Sensation and Perception (Chapter 3) PDF

134 CHAPTER 3 The ABCs of Sensation Information about the world has to have a way to get into the brain, where it can be used to determine actions and re...

134 CHAPTER 3 The ABCs of Sensation Information about the world has to have a way to get into the brain, where it can be used to determine actions and responses. The way into the brain is through the sensory organs and the process of sensation. TRANSDUCTION 3.1 Describe how we get information from the outside world into our brains. Sensation occurs when special receptors in the sense organs—the eyes, ears, nose, skin, and taste buds—are activated, allowing various forms of outside stimuli to become neu- ral signals in the brain. This process of converting outside stimuli, such as light, into neu- ral activity is called transduction. sensation The sensory receptors are specialized forms of neurons, the cells that make up the the process that occurs when special nervous system. Instead of receiving neurotransmitters from other cells, these receptor receptors in the sense organs are cells are stimulated by different kinds of energy—for example, the receptors in the eyes activated, allowing various forms are stimulated by light, whereas the receptors in the ears are activated by vibrations. of outside stimuli to become neural signals in the brain. Touch receptors are stimulated by pressure or temperature, and the receptors for taste and smell are triggered by chemical substances. Each receptor type transduces the phys- transduction ical information into electrical information in different ways, which then either depolar- the process of converting outside izes or hyperpolarizes the cell, causing it to fire more or to fire less based on the timing stimuli, such as light, into neural and intensity of information it is detecting from the environment (Gardner & Johnson, activity. 2013). In some people, the sensory information gets processed in unusual, but fasci- synesthesia nating ways. Taria Camerino is a pastry chef who experiences music, colors, shapes, disorder in which the signals from the and emotions as taste; Jamie Smith is a sommelier, or wine steward, who experiences various sensory organs are processed smells as colors and shapes; and James Wannerton is an information technology con- in the wrong cortical areas, resulting sultant who experiences sounds, words, and colors as tastes and textures (Carlsen, in the sense information being inter- 2013, March 18). All three of these individuals have a condition known as synesthe- preted as more than one sensation. sia, which literally means “joined sensation.” Studies suggest at least 4 to 5 percent of just noticeable difference the population may experience some form of synesthesia (Hubbard & Ramachandran, (jnd or the difference threshold) 2005; Simner, 2013; Simner et al., 2006). While the causes of synesthesia are still being the smallest difference between two investigated, it appears that in some forms, signals that come from the sensory organs, stimuli that is detectable 50 percent of such as the eyes or the ears, either go to places in the brain where they weren’t origi- the time. nally meant to be or they are processed differently. Overall, there is increased commu- nication between sensory regions that results in synesthetes experiencing the world differently than others. SENSORY THRESHOLDS 3.2 Describe the difference and absolute thresholds. Ernst Weber (1795–1878) did studies trying to determine the smallest difference between two weights that could be detected. His research led to the formulation known as Weber’s law of just noticeable differences (jnd, or the difference threshold). A jnd is the smallest difference between two stimuli that is detectable 50 percent of the time, and Weber’s law simply means that whatever the difference between stimuli might be, it is always a con- stant. If to notice a difference the amount of sugar a person would need to add to a cup of coffee that is already sweetened with 5 teaspoons is 1 teaspoon, then the percentage of change needed to detect a just noticeable difference is one fifth, or 20 percent. So if the coffee has 10 teaspoons of sugar in it, the person would have to add another 20 percent, or In some parts of the United States, “coffee regular” refers to coffee with two creams 2 teaspoons, to be able to taste the difference half of the time. Most people would not typi- and two sugars. How much more sugar cally drink a cup of coffee with 10 teaspoons of sugar in it, let alone 12 teaspoons, but you would you need to add to taste a difference? get the point.  Sensation and Perception 135 Table 3.1 Examples of Absolute Thresholds Sense Threshold Sight A candle flame at 30 miles on a clear, dark night Hearing The tick of a watch 20 feet away in a quiet room Smell One drop of perfume diffused throughout a three-room apartment Taste 1 teaspoon of sugar in 2 gallons of water Touch A bee’s wing falling on the cheek from 1 centimeter above To see a visual example of this, participate in the experiment Weber’s Law and dis- cover the amount of change needed to detect a just noticeable difference between two circles of light. Gustav Fechner (1801–1887) expanded on Weber’s work by studying something he called the absolute threshold (Fechner, 1860). An absolute threshold is the lowest level of stimulation that a person can consciously detect 50 percent of the time the stimulation is present. (Remember, the jnd is detecting a difference between two stimuli.) For example, assuming a very quiet room and normal hearing, how far away can someone sit and you might still hear the tick of their analog watch on half of the trials? For some examples of absolute thresholds for various senses, see Table 3.1. I’ve heard about people being influenced by stuff in movies and on television, things that are just below the level of conscious awareness. Is that true? Stimuli that are below the level of conscious awareness are called subliminal stim- uli. (The word limin means “threshold,” so sublimin means “below the threshold.”) These stimuli are just strong enough to activate the sensory receptors but not strong enough for people to be consciously aware of them. Many people believe that these stimuli act upon the unconscious mind, influencing behavior in a process called sublim- inal perception. At one time, many people believed that a market researcher named James Vicary had demonstrated the power of subliminal perception in advertising. In 1957, Vicary claimed that over a 6-week period, 45,699 patrons at a movie theater in Fort Lee, New Jersey, were shown two advertising messages, Eat Popcorn and Drink Coca-Cola, while they watched the film Picnic. According to Vicary, these messages were flashed for 3 milliseconds once every 5 seconds. Vicary claimed that over the 6-week period the sales of popcorn rose 57.7 percent and the sales of Coca-Cola rose 18.1 percent. It was 5 years before Vicary finally admitted that he had never conducted a real study (Merikle, 2000; Pratkanis, 1992). Furthermore, many researchers have gathered scien- tific evidence that subliminal perception does not work in advertising (Bargh et al., 1996; Broyles, 2006; Moore, 1988; Pratkanis & Greenwald, 1988; Trappey, 1996; Vokey & Read, 1985). This is not to say that subliminal perception does not exist—there is a growing body of evidence that we process some stimuli without conscious awareness, espe- cially stimuli that are fearful or threatening (LeDoux & Phelps, 2008; Öhman, 2008). In this effort, researchers have used event-related potentials (ERPs) and functional absolute threshold magnetic resonance imaging (fMRI) to verify the existence of subliminal perception the lowest level of stimulation that and associated learning in the laboratory (Babiloni et al., 2010; Bernat et al., 2001; a person can consciously detect 50 Fazel-Rezai & Peters, 2005; Sabatini et al., 2009). to Learning Objective 2.9. percent of the time the stimulation is The stimuli used in these studies are detectable by our sensory systems but below present. 