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ASAS PSIKOLOGI sensation and perception

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Perception and sensation
Perception and sensation
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ASAS PSIKOLOGI sensation and perception

  1. 1. Chapter 3: Sensation and Perception
  2. 2. Sensation and Perception  Sensation – The process by which our sense organs receive information from the environment  Perception – The sorting out, interpretation, analysis, and integration of stimuli involving our sense organs and brain
  3. 3. Sensing the World Around Us  Stimulus – Energy that produces a response in a sense organ – Varies in both type and intensity  Psychophysics – The study of the relationship between the physical aspects of stimuli and our psychological experience of them
  4. 4. Sensing the World Around Us  Absolute threshold – The smallest intensity of a stimulus that must be present for it to be detected
  5. 5. Sensing the World Around Us  Noise – Background stimulation that interferes with the perception of other stimuli
  6. 6. Noticing Distinctions Between Thresholds  Just-noticeable difference – The smallest level of stimulation required to sense that a change in stimulation has occurred  Weber’s law – Basic law of psychophysics that states “a just noticeable difference is a constant proportion of the intensity of an initial stimulus
  7. 7. Sensing Our World  Sensory adaptation – An adjustment in sensory capacity following prolonged exposure to stimuli
  8. 8. Vision: Structure of The Eye
  9. 9. Vision: Reaching the Retina Rods are thin, cylindrical receptor cells highly sensitive to light Cones are cone-shaped, light-sensitive receptor cells that are responsible for sharp focus and color perception, particularly in bright light
  10. 10. Vision: Sending the Message From the Eye to the Brain  Bipolar cells – Receive information directly from rods and cones and then communicates this information to ganglion cells  Ganglion cells – Collect and summarize visual information, which is gathered and moved out of the back of the eyeball through a bundle of ganglion axons called the optic nerve
  11. 11. Vision: Processing the Visual Message  Optic chiasm – Juncture where the optic nerves of both eyes meet and then split  Feature detection – Some neurons in the cortex are activated only by visual stimuli of a particular shape or pattern
  12. 12. Color Vision and Color Blindness  Trichromatic theory of color vision – Suggests that there are three kinds of cones in the retina, each of which responds primarily to a specific range of wavelengths  Opponent-process theory of color vision – Proposes that receptor cells are linked in pairs, working in opposition to each other
  13. 13. Color Blindness  The trichromatic theory of color vision proposes that color blindness is due to one of the three cone systems is malfunctioning, and colors covered by that range are misperceived
  14. 14. Hearing: Sensing Sound  Sound – The movement of air molecules brought about by the vibration of an object  Eardrum – The part of the ear that vibrates when sound waves hit it  Middle ear – Tiny chamber containing three bones (stirrup, anvil, and hammer) that acts as a tiny mechanical amplifier
  15. 15. Hearing: Sensing Sound  Cochlea – Coiled tube that looks something like a snail and is filled with fluid that can vibrate in response to sound  Basilar membrane – Structure that runs through the center of the cochlea, dividing it into an upper and lower chamber  Hair cells – Tiny cells located on the basilar membrane that are bent by the vibrations entering the cochlea and transmit a neural message
  16. 16. Hearing: Physical Aspects of Sound  Frequency – Number of wave cycles that occur in a second  Pitch – Characteristic of the sound that makes sound high or low
  17. 17. Hearing: Physical Aspects of Sound  Intensity – Feature of wave patterns that allows us to distinguish between loud and soft sounds  Decibels – Measurement of the intensity of the sound within our range of hearing
  18. 18. Hearing: Sorting Out Theories of Sound  Place theory of hearing – Different areas of the basilar membrane respond to different frequencies  Frequency theory of hearing – The entire basilar membrane acts like a microphone, vibrating as a whole in response to a sound
  19. 19. Balance  Semicircular canals – Structures of the inner ear consisting of three tubes containing fluid that sloshes through them when the head moves, signaling rotational or angular movement of the brain  Otoliths – Tiny, motion-sensitive crystals that sense bodily acceleration and gravity within the semicircular canals
  20. 20. Smell  Olfaction – Can detect more than 10,000 different smells – Can identify gender by smell – Can evoke memories  Olfactory cells – Receptor cells of the nose  Pheromones – Pollen-like chemicals that are released by non- humans that have an effect on other’s behavior
  21. 21. Taste: Gustation  Taste buds – Receptor cells located within the tongue, as well as other parts of the mouth and throat – Constantly reproduce every 10 days – “Supertasters” v “Nontasters” Bitter Sour Salty Sweet and Fatty
  22. 22. The Skin Senses  Touch, pressure, temperature, and pain  Gate-control theory of pain – Particular nerve receptors in the spinal cord lead to specific areas of the brain related to pain
  23. 23. Perceptual Organization  Figure – The object being perceived  Ground – The background or spaces within the object
  24. 24. Perceptual Organization: The Gestalt Laws of Organization Proximity Simplicity Closure Similarity
  25. 25. Perceptual Organization: Feature Analysis  An approach that considers how we perceive a shape, pattern, object, or scene by reacting first to the individual elements that make it up
  26. 26. Perceptual Organization  Top-down processing – Perception that is guided by higher- level knowledge, experience, expectations, and motivations  Bottom-up processing – Perception that consists of recognizing and processing information about the individual components of the stimuli
  27. 27. Perceptual Organization  Perceptual constancy – Phenomena in which physical objects are perceived as unvarying and consistent, despite changes
  28. 28. Perceptual Organization: Depth Perception  Binocular disparity – The ability of the brain to integrate the two images received from the eyes into one composite view  Monocular cues – Cues that allow us to obtain a sense of depth and distance with just one eye • Motion parallax • Relative size • Linear perspective
  29. 29. Perceptual Illusions •Visual illusions are physical stimuli that consistently produce errors in perception.
