Introduction to Psychology: Can't You Hear the Bats? PDF

preview Can’t You Hear the Bats? One of your authors once stood at dusk at the mouth of a huge oversimplifies how we sense and perceive the world. Certainly, limestone cave on the island of Borneo. For some 20 minutes, as our bat story...

preview Can’t You Hear the Bats? One of your authors once stood at dusk at the mouth of a huge oversimplifies how we sense and perceive the world. Certainly, limestone cave on the island of Borneo. For some 20 minutes, as our bat story shows, the senses do not capture complete infor- fruit bats streamed out of the cave until a dense cloud of half mation about the environment. While this may seem to be some a million bats swirled overhead in the gathering darkness. sort of evolutionary oversight, just the opposite is the case. We Besides the sheer spectacle, what most struck him was the collect enough sensory information to survive while reducing silence. Aside from the occasional whisper of bat wings swish- the amount of sensory information bombarding the brain. This ing by his ear, he heard nothing. And yet every single bat was selectivity helps prevent the brain from overloading. shouting — at a pitch so high a human being couldn’t hear it. Regardless, at this very moment you are bathed in a swirl- Echoes from those shouts allowed the bats to avoid colliding ing kaleidoscope of electromagnetic radiation, heat, pressure, with each other. At that moment, he was keenly aware of the vibrations, molecules, and mechanical forces. Without the limitations of his senses. senses, all of this would seem like nothing more than a void of Do you imagine that the ear is like a digital recorder or the silence and darkness. The next time you drink in the beauty of eye is like a camera, giving us a completely accurate “picture” of a sunset, a flower, or a friend, remember this: Sensation makes reality? Perhaps you already understand that this view greatly it all possible. Psychophysics — The tions into sound waves. Pluck a string and the guitar will produce a sound. However, stimuli that don’t cause the string to move will have Limits of Sensibility no effect. For instance, if you shine a light on the string, or pour cold Gateway Question: In what ways are our senses limited? water on it, the guitar will remain silent. (The owner of the guitar, Physical energy, in the form of light waves or heat or sound waves, however, might get quite loud at this point!) Thus, the eye trans- stimulates your senses. An instant later you see a snowball whiz duces electromagnetic radiation, the ear transduces sound waves, and past your nose or feel the warmth of the sun on your face or hear a so on. Many other types of stimuli cannot be sensed directly because catchy tune on the radio. In that instant, a remarkable series of we have no sense receptors to transduce their energy. events will have transpired as you detect, analyze, and interpret For example, humans cannot sense the bioelectric fields of sensory information. other living beings, but sharks have special organs that can (Fields, In an approach called psychophysics, physical energy (such as 2007). (Do they hear the fields or feel them or what?) Similarly, sound waves or electromagnetic radiation) is measured and related humans can transduce only visible light, which is a tiny slice of the to dimensions of the resulting sensations we experience (such as electromagnetic spectrum (entire spread of electromagnetic wave- loudness or brightness). Psychophysical investigations have lengths). The spectrum also includes infrared and ultraviolet light, revealed much about our sensory capacities and their limits. radio waves, television broadcasts, gamma rays, and other energies Our eyes, for example, provide us with stunningly wide access (look ahead at Figure 4.3). In contrast, the eyes of honeybees to the world. In one instant you can view a star light-years away, transduce parts of the electromagnetic spectrum invisible to and in the next, you can peer into the microscopic universe of a humans. As you can see, our rich sensory experience is only a small dewdrop. Yet, vision also dramatically narrows what we can part of what could be sensed and what some animals can sense. observe. Like the other senses, vision acts as a data reduction sys- tem. Your senses send only the most important data to your brain Absolute Thresholds (Sekuler & Blake, 2006). Before we examine specific senses in Before you can experience a sensation, a stimulus (physical energy) more detail, let’s explore how the senses reduce the amount of must be above a certain minimum intensity. The necessary mini- information the brain must process. mum defines the absolute threshold for a sensory system. For Transduction How does data reduction take place? To begin, our senses limit what Psychophysics Study of the relationship between physical stimuli and we can experience because they do not transduce all the physical ener- the sensations they evoke in a human observer. gies surrounding us. Sensory receptors, such as the eyes and ears, are Transducers Devices that convert one kind of energy into another. biological transducers, devices that convert one kind of energy into Absolute threshold The minimum amount of physical energy another (Fain, 2003). For example, a guitar transduces string vibra- necessary to produce a sensation. 119 120 CHAPTER 4 Table 4.1 Absolute Thresholds Sensory Modality Absolute Threshold Vision Candle flame seen at 30 miles on a clear dark night Hearing Tick of a watch under quiet conditions at 20 feet Taste 1 teaspoon of sugar in 2 gallons of water Smell 1 drop of perfume diffused into a three-room apartment Touch A bee’s wing falling on your cheek from 1 centi- meter above © Robert Rattner From Galanter, 1962. example, very soft sounds (which could be heard if they were just a little louder) fall below the absolute threshold for human hear- ing. Of course, owls have much lower absolute thresholds for hear- Absolute thresholds define the sensory worlds of humans and animals, some- times with serious consequences. The endangered Florida manatee (“sea cow”) ing, which allows them to hunt at night. is a peaceful, plant-eating creature that can live for more than 60 years. For the What is the quietest sound that humans can hear? The weakest light last decade, the number of manatees killed by boats has climbed alarmingly. The that we can see? The lightest touch that we can feel? Testing for absolute problem? Manatees have poor sensitivity to the low-frequency sounds made by thresholds shows just how sensitive we are. For example, it only takes slow-moving boats. Current laws require boats to slow down in manatee habi- tats, which may actually increase the risk to these gentle giants (Gerstein, 2002). three photons of light striking visual receptors at the back of the eye to produce a sensation. A photon (FOE-tahn: one quantum of energy) is the smallest possible “package” of light. Responding to students saw photos of a person flashed on a screen. Each time three photons is like seeing a candle flame 30 miles away! Table 4.1 before the face appeared, it was preceded by a subliminal image that was shown for just a fraction of a second. Some were images that gives approximate absolute thresholds for the five major senses. Some sensory systems have upper limits as well as lower ones. made viewers feel good (such as cute kittens). Others made them For example, if we test for pitch sensitivity (higher and lower feel bad (for example, a face on fire). All the emotional images were tones), we find