CHAPTER 4. Test Yourself, p. 151

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1 CHAPTER 4 Test Yourself, p What is sensation, and how does it differ from perception? Visual perception is accomplished in two broad phases: 1. First, a person registers visual sensations (the awareness of properties of an object or event that occurs when a type of receptor is stimulated). Sensations arise when enough physical energy strikes a sense organ so that receptor cells send neural impulses to the brain. 2. Second, a person actually perceives the stimulus; perception occurs when a person has organized and interpreted the sensory input as signaling a particular object or event. Perception relies on two types of processing a. At the outset, a person must organize patches of color, texture, edges, and other basic visual elements into coherent units, which usually correspond to surfaces and objects. As part of this process, the sizes and locations of objects must be specified. b. The final task of perception is to recognize and identify what is seen, for example, to realize that the object is a haystack and not a honeycomb. 2. What is the nature of light, and how do we see color, shape, and motion? Visual Sensation: More Than Meets the Eye 1. People know the world only as it is filtered though their senses and senses are not always accurate. 2. Scientists have developed careful methods for discovering the relation between what's actually out there and what people sense and perceive; this field is known as psychophysics (the study of the relation between physical events and the corresponding experience of those events). a. Over 100 years ago, scientists began trying to discover the relation between the properties of events in the world and people's sensations and perceptions of them. b. German scientist Gustav Theodor Fechner ( ) founded the field of psychophysics, which studies the relation between physical events and the corresponding experience of those events. c. Researchers in psychophysics made a series of discoveries, which apply to all the senses. 3. A person crosses a threshold when physical stimulation becomes strong enough to be noticed. a. An absolute threshold is the smallest amount of a stimulus needed in order to notice that the stimulus is present at all. 1) The absolute threshold is the magnitude of the stimulus needed to make it noticeable to the observer half the time. 2) In establishing absolute thresholds, a person is distinguishing between the background and the stimulus. The stimulus must have enough of its defining quality that a person can notice it. b. Sometimes a person doesn't simply need to detect the presence of a stimulus, but rather to distinguish among stimuli. A just-noticeable difference (JND) is the size of the difference in a stimulus property (such as the brightness of light) needed for the observer to notice a difference. 1) The size of a JND depends on the overall magnitude of the stimulus. 2) Weber's law (named after another German researcher, Ernst Weber) states that a constant percentage of a magnitude change is necessary to detect a difference. 4. Signal detection theory seeks to explain why people detect signals in some situations but miss them in others. a. Signals are always embedded in noise (events that could be mistaken for the stimulus); thus, the challenge is to distinguish signal from noise. b. Two concepts explain how signals are detected or missed: 1) Sensitivity (the threshold level for distinguishing between a stimulus and noise); the lower the threshold, the greater the sensitivity.

2 2) Bias (a person's willingness to report noticing a stimulus). People change their bias by adjusting their criterion how strong the signal needs to be before they say they have detected it. c. Sensitivity and bias can be assessed by comparing the occasions when people say a stimulus is or is not present with the occasions when the stimulus actually is present or not present. d. There are 4 possible outcomes in signal detection theory: 1) If there is a signal and it is reported, it is called a hit. 2) If there is a signal and it is not reported, it is called a miss. 3) If there is no signal, but a signal is reported, it is called a false alarm. 4) If there is no signal and no signal is reported, it is called a correct rejection. 5. Eyes work by registering light that is reflected from, or is produced by, objects in the line of sight. a. Light is a form of electromagnetic radiation. All people swim in a sea of electromagnetic radiation. This sea has waves: some large, some small, some that come in rapid succession, some spaced far apart. 1) The height of a wave is its amplitude. 2) The rate at which the peaks of the waves move past a given point is its frequency. With higher frequency, the peaks of the waves arrive more often; thus the length of time between peaks is shorter. 3) When the length of time between arrivals of the peaks is shorter, the light is said to have a shorter wavelength. b. In the electromagnetic spectrum, there is a narrow band of radiation perceived as visible light. 1) An almost uncountable number of colors are conveyed by this light. 2) The traditional seven that are readily distinguished are red, orange, yellow, green, blue, indigo, and violet. a) The lower frequencies (and longer wavelengths--larger nanometers) are toward the red end of the spectrum. b) The higher frequencies (and shorter wavelengths) are toward the violet end. 6. Sensory neurons in the eye convert physical energy into neural impulses ; this process is called transduction. a. Light enters the eye through the pupil (the opening in the eye through which light passes). b. Surrounding the pupil is a circular muscle called the iris, which changes the size of the pupil to let in more or less light. c. The light is focused mostly by the cornea, the transparent covering over the eye, and then focused even more by the lens. d. Unlike a camera lens, the lens in a human eye flexes. In fact, muscles can adjust the lens into a more or less round shape to focus light from objects that are different distances away. Accommodation is the automatic adjustment of the eye for seeing at different distances. e. The world appears sharp because people constantly move their eyes and what they choose to look at can quickly be moved into focus. f. With age, the lens thickens and becomes less flexible, often causing older people to have trouble seeing nearer objects, such as reading material. g. The critical part of transduction occurs at the retina, a sheet of tissue at the back of the eye containing cells that convert light to neural impulses. 1) The central part of the retina, the fovea, contains densely packed cells that transform light to nerve impulses. This area has the highest density of cones and the highest resolution. 2) Two kinds of cells in the retina convert light into neural responses that produce visual sensations. These cells are at the very back of the eye. a) Rods : Retinal cells that are very sensitive to light but register only shades of gray.? Rods actually look like little rods.