  30. 30. Poggendorf Illusion
  31. 31. Perception: Outer Limits  Subliminal perception – The perception of messages about which we have no awareness  Extrasensory perception (ESP)

Notes de l'éditeur

  • It is important to point out the fundamental differences between sensation/perception – that our sensory organs transform environmental stimuli into neurochemical impulses while our brain “perceives” the image. A discussion of “hallucinations” helps to bring this point across.
    As outlined in the chapter – the primary difference is that sensation can be thought of as an organism’s first encounter with a raw sensory stimulus, while perception is the process by which the stimulus is interpreted, analyzed, and integrated with other sensory information.
    The chapter gives an example of a fire alarm – when considering sensation we may ask about the loudness of the alarm whereas in considering perception we may ask if one recognizes the ringing sound as a fire alarm
    Also, the chapter has a nice quote examining the interaction of sensation and perception – “…both sensation and perception are necessary for transforming the physical world into our psychological reality.”
  • Both terms provide a “working vocabulary” for sensation and perception
    Identify a variety of stimuli – e. g. molecules for smell/taste, pressure for touch, light & sound
    “Each sort of stimulus that is capable of activating a sense organ can also be considered in terms of its strength, or intensity. Questions such as how intense a light stimulus needs to be before it is capable of being detected or how much perfume a person must wear before it is noticed by others relate to stimulus intensity.”
    “Psychophysics played a central role in the development of the field of psychology, and many of the first psychologists studied issues related to psychophysics (Baird, 1997; Gescheider, 1997).”
  • “thresholds permit our sensory apparatus to detect a wide range of sensory stimulation. In fact, the capabilities of our senses are so fine-tuned that we might have problems if they were any more sensitive. For instance, if our ears were just slightly more acute, we would be able to hear the sound of air molecules in our ears knocking into our eardrum—a phenomenon that would surely prove distracting and might even prevent us from hearing sounds outside our bodies.”
  • “Normally our senses cannot detect stimulation quite as well because of the presence of noise. Noise, as defined by psychophysicists, is background stimulation that interferes with the perception of other stimuli.”
    “noise refers not just to auditory stimuli, the most obvious example, but also to stimuli that affect the other senses”
    “a talkative group of people crammed into a small, crowded, smoke-filled room at a party. The din of the crowd makes it hard to hear individual voices, and the smoke makes it difficult to see, or even taste, the food. In this case, the smoke and crowded conditions would be considered “noise,” since they are preventing sensation at more discriminating levels.”
  • the difference threshold is the minimum stimulation required to detect the difference between two stimuli, or a just noticeable difference.
    The stimulus value that constitutes a just noticeable difference depends on the initial intensity of the stimulus. For instance, when the moon is visible during the late afternoon, it appears relatively dim—yet against a dark night sky, it seems quite bright.
    “The relationship between changes in the original value of a stimulus and the degree to which the change will be noticed forms one of the basic laws of psychophysics: Weber’s law.”
    “Weber found that the just noticeable difference for weight is 1:50. Consequently, it takes a 1-ounce increase in a 50-ounce weight to produce a noticeable difference, and it would take a 10-ounce increase to produce a noticeable difference if the initial weight were 500 ounces. In both cases, the same proportional increase is necessary to produce a just noticeable difference—1:50 = 10:500.”
