions also show that humans are highly sensitive to differences in the pitch of sounds but far less sensitive to loudness differences. 8. What accounts for sensory adaptation? Of what survival value is adaptation? Sensory Adaptation Because changes in our environment are often most newsworthy, sensory systems are finely attuned to changes in stimulation (Rensink, 2002). Sensory neurons are engineered to respond to a constant stimulus by decreasing their activity, and the diminishing sensitivity to an unchanging stimulus is called sensory adaptation. Adaptation (sometimes called habituation) is a part of everyday experience. After a while, monotonous background sounds are largely unheard. The feel of your wristwatch against your skin recedes from awareness. When you dive into a swimming pool, the water may feel cold at first because your body’s temperature sensors respond to the change in temperature. With time, however, you become used to the water temperature. Adaptation occurs in all sensory modalities, including vision. Indeed, were it not for tiny involuntary eye movements that keep images moving about the retina, stationary objects would simply fade from sight if we stared at them (MartinezConde, MacKnik, & Hubel, 2004). In an ingenious demonstration of this variety of adaptation, R.M. Pritchard (1961) attached a tiny projector to a contact lens worn by the participant (Figure 5.5a). This procedure guaranteed that visual images presented through the projector would maintain a constant position on the retina, even when the eye moved. When a stabilized image was projected through the lens onto the retina, participants reported that the image appeared in its entirety for a time, then began to vanish and reappear as parts of the original stimulus (Figure 5.5b). Sensation and Perception In Review • Sensation refers to the activities by which our sense organs receive and transmit information, whereas perception involves the brain’s processing and interpretation of the information. perceptions and behaviour in subtle ways, but not strongly enough to justify concerns about the subconscious control of behaviour through subliminal messages. • Psychophysics is the scientific study of how the physical properties of stimuli are related to sensory experiences. Sensory sensitivity is concerned in part with the limits of stimulus detectability (absolute threshold) and the ability to discriminate between stimuli (difference threshold). The absolute threshold is the intensity at which a stimulus is detected 50 percent of the time. Signal detection theory is concerned with factors that influence decisions about whether or not a stimulus is present. • The difference threshold, or just noticeable difference (jnd), is the amount by which two stimuli must differ for them to be perceived as different 50 percent of the time. Studies of the jnd led to Weber’s law, which states that the jnd is proportional to the intensity of the original stimulus and is constant within a given sense modality. • Sensory systems are particularly responsive to changes in stimulation, and adaptation occurs in response to unchanging stimuli. • Research indicates that subliminal stimuli, which are not consciously perceived, can influence (a) Original scene Perceptions Although sensory adaptation may reduce our overall sensitivity, it is adaptive because it frees our senses from the constant and the mundane to pick up informative changes in the environment. Sensitivity to such changes may turn out to be important to our well-being or survival—for example, by alerting us to potential threats. Sensory adaptation may be a “back-up measure” of sorts, for when we are not actively and consciously processing sensory stimuli in our environment. In one study, CastroAlamancos (2004) reported that sensory adaptation was mostly absent in animals while they were alert and engaged in a behavioural learning task, whereas after the task was learned and had become routine, levels of alertness lowered and sensory adaptation returned. THE SENSORY SYSTEMS Vision (b) FIGURE 5.5 (a) To create a stabilized retinal image, a person wears a contact lens to which a tiny projector has been attached. Despite eye movements, images will be cast on the same region of the retina. (b) Under these conditions, the stabilized image is clear at first, and then begins to fade and reappear in meaningful segments as the receptors fatigue and recover. Adapted from Pritchard, 1961. The normal stimulus for vision is electromagnetic energy, or light waves, which are measured in nanometres (or one billionths of a metre). In addition to that tiny portion that humans can perceive, the electromagnetic spectrum includes X-rays, TV and radio signals, and infrared and ultraviolet rays ( Figure 5.6 ). Bees are able to “see” ultraviolet light, and rattlesnakes can detect infrared energy. Our visual system is sensitive only to wavelengths extending from about 700 nanometres (red) down to about 400 nanometres (blue-violet). (You can remember the order of the spectrum, from higher wavelengths to lower ones, with the name ROY G. BIV—red, orange, yellow, green, blue, indigo, and violet.) 