136 CHAPTER 3 the level of full conscious perception. Participants are not aware or conscious that they have been exposed to the stimuli due to masking or manipulation of attention. Furthermore, the stimuli typically influence automatic reactions (such as an increase in facial tension) rather than direct voluntary behaviors (such as going to buy some- thing suggested by advertising). Another useful way of analyzing what stimuli we respond to is based on signal detection theory. Signal detection theory is used to compare our judgments, or the deci- sions we make, under uncertain conditions. The ability to detect any physical stimulus is based on how strong it is and how mentally and physically prepared the individual is. It was originally developed to help address issues associated with research participants guessing during experiments and is a way to measure accuracy (Green & Swets, 1966; Macmillan & Creelman, 1991). For example, a stimulus can be either present or absent. In turn, an individual can either detect a stimulus when present, a “hit,” or say it is not there, a “miss.” He or she can also falsely report a stimulus as present when it actually isn’t, a “false alarm,” or cor- rectly state it isn’t there, a “correct rejection.” HABITUATION AND SENSORY ADAPTATION 3.3 Explain why some sensory information is ignored. Some of the lower centers of the brain filter sensory stimulation and “ignore” or prevent conscious attention to stimuli that do not change. The brain is primarily interested in changes in information. That’s why people don’t really “hear” the noise of the air con- ditioner unless it suddenly cuts off, or the noise made in some classrooms, unless it gets very quiet or someone else directs their attention toward it. Although they actually are hearing it, they aren’t paying attention to it. This is called habituation, and it is the way the brain deals with unchanging information from the environment. to Learn- ing Objective 2.10. This young woman does not feel the Sometimes I can smell the odor of the garbage can in the kitchen piercings on her ear and nose because when I first come home, but after a while the smell seems to go sensory adaptation allows her to ignore a away—is this also habituation? constant, unchanging stimulation from the metal rings. What else is she wearing that would cause sensory adaptation? Although different from habituation, sensory adaptation is another process by which constant, unchanging information from the sensory receptors is effectively signal detection theory ignored. In habituation, the sensory receptors are still responding to stimulation, but the provides a method for assessing the lower centers of the brain are not sending the signals from those receptors to the cortex. accuracy of judgments or decisions The process of sensory adaptation differs because the receptor cells themselves become under uncertain conditions; used in less responsive to an unchanging stimulus—garbage odors included—and the receptors perception research and other areas. no longer send signals to the brain. An individual’s correct “hits” and For example, when you eat, the food that you put in your mouth tastes strong at rejections are compared against their first, but as you keep eating the same thing, the taste does fade somewhat, doesn’t it? “misses” and “false alarms.” Generally speaking, all of our senses are subject to sensory adaptation. You might think, then, that if you stare at something long enough, it would also habituation disappear, but the eyes are a little different. Even though the sensory receptors in the tendency of the brain to stop attending back of the eyes adapt to and become less responsive to a constant visual stimulus, under to constant, unchanging information. ordinary circumstances, the eyes are never entirely still. There’s a constant movement of sensory adaptation the eyes, tiny little vibrations called “microsaccades” or “saccadic movements,” that peo- tendency of sensory receptor cells to ple don’t consciously notice. These movements keep the eyes from adapting to what they become less responsive to a stimulus see. (That’s a good thing, because otherwise many students would no doubt go blind that is unchanging. from staring off into space.)  Sensation and Perception 137 Concept Map L.O. 3.1, 3.2, 3.3 The ABCs of Sensation related to the activation of receptors in the various sense organs and sensation transduction of that information into neural signals process by which information related to changes in physical stimuli from the outside world enters the brain detected by sensory receptors influenced by both absolute and difference thresholds; responses can also be examined through signal detection theory sometimes "ignored" through sensory adaptation or cognitive habituation Practice Quiz How much do you remember? Pick the best answer. 1. ________ involves the detection of physical stimuli from our envi- 3. After being in class for a while, ____________ is a likely explana- ronment and is made possible by the activation of specific receptor tion for not hearing the sound of the lights buzzing above you until cells. someone says something about it. a. Perception c. Adaptation a. accommodation c. sublimation b. Sublimation d. Sensation b. adaptation d. habituation 2. The lowest level of stimulation that a person can consciously detect 4. You are drinking a strong cup of coffee that is particularly bitter. 50 percent of the time the stimulation is present is called After a while, the coffee doesn’t taste as strong as it did when you a. absolute threshold. c. sensation. first tasted it. What has happened? b. just noticeable difference. d. sensory adaptation. a. sensory adaptation c. habituation b. subliminal perception d. perceptual defense The Science of Seeing I’ve heard that light is waves, but I’ve also heard that light is made of particles—which is it? Light is a complicated phenomenon. Although scientists have long argued over the nature of light, they finally have agreed that light has the properties of both waves and particles. The following section gives a brief history of how scientists have tried to “shed light” on the mystery of light. LIGHT AND THE EYE 3.4 Describe how light travels through the various parts of the eye. It was Albert Einstein who first proposed that light is actually tiny “packets” of waves. These “wave packets” are called photons and have specific wavelengths associated with them (Lehnert, 2007; van der Merwe & Garuccio, 1994). When people experience the physical properties of light, they are not really aware of its dual, wavelike and particle-like, nature. With regard to its psychological properties, there are three aspects to our perception of light: brightness, color, and saturation. Brightness is determined by the amplitude of the wave—how high or how low the wave actually is. The higher the wave, the brighter the light appears to be. Low waves are dimmer. Color, or hue, is largely determined by the length of the wave. Short wave- lengths (measured in nanometers) are found at the blue end of the visible spectrum (the portion of the whole spectrum of light that is visible to the human eye; see Figure!3.1), whereas longer wavelengths are found at the red end. 138 CHAPTER 3 CC Figure 3.1 The Visible Spectrum. (Watch this video on MyPsychLab.) Saturation refers to the purity of the color people perceive: A highly saturated red, for example, would contain only red wavelengths, whereas a less-saturated red might contain a mixture of wavelengths. For example, when a child is using the red paint from a set of poster paints, the paint on the paper will look like a pure red, but if the child mixes in some white paint, the paint will look pink. The hue is still red, but it will be less of a saturated red because of the presence of white wavelengths. Mixing in black or gray would also lessen the saturation. (Note that when combining different colors, light works differently than pigments or paint. We will look at this distinction when we examine perception of color.) THE STRUCTURE OF THE EYE The best way to talk about how the eye processes light is to talk about what happens to an image being viewed as the photons of light!from!that image travel through the eye. Refer to Figure 3.2 to follow the path of the image. Retina Innermost layer of the eye, Fovea where incoming light is converted Central area of retina where into nerve impulses; contains light rays are most sharply focused; photoreceptor cells greatest density of cones Vitreous humor Blind spot Jelly-like liquid that (optic disc) Where the optic nourishes and gives nerve leaves the eye; there shape to the eye are no photoreceptor cells Iris here Colored area containing muscles that control the pupil Aqueous humor Clear liquid that Optic nerve nourishes the eye Transmits visual information from the retina to the brain Blood vessels Pupil Opening in the center of the iris that changes size depending on the amount of light in the environment Cornea Curved, transparent dome that bends incoming light Eye muscle waves so the image can One of six surrounding be focused on retina Lens muscles that rotate the eye Transparent disc that changes in all directions shape to bring objects into focus CC Figure 3.2 Structure of the Eye. (Watch this video on MyPsychLab.) Light enters the eye through the cornea and pupil. The iris controls the size of the pupil. From the pupil, light passes through the lens to the retina, where it is transformed into nerve impulses. The nerve impulses travel to the brain along the optic nerve.  