    “the just noticeable difference distinguishing changes in loudness between sounds is larger for sounds that are initially loud than for sounds that are initially soft. This principle explains why a person in a quiet room is more apt to be startled by the ringing of a telephone than a person in an already-noisy room. In order to produce the same amount of reaction in a noisy room, a telephone ring might have to approximate the loudness of cathedral bells.”
  • “You enter a bar, and the odor of cigarettes assaults you. A few minutes later, though, you barely notice the smell.”
    “Adaptation occurs as people get used to a stimulus and change their frame of reference. In a sense, our brains mentally turn down the volume of the stimulation it’s experiencing.”
    “One example of adaptation is the decrease in sensitivity that occurs after repeated exposure to a strong stimulus. If, for example, you were to hear a loud tone over and over again, eventually it would begin to sound softer. Similarly, although jumping into a cold lake may be temporarily unpleasant, eventually we get used to the temperature.”
    “This apparent decline in sensitivity to sensory stimuli is due to the inability of the sensory nerve receptors to constantly fire off messages to the brain. Because these receptor cells are most responsive to changes in stimulation, constant stimulation is not effective in producing a reaction.”
    “You can demonstrate this for yourself by trying a simple experiment. Take two envelopes, one large and one small, and put fifteen nickels in each. Now lift the large envelope, put it down, and lift the small one. Which seems to weigh more? Most people report that the small one is heavier, although, as you know, the weights are nearly identical. The reason for this misconception is that the physical context of the envelope interferes with the sensory experience of weight. Adaptation to the context of one stimulus (the size of the envelope) alters responses to another stimulus (the weight of the envelope) (Coren & Ward, 1989).”
  • Light waves coming from some object outside the body (imagine the light reflected off the flower in Figure 4-2). Our eyes shape light into a form that can be used by the neurons that will serve as messengers to the brain.
    The ray of light we are tracing as it is reflected off the flower first travels through the cornea, a transparent, protective window. The cornea bends (or refracts) light as it passes through in order to more sharply focus it. After moving through the cornea, the light traverses the pupil. The pupil is a dark hole in the center of the iris, the colored part of the eye, which ranges in humans from a light blue to a dark brown.
    A small pupil greatly increases the range of distances at which objects are in focus. The eye takes advantage of bright light by decreasing the size of the pupil and thereby becoming more discerning. In dim light the pupil expands to enable us to view the situation better—but at the expense of visual detail.
    Once light passes through the pupil, it enters the lens, which is located directly behind the pupil. The lens acts to bend the rays of light so that they are properly focused on the rear of the eye. The lens focuses light by changing its own thickness, a process called accommodation. The lens becomes flatter when viewing distant objects and rounder when looking at closer objects.
    Having traveled through the pupil and lens, our image of the flower finally reaches its ultimate destination in the eye—the retina. Here the electromagnetic energy of light is converted into the neural codes that the brain uses. It is important to note that because of the physical properties of light, the image has reversed itself in traveling through the lens, and it reaches the retina upside down (relative to its original position).
  • “The retina consists of a thin layer of nerve cells at the back of the eyeball (see Figure 4-3). There are two kinds of light-sensitive receptor cells found in the retina. Rods are thin, cylindrical receptor cells highly sensitive to light. Cones are cone-shaped, light-sensitive receptor cells that are responsible for sharp focus and color perception, particularly in bright light”
    “The greatest concentration of cones is on the part of the retina called the fovea “ “The fovea is a particularly sensitive region of the retina. If you want to focus in on something of particular interest, you will automatically try to center the image from the lens onto the area of the fovea to see it more sharply.”
    “Cones are primarily responsible for the sharply focused perception of color, particularly in brightly lit situations, while rods are related to vision in dimly lit situations and are largely insensitive to color and to details as sharp as those the cones are capable of recognizing”
    “The rods play a key role in peripheral vision—seeing objects that are outside the main center of focus—and in night vision.”
    “Rods and cones also are involved in dark adaptation, the phenomenon of adjusting to dim light after being in brighter light. The speed at which dark adaptation occurs is a result of the rate of change in the chemical composition of the rods and cones. The opposite phenomenon—light adaptation­­, or the process of adjusting to bright light after exposure to dim light—occurs much faster, taking only a minute or so.”
    “The distinctive abilities of rods and cones make the eye analogous to a camera that is loaded with two kinds of film. One type is a highly sensitive black-and- white film (the rods). The other type is a somewhat less sensitive color film (the cones).”
  • “Stimulation of the nerve cells in the eye triggers a neural response that is transmitted to other nerve cells, called bipolar cells and ganglion cells, leading to the brain.”