that humans can hear sounds down to 20 hertz flashed too briefly to be recognized. Nevertheless, they altered the (vibrations per second) and up to about 20,000 hertz. This is an impressions students formed of the target person (Krosnick et al., impressive range — from the lowest rumble of a pipe organ to the 1992). Apparently, some emotional impact gets through, even when highest squeak of a stereo “tweeter.” On the lower end, the thresh- a stimulus is below the level of conscious awareness (Arndt, Allen, & old is as low as practical. If your ears could sense tones below Greenberg, 2001). To find out if such effects could be applied to 20 hertz, you would hear the movements of your own muscles. advertising, read “Subliminal Seduction or Subliminal Myths?” Imagine how disturbing it would be to hear your body creak and groan like an old ship as you move. Difference Thresholds The 20,000 hertz upper threshold for human hearing, on the Psychophysics also involves the study of difference thresholds. other hand, could easily be higher. Bats, dogs, cats, and other ani- Here we are asking, “How much must a stimulus change (increase mals can hear sounds well above this limit. That’s why a “silent” dog or decrease) before it becomes just noticeably different?” The whistle (which may make sounds as high as 40,000 to 50,000 hertz) study of just noticeable differences (JNDs) led to one of psychol- can be heard by dogs but not by humans. For dogs the sound exists. ogy’s first natural “laws.” Weber’s (VAY-bears) law can be roughly For humans it is beyond awareness. It’s easy to see how thresholds stated as follows: The amount of change needed to produce a JND define the limits of the sensory world in which we live. (If you want is a constant proportion of the original stimulus intensity. Here are to buy a stereo system for your dog, you will have a hard time find- some Weber’s proportions for common judgments: ing one that reproduces sounds above 20,000 hertz!) Pitch 1/333 (1/3 of 1 percent) Wouldn’t the absolute threshold be different for different people? Weight 1/50 (2 percent) Not only do absolute thresholds vary for different people, they Loudness 1/10 (10 percent) also change from time to time for a single person. The type of Taste 1/5 (20 percent) stimulus, the state of your nervous system, and the costs of false “detections” all make a difference (Goldstein, 2007). Emotional Notice how much more sensitive hearing is than taste. Very small factors are also important. Unpleasant stimuli, for example, may changes in pitch and loudness are easy to detect. If a voice or a musi- raise the threshold for recognition. cal instrument is off pitch by 1/3 of 1 percent, you’ll probably notice Is “subliminal” perception possible? Yes, under limited circum- it. For taste, we find that a 20-percent change is necessary to produce stances. Anytime information is processed below the normal limen a JND. If a cup of coffee has 5 teaspoons of sugar in it, you will have (LIE-men: threshold or limit) for awareness, it is subliminal. Sub- to add 1 more (1/5 of 5) before it will be noticeably sweeter. If you’re liminal perception was demonstrated by a study in which college salting soup, it takes a lot of cooks to spoil the broth!  Sensation and Reality 121 CRIT ICA L T H I N KI N G Subliminal Seduction or Subliminal Myth? Could subliminal perception ever be used performed no better than students who They suggest that only simple messages, against us? The sensationalistic book just listened to relaxing ocean sounds with- such as single words, can be processed sub- Subliminal Seduction (Key, 1973) voiced pop- out subliminal messages and students who liminally, which is why subliminal self-help ular fears of attempts to influence us through listened to nothing at all (Russell, Rowe, & materials are usually ineffective. They also subliminal messages embedded in advertis- Smouse, 1991). In this instance, subliminal provide evidence that subliminally flashing ing. But could it work? messages had no effect, even when people the brand name of a drink can increase the In a famous early attempt, a New Jersey wanted to be influenced. In most cases, likelihood that people will buy it, but only if theater flashed the words Eat popcorn and people who think they have been helped by they are already thirsty. Perhaps subliminal Drink Coca-Cola on the screen for 1/3,000 sec- subliminal messages have likely experienced advertising can work under limited circum- ond every 5 seconds during movies. Dramatic nothing more than a placebo effect (Froufe & stances. Nevertheless, advertisers are still claims that popcorn and Coke sales increased Schwartz, 2001). better off using the loudest, clearest, most as a result later turned out to be falsehoods. However, before writing off subliminal attention-demanding stimuli available — as The advertising “expert” responsible admit- advertising altogether, let’s consider the work most do (Smith & Rogers, 1994). ted he faked the whole thing. By lying about of psychologist Johan Karremans and his col- his ability to control audiences, he had hoped leagues (Karremans, Stroebe, & Claus, 2006). to gain customers for his marketing business (Pratkanis, 1992; Shrum, 2004). Regardless, the possibility of subliminal seduction was so horrifying that such advertising was banned in the United States, Britain, and Australia (Karremans, Stroebe, & Claus, 2006). Despite these fears, some businesses actually sell subliminal messages to people who want them to work. Each year, con- © Michael Maslin from cartoon bank.com. All Rights Reserved. sumers spend many millions of dollars on so-called subliminal self-help tapes and CDs (Pratkanis & Aronson, 2001). “Subliminal mes- sages” embedded in relaxing music or the soothing sounds of ocean waves purport- edly influence “subconscious motivation” to help listeners lose weight, relieve pain, find romance, succeed financially, improve grades, and so forth. In one study, students who listened to subliminal messages meant to improve their study habits, and, hence, their grades, Sensory Analysis and Coding What we experience is greatly influenced by sensory analysis. As they process information, the senses divide the world into impor- Subliminal perception Perception of a stimulus below the threshold tant perceptual features (basic stimulus patterns). For vision, for conscious recognition. such features include lines, shapes, edges, spots, colors, and other Difference threshold A change in stimulus intensity that is detectable patterns (Hubel & Wiesel, 2005). Look at Figure 4.1 and notice to an observer. how eye-catching the single vertical line is among a group of Just noticeable difference (JND) Any noticeable difference in a stimulus. slanted lines. This effect, which is called pop-out, occurs because Weber’s law The just noticeable difference is a constant proportion of your visual system is highly sensitive to perceptual features such as the original stimulus intensity. line orientation (Adler & Orprecio, 2006). Sensory analysis Separation of sensory information into important In some instances, the senses act as feature detectors because elements. they are attuned to very specific stimuli. Frog eyes, for example, Perceptual features Basic elements of a stimulus, such as lines, shapes, are highly sensitive to small, dark, moving spots. In other words, edges, or colors. 