Sensation and Perception 139 FROM FRONT TO BACK: THE PARTS OF THE EYE Light enters the eye directly from a source (such as the sun) or indirectly by reflecting off of an object. To see clearly, a single point of light from a source or reflected from an object must travel through the structures of the eye and end up on the retina as a single point. Light bends as it passes through substances of different densities, through a process known as refraction. For example, have you ever looked at a drinking straw in a glass of water through the side of the glass? It appears that the straw bends, or is broken, at the surface of the water. That optical illusion is due to the refraction of light. The structures of the eye play a vital role in both collecting and focusing of light so we can see clearly. The surface of the eye is covered in a clear membrane called the cornea. The cor- nea not only protects the eye but also is the structure that focuses most of the light coming into the eye. The cornea has a fixed curvature, like a camera that has no option to adjust the focus. However, this curvature can be changed somewhat through vision- This photo illustrates an optical illusion caused improving techniques that change the shape of the cornea. For example, ophthalmolo- by the refraction of light. The straw is not really gists, physicians who specialize in medical and surgical treatment of eye problems, can broken, although it appears that way. use both photoreactive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK) procedures to remove small portions of the cornea, changing its curvature and thus the focus in the eye. The next visual layer is a clear, watery fluid called the aqueous humor. This fluid is continually replenished and supplies nourishment to the eye. The light from the visual image then enters the interior of the eye through a hole, called the pupil, in a round mus- cle called the iris (the colored part of the eye). The iris can change the size of the pupil, letting more or less light into the eye. That also helps focus the image; people try to do the same thing by squinting. Behind the iris, suspended by muscles, is another clear structure called the lens. The flexible lens finishes the focusing process begun by the cornea. In a process called visual accommodation, the lens changes its shape from thick to thin, enabling it to focus on objects that are close or far away. The variation in thickness allows the lens to project a sharp image on the retina. People lose this ability as the lens hardens through aging (a disorder called presbyopia). Although people try to compensate* for their inability to focus on things that are close to them, eventually they usually need bifo- cals because their arms just aren’t long enough anymore. In nearsightedness, or myopia, visual accommodation may occur, but the shape of the eye causes the focal point to fall short of the retina. In farsightedness, or hyperopia, the focus point is beyond the retina (see Figure 3.3). Glasses, contacts, or corrective surgery like LASIK or PRK can correct these issues. visual accommodation Once past the lens, light passes through a large, open space filled with a clear, the change in the thickness of the lens jelly-like fluid called the vitreous humor. This fluid, like the aqueous humor, also nour- as the eye focuses on objects that are ishes the eye and gives it shape. far away or close. Nearsighted eye Farsighted eye Figure 3.3 Nearsightedness and Farsightedness *compensate: to correct for an error or defect. 140 CHAPTER 3 RETINA, RODS, AND CONES The final stop for light within the eye is the retina, a light- sensitive area at the back of the eye containing three layers: ganglion cells, bipolar cells, and the rods and cones, special receptor cells (photoreceptors) that respond to the various wavelengths of light. The video Rods and Cones provides an overview. CC Watch the Video Rods and Cones on MyPsychLab While the retina is responsible for absorbing and processing light informa- tion, the rods and the cones are the business end of the retina—the part that actu- ally receives the photons of light and turns them into neural signals for the brain, sending them first to the bipolar cells (a type of interneuron; called bipolar or “two- ended” because they have a single dendrite at one end and a single axon on the other; to Learning Objective!2.1) and then to the retinal ganglion cells whose axons form the optic nerve. The rods and cones are responsible for different aspects of vision. There are 6 mil- lion cones in each eye; of these, 50,000 have a private line to the optic nerve (one bipolar cell for each cone). This means that the cones are the receptors for visual acuity, or ability to see fine detail. Cones are located all over the retina but are more concentrated at its very center where there are no rods (the area called the fovea). Cones also need a lot more light to function than the rods do, so cones work best in bright light, which is also when people see things most clearly. Cones are also sensitive to different wavelengths of light, so they are responsible for color vision. The rods (about 100 million of them in each eye) are found all over the retina except the fovea but are concentrated in the periphery. Rods are sensitive to changes in brightness but not to a variety of wavelengths, so they see only in black and white and shades of gray. They can be very sensitive because many rods are connected to a single bipolar cell, so that rods if even only one rod is stimulated by a photon of light, the brain perceives the whole area visual sensory receptors found at the of those rods as stimulated. But because the brain doesn’t know exactly what part of the back of the retina, responsible for non- area (which rod) is actually sending the message, the visual acuity (sharpness) is quite low. color sensitivity to low levels of light. That’s why things seen in low levels of light, such as twilight or a dimly lit room, are fuzzy cones and grayish. Because rods are located on the periphery of the retina, they are also responsi- visual sensory receptors found at the ble for peripheral vision. back of the retina, responsible for color The eyes don’t adapt to constant stimuli under normal circumstances because of vision and sharpness of vision. saccadic movements. But if people stare with one eye at one spot long enough, small  Sensation and Perception 141 Figure 3.4 The Blind Spot Hold the book in front of you. Close your right eye and stare at the picture of the dog with your left eye. Slowly bring the book closer to your face. The picture of the cat will disappear at some point because the light from the picture of the cat is falling on your blind spot. If you cannot seem to find your blind spot, trying moving the book more slowly. objects that slowly cross their visual field may at one point disappear briefly because there is a “hole” in the retina—the place where all the axons of those ganglion cells leave the retina to become the optic nerve, the optic disk. There are no rods or cones here, so this is referred to as the blind spot. You can demonstrate the blind spot for yourself by following the directions in Figure 3.4. THE VISUAL PATHWAY 3.5 Explain how light information reaches the visual cortex. You may want to first look at Figure 3.5 for a moment before reading this section. Light enter- ing the eyes can be separated into the left and right visual fields. Light from the right visual field falls on the left side of each eye’s retina; light from the left visual field falls on the right side of each retina. Light travels in a straight line through the cornea and lens, resulting in the image projected on the retina actually being upside down and reversed from left to right as compared to the visual fields. Thank goodness our brains can compensate for this! The areas of the retina can be divided into halves, with the halves toward the tem- ples of the head referred to as the temporal retinas and the halves toward the center, or nose, called the