    “Bipolar cells receive information directly from the rods and cones. This information is then communicated to the ganglion cells. Ganglion cells collect and summarize visual information, which is gathered and moved out of the back of the eyeball through a bundle of ganglion axons called the optic nerve.”
    “Because the opening for the optic nerve passes through the retina, there are no rods or cones in the area, which creates a blind spot. Normally, however, this absence of nerve cells does not interfere with vision, because you automatically compensate for the missing part of your field of vision (Ramachandran, 1995; Churchland & Ramachandran, 1995).”
  • Once beyond the eye itself, the neural signals relating to the image move through the optic nerve. As the optic nerve leaves the eyeball, its path does not take the most direct route to the part of the brain right behind the eye. Instead, the optic nerves from each eye meet at a point roughly between the two eyes—called the optic chiasm—where each optic nerve then splits.
    When the optic nerves split, the nerve impulses coming from the right half of each retina are sent to the right side of the brain, and the impulses arriving from the left half of each retina are sent to the left side of the brain. Because the image on the retinas is reversed and upside down, however, those images coming from the right half of each retina actually originated in the field of vision to the person’s left, and images coming from the left half of each retina originated in the field of vision to the person’s right. In this way, our nervous system ultimately produces the phenomenon introduced in Chapter 3, in which each half of the brain is associated with the functioning of the opposite side of the body.
    Psychologists David Hubel and Torsten Wiesel won the Nobel prize for their discovery that many neurons in the cortex are extraordinarily specialized, being activated only by visual stimuli of a particular shape or pattern—a process known as feature detection They found that some cells are activated only by lines of a particular width, shape, or orientation. Other cells are activated only by moving, as opposed to stationary, stimuli (Hubel & Wiesel, 1979; Patzwahl, Zanker, & Altenmuller, 1994).
    More recent work has added to our knowledge of the complex ways in which visual information coming from individual neurons is combined and processed. Different parts of the brain seem to process nerve impulses in several individual systems simultaneously. For instance, one system relates to shapes, one to colors, and others to movement, location, and depth (Moutoussis & Zeki, 1997).
  • trichromatic theory of color vision. The theory suggests that there are three kinds of cones in the retina, each of which responds primarily to a specific range of wavelengths. One is most responsive to blue-violet colors, one to green, and the third to yellow-red (Brown & Wald, 1964). According to trichromatic theory, perception of color is influenced by the relative strength with which each of the three kinds of cones is activated. “
    “there are phenomena that the trichromatic theory is less successful at explaining. For example, the theory does not explain what happens after you stare at something like the flag shown in Figure 4-7 for about a minute. Try this yourself, and then move your eyes to a blank white page. You’ll see an image of the traditional red, white, and blue U.S. flag. “The phenomenon you have just experienced is called an afterimage. It occurs because activity in the retina continues even when you are no longer staring at the original picture.
    “Because trichromatic processes do not provide a full explanation of color vision, alternative explanations have been proposed. According to the opponent-process theory of color vision, receptor cells are linked in pairs, working in opposition to each other. Specifically, there is a blue-yellow pairing, a red-green pairing, and a black-white pairing. If an object reflects light that contains more blue than yellow, it will stimulate the firing of the cells sensitive to blue, simultaneously discouraging or inhibiting the firing of receptor cells sensitive to yellow—and the object will appear blue.
    “Both opponent processes and trichromatic mechanisms are at work in allowing us to see color. However, they operate in different parts of the visual sensing system. Trichromatic processes work within the retina itself, while opponent mechanisms operate both in the retina and at later stages of neuronal processing (Leibovic, 1990; Gouras, 1991; de Valois & de Valois, 1993).”
  • The trichromatic theory provides a straightforward explanation of color blindness. It suggests that one of the three cone systems malfunctions, and colors covered by that range are perceived improperly (Nathans et al., 1989).”
    “For most people with color-blindness, the world looks quite dull. Red fire engines appear yellow, green grass seems yellow, and the three colors of a traffic light all look yellow. In fact, in the most common form of color blindness, all red and green objects are seen as yellow. There are other forms of color blindness as well, but they are quite rare. In yellow-blue blindness, people are unable to tell the difference between yellow and blue, and in the most extreme case an individual perceives no color at all. To such a person the world looks something like the picture on a black-and-white television set.”