122 CHAPTER 4 including pressure — into visual features. As a result, you experi- ence light sensations, not pressure. Also important in producing this effect is sensory localization in the brain. Sensory localization means that the type of sensation you experi- ence depends on which brain area is activated. Some brain areas receive visual information, others receive auditory information, and still others receive taste or touch. (See Chapter 2, pages 63–67.) Knowing which brain areas are active tells us, in general, what kinds of sensations you are feeling. Sensory localization makes it possible to artificially restore sight, hearing, or other senses. For example, in July 2006, a woman named Cheri Robinson became the sixteenth person in the world with an implant that allows a miniature television camera to send electrical signals to her brain’s visual cortex ( Figure 4.2). She can now “see” 100 dots of light. Like a sports scoreboard, these lights can be used to form crude letters (Warren & Normann, 2005). Eventually, increasing the number of dots could make reading and “seeing” large objects, such as furniture and doorways, possible. months Figure 4.1 Visual pop-out. Pop-out is so basic that babies as young as 3 respond to it. (Adapted from Adler & Orprecio, 2006.) It is fascinating to realize that “seeing” and “hearing” take place in the brain, not in the eye or ear. Information arriving from the sense organs creates sensations. When the brain organizes sensa- they are basically “tuned” to detect bugs flying nearby (Lettvin, tions into meaningful patterns, we speak of perception, which 1961). But the insect (spot) must be moving, or the frog’s “bug will be covered in Chapter 5. It’s now time to examine each of the detectors” won’t work. A frog could starve to death surrounded senses in more detail. In the next section, we will begin with vision, by dead flies. which is perhaps the most magnificent sensory system of all. After they have selected and analyzed information, sensory Before you read more, it might be a good idea to stop and review systems must code it. Sensory coding refers to changing important some of the ideas we have covered. features of the world into messages understood by the brain (Hubel & Wiesel, 2005). To see coding at work, try closing your KNOWL E DG E B U I L DE R eyes for a moment. Then take your fingertips and press firmly on your eyelids. Apply enough pressure to “squash” your eyes slightly. Sensation and Psychophysics Do this for about 30 seconds and observe what happens. (Readers RECITE 1. Sensory receptors are biological ___________________, or devices with eye problems or contact lenses should not try this.) for converting one type of energy to another. Did you “see” stars, checkerboards, and flashes of color? These 2. The minimum amount of stimulation necessary for a sensation to are called phosphenes (FOSS-feens: visual sensations caused by occur defines the ___________________ ______________________. mechanical excitation of the retina). They occur because the eye’s Continued receptor cells, which normally respond to light, are also Visual somewhat sensitive to pressure. Notice though, that cortex the eye is only prepared to code stimulation — Electrodes Actual image Cameras Perceived image Figure 4.2 An artificial visual system.  Sensation and Reality 123 INVISIBLE LONG WAVES VISIBLE LIGHT SPECTRUM INVISIBLE SHORT WAVES Infrared rays Ultraviolet rays (beyond red) (beyond violet) 1500 1000 700 600 500 400 300 Infra- Gamma Cosmic Radio TV Microwaves U-V X-rays red rays rays Figure 4.3 The visible spectrum. 3. Subliminal stimuli have been shown to have an effect on the behav- Vision — Catching Some Rays ior of viewers. T or F? Gateway Question: How does the visual system function? 4. Lettvin found that a frog’s eyes are especially sensitive to phos- phenes. T or F? In the morning when you awaken and open your eyes you effort- 5. Important features of the environment are transmitted to the brain lessly become aware of the visual richness of the world around you. through a process known as But the ease with which normally sighted people can see conceals a. phosphenation incredible complexity. Vision is an impressive sensory system, b. coding c. detection worthy of a detailed discussion. d. programming What are the basic dimensions of light and vision? As we have noted, various wavelengths of light make up the visible spectrum REFLECT (the narrow spread of electromagnetic energies to which the eyes Critical Thinking 6. Is a doorbell a transducer? respond). Visible light starts at “short” wavelengths of 400 nano- 7. If the human ear were more sensitive than it is now, our hearing meters (nan-OM-et-er: one billionth of a meter), which we sense would be impaired. How could this be true? as purple or violet. Longer light waves produce blue, green, yel- 8. When promoters of self-help “subliminal tapes” are challenged to low, orange, and red, which has a wavelength of 700 nanometers provide evidence that their products work, what study do you think they most often cite? ( Figure 4.3). Relate The term hue refers to the basic color categories of red, orange, How does sensation affect what you are experiencing right now? What if yellow, green, blue, indigo, and violet. As just noted, various hues data reduction didn’t occur? What if you could transduce other energies? (or color sensations) correspond to the wavelength of the light What if your senses were tuned to detect different perceptual features? that reaches our eyes (Sekuler & Blake, 2006). White light, in What if your absolute thresholds were higher or lower for each sense? How would the sensory world you live in change? contrast, is a mixture of many wavelengths. Hues (colors) from a faked “Eat popcorn/drink Coca-Cola” study (Pratkanis, 1992). narrow band of wavelengths are very saturated, or “pure.” (An constant roaring or hissing noise. 8. Good guess! That’s right, it’s still the tive, they would convert the random movement of air molecules into a hydrogen atom) can be heard. Therefore, if your ears were more sensi- as small as one billionth of a centimeter (one tenth the diameter of a Sensory coding Codes used by the sense organs to transmit person in the house. 7. Under ideal conditions, vibrations of the eardrum information to the brain. sound waves that are transduced into nerve impulses by the ears of the Sensation A sensory impression; also, the process of detecting physical in order to strike a bell; the physical vibrations of the bell then produce energies with the sensory organs. into an electrical signal that is converted again into mechanical energy sense, it is. The button converts mechanical energy from your finger Perception The mental process of organizing sensations into Answers: 1. transducers 2. absolute threshold 3. T 4. F 5. b 6. In a broad meaningful patterns. Visible spectrum The narrow spread of the electromagnetic spectrum to which the eyes are sensitive. 