3 ? Each eye contains between 100 million and 120 million rods.? The rods are everywhere within the retina except in the fovea.? At night, there isn't enough light for the less light-sensitive cones to work, so night vision is based on the firing of the rods alone. b) Cones: Retinal cells that respond most strongly to one of three wavelengths and that play key role in producing color vision.? The cones look like cones.? They are not as sensitive to light as are the rods.? Each eye contains between 5 million and 6 million cones.? The cones are densest near the fovea. 3) The axons from retinal cells in each eye are gathered into a single large cord called the optic nerve. a) The optic nerve carries impulses from the retina into the brain. b) It is about as thick as a little finger. c) There are no rods or cones at the place where the optic nerve exits the retina, which causes a "blind spot" in what people can see laid out in front of them. d) Because the brain completes patterns that fall across this blind spot, people are not aware of it as they look around every day. 7. Dark adaptation is the process whereby exposure to darkness causes the eyes to become more sensitive, allowing for better vision in the dark. a. After about 30 minutes in the dark, a person is about 100,000 times more sensitive to light than during full daylight. b. In ideal conditions, the rods can respond when they receive a single photon, the smallest unit of light. c. Part of the increased sensitivity to what light there is arises because the pupil enlarges when a person is in darkness; in fact, it can expand to let in about 16 times as much light as enters in full daylight. d. The rods actually become more sensitive as the eyes remain in the dark because the genes cause the production of a crucial chemical, called rhodopsin, that responds to light. 8. Researchers have recently discovered evidence for a third kind of receptor in the eye that also registers light. a. Freedman and colleagues found that mice whose eyes had no rods or cones still shifted their circadian behavior when light was shined on them. However, when the eyes were removed, the mice no longer shifted circadian behavior. Thus, the third kind of lightsensitive receptor mu st be in the eyes. b. Certain ganglion cells in the retina respond to illumination in just the right ways to explain the observed effects of light on behavior. 9. One of the remarkable aspects of human vision is the huge range of colors that people can use and see. a. Colors vary in three separate ways. 1) Different wavelengths of light produce the sensation of different colors. This aspect of color is called hue. 2) The purity of the input produces the perception of saturation; that is, how deep the color appears. 3) The amplitude of the light waves produces the perception of lightness or (if the object, such as a television screen, produces light) brightness--how much light is present b. Color arises through the operation of two distinct types of processes, described by different theories. 1) Trichromatic theory of color vision is the theory that color vision arises from the combinations of neural impulses from 3 different kinds of sensors, each of which responds maximally to a different wavelength. a) This camp took its lead from observations reported by Thomas Young and Hermann von Helmholz in the 19th century.

4 b) This approach focused on phenomena such as the mixing of colors to produce new colors. c) These researchers, arguing by analogy, believed that the brain registers color by combining responses to separate wavelengths. d) They argued that the eye contains 3 kinds of color sensors, each most sensitive to a particular range of wavelengths: long, medium, and short. e) This theory turns out to be essentially correct, but it describes processing at only some places in the eye and brain. f) Consistent with trichromatic theory, our perception of hue arises because most of us possess 3 different types of cones.? One type of cone is most responsive to light in the wavelength seen as a shade of yellow.? Another type of cone is most responsive to light in the wavelength seen as green.? A third type is most sensitive to light violet. g) At least 2 of the 3 types of cones usually respond to any wavelength of visible light, but to different degrees. h) The mixture of the 3 types of response signals is different for each of a huge range of wavelengths. It is this mixture that is the crucial signal to the brain. The brain responds to the mixture, not the outputs from individual cones. i) If red, green, and blue are mixed in an image, stimulating all the cones, the person will see white. j) On the other hand, when people see paints, they see the light that is reflected from the paints; what is seen is what is not absorbed by the paints. So, if red, green, and blue were mixed, a person would see black: The combination of paints absorbs all the wavelengths that give rise to the perception of hue. k) Recent genetic research has shown that some women possess not 3, but 4 types of cones. These women see color differently from the rest of us. For example, they see more bands in a rainbow than people who have only 3 sorts of cones. These differences arise from the same sort of mixing that underlies trichromatic theory. 2) Opponent process theory of color vision is the theory that if a color is present, it causes cells that register it to inhibit the perception of the complementary color. a) The other camp in the debate followed the lead of German physician Ewald Hering, who worked at the end of the 19th century and the beginning of the 20th. b) Hering noticed that some colors cannot be mixed: It isn t possible to make reddish-green or yellowish-blue. This and similar observations led Hering to develop the opponent process theory of color vision, which states that the presence of one color of a pair (red/green, yellow/blue, and black/white) inhibits the perception of the other color. c) In addition, people experience afterimages, images left behind by a previous perception.? A key fact is that the cones feed into special types of cells in the retina and the lateral geniculate nucleus of the brain (part of the thalamus): red/green, yellow/blue, and black/white opponent cells. These cells are set up to pit the colors of each pair against each other.? For example, when wavelengths that produce blue are registered, they inhibit the perception of wavelengths that produce yellow and vice versa.? This is why people can't see greenish-red or yellowish-blue: Seeing 1 member of a pair inhibits seeing the other.

