Discovering Psychology: The Science of Mind (2022) - PDF

5 The Perceiving Mind SENSATION AND PERCEPTION LEARNING OBJECTIVES 1. Explain the basic 2. Identify the process 4. Describe the process by 5. Explain the mechanisms by c...

5 The Perceiving Mind SENSATION AND PERCEPTION LEARNING OBJECTIVES 1. Explain the basic 2. Identify the process 4. Describe the process by 5. Explain the mechanisms by concepts of sensation by which the physical which physical structures which the somatosensory and perception, structures of the eye of the ear transduce and chemical sense systems including transduction transduce light waves sound waves into neural produce perception of of stimuli into neural into neural signals, signals, producing body position, touch, skin signals, distinctions producing the sense of perception of pitch, temperature, pain, smell, between bottom-up vision. loudness, and spatial and taste. and top-down location in hearing. perceptual processing, 3. Summarize the 6. Analyze the causes thresholds, and processes responsible of various individual measurement. for color vision, object differences in perception, recognition, and depth including development and perception. culture, in terms of biology, experience, and their interaction. WE LIKE TO THINK WE UNDERSTAND REALITY. After all, we can see, hear, touch, smell, and taste it. We don’t live in some science fiction universe where things are not how they appear. Or do we? The human eye can see many different colors, but what does it mean to “see” a color? Is color something that is a fixed quality of an object? Is the sky really blue? Is an apple really red? Or does the human mind construct these colors from the light reflected from these objects into the eye? Consider the image of the blue/black or white/gold dress that became an Internet sensation in February 2015. A friend of a Scottish bride posted the dress worn by the bride’s mother on her Tumblr blog, leading to a discussion that engaged everyone from Justin Bieber to esteemed neuroscientists. Why do people see this photo so differently? Neuroscientists disagree about why the dress produced such different responses. The Journal of Vision prepared an entire issue (“A Dress Rehearsal for Vision Science”) devoted to explaining the dress phenomenon. A survey of 1,400 people found that 57% described the dress as blue/black (which is Laura Freberg correct), 30% as white/gold, 11% as blue/brown, and 2% as something else (Lafer-Sousa et al., 2015). Older individuals and women were more likely to choose white/gold. These researchers believe that people choose dress colors based on their expectations regarding the lighting. If you think the dress is seen in daylight, you make different conclusions than if you think the dress is seen under artificial light. “The Dress” became an Internet phenomenon as people debated its true Other scientists believe there is something special about the color blue due to our considerable colors. Do you see it as black and blue? experience with natural lighting (Winkler et al., 2015). Because indirect lighting and shadows are usually White and gold? Something else? 149 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  blue, participants are more likely to confuse blue objects with blue lighting. If you assume the light falling on the dress is somewhat blue, you will probably see it as white. Would having a color vision deficiency change the way a person sees the dress? See for yourself. We can reconstruct how the image would look to a person with a rare type of blue-yellow color vision deficiency. Surprisingly, this has little effect, although the blue looks somewhat gray. Is the reality seen by a person with a color vision deficiency different from your reality? We’re going to argue that it is not reality that changes but rather the way the brain views that reality. Laura Freberg As you’ll see in this chapter, we construct models of reality from the information obtained through our senses. We like to think that we are aware of the world around us, and it is unsettling to realize that the world might be different from the representations of reality formed by the human mind. You will learn how “Tritanopes” have a rare type of blue-yellow color vision deficiency. the models built by the human mind have promoted our survival over many generations. Our models of We can simulate how the mysterious reality are distinct from those built by the minds of other animals, whose survival depends on obtaining dress would look to a tritanope. The different types of information from their environments. differences are surprisingly subtle, which explains why we use the term “color vision deficiency” rather than “colorblindness.” This example also reminds us that a single reality (the How Does Sensation Lead to dress) can be sensed and perceived very differently by individual minds. Perception? Our bodies are bombarded with information during wakefulness and sleep. This information takes many forms, from the electromagnetic energy of the sun to vibrations in the air to mol- ecules dissolved in saliva on our tongues. The process of sensation brings information to the brain that arises in the reality outside our bodies, like a beautiful sunset, or originates from within, like an upset stomach. Sensory systems have been shaped by natural selection, described in Chapter 3, to provide information that enhances survival within a particular niche. We sense a uniquely human reality, and one that is not shared by other animals. Your dog howls seconds before you hear the siren from an approaching ambulance because the dog’s hearing is better than yours for high-pitched sounds. Horses bolt at the slightest provocation, but they may be reacting to the vibration of an approaching car or an animal that they sense through their front hooves, a source of information that is not available to the rider. Some animals sense light energy out- side the human visible spectrum. Insects can see ultraviolet light, and some snakes use infrared energy to detect their prey. Differences in sensation do occur from person to person, such as the need to wear cor- rective glasses or not, but they are relatively subtle. However, once we move from the process sensation The process of detecting of sensation to that of perception, or the interpretation of sensory input, individual differ- environmental stimuli or stimuli arising ences become more evident. For example, friends voting for different presidential candidates from the body. will come to different conclusions about who won a debate. Everyone watching the exchange perception The process of interpreting sensed similar information, but each person’s perceptions are unique. sensory information. Sensory Information Travels to the Brain Sensation begins with the interaction between a physical stimulus and our biological sensory systems. A stimulus is anything that elicits a reaction from our sensory systems. For example, we react to light energy that falls within our visual range, as we will see later in this chapter, but we cannot see light energy that falls outside that range, such as the microwaves that cook our dinner or the ultraviolet waves that harm our skin (see Figure 5.1). 