124 CHAPTER 4 intense “fire-engine” red is more saturated than a muddy “brick” process is called accommodation. In cameras, focusing is done red.) A third dimension of vision, brightness, corresponds roughly more simply — by changing the distance between the lens and the to the amplitude, or height of light waves. Waves of greater ampli- image sensor. tude are “taller,” carry more energy, and cause the colors we see to appear brighter or more intense. For example, the same “brick” red Visual Problems would look bright under intense, high-energy illumination and Focusing is also affected by the shape of the eye. If your eye is too drab under dim light. short, nearby objects will be blurred, but distant objects will be sharp. This is called hyperopia (HI-per-OPE-ee-ah: farsighted- Structure of the Eye ness). If your eyeball is too long, images fall short of the retina and Although the visual system is much more complex than any digital you won’t be able to focus distant objects. This results in myopia camera, both cameras and eyes have a lens to focus images on a (my-OPE-ee-ah: nearsightedness). When the cornea or the lens is light-sensitive layer at the back of a closed space. In a camera, this misshapen, part of vision will be focused and part will be fuzzy. In layer is the digital image sensor. In the eye, it is a layer of photore- this case, the eye has more than one focal point, a problem called ceptors (light-sensitive cells) in the retina, an area about the size astigmatism (ah-STIG-mah-tiz-em). All three visual defects can and thickness of a postage stamp ( Figure 4.4). be corrected by reshaping the cornea through laser eye surgery or by placing glasses (or contact lenses) in front of the eye to change How does the eye focus? Most focusing is done at the front of the eye by the cornea, a clear membrane that bends light inward. The the path of light ( Figure 4.5). lens makes additional, smaller adjustments. Your eye’s focal point As people age, the lens becomes less flexible and less able to changes when muscles attached to the lens alter its shape. This accommodate. The result is presbyopia (prez-bee-OPE-ee-ah: old vision, or farsightedness due to aging). Perhaps you have seen Ciliary a grandparent or older friend reading a newspaper at arm’s length muscle because of presbyopia. If you now wear glasses for nearsightedness, you may need bifocals as you age. ( Just like your authors. Sigh.) Aqueous humor Bifocal lenses correct near vision and distance vision. Fovea Iris Blind spot Light Control Pupillary opening There is one more major similarity between the eye and a camera. Optic nerve Cornea In front of the lens in both is a mechanism that controls the amount of light entering. In the eye, this mechanism is the iris; in a camera, it is the diaphragm ( Figure 4.6). The iris is a colored Retinal arteries circular muscle that gives your eyes their blue or brown or green and veins Lens color. By expanding and contracting, the iris changes the size of the pupil (the opening at the center of the eye). Retina With some cameras, you can adjust the sensitivity of the digital Figure 4.4 The human eye, a simplified view. image sensor. The sensitivity of the retina also changes in brighter Point of focus Distant point Near point Misshapen cornea Misshapen lens Part of image is focused; part is not Concave Convex Nonsymmetrical lens lens lens (a) Nearsighted Eye (b) Farsighted Eye (c) Astigmatic Eye Figure 4.5 Visual defects and corrective lenses: (a) A myopic (longer than usual) eye. The concave lens spreads light rays just enough to increase the eye’s focal length. (b) A hyperopic (shorter than usual) eye. The convex lens increases refraction (bending), returning the point of focus to the retina. (c) An astigmatic (lens or cornea not symmetrical) eye. In astigmatism, parts of vision are sharp and parts are unfocused. Lenses to correct astigmatism are unsymmetrical.  Sensation and Reality 125 120 million, can’t detect colors ( Figure 4.7). Pure rod vision is black and white. However, the rods are much more sensitive to light than the cones are. The rods therefore allow us to see in very dim light. Notice in Figure 4.7 that light does not fall directly on the rods and cones. It must pass through the outer layers of the retina. Figure 4.6 The iris and diaphragm. Note, too, that the rods and cones face the back of the eye! Only about one half of the light falling on the front of the eye ever reaches the rods and cones — testimony to the eye’s amazing light sensitivity. Direction of light Surprisingly, the retina has a “hole” in it: Each eye has a blind spot because there are no receptors where the optic nerve passes out of the eye and blood vessels enter ( Figure 4.7). The blind spot shows that vision depends greatly on the brain. If you close one eye, part of what you see will fall on the blind spot of your open eye ( Figure 4.8a). Why isn’t there a gap in your vision? The answer is that the visual cortex of the Fibers of the brain actively fills in the gap with patterns from optic nerve surrounding areas ( Figure 4.8b). By closing Ganglion cell one eye, you can visually “behead” other peo- ple by placing their images on your blind Dire e impu Amacrine cell nerv Retina spot. (Just a hint for some classroom fun.) ctio The brain can also “erase” distracting infor- n of ses Bipolar mation. Roll your eyes all the way to the l neuron right and then close your right eye. You Horizontal should clearly see your nose in your Omikron/Photo Researchers, Inc. cell left eye’s field of vision. Now, open Retina your right eye again and your nose Photoreceptor will nearly disappear as your brain cells: Cone disregards its presence. Optic Rod nerve It’s tempting to think of vision Pigment layer as a movie-like projection of “pic- of retina tures” to the brain. However, this mistaken notion Choroid layer immediately raises the question, “Who’s watching Sclera the movie?” From the retina on, vision becomes quite Figure 4.7 Anatomy of the retina. The retina lies behind the vitreous humor, which is the jelly-like substance that fills the eyeball. The rods and cones are much unlike a digital camera, which merely records and displays the digital images it has captured. Instead, vision is a complex system smaller than implied here. The smallest are 1 micron (one millionth of a meter) wide. The lower-left photograph shows rods and cones as seen through an electron microscope. In the photograph the cones are colored green and the rods blue. Retina The light-sensitive layer of cells at the back of the eye. or dimmer light, but the retina can only adapt slowly. By making Accommodation Changes in the shape of the lens of the eye. rapid adjustments, the iris allows us to move quickly from dark- Hyperopia Difficulty focusing nearby objects (farsightedness). ness to bright sunlight, or the reverse. In dim light the pupils dilate Myopia Difficulty focusing distant objects (nearsightedness). (enlarge), and in bright light they constrict (narrow). When the Astigmatism Defects in the cornea, lens, or eye that cause some areas iris is wide open, the pupil is 17 times larger than at its smallest. of vision to be out of focus. Were it not for this, you would be blinded for some time after Presbyopia Farsightedness caused by aging. walking into a darkened room. Iris Circular muscle that controls the amount of light entering the eye. Rods and Cones Pupil The opening at the front of the eye through which light passes. The eye has two types of “image sensors,” consisting of receptor Cones Visual receptors for colors and daylight visual acuity. cells called rods and cones (Goldstein, 2007). The 5 million cones Rods Visual receptors for dim light that produce only black and white in each eye work best in bright light. They also produce color sensations. sensations and fine details. In contrast, the rods, numbering about Blind spot An area of the retina lacking visual receptors. 