5 c. People who have color blindness are either unable to distinguish hues from one another or, in more serious cases, are unable to see hue at all. 1) Most color blindness is present from birth. 2) Depending on the specific group, as many as 8% of European men but less than 0.5% of European women are born color-blind. 3) Rather than being completely insensitive to hue, most color-blind people are unable to distinguish red from green. 4) Researchers have found that people with the most common type of color blindness possess genes that produce similar pigments in their cones, thus the cones do not respond to wavelengths as they should. A small number of people (roughly 2% of males, and a very small number of females) are actually missing a type of cone. Even more severe deficits occur when more than 1 type of cone is affected. 5) Some people only become color blind after a particular part of their brains is damaged. This acquired color blindness is called acquired achromotopsia. 10. It is estimated that less than a third of the world's population has perfect vision. These problems affect both absolute and relative thresholds, and also can distort the stimulus. a. People with myopia, or nearsightedness, have difficulty focusing on distant objects. 1) Myopia is usually caused by an eyeball that is too long to focus the image on the retina properly. 2) This problem, which in the United States affects about 1 in 5 people, can be corrected by external lenses, either eyeglasses or contact lenses, that focus the image correctly on the retina. 3) Laser surgery often can correct the lens of the eye itself. 4) Time spent reading is correlated with myopia, but correlation does not imply causation. b. In contrast to nearsightedness, which involves problems with distance vision, people with hypermetropia have difficulty focusing on near objects. Such farsightedness usually results from an eyeball that is too short, or a lens that is too thin, to allow the image on the retina to focus properly. c. Astigmatism is a defect in the curvature of the cornea or lens, causing blurriness. Astigmatism, like nearsightedness and farsightedness, can be corrected with eyeglasses (and sometimes with contact lenses). d. A cataract is a cloudy part of the lens of the eye, which can cause blurred vision, distorted images, and sensitivity to light and glare. 1) About 70% of Americans over age 75 have (or have had) a cataract. 2) Cataracts are responsible for at least half of all incidents of blindness. 3) Surgery can correct cataracts by removing the lens and replacing it with a substitute lens. 3. How are we able to separate figure from ground and see the world as a stable collection of objects? First Steps of Visual Perception: Organizing the World 1. Once a stimulus has been sensed, the first task of visual perception is to organize the input into shapes that correspond to surfaces and objects and to specify their sizes and locations. Part of this phase of processing is to separate figure from ground. a. A figure is a set of characteristics (such as shape, color, texture) that corresponds to an object. b. Ground is the background, which must be distinguished in order to pick out figures. c. When figure and ground are similar, the figure is said to be camouflaged. 2. Separating figure from ground involves organizing regions into shapes that are likely to correspond to objects or their parts. A critical part of this process is finding edges. a. Hubel and Weisel discovered that neurons in the first part of the cortex to process visual input (the primary visual cortex, in the occipital lobe) are arranged into columns. 1) The neurons in each column fire selectively to edges that have a specific orientation.

6 2) These columns are in turn arranged into sets that are driven by input from either the left eye or the right eye. 3) These sets are arranged into a hypercolumn. a) The neurons in each hypercolumn respond to input from a single spot on the retina. b) Hypercolumns in the visual areas of the occipital lobe are topographically organized--that is, the pattern falling on the retina is spatially laid out on the cortex. c) This arrangement helps the brain to delineate the edges of objects. b. Gestalt psychologists discovered a set of laws that describe how the brain organizes the input from the eyes. 1) Gestalt laws of organization: A set of rules describing the circumstances under which marks will be grouped into perceptual units, such as proximity, good continuation, similarity, closure, and good form. 2) The most important of these Gestalt laws of organization are: a) Proximity: Marks that are near one another tend to be grouped together. b) Continuity: Marks that tend to fall alone a smooth curve or a straight line tend to be grouped together. c) Similarity: Marks that look alike tend to be grouped together. d) Closure: Gaps in a figure tend to be closed. e) Good form: Marks that form a single shape tend to be grouped together. c. Sometimes a figure can be organized and perceived in more than 1 way. Even when visual patterns are organized automatically, they are organized not only on the basis of their physical properties but also as a result of learning. After extensive practice, the visual system becomes tuned so that people can automatically organize what they see in a new way. 3. Perceptual constancy is the perception of characteristics that occurs when an object or quality (such as shape or color) looks the same even though the sensory information striking the eyes changes. a. Size constancy occurs when a person sees an object (such as a car) as the same actual size even when it is at different distances, so that its image (as in a photograph) is at different sizes. b. Shape constancy occurs when a person sees an object as the s ame shape, even when viewed from different angles. This stabilization occurs not in the eyes, but in the brain, and is fundamental to the ability to recognize objects and know how to interact with them. c. Color constancy refers to people s ability to see colors as constant even when the lighting changes; color constancy may arise because we see the lightest thing in a scene as white and everything else relative to that color. 4. Because the world is 3-dimensional, people need to register distances; this is necessary to appreciate how parts of an object are arranged, as well as to reach and navigate properly. a. Eyes project images onto the 2-dimensional surface of our retinas, but people need to see objects in 3 dimensions. b. The brain uses different types of cues to derive 3 dimensions from the 2-dimensional images on the retinas of the eyes. 1) Static ("unmoving") information plays a large role in allowing people to determine how far away something is. a) Binocular cues arise from both eyes working together.? The brain uses slight differences in the images striking each of the eyes to assess the distance of an object.? Because the eyes are separated, people need to cross them in order to focus on an object (so that the same image appears on the central, high-resolution fovea in each eye).