nasal retinas. Look at Figure 3.5 again. Notice that the information from the left visual field (falling on the right side of each retina) goes to the right visual cortex, while the information from the right visual field (falling on the left side of each retina) goes to the left visual cortex. This is because the axons from the temporal halves of each retina project to the visual cortex on the same side of the brain, while the axons from the nasal halves cross over to the visual cortex on the opposite side of the brain. The optic chiasm is the point of crossover. Because rods work well in low levels of light, they are also the cells that allow the eyes to adapt to low light. Dark adaptation occurs as the eye recovers its ability to see when going from a brightly lit state to a dark state. (The light-sensitive pigments that allow us to see are able to regenerate or “recharge” in the dark.) The brighter the light was, blind spot the longer it takes the rods to adapt to the new lower levels of light (Bartlett, 1965). This is area in the retina where the axons of why the bright headlights of an oncoming car can leave a person less able to see for a while the three layers of retinal cells exit the after that car has passed. Fortunately, this is usually a temporary condition because the eye to form the optic nerve; insensitive bright light was on so briefly and the rods readapt to the dark night relatively quickly. Full to light. dark adaptation, which occurs when going from more constant light to darkness, such as turning out one’s bedroom lights, takes about 30 minutes. As people get older this process dark adaptation takes longer, causing many older persons to be less able to see at night and in darkened the recovery of the eye’s sensitivity rooms (Klaver et al., 1998). This age-related change can cause night blindness, in which a to visual stimuli in darkness after person has difficulty seeing well enough to drive at night or get around in a darkened exposure to bright lights. 142 CHAPTER 3 Left visual field Right visual field Left eye Right eye Optic nerve Optic chiasm Optic tract Lateral Optic radiations geniculate nucleus Left visual cortex Right visual cortex Figure 3.5 Crossing of the Optic Nerve Light falling on the left side of each eye’s retina (from the right visual field, shown in yellow) will stimulate a neural message that will travel along the optic nerve to the thalamus and then on to the visual cortex in the occipital lobe of the left hemisphere. Notice that the message from the temporal half of the left retina goes to the left occipital lobe, while the message from the nasal half of the right retina crosses over to the left hemi- sphere (the optic chiasm is the point of crossover). The optic nerve tissue from both eyes joins together to form the left optic tract before going on to the lateral geniculate nucleus of the thalamus, the optic radiations, and then the left occipital lobe. For the left visual field (shown in blue), the messages from both right sides of the retinas will travel along the right optic tract to the right visual cortex in the same manner. room or house. Some research indicates that taking supplements such as vitamin A can reverse or relieve this symptom in some cases (Jacobson et al., 1995). When going from a darkened room to one that is brightly lit, the opposite process occurs. The cones have to adapt to the increased level of light, and they accomplish this light adaptation much more quickly than the rods adapt to darkness—it takes a few seconds at most (Hood, 1998). While this deer may seem to see relatively well at night, the oncoming headlights of a PERCEPTION OF COLOR car will briefly blind it. It may only take a few 3.6 Compare and contrast two major theories of color vision, and explain how seconds for light adaption to occur, but until color-deficient vision occurs. it does, the deer is unable to fully see, so it does not move. Earlier you said the cones are used in color vision. There are so light adaptation many colors in the world—are there cones that detect each color? Or the recovery of the eye’s sensitivity to do all cones detect all colors? visual stimuli in light after exposure to darkness. Although experts in the visual system have been studying color and its nature for many years, at this point in time there is an ongoing theoretical discussion about the role the trichromatic theory cones play in the sensation of color. theory of color vision that proposes three types of cones: red, blue, and TRICHROMATIC THEORY Two theories about how people see colors were originally green; “three colors” theory. proposed in the 1800s. The first is called the trichromatic (“three colors”) theory. First  Sensation and Perception 143 proposed by Thomas Young in 1802 and later modified by Hermann von Helmholtz in 1852, this theory proposed three types of cones: red cones, blue cones, and green cones, one for each of the three primary colors of light. Most people probably think that the primary colors are red, yellow, and blue, but these are the primary colors when talking about painting—not when talking about light. Paints reflect light, and the way reflected light mixes is different from the way direct light mixes. For example, if an artist were to blend red, yellow, and blue paints together, the result would be a mess—a black mess. The mixing of paint (reflected light) is subtrac- tive, removing more light as you mix in more colors. As all of the colors are mixed, more Figure 3.6 Mixing Light light waves are absorbed and we see black. But if the artist were to blend a red, green, The mixing of direct light is different than the and blue light together by focusing lights of those three colors on one common spot, mixing of reflected light. The mixing of red, the result would be white, not black (see Figure 3.6). The mixing of direct light is addi- blue, and green light is additive, resulting in white light. The mixing of multiple colors of tive, resulting in lighter colors, more light, and when mixing red, blue, and green, we see paint (reflected light) is subtractive, resulting white, the reflection of the entire visual spectrum. in a dark gray or black color. In the trichromatic theory, different shades of colors correspond to different amounts of light received by each of these three types of cones. These cones then fire their message to the brain’s vision centers. It is the combination of cones and the rate at which they are firing that determine the color that will be seen. For example, if the red and green cones are firing in response to a stimulus at fast enough rates, the color the person sees is yellow. If the red and blue cones are firing fast enough, the result is magenta. If the blue and green cones are firing fast enough, a kind of cyan color (blue-green) appears. Paul K. Brown and George Wald (1964) identified three types of cones in the ret- ina, each sensitive to a range of wavelengths, measured in nanometers (nm), and a peak sensitivity that roughly corresponds to three different colors (although hues/colors can vary depending on brightness and saturation). The peak wavelength of light the cones seem to be most sensitive to turns out to be just a little different from Young and von Helmholtz’s original three corresponding colors: Short-wavelength cones detect what we see as blue-violet (about 420 nm), medium-wavelength cones detect what we see as green (about 530 nm), and long-wavelength cones detect what we see as green-yellow (about 560 nm). Interestingly, none of the cones identified by Brown and Wald have a peak sen- sitivity to light where most of us see red (around 630 nm). Keep in mind, though, each cone responds to light across a range of wavelengths, not just its wavelength of peak sen- sitivity. Depending on the intensity of the light, both the medium and long wavelength cones respond to light that appears red, as shown in Figure 3.7. “Blue” 419 496 531 559 nm 1.0 cone Rod Relative absorbance “Green” cone “Red” 0.5 cone 400 500 600 Wavelength (nm) Figure 3.7 Absorbance of Light from Rods and Three Types of Cones 144 CHAPTER 3 OPPONENT-PROCESS THEORY The trichromatic theory would, at first glance, seem to be more than adequate to explain how people perceive color. But there’s an inter- esting phenomenon that this theory cannot explain. If a person stares at a picture of the American flag for a little while—say, a minute—and then looks away to a blank white wall or sheet of paper, that person will see an afterimage of the flag. Afterimages occur when a visual sensation persists for