126 CHAPTER 4 (a)Figure 4.8 Experiencing the blind spot. With your right eye closed, stare at the upper-right cross. Hold the book about 1 foot from your eye and slowly move it back and (a) forth. You should be able to locate a position that causes the black spot to disappear. When it does, it has fallen on the blind spot. With a little practice you can learn to make people or objects you dislike disappear too! (b) Repeat the procedure described, but stare at the lower cross. When the white space falls on the blind spot, the black lines will appear to be continu- ous. This may help you understand why you do not usually experience a blind spot in your (b) visual field. for analyzing patterns of light. Thanks to the Nobel Prize– mation takes through the brain, see “Blindsight: The ‘What’ and winning work of biopsychologists David Hubel and Torsten the ‘Where’ of Vision.”) Wiesel, we now know that vision acts more like an incredibly advanced computer than a television or movie camera. Visual Acuity Hubel and Wiesel directly recorded the activities of single cells The rods and cones also affect visual acuity, or sharpness. The in the brain’s visual cortex in cats and monkeys. As they did, they cones lie mainly at the center of the eye. In fact, the fovea (FOE- noted the area of the retina to which each cell responded. Then vee-ah: a small cup-shaped area in the middle of the retina) con- they aimed lights of various sizes and shapes at the retina and tains only cones — about 50,000 of them. Like a newspaper photo- recorded how often the corresponding brain cell fired nerve graph made of many small dots, the tightly packed cones in the impulses ( Figure 4.9a). fovea produce the sharpest images. Normal acuity is designated as The results were fascinating. Many brain cells responded only 20/20 vision: At 20 feet in distance, you can distinguish what the to lines of a certain width or orientation. These same cells didn’t average person can see at 20 feet ( Figure 4.11). If your vision is get the least bit “excited” over a dot of light or overall illumination 20/40, you can only see at 20 feet what the average person can see ( Figure 4.9b). Other cells responded only to lines at certain at 40 feet. If your vision is 20/200, everything is a blur and you angles, or lines of certain lengths, or lines moving in a particular need glasses! Vision that is 20/12 would mean that you can see at direction (Hubel & Wiesel, 1979). 20 feet what the average person must be 8 feet nearer to see, indi- The upshot of such findings is that cells in the brain, like the cating better than average acuity. American astronaut Gordon frog’s retina described earlier, act as feature detectors. The brain Cooper, who claimed to see railroad lines in northern India from seems to first analyze information into lines, angles, shading, 100 miles above, had 20/12 vision. movement, and other basic features. Then, other brain areas com- bine these features into meaningful visual experiences. (This con- Peripheral Vision cept is discussed further in Chapter 5.) Reading this page is a What is the purpose of the rest of the retina? Areas outside the fovea direct result of such feature analysis. Given the size of the task, it’s also get light, creating a large region of peripheral (side) vision. little wonder that as much as 30 percent of the human brain may The rods are most numerous about 20 degrees from the center of be involved in vision. (To further follow the pathways visual infor- the retina, so much of our peripheral vision is rod vision. Although Stimulus High Frequency of nerve impulses Receptive field for Total visual field single cell in cortex cellFigure 4.9 (a) A “typical” brain responds to only a small area of the total field of vision. In this example, the cell responds to stimuli that fall above Low and left of the center of vision. The bar graph (b) illustrates how a brain cell may act as a feature detector. Notice how the Test stimuli cell primarily responds to just one type of Center of vision stimulus. (Adapted from Hubel, 1979.) (a) (b)  Sensation and Reality 127 B RAIN W AVES Blindsight: The “What” and the “Where” of Vision As you read this book, you may find yourself was, but she had enough sight to know where What happens if someone suffers brain wondering why psychologists are so inter- it was in her visual field (James et al., 2003). damage to the dorsal pathway? In a rare case, ested in the brain. Brainwaves boxes like this What patients like D. F. teach us is that the a woman with just such damage had great one are designed to help you think about brain has assigned the job of seeing to dif- difficulty crossing the street. Although she how the biopsychological perspective con- ferent brain regions (Deco, Rolls, & Horowitz, had no trouble recognizing cars (the what), tributes to a better understanding of human 2004). One series of regions, the ventral path- she could not tell where they were. She could behavior. way, is responsible for the “what” of vision, not even distinguish approaching cars from Meet a woman we will call D. F., who suf- whereas another series of regions, the dor- parked cars (Zeki, 1991). fered brain damage that caused severe visual sal pathway, is responsible for the “where” of agnosia (Goodale et al., 1991). If D. F. was vision ( Figure 4.10). D. F. suffered damage shown an object, she could not recognize it. in her ventral pathway so she could not Remarkably, even though she couldn’t recog- process the “what” of vision, but her D (W or nize objects, D. F. could successfully manipu- intact dorsal pathway could still sa here lp late them. For example, in one test she was process the “where” of vision. ath Primary way given a card and asked to insert it into a slot ) visual at a certain angle. Although she could not cortex pathway describe the slot’s orientation, she had no dif- Ventral t) (Wha ficulty inserting the card into it. You could say that D. F. displayed blindsight: When shown an object, she was blind to what the object Figure 4.10 The ventral and dorsal visual pathways. The rods are also highly responsive to dim light. Because most rods are 20 degrees to each side of the fovea, the best night vision F comes from looking next to an object you wish to see. Test this yourself some night by looking at, and next to, a very dim star. L C B K E S T Color Vision — There’s More (a) (b) (c) to It Than Meets the Eye Gateway Question: How do we perceive colors? ity.Figure 4.11 Tests of visual acuity. Here are some common tests of visual acu- In (a) sharpness is indicated by the smallest grating still seen as individual lines. What would you say is the brightest color? Red? Yellow? Blue? The Snellen chart (b) requires that you read rows of letters of diminishing size until Actually, there are two answers to this question, one for the rods and you can no longer distinguish them. The Landolt rings (c) require no familiarity with one for the cones. The cones are most sensitive to the yellowish green letters. All that is required is a report of which side has a break in it. part of the spectrum. In other words, if all colors are tested in day- light (with each reflecting the same amount of light), then yellowish green appears brightest. Yellow-green fire trucks and the bright yel- rod vision is not very sharp, the rods are quite sensitive to move- low vests worn by roadside work crews are a reflection of this fact. ment in peripheral vision. To experience this characteristic of the To what color are the rods most sensitive? Remember that the rods rods, look straight ahead and hold your hand beside your head, at do not produce color sensations. If you were looking at a very dim about 90 degrees. Wiggle your finger and slowly move your hand colored light, you wouldn’t see any color. Even so, one light would forward until you can detect motion. You will become aware of the appear brighter than the others. When tested this way, the rods are movement before you can actually “see” your finger. most sensitive to blue-green lights. Thus, at night or in dim light, Seeing “out of the corner of the eye” is important for sports, when rod vision prevails, the brightest-colored light will be one that driving, and walking down dark alleys. People who suffer from tunnel vision (a loss of peripheral vision) feel as if they are wearing blinders (Godnig, 2003). Tunnel vision can also occur temporarily Visual acuity The sharpness of visual perception. when we are overloaded by a task. For example, if you were playing a demanding video game you might be excused for not noticing Fovea An area at the center of the retina containing only cones. that a friend had walked up beside you. Peripheral vision Vision at the edges of the visual field. 