143 144 CHAPTER FIVE 101 Ultraviolet Rays 103 105 107 Infra- Radar red Rays 109 FM Radio 1015 1011 1013 AC Circuits TV AM Radio Infrared Ultraviolet 10⫺3 10⫺1 Gamma X-Rays Rays 400 500 600 Wavelength (nanometres) 700 FIGURE 5.6 The full spectrum of electromagnetic radiation. Only the narrow band between 400 and 700 nanometres is visible to the human eye. One nanometre 5 1 000 000 000th of a metre. 9. How does the lens affect visual acuity, and how does its dysfunction cause the visual problems of myopia and hyperopia? The Human Eye Light waves enter the eye through the cornea, a transparent protective structure at the front of the eye (Figure 5.7a). Behind the cornea is the pupil, an adjustable opening that can dilate or constrict to control the amount of light that enters the eye. The pupil’s size is controlled by muscles in the coloured iris that surrounds the pupil. Low levels of illumination cause the pupil to dilate, letting more light into the eye to improve optical clarity; bright light triggers constriction of the pupil. Behind the pupil is the lens, an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects. Just as the lens of a camera focuses an image on a photosensitive material (film), so the lens of the eye focuses the visual image on the light-sensitive retina, a multi-layered tissue at the rear of the fluid-filled eyeball. As seen in Figure 5.7a, the lens reverses the image from right to left and top to bottom when it is projected on the retina, but the brain reconstructs the visual input into the image that we perceive. The ability to see clearly depends on the lens’s ability to focus the image directly onto the retina (Pedrotti & Pedrotti, 1997). If you have good vision for nearby objects but have difficulty seeing faraway objects, then you probably suffer from myopia (nearsightedness). In nearsighted people, the lens focuses the visual image in front of the retina (too near the lens), resulting in a blurred image for faraway objects. This condition generally occurs because the eyeball is longer (front to back) than normal. In contrast, some people have excellent distance vision but have difficulty seeing closeup objects clearly. Hyperopia (farsightedness) occurs when the lens does not thicken enough and the image is therefore focused on a point behind the retina (too far from the lens). The aging process typically causes the eyeball to become shorter over time, contributing to the development of hyperopia and the need for many middle-aged people to acquire reading glasses (after complaining that their arms are not long enough to read newspapers and telephone books). Ironically, this age-related shortening of the eyeball often improves the vision of myopic people, for, as the retina moves closer to the lens, it approaches the point where the “nearsighted” lens is projecting the image (Orr, 1998). Eyeglasses and contact lenses are designed to correct for the natural lens’s inability to focus the visual image directly onto the retina. Recent research (Li, Polat, & Bavelier, 2009) suggests that playing action video games might also be effective in improving eyesight, though it’s unlikely that playing video games will replace the need for corrective lenses! Photoreceptors: The Rods and Cones The retina, a multi-layered screen that lines the back surface of the eyeball and contains specialized sensory neurons, is actually an extension of the brain (Bullier, 2002). The retina contains two types of light-sensitive receptor cells, called rods and cones because of their shapes (Figure 5.7b). There are about 120 million rods and 6 million cones in the human eye. Sensation and Perception Ganglion Amacrine cells cells Iris Bipolar Horizontal cells cells 145 Cone Rod Retina Light Cornea Fovea Back of eye Light Pupil Light Lens Ciliary muscles Optic nerve to the brain Blind spot (optic disk) (a) Optic nerve fibres (to brain) Ganglion cell layer Bipolar cell layer Photoreceptor layer (b) FIGURE 5.7 (a) This cross-section shows the major parts of the human eye. The iris regulates the size of the pupil. The ciliary muscles regulate the shape of the lens. The image entering the eye is reversed by the lens and cast on the retina, which contains the photoreceptor cells. The optic disk, where the optic nerve exits the eye, has no receptors and produces a “blind spot” as demonstrated in Figure 5.8. (b) Photoreceptor connections in the retina. The rods and cones synapse with bipolar cells, which in turn synapse with ganglion cells, whose axons form the optic nerve. The horizontal and amacrine cells allow sideways integration of retinal activity across areas of the retina. The rods, which function best in dim light, are primarily black-and-white brightness receptors. They are about 500 times more sensitive to light than are the cones, but they do not give rise to colour sensations. The retinas of some night creatures, such as the owl, contain only rods, so they have exceptional vision in very dim light but no colour