a brief time even after the original stimulus is removed. The person would also notice rather quickly that the colors of the flag in the afterimage are all wrong—green for red, black for white, and yellow for blue. If you follow the directions for Figure 3.8, in which the flag is yellow, green, and black, you should see a flag with the usual red, white, and blue. Interactive Figure 3.8 Color Afterimage Hey, now the afterimage of the flag has normal colors! Why does this happen? The phenomenon of the color afterimage is explained by the second theory of color perception, called the opponent-process theory (De Valois & De Valois, 1993; Hurvich & Jameson, 1957), based on an idea first suggested by Edwald Hering in 1874 (Finger, 1994). In opponent-process theory, there are four primary colors: red, green, blue, and yellow. The colors are arranged in pairs, with each member of the pair as opponents. Red is paired with its opponent green, and blue is paired with its opponent yellow. If one member of a pair is strongly stimulated, the other member is inhibited and cannot be working—so there are no reddish-greens or bluish-yellows. So how can this kind of pairing cause a color afterimage? From the level of the bipolar and ganglion cells in the retina, all the way through the thalamus, and on to the visual corti- cal areas in the brain, some neurons (or groups of neurons) are stimulated by light from one afterimages part of the visual spectrum and inhibited by light from a different part of the spectrum. For images that occur when a visual example, let’s say we have a red-green ganglion cell in the retina whose baseline activity is sensation persists for a brief time rather weak when we expose it to white light. However, the cell’s activity is increased by red even after the original stimulus is light, so we experience the color red. If we stimulate the cell with red light for a long enough removed. period of time, the cell becomes fatigued. If we then swap out the red light with white light, opponent-process theory the fatigued cell responds even less than the original baseline. Now we experience the color theory of color vision that proposes green, because green is associated with a decrease in the responsiveness of this cell. visual neurons (or groups of neurons) So which theory is the right one? Both theories play a part in color vision. Trichro- are stimulated by light of one color matic theory can explain what is happening with the raw stimuli, the actual detection and inhibited by light of another color. of various wavelengths of light. Opponent-process theory can explain afterimages and  Sensation and Perception 145 other aspects of visual perception that occur after the initial detection of light from our environment. In addition to the retinal bipolar and ganglion cells, opponent-process cells are contained inside the thalamus in an area called the lateral geniculate nucleus (LGN). The LGN is part of the pathway that visual information takes to the occipital lobe. It is when the cones in the retina send signals through the retinal bipolar and ganglion cells that we see the red versus green pairings and blue versus yellow pairings. Together with the retinal cells, the cells in the LGN appear to be the ones responsible for opponent- processing of color vision and the afterimage effect. So which theory accounts for color blindness? I’ve heard that there are two kinds of color blindness, when you can’t tell red from green and when you can’t tell blue from yellow. COLOR BLINDNESS From the mention of red-green and yellow-blue color blindness, one might think that the opponent-process theory explains this problem. But in reality, “color blindness” is caused by defective cones in the retina of the eye and, as a more general term, color-deficient vision is more accurate, as most people with “color blindness” have two types of cones working and can see many colors. There are really three kinds of color-deficient vision. In a very rare type, monochrome color blindness, people either have no cones or have cones that are not working at all. Essentially, if they have cones, they only have one type and, therefore, every- thing looks the same to the brain—shades of gray. The other types of color-deficient vision, or dichromatic vision, are caused by the same kind of problem—having one cone that does not work properly. So instead of experiencing the world with nor- mal vision based on combinations of three cones or colors, Figure 3.9 The Ishihara Color Test trichromatic vision, individuals with dichromatic vision expe- In the circle on the left, the number 8 is visible only to those with normal color vision. rience the world with essentially combinations of two cones or In the circle on the right, people with normal vision will see the number 96, while colors. Red-green color deficiency is due to the lack of function- those with red-green color blindness will see nothing but a circle of dots. ing red or green cones. In both of these, the individual confuses reds and greens, seeing the world primarily in blues, yellows, and shades of gray. In one real- world example, a November 2015 professional American football game had one team in all green uniforms and the other in all red uniforms. The combination caused problems for some viewers, who were unable to tell the teams apart! A lack of functioning blue cones is much less common and causes blue-yellow color deficiency. These individuals see the world primarily in reds, greens, and shades of gray. To get an idea of what a test for color-deficient vision is like, look at Figure 3.9. Why are most of the people with color-deficient vision men? Color-deficient vision involving one set of cones is inherited in a pattern known as sex- linked inheritance. The gene for color-deficient vision is recessive. To inherit a recessive trait, you normally need two of the genes, one from each parent. to Learning Objective!8.3. But the gene for color-deficient vision is attached to a particular chromosome (a package of genes) that helps determine the sex of a person. Men have one X!chromosome and one smaller Y chromosome (named for their shapes), whereas women have two X chromosomes. The smaller Y has fewer genes than the larger X, and one of the genes missing is the one that would suppress the gene for color-deficient vision. For a woman to have color-deficient vision, she must inherit two recessive genes, one from each parent, but a man only needs to inherit one recessive gene—the one passed on to him on his mother’s X chromosome. His odds are greater; therefore, more males than females have color-deficient vision. 146 CHAPTER 3 Concept Map L.O. 3.4, 3.5, 3.6 is a form of electromagnetic radiation with properties of both waves and particles is a physical stimulus cornea processed pupil by the eye rods lens contains photoreceptors light retina cones brightness has a blind spot has psychological color/hue properties saturation The Science of Seeing found in periphery of retina “see” black and white or shades of gray rods work well in low light begins with retinal found all over but greatest density receptor cells in center of retina (fovea) cones “see” colors work best in bright light trichromatic theory — processing primarily responsible by cones for color vision: two theories opponent-process theory — processing seeing beyond cones (bipolar or ganglion cells to LGN of thalamus) right visual field → left side of each retina; retina → optic nerve → optic chiasm → optic left visual field → right side of each retina visual pathway tract → LGN of thalamus → optic radiations → primary visual cortex axons from temporal halves of each retina project to visual cortex on same side of the brain; axons from nasal halves project to visual cortex on opposite side of the brain; optic chiasm is point of crossover Practice Quiz How much do you remember? Pick the best answer. 1. Which of the following is largely determined by the length of a light 4. Colleen stares at a fixed spot in her bedroom using only one eye. wave? After a while, what might happen to her vision? a. color a. Any small object that crosses her visual field very slowly may at b. brightness one point disappear. c. saturation b. Any object that she focuses on will begin to rotate, first clock- d. duration wise, then counterclockwise. 