128 CHAPTER 4 is blue or blue-green. For this reason, police and highway patrol cars chemical called rhodopsin (row-DOP-sin), another light-sensitive in many states now have blue emergency lights for night work. Also, visual pigment.) As predicted, each form of iodopsin is most sensi- you may have wondered why the taxiway lights at airports are blue. It tive to light in roughly the red, green, or blue region. The three seems like a poor choice, but blue is actually highly visible to pilots. types of cones fire nerve impulses at different rates to produce various color sensations ( Figure 4.13). Color Theories In contrast, the opponent-process theory better explains what How do the cones produce color sensations? The trichromatic (TRY- happens in optic pathways and the brain after information leaves kro-MAT-ik) theory of color vision holds that there are three the eye. For example, some nerve cells in the brain are excited by types of cones, each most sensitive to either red, green, or blue. the color red and inhibited by the color green. So both theories are Other colors result from combinations of these three. “correct.” One explains what happens in the eye itself. The other A basic problem with the trichromatic theory is that four colors explains how colors are analyzed after messages leave the eye of light — red, green, blue, and yellow — seem to be primary (you (Gegenfurtner & Kiper, 2003). can’t get them by mixing other colors). Also, why is it impossible to have a reddish green or a yellowish blue? These problems led to the Constructing Colors development of a second view, known as the opponent-process The preceding explanations present a fairly mechanical view of how theory, which states that vision analyzes colors into “either-or” colors are sensed. In reality, color experiences are more complex. For messages (Goldstein, 2007). That is, the visual system can produce example, the apparent color of an object is influenced by the colors messages for either red or green, yellow or blue, black or white. of other nearby objects. This effect is called simultaneous color Coding one color in a pair (red, for instance) seems to block the contrast. It occurs because brain cell activity in one area of the cere- opposite message (green) from coming through. As a result, a red- bral cortex can be altered by activity in nearby areas. Simultaneous dish green is impossible, but a yellowish red (orange) can occur. contrast can make it difficult to paint a picture or decorate a room. According to opponent-process theory, fatigue caused by making If you add a new color to a canvas or a room, all the existing colors one response produces an afterimage of the opposite color as the will suddenly look different. Typically, each time a new color is system recovers. Afterimages are visual sensations that persist after a added, all the other colors must be adjusted ( Figure 4.14). stimulus is removed—like seeing a spot after a flashbulb goes off. To More striking than simultaneous contrast is the fact that color see an afterimage of the type predicted by opponent-process theory, experiences are actively constructed in the brain. The brain does not look at Figure 4.12 and follow the instructions there. simply receive prepackaged color messages. It must generate color from the data it receives. As a result, it is possible to experience color Which color theory is correct? Both! The three-color theory applies to the retina, where three different types of cone have been where none exists. (See Figure 4.15 for an example.) Indeed, all our found. Each contains a different type of iodopsin (i-oh-DOP-sin), experiences are at least partially constructed from the information a light-sensitive pigment that breaks down when struck by light. surrounding us. (We’ll explore this idea further later in this chapter.) This triggers action potentials and sends neural messages to the brain. The three types of cones are most sensitive to red, green, or Firing rates of cones Color experienced blue. Other colors result from combinations of these three. (Black and white sensations are produced by the rods, which contain a Blue Green Red Yellow Orange Purple White Figure 4.12 Negative afterimages. Stare at the dot near the middle of the flag for at least 30 seconds. Then look immediately at a plain sheet of white paper or B G R a white wall. You will see the American flag in its normal colors. Reduced sensitivity to yellow, green, and black in the visual system, caused by prolonged staring, results entFigure 4.13 Firing rates of blue, green, and red cones in response to differ- colors. The taller the colored bar, the higher the firing rates for that type of cone. in the appearance of complementary colors. Project the afterimage of the flag on As you can see, colors are coded by differences in the activity of all three types of other colored surfaces to get additional effects. cones in the normal eye. (Adapted from Goldstein, 2007.)  Sensation and Reality 129 (a) (b) onFigure 4.14 Notice how different the gray-blue color looks when it is placed different backgrounds. Unless you are looking at a large, solid block of color, simultaneous contrast is constantly affecting your color experiences. (c) Michael Newman/PhotoEdit areFigure 4.15 On the left is a “star” made of red lines. On the right, the red lines placed on top of longer black lines. Now, in addition to the red lines, you will see a glowing red disk, with a clear border. Of course, no red disk is printed on this page. No ink can be found between the red lines. The glowing red disk exists only in your mind. (After Hoffman, 1999, p. 111.) Figure 4.16 Color blindness and color weakness. (a) Photograph illustrates nor- mal color vision. (b) Photograph is printed in blue and yellow and gives an impression of what a red-green color-blind person sees. (c) Photograph simulates total color blind- Color Blindness and Color Weakness ness. If you are totally color blind, all three photos will look nearly identical. Do you know anyone who regularly draws hoots of laughter by wearing clothes of wildly clashing colors? Or someone who sheep- weakness, involving yellow and blue, is extremely rare (Hsia & ishly tries to avoid saying what color an object is? If so, you prob- Graham, 1997). (See “Are You Color Blind?”) ably know someone who is color blind. Color blindness is caused by changes in the genes that control What is it like to be color blind? What causes color blindness? A red, green, and blue pigments in the cones. Red-green color weak- person who is color blind cannot perceive colors. It is as if the world is a black-and-white movie. The color-blind person either lacks cones or has cones that do not function normally (Deeb, Trichromatic theory Theory of color vision based on three cone types: red, green, and blue. 2004). Such total color blindness is rare. In color weakness, or partial color blindness, a person can’t see certain colors. Approxi- Opponent-process theory Theory of color vision based on three coding systems (red or green, yellow or blue, black or white). mately 8 percent of Caucasian males are red-green color blind (but fewer Asian American, African American, and Native American Simultaneous color contrast Changes in perceived hue that occur when a colored stimulus is displayed on backgrounds of various colors. males, and less than 1 percent of women, are) (Delpero et al., 2005). These people see both reds and greens as the same color, Color blindness A total inability to perceive colors. usually a yellowish brown ( Figure 4.16). Another type of color Color weakness An inability to distinguish some colors. 