vision during the day (Dossenbach & Dossenbach, 1998). The cones, which are colour receptors, function best in bright illumination. Some creatures that are active only during the day, such as the pigeon and the chipmunk, have only cones in their retinas, so they see the world in living colour but have very poor night vision (Dossenbach & Dossenbach, 1998). Animals that are active during both day and night, as humans are, have a mixture of rods and cones. In humans, rods are found throughout the retina except in the fovea, a small area in the centre of the retina that contains only cones. Cones decrease in concentration as one moves away from the centre of the retina, and the periphery of the retina contains mainly rods. Rods and cones send their messages to the brain via two additional layers of cells. Bipolar cells have synaptic connections with the rods and cones. The bipolar cells, in turn, synapse with a layer of about one million ganglion cells, whose axons are collected into a bundle to form the optic nerve. Thus, input from more than 126 million rods and cones is eventually funnelled into only one million traffic lanes leading out of the retina toward higher visual centres. Figure 5.7b shows how the rods and cones are connected to the bipolar and ganglion cells. One interesting aspect of these connections is the fact that the rods and cones not only form the rear layer of the retina, but their light-sensitive ends actually point away from the direction of the entering light so that they receive only a fraction of the light energy that enters the eye. Furthermore, the manner in which the rods and cones are connected to the bipolar cells accounts for both the greater importance of rods in dim light and our greater ability to see fine detail in bright illumination, when the cones are most active. Typically, many rods are connected to the same bipolar cell. They therefore can combine or “funnel” their individual electrical messages to the bipolar cell, where the additive effect of the many signals may be enough to fire it. That is why we can more easily detect a faint stimulus, such as a dim star, if we look slightly to one side so that its image falls not on the fovea but on the peripheral portion of the retina, where the rods are packed most densely. Like the rods, the cones that lie in the periphery of the retina also share bipolar cells. In the fovea, however, the densely packed cones each have their own “private line” to a single bipolar cell. As a result, our visual acuity, or ability to see fine detail, is greatest when the visual image projects directly onto the fovea. Such focusing results in the firing of a large number of cones and their private-line bipolar cells. Some birds of prey, such as eagles and hawks, are blessed with not one, but two foveas in each eye, contributing to a visual acuity that allows them to see small prey on the ground as they soar high above the earth (Tucker, 2000). The optic nerve formed by the axons of the ganglion cells exits through the back of the eye not far from the fovea, producing a blind spot, where there are no photoreceptors. You can demonstrate the existence of your blind spot by following the 10. How are the rods and cones distributed in the retina, and how do they contribute to brightness perception, colour vision, and visual acuity? CHAPTER FIVE X FIGURE 5.8 Close your left eye and, from a distance of about 30 centimetres, focus steadily on the dot with your right eye as you slowly move the book toward your face. At some point, the image of the X will cross your optic disk (blind spot) and disappear. It will reappear after it crosses the blind spot. Note how the checkerboard remains wholly visible even though part of it falls on the blind spot. Your perceptual system “fills in” the missing information. directions for the demonstration in Figure 5.8. Ordinarily, we are unaware of the blind spot because our perceptual system “fills in” the missing part of the visual field (Rolls & Deco, 2002). 11. What is transduction, and how does this process occur in the photoreceptors of the eye? 12. How is brightness sensitivity in rods and cones affected by the colour spectrum? 13. What is the physiological basis for dark adaptation? What are the two components of the dark adaptation curve? Visual Transduction: From Light to Nerve Impulses The process whereby the characteristics of a stimulus are converted into nerve impulses is called transduction. Rods and cones translate light waves into nerve impulses through the action of protein molecules called photopigments (Stryer, 1987; Wolken, 1995). The absorption of light by these molecules produces a chemical reaction that changes the rate of neurotransmitter release at the receptor’s synapse with the bipolar cells (Burns & Arshavsky, 2005). The greater the change in transmitter release, the stronger the signal passed on to the bipolar cell and, in turn, to the ganglion cells whose axons form the optic nerve. If nerve responses are triggered at each of the three levels (rod or cone, bipolar cell, and ganglion