7 ? When the eyes are crossed, however, the images of other objects fall on slightly different parts of the retinas of the 2 eyes.? This difference between the images on the 2 eyes is called retinal disparity (also called binocular disparity).? The brain uses the amount of disparity to determine which objects are in front of and which are behind others. The process of figuring out depth from retinal disparity is called stereopsis. b) Monocular (static or "one-eyed") cues for distance can be picked up with 1 eye, and operate even for far distances.? One of these is the texture gradient, a progressive change in the texture of an object. Objects that are closer give rise to larger images (they are larger in the photograph) and this information is used to determine distance.? In drawing pictures, artists also use linear perspective, or foreshortening, making the parts of objects that are farther away from the viewer actually smaller on the page.? Distance can be inferred if 1 object partially covers another, indicating that the obscured object is behind the other and thus farther away; this relation is called an occlusion cue.? If the base of an object appears higher on the horizon than the base of another object, a person takes that as a cue that the higher one is farther away.? The way such cues are depicted in drawings sometimes relies on cultural conventions, which must be learned. c) Motion cues specify the distance of an object on the basis of its movement. These cues work as well with 1 eye as with 2.? Objects closer than the 1 on which a person is fixated seem to move in the opposite direction to his or her movements, whereas those farther away than the fixation point seem to move in the same direction.? Depending on the distance, the objects will seem to shift at different speeds. This difference in shifting provides information about relative distance, a cue called motion parallax.? Motion not only helps a person perceive depth, but also signals that an object is changing position relative to the viewer. 4. How do we make sense of what we see? Visual Perception: Recognition and Identification 1. The processes of visual perception accomplish 2 major goals, which are handled by separate mechanisms in the brain. They allow people to a. assign meaning to the shapes they see. b. map space so that they can guide their movements according to their goals. 2. Evidence for the existence of these distinct mechanisms comes from: a. Studies of human pathways of people who have had strokes. A stroke that affects the bottom parts of the temporal lobes impairs the patient's ability to recognize objects by sight. A stroke that affects the back parts of the parietal lobes impairs the patient's ability to register locations. b. Neuroimaging studies in which normal people are asked to distinguish shapes or locations. 3. The 2 pathways come together in the frontal lobes, where information about an object's identity and location is used to make decisions.

8 4. When a person sees an object: a. The processes that underlie visual sensation respond to basic characteristics of the input, such as its wavelength and contours. b. Outputs from these processes are then organized in the first phases of perceptual processing. c. To know more about these objects, the final phases of perceptual processing must occur, which allow the visual input to activate information stored in memory from prior experience. d. To interpret what is seen, input needs to be compared to stored information. 1) If the input matches something stored in memory, that information can be applied to the present case. 2) If the object is recognized, it seems familiar. 3) If the object is identified, additional facts are known about it. 5. Objects are sometimes recognized as collections of parts. a. Objects may be represented in the brain as collections of simple shapes, with each shape representing a part of the object, such as the head, neck, body, and legs of a dog. b. It is clear that people can see parts of objects individually. c. But, objects are not always recognized on the basis of their individual parts. 6. The progression from sensation, to organization, to recognition and identification may seem to indicate a simple sequential process, but more is happening. a. Processing may be: 1) Bottom-up, initiated by the stimulus. 2) Top-down, guided by knowledge, expectation, or belief. b. The two sorts of processes may be in play at the same time. c. Bottom-up processing can sometimes produce remarkably complex experiences. For example, perhaps as many as 1 in 2,000 people experience synesthesia, where they simultaneously experience more than 1 perception when given certain stimuli. d. Top-down processing can alter the mechanisms used in bottom-up processing. For example, in 1 study, the knowledge of the usual color of the objects affected how participants saw the actual color. e. A perceptual set is the sum of a person s assumptions and beliefs that lead him or her to expect to perceive certain objects or characteristics in particular contexts. People often use context to form such perceptual expectancies, perceptions dependent on their previous experience. 7. Researchers have also studied the final phases of perceptual processing in the "where" pathway. a. The goal of this processing is to use the sum total of the distance cues (discussed earlier) to identify the distance and direction of objects, either relative to oneself or to other objects. b. The brain appears to use 2 different ways to code space: 1) Categorical spatial relations code positions with categories such as "above" or left of or inside. The left cerebral hemisphere may be better at specifying categorical spatial relations, which are easily named by a word or 2 (and thus are compatible with the left hemisphere's facility at labeling). 2) Categorical spatial relations are useless for the other main tasks of vision: navigation and reaching. For these tasks, people need precise information about the distance and direction of objects. Coordinate spatial relations specify continuous distance from the viewer s body or another object that serves as an "origin" of a coordinate space. The right hemisphere may be better at specifying coordinate spatial relations, which are essential for navigation (and the right hemisphere typically is better at this ability). 8. Attention is the act of focusing on particular information, which allows that information to be processed more fully than information that is not attended to. a. Research has shown that paying attention increases sensitivity to the attended events. b. Selective attention allows people to pick out a particular characteristic, object, or event. c. People are aware only of what they pay attention to. d. People pay attention to something for 1 of 2 reasons:

9 1) Something about an event grabs us, such as a sudden change in illumination or movement. Certain qualities or features of displays, such as advertisements, automatically (via bottom-up processes) leap out--a phenomenon psychologists refer to as pop-out. 2) The other reason people pay attention is that they are actively searching for a particular characteristic, object, or event (via top-down processes). e. Vigilance occurs when one anticipates a particular event and thus maintains attention while waiting for it. Vigilance also relies on the ability to focus attention voluntarily. f. Allocation of attention via bottom-up versus top-down processes is accomplished by different parts of the brain. 1) In the case of a sudden change in the environment, such as a bright light or a quick movement, the superior colliculus (a small subcortical structure) acts like a reflex, shifting attention automatically to that event. Moreover, a brain region standing between the right temporal and parietal lobe and another region in the lower right frontal lobe are used in bottom-up processing. 2) While searching for something or someone (or remaining vigilant), the frontal eye fields (in the frontal lobes) and regions of the parietal lobes are active. g. Using magnetic pulses to disrupt the parietal lobe, researchers found that while the ability to search for arrangements of features was impaired, the ability to experience pop-out was not. h. In addition, the 2 types of attention allocation operate differently. When a person is searching for an object that is not distinct from the others around it, he or she must look at each possible candidate 1 at a time. i. Attention arises from the joint action of events at the different levels of analysis. In addition to the events at the level of the brain noted above: 1) At the level of the person, it is influenced by a person s beliefs. 2) At the level of the group, what a person s believes in part depends on his or her previous interactions with other people and knowledge of the surrounding culture. j. There are 3 limits on attention. 1) People cannot pay attention to more than 1 task at the same instant in time. If people must perform 2 tasks that require attention, they must divide attention. a) Divided attention occurs when a person shifts back and forth between different stimuli or tasks. b) Divided attention usually has a cost, with performance on 1 or both of the tasks being worse than if the person focused on 1 task alone. This problem may arise either because:? Additional processes in the frontal lobes must be brought to bear when attention is shifted back and forth, or? The same mechanisms are being used in competing tasks. c) However, if the tasks are different enough, so that they can be accomplished by different mechanisms, they won't interfere as much with each other. d) If the tasks or signals don't compete with each other, but instead lead to the same response, a person might actually do better when he or she has to pay attention to 2 tasks or 2 signals at once. For example, the redundant signal effect occurs when a person is asked to respond as soon as he or she perceives a signal: Response time will decrease. 2) People can focus attention only within a limited region of space and, if objects are moving, only when they move below a certain speed. 3) Attention is also limited in its ability to filter out information. a) The Stroop effect illustrates a classic example of such a limit of attention. b) If a person is asked to pay attention only to the color of the ink (and not the word itself), and say this color aloud, he or she will nevertheless have trouble if the word names another color.

10 c) If the meaning of the word is different from the color of the ink, a person will experience interference when trying to name the color of the ink. d) Bottom-up processes lead a person to read the meaning of the word, and attention cannot simply turn off such processes. 9. Some people who suffer strokes that leave them blind (i.e., they have no conscious experience of visual perception) can nevertheless use vision to report accurately when spots of light are presented, and will even know where they are. Such ability has been called blindsight. a. Multiple pathways from the eye lead to many places in the brain. b. Some of these pathways can function even though they do not pass through brain areas that give rise to conscious experience. c. Thus, even when the areas or connections from these areas that are crucial for consciousness are damaged, some visual function persists. 10. People s brains can respond when they see an object even if they are not aware of seeing it. Perception of events outside awareness is called subliminal perception. After many years of unreliable findings, researchers have not only documented that subliminal perception exists, but they are even tracking down the brain events responsible for this effect. a. However, more often than not, instead of perceiving more than a person are aware of, he or she actually perceives less than thought. b. A large body of research shows that people can perceive with high accuracy only those stimuli to which they pay attention. Conversely, people are remarkably bad at noticing even large changes in stimuli if they are not paying attention to the relevant parts. c. Repetition blindness is the inability to see the the second occurrence of a stimulus that appears twice in succession (such as the word "the" in this sentence). 1) This is said to be the hardest error for a proofreader to catch. 2) Repetition blindness appears to result because the stimuli are not registered as individual events, but simply as a type of event. d. Attentional blink is a rebound period in which a person cannot pay attention to 1 thing after having just paid attention to another. 1) The attentional blink can occur for a different stimulus, not necessarily a second instance of the same or a closely related one. 2) The effect may actually be larger for stimuli that occur a few items after the 1 attended to. 3) The attentional blink may occur because the act of registering information in detail may "lock up" certain neural processes for a brief period, during which attention cannot easily be reengaged. Test Yourself, p How do the ears register auditory sensation? Auditory Sensation: If a Tree Falls but Nobody Hears It, Is There a Sound? 1. About 40% of normal people emit a detectable soft humming sound from their ears, of which they are unaware. In some cases, the humming is loud enough that other people can hear it. These sounds play no role in hearing. They are probably caused by feedback from the brain to the ear; feedback (not the sounds produced by it) helps us hear slight differences in sounds. As in vision, there are many feedback connections between areas of the brain that process sound, and between these areas and the ear. This is not the only similarity between hearing and seeing. 2. Like vision, auditory processing occurs in 2 major phases: sensation and perception. And, like vision, perception itself can be divided into processes used at the beginning and at the end. 3. Hearing begins with the sensation of sound. a. Sound usually arises when something vibrates, creating waves of moving air that enter our ears. 1) Sound can arise when any type of molecules--gas, liquid, or solid--move and create pressure waves.