130 CHAPTER 4 D ISC O VERIN G P S YCH OLOG Y Are You Color Blind? How can I tell if I am color blind? Surprisingly, dots ( Figure 4.17). The background and Figure 4.17 lists what people with normal it is not as obvious as you might think; some the numbers are of different colors (red and color vision and color blindness see. Because of us reach adulthood without knowing. The green, for example). A person who is color this chart is just a replica, it is not a definitive Ishihara test is commonly used to measure blind sees only a jumble of dots. If you have test of color blindness. Nevertheless, if you color blindness and weakness. In the test, normal color vision you can detect the num- can’t see all the embedded designs, you may numbers and other designs made of dots bers or designs (Birch & McKeever, 1993; be color blind or color weak. are placed on a background also made of Coren, Ward, & Enns, 2004). The chart below Ishihara Figure 4.17 A replica of the test for color blindness.  Sensation and Reality 131 ness is a recessive, sex-linked trait. That means it is carried on the As you might have noticed, a few seconds of exposure to bright X, or female, chromosome. Women have two X chromosomes, so white light can completely wipe out dark adaptation. That’s why if they receive only one defective color gene, they still have normal you should be sure to avoid looking at oncoming headlights when vision. Color-weak men, however, have only one X chromosome, you are driving at night — especially the new bluish-white xenon so they can inherit the defect from their mothers (who usually lights. don’t display any color weakness). Under normal conditions, glare recovery takes about 20 sec- How can color-blind individuals drive? Don’t they have trouble onds, plenty of time for an accident. After a few drinks, it may take with traffic lights? Red-green color-blind individuals have normal 30 to 50 percent longer because alcohol dilates the pupils, allow- vision for yellow and blue, so the main problem is telling red lights ing more light to enter. Note, too, that dark adaptation occurs from green. In practice, that’s not difficult. The red light is always more slowly as we grow older. This is one reason why injuries on top, and the green light is brighter than the red. Also, “red” caused by falling in the dark become more common among the traffic signals have yellow light mixed in with the red and a “green” elderly ( Jackson, Owsley, & McGwin, 1999). light that is really blue-green. Is there any way to speed up dark adaptation? The rods are insen- sitive to extremely red light. That’s why submarines, airplane cock- pits, and ready rooms for fighter pilots are illuminated with red Dark Adaptation — Let There Be Light! light. In each case, people can move quickly into the dark without Gateway Question: How do we adjust to the dark? having to adapt. Because the red light doesn’t stimulate the rods, it is as if they had already spent time in the dark. What happens when the eyes adjust to a dark room? Dark adapta- Can eating carrots really improve vision? One chemical “ingredi- tion is the dramatic increase in retinal sensitivity to light that ent” of rhodopsin is retinal, which the body makes from vitamin occurs after a person enters the dark (Goldstein, 2007). Consider A. (Retinal is also called retinene.) When too little vitamin A is walking into a theater. If you enter from a brightly lighted lobby, available, less rhodopsin is produced. Thus, a person lacking vita- you practically need to be led to your seat. After a short time, min A may develop night blindness. In night blindness, the per- however, you can see the entire room in detail (including the son can see normally in bright light while using the cones, but couple kissing over in the corner). It takes about 30 to 35 minutes becomes blind at night when the rods must function. Carrots are of complete darkness to reach maximum visual sensitivity ( Fig- an excellent source of vitamin A, so they could improve night ure 4.18). At that point, your eye will be 100,000 times more sensi- vision for someone suffering a deficiency, but not the vision of tive to light. anyone with an adequate diet (Carlson, 2005). What causes dark adaptation? Remember that both rods and cones contain light-sensitive visual pigments. When struck by light, visual pigments bleach, or break down chemically. The afterimages you have seen after looking at a flashbulb are a result of this bleach- KNOWL E DG E B U I L DE R ing. To restore light sensitivity, the visual pigments must recom- Vision bine, which takes time. Night vision is due mainly to an increase in RECITE rhodopsin, the rod pigment. When completely dark adapted, the 1. The __________________ ___________________ is made up of human eye is almost as sensitive to light as the eye of an owl. electromagnetic radiation with wavelengths between 400 and 700 nanometers. 2. Hyperopia is related to a. farsightedness Low Rods only b. having an elongated eye c. corneal astigmatism Sensitivity to light d. lack of cones in the fovea Cones only 3. Hubel and Wiesel found that cells in the visual cortex of the brain function as ________________________ ______________________. 4. In dim light, vision depends mainly on the ____________________. In brighter light, color and fine detail are produced by the ____________________. 5. The fovea has the greatest visual acuity due to the large concentra- tion of rods found there. T or F? High 6. The term “20/20 vision” means that a person can see at 20 feet what can normally be seen from 20 feet. T or F? 0 5 10 15 20 25 30 Time in the dark (minutes) Continued threshold Figure 4.18 Typical course of dark adaptation. The black line shows how the for vision lowers as a person spends time in the dark. (A lower threshold means that less light is needed for vision.) The green line shows that the cones adapt first, but they soon cease adding to light sensitivity. Rods, shown by the red Dark adaptation Increased retinal sensitivity to light. line, adapt more slowly. However, they continue to add to improved night vision long after the cones are fully adapted. Night blindness Blindness under conditions of low illumination. 