  • Sound is the movement of air molecules brought about by the vibration of an object. Although many of us think primarily of the outer ear when we consider hearing, this part functions simply as a reverse megaphone, designed to collect and bring sounds into internal portions of the ear, (see Figure 4-8). However, the location of the outer ears on different sides of the head helps with sound localization, the process by which we identify the origin of a sound. Wave patterns in the air enter each ear at a slightly different time, permitting the brain to use the discrepancy to locate the place from which the sound is originating
    In addition, the two outer ears delay or amplify sounds of particular frequencies to different degrees. Sounds arriving at the outer ear in the form of wave vibrations are funneled into the auditory canal, a tube like passage that leads to the eardrum
    The eardrum is aptly named because it operates like a miniature drum, vibrating when sound waves hit it. These vibrations are then transferred into the middle ear, a tiny chamber containing three bones (the hammer, the anvil, and the stirrup) that transmit vibrations to the oval window, a thin membrane leading to the inner ear. Because the hammer, anvil, and stirrup act as a set of levers, they not only transmit vibrations but actually increase their strength. The middle ear, acts as a tiny mechanical amplifier.
  • When sound enters the inner ear through the oval window, it moves into the cochlea, a coiled tube that looks something like a snail and is filled with fluid that can vibrate in response to sound.
    Inside the cochlea is the basilar membrane, a structure that runs through the center of the cochlea, dividing it into an upper and a lower chamber
    The basilar membrane is covered with hair cells. When these hair cells are bent by the vibrations entering the cochlea, a neural message is transmitted to the brain
    Although sound typically enters the cochlea via the oval window, there is an additional method of entry: bone conduction. Because the ear rests on a maze of bones within the skull, the cochlea is able to pick up subtle vibrations that travel across the bones from other parts of the head. For instance, one of the ways you hear your own voice is through bone conduction. This explains why you sound different to yourself than to other people who hear your voice. (Listen to yourself on a tape recorder sometime to hear what you really sound like!) The sound of your voice reaches you both through the air and via bone conduction and therefore sounds richer to you than to everyone else.
  • sound is actually the physical movement of air molecules in regular, wavelike patterns caused by the vibration of an object
    Sometimes it is even possible to view these vibrations, as in the case of a stereo speaker that has no enclosure. If you have ever seen one, you know that, at least when the lowest notes are playing, you can see the speaker moving in and out. What is less obvious is what happens next: The speaker pushes air molecules into waves with the same pattern as its movement. These wave patterns soon reach your ear, although their strength has been weakened considerably during their travels. All other stimuli that produce sound work in essentially the same fashion, setting off wave patterns that move through the air to the ear. Air—or some other medium, such as water—is necessary to make the vibrations of objects reach us. This explains why there can be no sound in a vacuum.
    Frequency is the number of wave cycles that occur in a second. With very low frequencies there are relatively few, and therefore slower, up-and-down wave cycles per second. These are visible to the naked eye as vibrations in the speaker
    Low frequencies are translated into a sound that is very low in pitch. (Pitch is the characteristic that makes sound “high” or “low.”) For example, the lowest frequency that humans are capable of hearing is 20 cycles per second. Higher frequencies translate into higher pitch. At the upper end of the sound spectrum, people can detect sounds with frequencies as high as 20,000 cycles per second.
  • Intensity is a feature of wave patterns that allows us to distinguish between loud and soft sounds. Intensity is produced by difference between the peaks and valleys of air pressure in a sound wave as it travels through the air. Waves with small peaks and valleys produce soft sounds, while those that are relatively large produce loud sounds
    The loudest sounds we are capable of hearing are about 10 million times as intense as the very weakest sound we can hear. This range is measured in decibels. When sounds get higher than 120 decibels, they become painful to the human ear.
    Although minor hearing impairment may be treated with hearing aids that increase the volume of sounds reaching the ear, more drastic measures are necessary in more severe cases. Certain forms of deafness, produced by damage to the hair cells, can be treated through a cochlear implant, like that received by Amy Ecklund, whose case was discussed in the chapter prologue. Implants consist of a tiny receiver inside the ear and an electrode that stimulates hair cells, controlled by a small external sound processor worn behind the ear.
    Although the restoration of hearing to a deaf person may seem like an unquestionably positive achievement, some advocates for the deaf suggest otherwise, especially when it comes to deaf children who are not old enough to provide informed consent. These critics suggest that deafness represents a legitimate culture—no better nor worse than the hearing culture—and that providing even limited hearing to deaf children robs them of their natural cultural heritage. It is, without doubt, a controversial position.
  • the place theory of hearing says that different areas of the basilar membrane respond to different frequencies
    place theory does not tell the full story of hearing, since very low frequency sounds trigger neurons across such a wide area of the basilar membrane that no single site is involved. Consequently, an additional explanation for hearing has been proposed: frequency theory
    The frequency theory of hearing suggests that the entire basilar membrane acts like a microphone, vibrating as a whole in response to a sound. According to this explanation, the nerve receptors send out signals that are tied directly to the frequency (the number of wave crests per second) of the sounds to which we are exposed, with the number of nerve impulses being a direct function of the sound’s frequency.