cell), the message is instantaneously on its way to the visual relay station in the thalamus, and then on to the visual cortex of the brain. Brightness Vision and Dark Adaptation As noted earlier, rods are far more sensitive than cones under conditions of low illumination. Nonetheless, the brightness sensitivity of both the rods and the cones depends in part on the wavelength of the light. Research has shown that rods have much greater brightness sensitivity than cones throughout the colour spectrum except at the red end, where rods are relatively insensitive. Cones are most sensitive to low illumination in the greenishyellow range of the spectrum (Valberg, 2006). These findings have prompted many cities to change the colour of their fire engines from the traditional red (which rods are insensitive to) to yellow-green in order to increase the vehicles’ visibility to both rods and cones in dim lighting. Similarly, airport landing lights are often blue because this wavelength is picked up particularly well by the rods during night vision, when the cones are relatively inoperative. Although the rods are by nature sensitive to low illumination, they are not always ready to fulfill their function. Perhaps you have had the embarrassing experience of entering a movie theatre from bright sunlight, groping around in the darkness, and finally sitting down on someone’s lap. Although one can meet interesting people this way, most of us prefer to stand in the rear of the theatre until our eyes adapt to the dimly lit interior. Dark adaptation is the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination. After absorbing light, a photoreceptor is depleted of its pigment molecules for a period of time. If the eye has been exposed to conditions of high illumination, such as bright sunlight, a substantial amount of photopigment will be depleted. During the process of dark adaptation, the photopigment molecules are regenerated, and the receptor’s sensitivity increases greatly. Vision researchers have plotted the course of dark adaptation as people move from conditions of bright light into darkness (Carpenter & Robson, 1999). By focusing light flashes of varying wavelengths and brightness on the fovea, which contains only cones, or on the periphery of the retina, where rods reside, they discovered the two-part curve shown in Figure 5.9. The first part of the curve is due to dark adaptation of the cones. As you can see, the cones gradually become sensitive to fainter lights as time passes, but after about 5 to 10 minutes Intensity of light to produce vision 146 Rods only Cones only 0 10 20 30 40 Time in dark (in minutes) FIGURE 5.9 The course of dark adaptation is graphed over time. The curve has two parts, one for the cones and one for the rods. The cones adapt completely in about 10 minutes, whereas the rods continue to increase their sensitivity for another 20 minutes. Sensation and Perception in the dark, their sensitivity has reached its maximum. The rods, whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about half an hour. It is estimated that after complete adaptation, rods are able to detect light intensities only 1/10 000 as great as those that could be detected before dark adaptation began (May, 2007; Stryer, 1987). During World War II, psychologists familiar with the facts about dark adaptation provided a method for enhancing night vision in pilots who needed to take off at a moment’s notice and see their targets under conditions of low illumination. Knowing that the rods are important in night vision and relatively insensitive to red wavelengths, they suggested that fighter pilots either wear goggles with red lenses or work in rooms lit only by red lights while waiting to be called for a mission. Because red light stimulates only the cones, the rods remain in a state of dark adaptation, ready for immediate service in the dark. That highly practical principle continues to be useful to this day (Figure 5.10). Colour Vision We are blessed with a world rich in colour. The majesty of a glowing sunset, the rich blues and greens of a tropical bay, the brilliant colours of fall foliage all produce visual delights for us. Human vision is finely attuned to colour; our difference thresholds for light wavelengths are so small that we are able to distinguish an estimated 7.5 million hue variations (a) FIGURE 5.10 Working in red light keeps the rods in a state of dark adaptation because rods are quite insensitive to that wavelength. Therefore, they retain high levels of photopigment and remain sensitive to low illumination. (Medieros, 2006). Historically, two different theories of colour vision have tried to explain how this occurs. The trichromatic theory. Around 1800, it was discovered that any colour in the visible spectrum can be produced by some combination of the wavelengths that correspond to the colours blue, green, and red in what is known as additive colour mixture (Figure 5.11a). This fact was the basis of an important trichromatic (three-colour) theory of colour vision advanced by Thomas Young, an