132 CHAPTER 4 D ISC O VERIN G P S YCH OLOG Y Going Biosonar This chapter opened with a story about echo- himself (!), it turns out that researchers have location (also known as biosonar), the remark- known about human echolocation for 50 able ability of bats to use the echoes of their years and have even proposed training blind own voices to judge distance. Not human people to echolocate (Kellogg, 1962). at all, right? Don’t tell that to teenager Ben Although blind people may be better at Underwood, who has been sightless since echolocating, there is no reason why the rest the age of 3, when retinal cancer claimed of us can’t do it as well (Rosenblum, Gordon, his eyes (Engber, 2006). In 2006, Ben proudly & Jarquin, 2000). Try this: Blindfold your- proclaimed during a television interview, “I’m self and have a friend move a large plate or not blind, I just can’t see.” Sure enough, Ben pan closer or farther away from you. All the can ride a bike, climb trees, skate, and even while, make some noise. Click like Ben does play video games. He does it by using echo- or sing a song or whistle. Don’t expect to location. hear your own echo as a separate sound. It turns out that bat echolocation is espe- Noticeable echoes only occur when sounds Felicia Rule cially powerful because bats use very high- bounce off objects far away. Instead, expect pitched sounds. But any sounds will do, and to hear slight differences when an object Ben makes clicking sounds with his tongue. is moved. With some practice, you can tell Although Ben Underwood has been blind since the age of 3, he has learned to use echolocation With practice he has learned to use his when the plate or pan is closer and when to do many things that normally sighted children own echoes to navigate through the world. it is farther away. Congratulations, you’ve take for granted. Although Ben discovered echolocation all by gone biosonar. 7. For the cones, the most visible color is Hearing — Good Vibrations a. reddish orange Gateway Question: What are the mechanisms of hearing? b. blue-green c. yellow-orange Rock, classical, jazz, rap, country, electronic, hip-hop — whatever d. yellowish green your musical taste, you have probably been moved by the riches of 8. The eyes become more sensitive to light at night because sound. Hearing also collects information from all around the of a process known as ________________________ body, such as detecting the approach of an unseen car (Yost, _________________________. 2007). Vision, in all its glory, is limited to stimuli in front of the REFLECT eyes (unless, of course, your “shades” have rearview mirrors Critical Thinking attached). 9. William James once said, “If a master surgeon were to cross the audi- tory and optic nerves, we would hear lightning and see thunder.” Can you explain what James meant? 10. Sensory transduction in the eye takes place first in the cornea, then in the lens, then in the retina. T or F? Compression Relate Rarefaction Pretend you are a beam of light. What will happen to you at each step as you pass into the eye and land on the retina? What will happen if the eye is not perfectly shaped? How will the retina know you’ve arrived? How will it tell what color of light you are? What will it tell the brain about you? the retina converts light to nerve impulses. another form of energy. No change in the type of energy takes place until and lens bend and focus light rays, but they do not change light to Amplitude to the visual area, a sensation of light would occur. 10. False. The cornea tion. Likewise, if the ears transduced a thunderclap and sent impulses activate auditory areas of the brain, we would experience a sound sensa- Time function: If a lightning flash caused rerouted messages from the eyes to Compression Wavelength 6. T 7. d 8. dark adaptation 9. The explanation is based on localization of Rarefaction Answers: 1. visible spectrum 2. a 3. feature detectors 4. rods, cones 5. F forFigure 4.19 Waves of compression in the air, or vibrations, are the stimulus hearing. The frequency of sound waves determines their pitch. The amplitude determines loudness.  Sensation and Reality 133 External Ear (air conduction) Inner Ear (fluid conduction) Figure 4.20 Anatomy of the ear. The entire ear is a mechanism for changing waves of air pressure into nerve impulses. The inset in the foreground (Cochlea “Unrolled”) shows that as the stapes moves (bone conduction the oval window, the round window bulges outward, allowing waves Auditory by ossicles) Vestibular to ripple through fluid in the cochlea. The waves move membranes canal apparatus near the hair cells, causing cilia or “bristles” on the tips of the cells Incus to bend. The hair cells then generate nerve impulses carried to the Malleus Stapes Auditory brain. (See an enlarged cross section of cochlea in Figure 4.21.) nerve Cochlea Pinna Scala vestibuli (with perilymph) Cochlear Round canal (with window endolymph) Oval Tympanic window Scala tympani membrane (eardrum) (with perilymph) Oval Cochlea in window Cross Section Stapes waves collide with the tympanic membrane Cochlear (eardrum), setting it in motion. This, in turn, canal causes three small bones (the auditory ossicles) (OSS-ih-kuls) to vibrate ( Figure 4.20). Round Cochlea “Unrolled” Perilymph The ossicles are the malleus (MAL-ee-us), window (fluid inside cochlea) incus, and stapes (STAY-peas). Their com- mon names are the hammer, anvil, and stir- rup. The ossicles link the eardrum with the What is the stimulus for hearing? If you throw cochlea (KOCK-lee-ah: a snail-shaped organ a stone into a quiet pond, a circle of waves will that makes up the inner ear). The stapes is spread in all directions. In much the same way, Basilar attached to a membrane on the cochlea Auditory nerve Hair cells sound travels as a series of invisible waves of com- fibers membrane called the oval window. As the oval window pression (peaks) and rarefaction (RARE-eh-fak- Organ of Corti moves back and forth, it makes waves in a shun: valleys) in the air. Any vibrating object — a fluid inside the cochlea. tuning fork, the string of a musical instrument, or the vocal Inside the cochlea tiny hair cells detect waves in the fluid. The cords — will produce sound waves (rhythmic movement of air hair cells are part of the organ of Corti (KOR-tee), which makes molecules). (To learn how to use sound waves to act like a bat, read up the center part of the cochlea ( Figure 4.21). A set of stereo- “Going Biosonar.”) Other materials, such as fluids or solids, can cilia (STER-ee-oh-SIL-ih-ah), or “bristles,” atop each hair cell also carry sound. But sound does not travel in a vacuum or the air- brush against the tectorial membrane when waves ripple through less realm of outer space. Movies that show characters reacting to fluid surrounding the organ of Corti. As the stereocilia are bent, the “roar” of alien starships or titanic battles in deep space are in transduction takes place and nerve impulses are triggered, which error. then flow to the brain. (Are your ears “bristling” with sound?) The frequency of sound waves (the number of waves per sec- How are higher and lower sounds detected? The frequency the- ond) corresponds to the perceived pitch (higher or lower tone) of ory of hearing states that as pitch rises, nerve impulses of a corre- a sound. The amplitude, or physical “height,” of a sound wave tells sponding frequency are fed into the auditory nerve. That is, an how much energy it contains. Psychologically, amplitude corre- 800-hertz tone produces 800 nerve impulses per second. (Hertz sponds to sensed loudness (sound intensity) ( Figure 4.19). How We Hear Sounds Hair cells Receptor cells within the cochlea that transduce vibrations How are sounds converted to nerve impulses? Hearing involves an into nerve impulses. elaborate chain of events that begins with the pinna (PIN-ah: the Organ of Corti Center part of the cochlea, containing hair cells, canals, visible, external part of the ear). In addition to being a good place to and membranes. hang earrings or balance pencils, the pinna acts like a funnel to con- Frequency theory Holds that tones up to 4,000 hertz are converted to centrate sounds. After they are guided into the ear canal, sound nerve impulses that match the frequency of each tone. 134 CHAPTER 4