2. Aside from the lens, damage to the ________ can affect the eye’s c. Objects will become more focused the longer she looks at them. ability to focus light. d. Objects will become more distorted the longer she looks at them. a. iris 5. What are the three primary colors as proposed by the trichromatic b. cornea theory? c. pupil a. red, yellow, blue c. white, black, brown d. retina b. red, green, blue d. white, black, red 3. In farsightedness, also known as _______________, the focal point 6. Which of the following best explains afterimages? is _____________ the retina. a. trichromatic theory a. presbyopia; above b. opponent-process theory b. myopia; below c. color-deficient vision c. hyperopia; beyond d. monochrome color blindness d. presbyopia; in front of  Sensation and Perception 147 The Hearing Sense: Can You Hear Me Now? If light works like waves, then do sound waves have similar properties? The properties of sound are indeed similar to those of light, as both senses rely on waves. But the similarity ends there, as the physical properties of sound are different from those of light. SOUND WAVES AND THE EAR 3.7 Explain the nature of sound, and describe how it travels through the various parts of the ear. Sound waves do not come in little packets the way light comes in photons. Sound waves are simply the vibrations of the molecules of air that surround us. Sound waves do have the same properties of light waves though—wavelength, amplitude, and purity. Wave- lengths are interpreted by the brain as frequency or pitch (high, medium, or low). Ampli- © The New Yorker Collection 1998 Charles Barsotti tude is interpreted as volume, how soft or loud a sound is. (See Figure!3.10.) Finally, what from cartoonbank.com. All Rights Reserved. Louder Sound/ Lower Pitch Amplitude Softer Sound/ Higher Pitch a. Time 135: Headphones turned to highest volume Pain 125: Jackhammer, 3 feet away threshold 120: Sound causes pain 110: Live rock music 100: Chain saw or subway train going by, 20 feet Potential away ear damage 85–90: Prolonged exposure to any sound above this level causes hearing loss 70: Vacuum cleaner 60: Normal conversation between persons 3 feet apart 40: Quiet office 30: Library 15: Normal breathing b. Figure 3.10 Sound Waves and Decibels (a) Two sound waves. The higher the wave, the louder the sound; the lower the wave, the softer the sound. If the waves are close together in time (high frequency), the pitch will be perceived as a high pitch. Waves that are farther apart (low frequency) will be perceived as having a lower pitch. (b) Decibels of various stimuli. A decibel is a unit of measure for loudness. Psychologists study the effects that noise has on stress, learn- ing, performance, aggression, and psychological and physical well-being. 148 CHAPTER 3 would correspond to saturation or purity in light is called timbre in sound, a richness in the tone of the sound. And just as people rarely see pure colors in the world around us, they also seldom hear pure sounds. The everyday noises that surround people do not allow them to hear many pure tones. Just as a person’s vision is limited by the visible spectrum of light, a person is also limited in the range of frequencies he or she can hear. Frequency is measured in cycles (waves) per second, or hertz (Hz). Human limits are between 20 and 20,000 Hz, with the most sensitivity from about 2,000 to 4,000 Hz, very important for conversational speech. (In comparison, dogs can hear between 50 and 60,000 Hz, and dolphins can hear up to 200,000 Hz.) To hear the higher and lower frequencies of a piece of music on their iPod® or iPhone®, for example, a person would need to increase the amplitude or volume— which explains why some people like to “crank it up.” THE STRUCTURE OF THE EAR: FOLLOW THE VIBES The ear is a series of structures, each of which plays a part in the sense of hearing, as shown in Figure 3.11. Vestibular organ Hammer (semicircular canals) Anvil Oval Pinna window Cochlea Auditory nerve Ear canal Eardrum Stirrup Outer ear Middle ear Inner ear CC Figure 3.11 The Structure of the Ear (Watch this video on MyPsychLab) THE OUTER EAR The pinna is the visible, external part of the ear that serves as a kind of concentrator, funneling* the sound waves from the outside into the structure of the ear. hertz (Hz) The pinna is also the entrance to the auditory canal (or ear canal), the short tunnel that cycles or waves per second, a mea- runs down to the tympanic membrane, or eardrum. When sound waves hit the eardrum, surement of frequency. they cause three tiny bones in the middle ear to vibrate. pinna THE MIDDLE EAR: HAMMER, ANVIL, AND STIRRUP The three tiny bones in the middle ear the visible part of the ear. are known as the hammer (malleus), anvil (incus), and stirrup (stapes), each name stem- ming from the shape of the respective bone. Collectively they are referred to as the ossicles auditory canal and they are the smallest bones in the human body. The vibration of these three bones short tunnel that runs from the pinna amplifies the vibrations from the eardrum. The stirrup, the last bone in the chain, causes to the eardrum. a membrane covering the opening of the inner ear to vibrate. cochlea THE INNER EAR This membrane is called the oval window, and its vibrations set off snail-shaped structure of the inner ear another chain reaction within the inner ear. The inner ear is a snail-shaped structure that is filled with fluid. called the cochlea, which is filled with fluid. When the oval window vibrates, it causes *funneling: moving to a focal point.  Sensation and Perception 149 the fluid in the cochlea to vibrate. This fluid surrounds a membrane running through the middle of the cochlea called the basilar membrane. The basilar membrane is the resting place of the organ of Corti, which contains the receptor cells for the sense of hearing. When the basilar membrane vibrates, it vibrates the organ of Corti, causing it to brush against a membrane above it. On the organ of Corti are special cells called hair cells, which are the receptors for sound. When these auditory receptors or hair cells are bent up against the other membrane, it causes them to send a neural message through the auditory nerve (which contains the axons of all the receptor neurons) and into the brain, where after passing through the thalamus, the auditory cor- tex will interpret the sounds (the transformation of the vibrations of sound into neural messages is transduction). The louder the sound in the outside world, the stronger the vibrations that stimulate more of those hair cells—which the brain interprets as loudness. I think I have it straight—but all of that just explains how soft and loud sounds get to the brain from the outside. How do we hear different kinds of sounds, like high pitches and low pitches? PERCEIVING PITCH 3.8 Summarize three theories of how the brain processes information about pitch. Pitch refers to how high or low a sound is. For example, the bass beats in the music pounding through the wall of your apartment from the neighbors next door are low pitch, whereas the scream of a 2-year-old child is a very high pitch. Very high. There are three primary theories about how the brain receives information about pitch. PLACE THEORY The oldest of the three theories, place theory, is based on an idea pro- posed in 1863 by Hermann von Helmholtz and elaborated on and modified by Georg von Békésy, beginning with experiments first published in 1928 (Békésy, 1960). In this auditory nerve theory, the pitch a person hears depends on where the hair cells that are stimulated are located on the organ of Corti. For example, if the person is hearing a high-pitched sound, bundle of axons from the hair cells in the inner ear. all of the hair cells near the oval window will be stimulated, but if the sound is low pitched, all of the hair cells that are stimulated will be located farther away on the organ pitch of Corti. psychological experience of sound FREQUENCY THEORY Frequency theory, developed by Ernest Rutherford in 1886, that corresponds to the frequency of states that pitch is related to how fast the basilar membrane vibrates. The faster this the sound waves; higher frequencies membrane vibrates, the higher the pitch; the slower it vibrates, the lower the pitch. (In are perceived as higher pitches. this theory, all of the auditory neurons would be firing at the same time.) place theory So which of these first two theories is right? It turns out that both are right—up to theory of pitch that states that a point. For place theory to be correct, the basilar membrane has to vibrate unevenly— different pitches are experienced which it does when the frequency of the sound is above 1,000 Hz. For the frequency the- by the stimulation of hair cells ory to be correct, the neurons associated