English physicist, and Hermann von Helmholtz, a German physiologist. According to the Young–Helmholtz trichromatic theory, there are three types of colour receptors in the retina. Although all cones can be stimulated by most wavelengths to varying degrees, individual cones are most sensitive to wavelengths (b) FIGURE 5.11 Additive and subtractive colour mixture are different processes. (a) Additive colour mixture. A beam of light of a specific wavelength directed onto a white surface is perceived as the colour that corresponds to that wavelength on the visible spectrum. If beams of light that fall at certain points within the red, green, or blue colour range are directed together onto the surface in the correct proportions, a combined or additive mixture of wavelengths will result and any colour in the visible spectrum can be produced (including white at the point where all three colours intersect). The Young–Helmholtz trichromatic theory of colour vision assumes that colour perception results from the additive mixture of impulses from cones that are sensitive to red, blue, and green (see text). (b) Subtractive colour mixture. Mixing pigments or paints produces new colours by subtraction—that is, by removing (i.e., absorbing) other wavelengths. Paints absorb (subtract) colours different from themselves while reflecting their own colour. For example, blue paint mainly absorbs wavelengths that correspond to nonblue hues. Mixing blue paint with yellow paint (which absorbs wavelengths other than yellow) will produce a subtractive mixture that emits wavelengths between yellow and blue (i.e., green). Theoretically, certain wavelengths of the three primary colours of red, yellow (not green, as in additive mixture), and blue can produce the whole spectrum of colours by subtractive mixture. Thus, in additive colour mixture, the primary colours are red, blue, and green; in subtractive colour mixture, they are red, yellow, and blue. 147 148 CHAPTER FIVE 14. Describe the Young–Helmholtz trichromatic theory of colour vision. What kinds of evidence support this theory, and what two phenomena challenge it? 15. Describe the opponent-process theory. What evidence supports it? that correspond to either blue, green, or red (Figure 5.12). Presumably, each of these receptor classes sends messages to the brain, based on the extent to which they are activated by the light energy’s wavelength. The visual system then combines the signals to recreate the original hue. If all three cones are equally activated, a pure white colour is perceived. Although the Young–Helmholtz theory was consistent with the laws of colour mixture, several facts did not fit the theory. For example, according to the theory, yellow is produced by activity of red and green receptors. Yet certain people with red-green colour blindness are able to experience yellow. This finding suggested to other scientists that there must be a different means of perceiving yellow. A second phenomenon that posed problems for the trichromatic theory was the colour afterimage, in which an image in a different colour appears after a colour stimulus has been viewed steadily and then withdrawn. To experience one yourself, stare steadily at the object in Figure 5.13 for a full minute, and then shift your gaze to a blank white space. Trichromatic theory cannot account for what you’ll see. Opponent-process theory. A second influential colour theory, formulated by Ewald Hering in 1870, also assumed that there are three types of cones. Hering’s opponent-process theory proposed that each of Trichromatic theory Retinal receptors Blue Green Red To brain Opponent-process theory Retinal receptors B Y G R the three cone types responds to two different wavelengths. One type responds to red or green, another to blue or yellow, and a third to black or white. For example, a red-green cone responds with one chemical reaction to a green stimulus and with its other chemical reaction (opponent process) to a red stimulus (Figure 5.12). You have experienced one of the phenomena that supports the existence of opponent processes if you did the exercise in Figure 5.13. The colour afterimage you saw in the blank space contains the colours specified by opponent-process theory: The black portion of the flag appeared as white, and the green portion “turned” red. According to opponent-process theory, as you stared at the black and green colours, the neural processes that register these colours became fatigued. Then, when you cast your gaze on the white surface, which reflects all wavelengths, a “rebound” opponent reaction occurred as each receptor responded with its opposing white or red reactions. Dual processes in colour transduction. Which theory—the trichromatic theory or the opponentprocess theory—is correct? Two centuries of research have yielded a win-win verdict for both sets of theorists. Today’s dual-process theory combines the trichromatic and opponent-process theories to account for the colour transduction process (Valberg, 2006). Trichromatic theorists, such as Young and Helmholtz, were right