    Neither place theory nor frequency theory provides the full explanation for hearing (Levine & Shefner, 1991; Hartmann, 1993; Luce, 1993; Hirsh & Watson, 1996). Place theory provides a better explanation for the sensing of high-frequency sounds, whereas frequency theory explains what happens when low-frequency sounds are encountered. Medium-frequency sounds incorporate both processes
  • Several structures of the ear are related more to our sense of balance than to our hearing.
    The semicircular canals of the inner ear consist of three tubes containing fluid that sloshes through them when the head moves, signaling rotational or angular movement to the brain. The pull on our bodies caused by the acceleration of forward, backward, or up-and-down motion, as well as the constant pull of gravity, is sensed by the otoliths, tiny, motion-sensitive crystals. When we move, these crystals shift like sands on a windy beach. The brain’s inexperience in interpreting messages from the weightless otoliths is the cause of the space sickness commonly experienced by two-thirds of all space travelers
  • Although many animals have keener abilities to detect odors than we do, our sense of smell (olfaction) permits us to detect more than 10,000 separate smells. We also have a good memory for smells, and long-forgotten events and memories can be brought back with the mere whiff of an odor associated with the memory (Schab, 1991; Bartoshuk & Beauchamp, 1994; Gillyatt, 1997).
    Our understanding of the mechanisms that underlie the sense of smell is just beginning to emerge. We do know that the sense of smell is sparked when the molecules of a substance enter the nasal passages and meet olfactory cells, the receptor cells of the nose, which are spread across the nasal cavity. More than 1,000 separate types of receptor cells have been identified so far. Each of these cells is so specialized that it responds only to a small band of different odors. The responses of the separate olfactory cells are then transmitted to the brain, where they are combined into recognition of a particular smell (Buck & Axel, 1991; Katz & Rubin, 1999).
    There’s increasing evidence that smell may also act as a hidden means of communication for humans. It has long been known that non-humans release pheromones, pollen-like chemicals that produce a reaction in other members of a species, permitting the transmission of such messages as sexual availability. For instance, certain substances in the vaginal secretions of female monkeys contain pheromones that stimulates sexual interest in male monkeys (Holy, Dulac, & Meister, 2000).
  • Unlike smell, which employs more than 1,000 separate types of receptor cells, the sense of taste (gustation) seems to make do with only a handful of fundamental types of receptors. Most psychologists believe that there are just four basic receptor cells, which specialize in either sweet, sour, salty, or bitter flavors. Every other taste is simply a combination of these four basic qualities, in the same way that the primary colors blend into a vast variety of shades and hues (McLaughlin & Margolskee, 1994).
    The receptor cells for taste are located in roughly 10,000 taste buds, which are distributed across the tongue, as well as other parts of the mouth and throat. The taste buds wear out and are replaced every 10 days or so. That’s a good thing, because if our taste buds weren’t constantly reproducing, we’d lose the ability to taste after we’d accidentally burned our tongues.
    The sense of taste differs significantly from one person to another, determined largely by genetic factors. Some people, dubbed “supertasters,” are highly sensitive to taste; they have twice as many taste receptors as “nontasters,” who are relatively insensitive to taste. Supertasters (who, for unknown reasons, are more likely to be female than male) find sweets sweeter, cream creamier, and spicy dishes spicier, and weaker concentrations of flavor are enough to satisfy any cravings they may have. On the other hand, because they aren’t so sensitive to taste, nontasters may seek out relatively sweeter and fattier foods in order to maximize the taste. As a consequence, they may be prone to obesity (Bartoshuk & Drewnowski, 1997; Bartoshuk, 2000).
  • all our skin senses—touch, pressure, temperature, and pain—play a critical role in survival, making us aware of potential danger to our bodies. Most of these senses operate through nerve receptor cells located at various depths throughout the skin, distributed unevenly throughout the body.
    Probably the most extensively researched skin sense is pain. Pain is a response to a great variety of different kinds of stimuli. A light that is too bright can produce pain, and sound that is too loud can be painful. One explanation is that pain is an outcome of cell injury; when a cell is damaged, regardless of the source of damage, it releases a chemical called substance P that transmits pain messages to the brain.
    According to the gate-control theory of pain, particular nerve receptors in the spinal cord lead to specific areas of the brain related to pain. When these receptors are activated because of some injury or problem with a part of the body, a “gate” to the brain is opened, allowing us to experience the sensation of pain.