with the hair cells would have to fire as fast as in different locations on the the basilar membrane vibrates. This only works up to 1,000 Hz, because neurons don’t organ of Corti. appear to fire at exactly the same time and rate when frequencies are faster than 1,000 times per second. Not to mention the maximum firing rate for neurons is approximately frequency theory 1,000 times per second due to the refractory period. theory of pitch that states that pitch is related to the speed of vibrations in VOLLEY PRINCIPLE The frequency theory works for low pitches, and place the- the basilar membrane. ory works for moderate to high pitches. Is there another explanation? Yes, and it is a third theory, developed by Ernest Wever and Charles Bray, called the volley principle volley principle (Wever, 1949; Wever & Bray, 1930), which appears to account for pitches from about theory of pitch that states that 400!Hz up to about 4,000 Hz. In this explanation, groups of auditory neurons take turns frequencies from about 400 Hz to firing in a process called volleying. If a person hears a tone of about 3,000!Hz, it means 4000 Hz cause the hair cells (auditory that three groups of neurons have taken turns sending the message to the brain—the neurons) to fire in a volley pattern, or first group for the first 1,000 Hz, the second group for the next 1,000 Hz, and so on. take turns in firing. 150 CHAPTER 3 TYPES OF HEARING IMPAIRMENTS 3.9 Identify types of hearing impairment and treatment options for each. Hearing impairment is the term used to refer to difficulties in hearing. A person can be partially hearing impaired or totally hearing impaired, and the treatment for hearing loss will vary according to the reason for the impairment. CONDUCTION HEARING IMPAIRMENT Conduction hearing impairment, or conductive hearing loss, refers to problems with the mechanics of the outer or middle ear and means that sound vibrations cannot be passed from the eardrum to the cochlea. The cause might be a damaged eardrum or damage to the bones of the middle ear (usually from an infection). In this kind of impairment, the causes can often be treated, for example, hearing aids may be of some use in restoring hearing. NERVE HEARING IMPAIRMENT In nerve hearing impairment, or sensorineural hearing loss, the problem lies either in the inner ear or in the auditory pathways and cortical areas of the brain. This is the most common type of permanent hearing loss. Normal aging causes loss of hair cells in the cochlea, and exposure to loud noises can damage hair cells. Tinnitus is a fancy word for an extremely annoying ringing in one’s ears, and it can also be caused by infections or loud noises—including loud music in head- phones. Prolonged exposure to loud noises further leads to permanent damage and hearing loss, so you might want to turn down that stereo or personal music player! Because the damage is to the nerves or the brain, nerve hearing impairment can- not typically be helped with ordinary hearing aids, which are basically sound amplifi- ers, or the hearing aids are not enough. A technique for restoring some hearing to those with irreversible nerve hearing impairment makes use of an electronic device called a cochlear implant. This device sends signals from a microphone worn behind the ear to a sound processor worn on the belt or in a pocket, which then translates those signals into electrical stimuli that are sent to a series of electrodes implanted directly into the cochlea, allowing transduction to take place and stimulating the auditory nerve. (See Figure 3.12.) The brain then processes the electrode information as sound. Cable to Headpiece speech processor Microphone Implant Cochlea Auditory nerve Speech processor Electrode array CC Figure 3.12 Cochlear Implant (Watch this video on MyPsychLab) In a cochlear implant, a microphone implanted just behind the ear picks up sound from the surrounding environment. A speech processor, attached to the implant and worn outside the body, selects and arranges the sound picked up by the microphone. The implant itself is a transmitter and receiver, converting the signals from the speech processor into electrical impulses that are collected by the electrode array in the cochlea and then sent to the brain.  Sensation and Perception 151 THINKING CRITICALLY How might someone who has had total hearing loss from birth react to being able to hear? Concept Map L.O. 3.7, 3.8, 3.9 have wavelengths The Hearing Sense composed of sound waves and wavelike properties that can be measured outer ear result of vibrations of air molecules is a physical stimulus processed by the ear middle ear inner ear processing can be impaired; hearing aids may help with conductive issues sound whereas cochlear implants may be used with nerve hearing impairment shorter wavelengths theories place theory frequency = more waves per of pitch frequency theory or pitch second = higher perception volley theory frequencies has psychological volume larger wave amplitudes associated with louder volume properties timbre increase in number of sounds results in greater richness Practice Quiz How much do you remember? Pick the best answer. 1. The part of the ear that can be seen is also called the 4. Yoshi has suffered minor damage to the bones in his left middle ear. a. pinna. c. organ of Corti. What treatment, if any, might help restore his hearing? b. oval window. d. cochlea. a. a hearing aid 2. The oval window is found in what part of the ear? b. a cochlear implant a. outer ear c. inner ear c. Both a hearing aid and a cochlear implant will be needed. b. middle ear d. The oval window is not a structure d. Such damage is permanent and cannot be remedied. of the ear. 5. Which is considered the most common type of permanent hearing loss? 3. Which theory cannot adequately account for pitches above 1,000 Hz? a. psychological hearing loss a. place c. volley b. conductive hearing loss b. frequency d. adaptive c. frequency-based hearing loss d. sensorineural hearing loss Chemical Senses: It Tastes Good and Smells Even Better The sense of taste (taste in food, not taste in clothing or friends) and the sense of smell are very closely related. As Dr. Alan Hirsch, a researcher on smell and taste, explains in the video Smell and Taste, about 90 percent of what we deem taste is really smell. Have you ever 152 CHAPTER 3 noticed that when your nose is all stopped up, your sense of taste is affected, too? That’s because the sense of taste is really a combination of taste and smell. Without the input from the nose, there are actually only four, or possibly five, kinds of taste sensors in the mouth. CC Watch the Video Smell and Taste on MyPsychLab GUSTATION: HOW WE TASTE THE WORLD 3.10 Explain how the sense of taste works. Our food preferences, or aversions, start to form very early in life, very early! Taste is one of our earliest developed senses. Research suggests developing babies are exposed to substances the mother inhales or digests, and these impart flavor to the amniotic fluid, which the baby also ingests. Along with exposure to different flavors early in life after we are born, these experiences may affect food choices and nutritional status, that is, picking certain foods over others, for a long time to come (Beauchamp & Mennella, 2011; Mennella & Trabulsi, 2012). TASTE BUDS Taste buds are the common name for the taste receptor cells, special kinds of neurons found in the mouth that are responsible for the sense of taste, or gustation. Most taste buds are located on the tongue, but there are a few on the roof of the mouth, the cheeks, under the tongue, and in the throat as well. How sensitive people are to var- ious tastes depends on how many taste buds they have; some people have only around 500, whereas others have 20 times that number. The latter are called “supertasters” and need far less seasoning in their food than those with fewer taste buds (Bartoshuk, 1993). So taste buds are those little bumps I can see when I look closely at my tongue? No, those “bumps” are called papillae, and the taste buds line the walls of these papillae. (See Figure 3.13.) Each taste bud has about 20 receptors that are very similar to the receptor sites on receiving neurons at the synapse. to Learning Objective 2.3. In fact, the receptors on taste buds work exactly like receptor sites on neurons—they receive molecules of vari- ous substances that fit into the receptor like a key into a lock. Taste is often called a chem- ical sense because it works with the molecules of foods people eat in the same way the gustation neural receptors work with neurotransmitters. When the molecules (dissolved in saliva) the sensation of a taste. fit into the receptors, a signal is fired to the brain, which then interprets the taste sensation.  