11 2) These pressure waves go up, and then down, repeatedly; each complete up- anddown movement is called a cycle. 3) As with light waves for vision, sound waves have both frequency and amplitude. a) People usually hear variations in frequency as differences in pitch (how high or low the sound seems). b) People hear variations in amplitude as differences in loudness. 4) The same psychophysical concepts that apply to vision, such as thresholds, JNDs, and so on, also apply to hearing and are measured in comparable ways. b. Sound is a psychological event and hence depends on a nervous system to transduce the physical energy of the vibrations to nerve energy. Without a brain to register the physical energy, there can be no sound. The situation is exactly analogous to the relationships of wavelength to hue and of amplitude to lightness. Physical properties lead to psychological events, but they are not the events themselves. 1) There are 3 parts to the ear. a) The outer ear. b) The middle ear, which contains:? The eardrum (tympanic membrane, which stretches across the inside end of the auditory canal). Waves move the eardrum, which in turn move the 3 bones of the middle ear.? The 3 bones are called the hammer, anvil, and stirrup (based on their appearance). c) The inner ear, which includes the semicircular canals and the cochlea. Inside the cochlea is:? The basilar membrane, where different frequencies of sound are coded into different nerve impulses.? Hair cells are inside the basilar membrane. These hairs trigger nerve impulses, which are then sent to the brain. 2) There are 2 main theories about the way the basilar membrane converts pressure waves to perceived sound: a) Frequency theory holds that higher frequencies produce greater neural firing. This theory cannot explain the full extent of our ability to hear because neurons can fire only approximately 1,000 times a second at most, so it doesn t explain how people hear sounds produced by much higher frequencies. b) According to place theory, different frequencies activate different places along the basilar membrane. This theory appears to be correct, at least for most frequencies; it is possible, however, that the rate of vibration does help us hear relatively low tones. As in vision, a number of brain areas working together allow people to sense sound:? The first part of the cortex to receive auditory information, the primary auditory cortex, is spatially organized.? Researchers have shown that as the pitch changes, activity shifts to different locations along this structure.? Just as in vision, in which the pattern of activation on the eye is in turn laid out on the brain, the spatial arrangement of vibration on the basilar membrane is mimicked in the brain. This sort of spatial arrangement is called tonotopic organization. 4. More than 28 million Americans have some sort of difficulty in hearing. a. Deafness is not a single difficulty. 1) Over 30 genes have now been linked to deafness; thus, it is no surprise to find that there are different forms of deafness and that some forms of deafness are inherited. 2) Not all forms of deafness are genetic, however. One of the most serious is nerve deafness, which typically occurs when the hair cells are destroyed by loud