    However, another set of neural receptors is able, when stimulated, to close the “gate” to the brain, thereby reducing the experience of pain. The gate may be shut in two different ways. First, other impulses can overwhelm the nerve pathways relating to pain, which are spread throughout the brain.
    Psychological factors account for the second way in which a gate may be shut. Depending on an individual’s current emotions, interpretation of events, and previous experience, the brain may close a gate by sending a message down the spinal cord to an injured area, producing a reduction in or relief from pain.
  • Because each figure is two-dimensional, the usual means we employ for distinguishing the figure (the object being perceived) from the ground (the background or spaces within the object) do not work.
    The fact that we can look at the same figure in more than one way illustrates an important point. We do not just passively respond to visual stimuli that happen to fall on our retinas. Instead, we actively try to organize and make sense of what we see.
    We turn now from a focus on the initial response to a stimulus (sensation) to what our minds make of that stimulus—perception. Perception is a constructive process by which we go beyond the stimuli that are presented to us and attempt to construct a meaningful situation
  • Some of the most basic perceptual processes operate according to a series of principles that describe how we organize bits and pieces of information into meaningful wholes. These are known as gestalt laws of organization, set forth in the early 1900s by a group of German psychologists who studied patterns, or gestalts (Wertheimer, 1923). They discovered a number of important principles that are valid for visual (as well as auditory) stimuli
    Closure. Groupings are usually made in terms of enclosed or complete figures rather than open ones. We tend to ignore the breaks and concentrate on the overall form.
    Proximity. Elements that are closer together are grouped together.
    Similarity. Elements that are similar in appearance are grouped together.
    Simplicity. In a general sense, the overriding gestalt principle is simplicity: When we observe a pattern, we perceive it in the most basic, straightforward manner that we can. For example, most of us see a square with lines on two sides, rather than as the block letter “W” on top of the letter “M.” If we have a choice of interpretations, we generally opt for the simpler one.
  • A more recent approach to perception, feature analysis, considers how we perceive a shape, pattern, object, or scene by reacting first to the individual elements that make it up. These individual components are then used to understand the overall nature of what we are perceiving. Feature analysis begins with the evidence that individual neurons in the brain are sensitive to specific spatial configurations, such as angles, curves, shapes, and edges, as discussed earlier in the chapter. The presence of these neurons suggests that any stimulus can be broken down into a series of component features.
    According to feature analysis, when we encounter a stimulus—such as a letter—our brain’s perceptual processing system initially responds to its component parts. Each of these parts is compared with information about components that is stored in memory. When the specific components we perceive match up with a particular set of components that we have encountered previously, we are able to identify the stimulus (Spillmann & Werner, 1990; Ullman, 1996).
  • If perception were based primarily on breaking down a stimulus into its most basic elements, understanding the sentence, as well as other ambiguous stimuli, would not be possible. The fact that you were probably able to recognize such an imprecise stimulus illustrates that perception proceeds along two different avenues, called top-down processing and bottom-up processing.
    In top-down processing, perception is guided by higher-level knowledge, experience, expectations, and motivations
    Even though top-down processing allows us to fill in the gaps in ambiguous and out-of-context stimuli, we would be unable to perceive the meaning of such stimuli without bottom-up processing
    Bottom-up processing consists of recognizing and processing information about the individual components of the stimuli.
    It should be apparent that top-down and bottom-up processing occur simultaneously, and interact with each other, in our perception of the world around us. It is bottom-up processing that permits us to process the fundamental characteristics of stimuli, whereas top-down processing allows us to bring our experience to bear on perception. And as we learn more about the complex processes involved in perception, we are developing a better understanding of how our brain continually interprets information from our senses and permits us to make responses appropriate to the environment (Egeth & Yantis, 1997; Rees, Frith, & Lavie, 1997).
  • Perceptual constancy is a phenomenon in which physical objects are perceived as unvarying and consistent, despite changes in their appearance or in the physical environment.
    One of the most dramatic examples of perceptual constancy involves the rising moon. When the moon first appears at night, close to the horizon, it seems to be huge—considerably larger than when it is high in the sky later in the evening. You may have thought that the apparent size of the moon was caused by the moon’s being physically closer to the earth when it first appears. In fact, though, this is not the case at all.
    Instead, the moon appears to be larger when it is close to the horizon primarily because of a misapplication of perceptual constancy. When the moon is near the horizon, the perceptual cues of intervening terrain and objects such as trees on the horizon produce a misleading sense of distance.