Sensation and Perception 153 Receptor cell Taste hair Taste pore Supporting cell Outer layer of tongue Nerve fiber CC Figure 3.13 The Tongue and Taste Buds. (Watch this video on MyPsychLab.) Taste buds are located inside the papillae of the tongue and are composed of small cells that send signals to the brain when stimulated by molecules of food. What happens to the taste buds when I burn my tongue? Do they repair themselves? I know when I have burned my tongue, I can’t taste much for a while, but the taste comes back. In general, the taste receptors get such a workout that they have to be replaced every 10 to 14 days (McLaughlin & Margolskee, 1994). And when the tongue is burned, the damaged cells no longer work. As time goes on, those cells get replaced and the taste sense comes back. THE FIVE BASIC TASTES In 1916 a German psychologist named Hans Henning proposed that there are four primary tastes: sweet, sour, salty, and bitter. Lindemann (1996) sup- ported the idea that there is a fifth kind of taste receptor that detects a pleasant “brothy” taste associated with foods like chicken soup, tuna, kelp, cheese, and soy products, among others. Lindemann proposed that this fifth taste be called umami, a Japanese word first coined in 1908 by Dr. Kikunae Ikeda of Tokyo Imperial University to describe the taste. Dr. Ikeda had succeeded in isolating the substance in kelp that generated the sen- sation of umami—glutamate (Beyreuther et al., 2007). to Learning Objective!2.3. Glutamate exists not only in the foods listed earlier but is also present in human breast milk and is the reason that the seasoning MSG—monosodium glutamate—adds a pleasant flavor to foods. Although not yet widely accepted, researchers have recently suggested there may be yet another basic taste. The proposed name for this potential sixth taste is oleogustus, the taste of fatty acids in the food we eat (Running et al., 2015). Although researchers used to believe that certain tastes were located on certain places on the tongue, it is now known that all of the taste sensations are processed all over Figure 3.14 The Gustatory Cortex the tongue (Bartoshuk, 1993). The taste information is sent to the gustatory cortex, found The gustatory cortex is found in the anterior in the front part of the insula and the frontal operculum. (See Figure 3.14.) These areas are insula and frontal operculum. The insula is an area of cortex covered by folds of overly- involved in the conscious perception of taste, whereas the texture, or “mouth-feel,” of ing cortex, and each fold is an operculum. In foods is processed in the somatosensory cortex of the parietal lobe (Buck & Bargmann, the coronal section of a human brain above, 2013; Pritchard, 2012; Shepherd, 2012). The five taste sensations work together, along with the gustatory cortex is found in the regions the sense of smell and the texture, temperature, and “heat” of foods, to produce thousands colored a light red. 154 CHAPTER 3 of taste sensations, which are further affected by our culture, personal expectations, and past learning experiences. For example, boiled peanuts are not an uncommon snack in parts of the southern United States, but the idea of a warm, soft and mushy, slightly salty peanut may not be appealing in other parts of the country. The cortical taste areas also project to parts of the limbic system, which helps explain why tastes can be used for both positive and negative reinforcement (Pritchard, 2012). to Learning Objective 5.5. Just as individuals and groups can vary on their food preferences, they can also vary on level of perceived sweetness. For example, obese individuals have been found to experience less sweetness than individuals who are not obese; foods that are both sweet and high in fat tend to be especially attractive to individuals who are obese (Bartoshuk et!al., 2006). Such differences (as well as genetic variations like the supertasters) compli- cate direct comparison of food preferences. One possible solution is to have individuals rate taste in terms of an unrelated “standard” sensory experience of known intensity, such as the brightness of a light or loudness of a sound or preference in terms of all plea- surable experiences, and not just taste (Bartoshuk et al., 2005; Snyder & Bartoshuk, 2009). Turning our attention back to how things taste for us as individuals, have you ever noticed that when you have a cold, food tastes very bland? Everything becomes bland or muted because you can taste only sweet, salty, bitter, sour, and umami—and because your nose is stuffed up with a cold, you don’t get all the enhanced variations of those tastes that come from the sense of smell. THE SENSE OF SCENTS: OLFACTION 3.11 Explain how the sense of smell works. Like the sense of taste, the sense of smell is a chemical sense. The ability to smell odors is called olfaction, or the olfactory sense. The outer part of the nose serves the same purpose for odors that the pinna and ear canal serve for sounds: Both are merely ways to collect the sensory information and get it to the part of the body that will translate it into neural signals. The part of the olfactory system that transduces odors—turns odors into signals the brain can understand—is located at the top of the nasal passages. This area of olfactory receptor cells is only about an inch square in each cavity yet contains about 10 million olfactory receptors. (See Figure 3.15.) Olfactory bulb Nerve fiber Olfactory bulb Cilia of olfactory receptor cell Olfactory epithelium Receptor cell Cilia Supporting cell CC Figure 3.15 The Olfactory Receptors. (Watch this video on MyPsychLab.) (a) A cross-section of the nose and mouth. This drawing shows the nerve fibers inside the nasal cavity that carry information about smell directly to the olfactory bulb just under the frontal lobe of the brain (shown in olfaction (olfactory sense) green). (b) A diagram of the cells in the nose that process smell. The olfactory bulb is on top. Notice the cilia, the sensation of smell. tiny hairlike cells that project into the nasal cavity. These are the receptors for the sense of smell.  Sensation and Perception 155 OLFACTORY RECEPTOR CELLS The olfactory receptor cells each have about a half dozen to a dozen little “hairs,” called cilia, that project into the cavity. Like taste buds, there are receptor sites on these hair cells that send signals to the brain when stimulated by the molecules of substances that are in the air moving past them. Wait a minute—you mean that when I can smell something like a skunk, there are little particles of skunk odor IN my nose? Yes. When a person is sniffing something, the sniffing serves to move molecules of whatever the person is trying to smell into the nose and into the nasal cavities. That’s okay when it’s the smell of baking bread, apple pie, flowers, and the like, but when it’s skunk, rotten eggs, dead animals—well, try not to think about it too much. Olfactory receptors are like taste buds in another way, too. Olfactory receptors also have to be replaced as they naturally die off, about every 5–8 weeks. Unlike the taste buds, there are many more than 5 types of olfactory receptors—in fact, there are at least 1,000 of them. Signals from the olfactory receptors in the nasal cavity do not follow the same path as the signals from all the other senses. Vision, hearing, taste, and touch all pass through the thalamus and then on to the area of the cortex that processes that particular sensory infor- mation. But the sense of smell has its own special place in the brain—the olfactory bulbs. THE OLFACTORY BULBS The olfactory bulbs are located right on top of the sinus cav- ity on each side of the brain directly beneath the frontal lobes. (Refer to Figure 3.15.) olfactory bulbs The olfactory receptors send their neural signals directly up to these bulbs, bypassing two bulb-like projections of the brain the thalamus, the relay center for all other sensory information. The olfactory infor- located just above the sinus cavity mation is then sent from the olfactory bulbs to higher cortical areas, including the pri- and just below the frontal lobes that mary olfactory cortex (the piriform cortex), the orbitofrontal cortex, and the amygdala receive information from the olfactory (remember from Chapter Two that the orbitofrontal cortex and amygdala play import-