12 sounds. A rock band heard at close range can produce sounds loud enough to cause this sort of damage. a) Nerve deafness may affect only certain frequencies. b) In those instances, a hearing aid can amplify the remaining frequencies, and hearing can be improved. c) Surgery may soon allow doctors to make an end run around a damaged ear and allow auditory input to stimulate the auditory cortex directly. 3) Conduction deafness can result from any accident or other cause that impairs the functioning of the external ear or middle ear. A broken eardrum, for example, can cause conduction deafness. b. Another form of hearing impairment, tinnitus, is signaled by a constant ringing or noise in the ears. c. Some drugs, including aspirin, can dull a person's hearing. Fortunately, the dulling effects of aspirin are only temporary. 5. It is worth noting that if someone becomes deaf as a child, other senses can eventually compensate. a. Researchers have found increased activation in the visual cortex of deaf people. b. By adolescence, the deaf can focus visual attention in many tasks better than can hearing people (particularly when they have to attend to something not currently being focused on). c. This is another example of the brain's plasticity. 2. What auditory cues allow us to organize sounds into coherent units and locate their sources? First Steps of Auditory Perception: Organizing the Auditory World 1. The processes engaged during the first phases of auditory perception allow us to organize sounds as coming from distinct objects and to locate the sources of sounds. 2. To make sense of what they hear, people first need to sort out individual sounds. As in vision, people need to distinguish figure from ground. a. Bregman calls this process auditory scene analysis, which relies on organization very much like what occurs in vision. b. The Gestalt laws help here, too. For example, people organize sounds partly based on: 1) Similarity (e.g., grouping sounds with the same pitch). 2) Good continuation (e.g., grouping the same pitch continued over time). c. Recognizing and identifying speech relies crucially on auditory scene analysis because the actual stimulus is continuous, with no indication of breaks to delineate the beginnings and endings of words. Yet, to communicate, people must identify individual words. This problem is the speech segmentation problem. d. People hear speech sounds as distinct categories. This categorical perception produces categories with remarkably sharp boundaries. Infants, monkeys, chinchillas, and various other animals also show categorical perception, which suggests that the perceptual system itself does this work, not the language systems of our various cultures. 3. In vision, our brains use slight differences in the images striking the 2 eyes to assess the distance of an object. Similarly, hearing makes use of differences in the stimuli reaching the 2 ears to assess the distance and direction of a sound source. 3 kinds of differences are particularly important. a. Sound waves reach the 2 ears at slightly different points in the wave cycle. The difference in phase reaching the 2 ears is particularly useful for detecting the source of relatively low- frequency sounds, which arise from longer waves. b. A difference in loudness at the 2 ears is used as a cue. In addition to all of their other functions, heads are useful because they block sound, and thus the amplitude of sound waves is smaller when it reaches the ear on the side of the head away from the sound source. This cue is particularly effective for high-frequency sounds. c. The sound wave will reach the 2 ears at slightly different times; this onset difference is tiny, but the brain uses it effectively.

13 4. As in vision, people use many different cues to assess where an object is. Some cues depend on only 1 ear, not 2. Consider 3 such cues. a. Especially for familiar objects, volume can be used as an indicator of distance. b. The way people s external ears are crinkled bends sound waves in different ways; these variations help us detect the location of the sound source. c. By moving their heads and bodies, people can compare the relative volume of a sound from different vantage points, which helps locate its source. 3. How do we make meaning out of sound? Auditory Perception: Recognition and Identification 1. As in vision, sounds become meaningful when they are matched to information already stored in memory, which is the job of auditory perceptual processing. 2. Just as in vision, people can adjust their criterion for "detecting a signal" and this adjustment would be based on what they expect to hear. But also, as in vision, what a person expects to hear actually influences what he or she does hear. a. People to listen to a tape-recorded sentence after part of a word had been replaced with the sound of a cough. Although part of the word was actually missing, all the participants claimed that they actually heard the entire word and denied that the cough covered part of it. In fact, the listeners were not exactly sure at what point the cough occurred. This effect, more obvious for words in sentences than for words standing alone, is called the phonemic restoration effect (a phoneme is the smallest segment of spoken speech, such as "ba" or "da"). b. This filling-in effect occurs not only with speech sounds but also with musical instruments. In fact, if a person sees someone bowing the strings of a cello at the same time he or she hears the strings being plucked, the sight is enough to distort the sound heard. 3. As happens in vision, people pick up some auditory information without being aware of it. a. Perhaps the most common experience of perception without awareness is the cocktail party phenomenon. At an event like a party, a person may not be aware of other people's conversation until someone mentions his or her name--which is heard immediately. To become aware of the sound of his or her name, the person must have been tracking the conversation all along (using bottom-up processing); he or she was simply not aware of the conversation until that important word was spoken. b. Treisman showed that when people listen to stimuli presented separately to the 2 ears (through headphones) and are instructed to listen to only 1 ear (a procedure known as dichotic listening) they still register some information from the ignored ear. To ensure that participants listen to only 1 voice, they often are asked to repeat it aloud--a practice known as shadowing. 1) This discovery spawned an industry that proclaimed that people can learn in their sleep, simply by playing tapes. 2) Unfortunately, it turns out that unless a person is paying attention, not much gets through. 3) Even when information does get through, it is retained very briefly; when tested hours later, people remember almost none of the information presented outside awareness. 4. Music is a part of virtually all cultures; in some, its importance is so great that governments have occasionally regulated what constitutes music itself. Aside from these cultural curiosities, however, the existence of music depends on the fact that the brain registers sounds relative to 1 another, not in isolation. a. For exa mple, if a frequency is doubled, people hear the same note but an octave higher (an octave in Western music is 8 consecutive notes). b. The continuous variation in frequency between octaves is divided into distinct intervals, which form a scale. The nature of scales varies in different cultures. 1) The Western scale relies on 12 half-steps for each octave, compared with more than 50 in Indian music. 2) Nevertheless, all humans hear notes an octave apart as more similar than consecutive notes.