    Although other factors help account for the moon illusion, perceptual constancy appears to be a primary ingredient in our susceptibility to the illusion. Furthermore, perceptual constancy occurs not just in terms of size (as with the moon illusion) but with shape and color as well. Despite the varying images on our retina as a plane approaches, flies overhead, and disappears, we do not perceive the plane as changing shape (Coren & Aks, 1990; Suzuki, 1991).
  • The ability to view the world in three dimensions and to perceive distance—a skill known as depth perception—is due largely to the fact that we have two eyes. Because there is a certain distance between the eyes, a slightly different image reaches each retina. The brain then integrates these two images into one composite view. But it does not ignore the difference in images, which is known as binocular disparity. The disparity allows the brain to estimate the distance of an object from us.
    In some cases, certain cues permit us to obtain a sense of depth and distance with just one eye (Burnham, 1983). These cues are known as monocular cues. One monocular cue—motion parallax—is the change in position of an object on the retina due to movement of the head. The brain is able to calculate the distance of the object by the amount of change in the retinal image. Similarly, experience has taught us that if two objects are the same size, the one that makes a smaller image on the retina is farther away than the one that provides a larger image—an example of the monocular cue of relative size.
    Finally, anyone who has ever seen railroad tracks that seem to join together in the distance knows that distant objects appear to be closer together than nearer ones, a phenomenon called linear perspective. People use linear perspective as a monocular cue in estimating distance, allowing the two-dimensional image on the retina to record the three-dimensional world (Bruce, Green, & Georgeson, 1997; Dobbins et al., 1998).
  • Visual illusions are physical stimuli that consistently produce errors in perception.
    Modern-day architects and designers also take visual distortions into account in their planning. For example, the New Orleans Superdome makes use of several visual tricks. Its seats vary in color throughout the stadium to give the appearance, from a distance, that there is always a full house. The carpeting in some of the sloping halls has stripes that make people slow their pace by producing the perception that they are moving faster than they actually are. The same illusion is used at toll booths on superhighways. Stripes painted on the pavement in front of the toll booths make drivers feel that they are moving more rapidly than they actually are and cause them to decelerate quickly.
  • The implications of visual illusions go beyond the attractiveness of buildings. For instance, suppose you were an air traffic controller watching a radar screen like the one shown in Figure 4-20a. You might be tempted to sit back and relax as the two planes, whose flight paths are indicated in the figure, drew closer and closer together. If you did, however, the result might be an air disaster. Although it looks as if the two planes will miss each other, they are headed for a collision. Investigation has suggested that some 70 to 80 percent of all airplane accidents are caused by pilot errors of one sort or another (O’Hare & Roscoe, 1990; Baker, et al., 1993).
    The flight-path illustration provides an example of a well-known visual illusion called the Poggendorf illusion. As you can see in Figure 4-20b, the Poggendorf illusion, when stripped down to its basics, gives the impression that line X would pass below line Y if it were extended through the pipe-like figure, instead of heading directly toward line Y as it actually does.
    The misinterpretations created by visual illusions are ultimately due, then, to errors in both fundamental visual processing and the way the brain interprets the information it receives. But visual illusions, by illustrating something fundamental about perception, become more than mere psychological curiosities. There is a basic connection between our prior knowledge, needs, motivations, and expectations about how the world is put together and the way we perceive it. Our view of the world is very much a function, then, of fundamental psychological factors. Furthermore, each person perceives the environment in a way that is unique and special—a fact that allows each of us to make our own special contribution to the world.
  • Subliminal perception refers to the perception of messages about which we have no awareness. The stimulus may be a word, a sound, or even a smell that activates the sensory system, but that is not intense enough to be reported as having been experienced by a person. For example, in some studies people are exposed to a descriptive label—called a prime—about a person (such as the word “smart” or “happy”) so briefly that they cannot report seeing the label. Later, however, they form impressions that are influenced by the content of the prime.
    Given the lack of evidence for subliminal perception, psychologists are even more skeptical of reports of extrasensory perception, or ESP—perception that does not involve our known senses. Most psychologists reject the existence of ESP, asserting that there is no sound documentation that the phenomenon exists (Swets & Bjork, 1990; Hyman, 1994).
    However, an ongoing debate in the last decade in one of the most prestigious psychology journals, Psychological Bulletin, has heightened interest in the area. According to proponents of ESP, reliable evidence exists for an “anomalous process of information transfer,” or psi (Bem & Honorton, 1994). These researchers, who painstakingly reviewed considerable evidence, argue that a cumulative body of research shows reliable support for the existence of psi. Because of questions about the quality of the research, as well as a lack of any credible theoretical explanation for how extrasensory perception might take place, most psychologists continue to believe that there is no reliable scientific support for ESP.

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