about the cones. The cones do indeed contain one of three different protein photopigments that are most sensitive to wavelengths roughly corresponding to the colours blue, red, and green (Valberg, 2006). Different ratios of activity in the red-, blue-, and green-sensitive cones can produce a pattern of neural activity that corresponds to any hue in the spectrum (Backhaus et al., 1998). This process is similar to that which occurs on your TV screen, where colour pictures (including white W B To brain FIGURE 5.12 Two classic theories of colour vision. The Young– Helmholtz trichromatic theory proposed three different receptors, one for blue, one for red, and one for green. The ratio of activity in the three types of cones in response to a stimulus yields our experience of colour. Hering’s opponent-process theory also assumed that there are three different receptors: one for yellow-blue, one for red-green, and one for black-white. Each of the receptors can function in two possible ways, depending on the wavelength of the stimulus. Again, the pattern of activity in the receptors yields our perception of the hue. FIGURE 5.13 Negative colour afterimages demonstrate opponent processes occurring somewhere in the visual system. Stare steadily at the black dot in the centre of the flag for about a minute, then shift your gaze to a blank, white page. The opponent colours should appear. Sensation and Perception hues) are produced by activating combinations of tiny red, green, and blue dots. Hering’s opponent-process theory was also partly correct, but opponent processes do not occur at the level of the cones, as he maintained. When researchers began to use microelectrodes to record from single cells in the visual system, they discovered that certain ganglion cells in the retina, as well as some neurons in visual relay stations and the visual cotrex, respond in an opponent-process fashion by altering their rate of firing (DeValois & DeValois, 1993; Gegenfurtner & Kiper, 2003; Knoblauch, 2002; Pridmore, 2013). For example, if a red light is shone on the retina, an opponentprocess ganglion cell may respond with a high rate of firing, but a green light will cause the same cell to fire at a very low rate. Other neurons respond in a similar opponent fashion to blue and yellow stimuli. The red-green opponent processes are triggered directly by input from the red- or green-sensitive cones in the retina (Figure 5.14). The blue-yellow opponent process is a bit more complex. Activity of blue-sensitive cones directly stimulates the “blue” process farther along in the visual system. And yellow? The yellow opponent process is triggered not by a “yellow-sensitive” cone, as Hering proposed, but rather by simultaneous input from the red- and green-sensitive cones (Valberg, 2006). Colour-deficient vision. People with normal colour vision are referred to as trichromats. They are sensitive to all three systems: red-green, yellow-blue, 500 600 16. How does the dual-process theory of colour vision combine the trichromatic and opponent-process theories? 17. What are the two major types of colour-blindness? How are they tested? Analysis and Reconstruction of Visual Scenes Once the transformation of light energy to nerve impulses occurs, the process of combining the messages received from the photoreceptors into the perception of a visual scene begins. As you read this page, nerve impulses from countless neurons are being analyzed and the visual image that you perceive is being reconstructed. Moreover, you know what these black squiggles on the page “mean.” How does this occur? Feature detectors. From the retina, the optic nerve sends nerve impulses to a visual relay station in the thalamus, the brain’s sensory switchboard. From there, the input is routed to various parts of the 700 Responsiveness of cone receptors 400 and black-white. However, about 7 percent of the male population and 1 percent of the female population have a deficiency in the red-green system, the yellow-blue system, or both. This deficiency is caused by an absence of hue-sensitive photopigment in certain cone types. A dichromat is a person who is colour-blind in only one of the systems (redgreen or yellow-blue). A monochromat is sensitive only to the black-white system and is totally colourblind. Most colour-deficient people are dichromats and have their deficiency in the red-green system. Tests of colour-blindness typically contain sets of coloured dots such as those in Figure 5.15. Depending on the type of deficit, a colour-blind person cannot discern certain numbers embedded in the circles. 149 Three kinds of cones (trichromatic) Yellow Blue Short-wavelength cones Medium-wavelength cones Long-wavelength cones or Green or Red Opponent-process mechanisms Input to brain FIGURE 5.14 Colour vision involves both trichromatic and opponent processes that occur at different places in the visual system. Consistent with trichromatic theory, three types of cones are maximally sensitive to short (blue), medium (green), and long (red) wavelengths, respectively. However, opp