14 c. This is another example of the way in which the physical nature of a stimulus differs from its psychological experience. 1) People can hear 2 different sequences of notes (which have different physical frequencies) as the same, provided that the notes are separated by the same intervals. 2) However, not all people can recognize or remember musical steps equally well. a) Interestingly, identical twins have very similar abilities to recognize incorrect notes in familiar popular tunes, which (when compared with the less similar abilities of fraternal twins) led researchers to conclude that genetic differences are responsible for at least 70% of the variation in this ability. b) Within a given culture, the ease of identifying the notes in a scale depends partly on whether a person has absolute pitch, the striking ability to identify a particular note by itself, not simply in relation to other notes.? Studies have shown that Americans with absolute pitch identify the intervals between notes from a standard Western scale better if an instrument is in tune (that is, the notes are set to the correct absolute frequencies).? Many people with absolute pitch developed the ability during childhood.? People with absolute pitch have an unusually large planum temporale, a part of the auditory cortex that lies on the top part of the temporal lobe, near the back. However, research has yet to establish whether this is a cause or an effect of havng this ability. Test Yourself, p How does the sense of smell work? Smell: A Nose for News? 1. Smell and taste are often grouped together as the chemical senses because both rely on registering the presence of specific chemicals. 2. People differ widely in their sense of smell, or olfaction. a. Some people are 20 times more sensitive to odors than are other people. b. Most people are remarkably poor at identifying odors, even though they often think they are good at it. 1) Cain (1979) found that people could correctly identify only about half of 80 common scents. 2) People may recognize that an odor is familiar, but be unable to identify it. 3) In general, women are better than men at detecting many types of odors. 4) Women are particularly sensitive to smell when they are ovulating--unless they take birth control pills, in which case their abilities do not fluctuate over the course of the month. 5) In addition, younger adults are better at detecting odors than either children (up to 14 years old) or middle -aged adults (between 40 and 50 years old). 3. The best theory of odor detection can be described using the lock and key metaphor. Molecules have different shapes, and the olfactory receptors are built so that only molecules with particular shapes will fit in particular places. The molecules are like keys, and the receptors like locks. a. When the right-shaped molecule arrives at a particular receptor, it sends a signal to the brain, and people sense the odor. b. Just as there is not a single type of cone for each color people can see, there is not a single receptor for each odor they can smell; rather, the overall pattern of activity signals a particular odor. 4. Two major neural tracks send signals about odor into the brain:

15 a. One passes through the thalamus and is particularly involved in memory b. The other, connected to the limbic system, is particularly involved in emotions. 5. The connections through the limbic system explain why odors often tap emotionally charged memories. 6. Smell serves to signal the presence of noxious substances; our brains are wired so that odors can quickly activate the "fight-or-flight" system. a. Smell is often the only signal that meat or other food is spoiled. b. Relatively few people have no olfactory sense, a deficit that can arise from brain damage or a virus. 7. Airborne chemicals released by female animals in heat arouse the male of the species. These are an example of pheromones, chemical substances that serve as a means of communication. Like hormones, they modulate the functions of various organs, including the brain. Unlike hormones, pheromones are released outside the body, in urine and sweat. a. Female roommates tend to synchronize their menstrual cycles. However, this effect depends on certain pheromones' reaching the nose. The receptors that are triggered by pheromones are accessed via the nose, and odors sometimes accompany these chemicals. b. Female pheromones can attract men. In 1 study, university women reported more petting, sexual intercourse, sleeping next to a partner, and formal dates after they began mixing a female pheromone into their perfume. They did not report that more men had approached them or that they had more informal dates or an increase in masturbation. 2. How does the sense of taste work? Taste: The Mouth Has It 1. When scientists discuss taste, they are talking about sensing via receptors located solely in the mouth. 2. Taste buds are microscopic structures mounted on the sides of the little bumps on one s tongue. a. People have taste buds in other places in the mouth as well, such as the back of the throat and cheeks. b. The taste buds on the tongue line the papillae, the visible bumps. c. Taste buds die and are replaced, on average, every 10 days. 1) Humans have more taste buds than some species, such as chickens. 2) But, they have fewer taste buds than other species. 3) Children have more sensitive taste buds than adults, and thus flavors are presumably stronger for them than for adults. d. The 4 types of taste buds are sensitive to sweet, sour, salty, and bitter tastes. e. Free nerve endings in the mouth appear to be irritated by spicy foods. These provide another source of information about taste, 1 that is not directly related to the taste buds. f. Different parts of the mouth and tongue are more or less sensitive to different tastes. 1) People can detect bitter flavors best in the back of their mouths. 2) However, these different sensitivities are a matter of degree: All kinds of taste buds are found in most locations on the tongue. 3) Curiously, there is a "taste hole" in the middle of the tongue, an area where there are no taste buds at all. g. For the most part, something cannot be tasted unless it can be at least partially dissolved by saliva. 1) This is why a marble doesn t taste. 2) An exception to this principle can occur if the stimulus changes the temperature of the tongue. a) For example, warming the front edge of the tongue can lead to a sweet taste. b) Cooling this region can lead to tastes of saltiness or sweetness. h. Finally, the tongue is sensitive to texture, which influences the way people experience different foods. i. In fact, the tongue is so sensitive that it is now being used to help blind people see. Researchers have devised machines that translate visual forms into patterns of stimulation