150 Chapter 5 THE PERCEIVING MIND: SENSATION AND PERCEPTION Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  FIGURE 5.1 All Species Experience an Adaptive Reality. Humans see only a small part of the electromagnetic energy emitted from the Sun. Some animals see even less. Dogs apparently do fine seeing blues, yellows, and grays, whereas humans have evolved to see a more colorful world. The dog’s view Leah Warkentin/AGE Fotostock of the world is simulated in the photo on the right. Wavelength in meters 10–15 10–14 10–13 10–12 10–11 10–10 10–9 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10–1 101 102 103 Visible Cosmic rays Gamma X-rays Ultraviolet Infrared Microwaves Radar TV FM AM Short rays Radio waves waves VISIBLE RANGE Human’s view 400 450 500 550 600 650 700 750 Wavelength in nanometers Dog’s view Before you can use information from your senses, it must be translated into a form the nervous system can understand. This process of translation from stimulus to neural signal is known as transduction. You might think of sensory transduc- tion as being similar to the processing of information by your computer. Modern computers transduce a variety of inputs, including voice, photos, keyboard, mouse clicks, and touch, into a programming language for further processing. The Brain Constructs Perceptions From Ted Kinsman/Science Source Sensory Information Once information from the sensory systems has been transduced into neural sig- nals and sent to the brain, the process of perception, or the interpretation of the sensory information, begins. Perception allows us to organize, recognize, and use the information provided by the senses. An important gateway to perception is the process of attention, defined as a narrow focus of Some types of snakes (vipers, boas, consciousness. As we discuss in Chapters 6, 9, and 10, attention often determines which features and pythons) can locate prey by of the environment influence our subsequent thoughts and behaviors. Which stimuli are likely to sensing infrared energy. grab our attention? Unfamiliar, changing, or high-intensity stimuli often affect our survival and have a high priority for our attention. Unfamiliar stimuli in our ancestors’ environment might have meant a new source of danger (an unknown predator) or a new source of food (an unfamil- transduction The translation of incom- iar fruit) that warranted additional investigation. Our sensory systems are particularly sensitive ing sensory information into neural signals. to change in the environment. Notice how you pay attention to the sound of your heating system HOW DOES SENSATION LEAD TO PERCEPTION? 151 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  cycling on or off but pay little attention to the noise it makes while running. This reduced re- sensory adaptation The tendency to pay less attention to a nonchanging source sponse to an unchanging stimulus is known as sensory adaptation. High-intensity stimuli, such of stimulation. as bright lights and loud noises, draw our attention because the situations that produce these bottom-up processing Perception stimuli, such as a nearby explosion, can have obvious consequences for our safety. based on building simple input into more We rarely have the luxury of paying attention to any single stimulus. In most cases, we complex perceptions. experience divided attention, in which we attempt to process multiple sources of sensory in- top-down processing A perceptual formation. Students walk to class without getting run over by a car while texting. These divided process in which memory and other cogni- attention abilities are limited. We simply cannot process all the information converging simulta- tive processes are required for interpreting neously on our sensory systems. To prioritize input, we use selective attention, or the ability to incoming sensory information. focus on a subset of available information and exclude the rest. These abilities may be disrupted in cases of attention deficit hyperactivity disorder (ADHD; Wimmer et al., If you think about the most memorable 2015; also see Chapter 14). advertisements you have seen lately on television We refer to the brain’s use of incoming signals to construct perceptions as or online, it is likely that they share the features of bottom-up processing. For example, we construct our visual reality from infor- mation about light that is sent from the eye to the brain. However, the brain also attention-getting stimuli: novelty (we don’t see imposes a structure on the incoming information, a type of processing known talking geckos every day), change (rapid movement, as top-down. In top-down processing, we use knowledge gained from prior use of changing colors, and the dreaded pop-up), experience with stimuli to perceive them. For example, a skilled reader has no and intensity (the sound is often louder than the trouble reading the following sentences, even though the words are jumbled: program you’re watching). All you hvae to do to mkae a snetnece raedalbe is to mkae srue taht the fisrt and lsat letrtes of ecah wrod saty the smae. Wtih prcatcie, tihs porcses becoems mcuh fsater and esaeir. How can we explain our abil- We have all had the experience ity to read these sentences? First, of watching events with others we require bottom-up processing to (sensation) and then being shocked by bring the sensations of the letter the different interpretations we hear shapes to our brain. From there, of what just happened (perception). however, we use knowledge and experience to recognize individual words. Many students have learned the hard way that term papers must AP Images/Roberto Pfeil be proofread carefully. As in our ex- ample, if the brain expects to see a particular word, you are likely to see that word, even if it is misspelled, a metamorworks/Shutterstock.com Divided attention abilities are limited. Some people believe that heads-up displays for cars assist drivers with divided attention, while others believe the displays are too distracting. 152 Chapter 5 THE PERCEIVING MIND: SENSATION AND PERCEPTION Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  mistake that is unlikely to be made by the literal, bottom-up processing of a com- Selective attention, or our focus on a puter spell-checker. subset of input, prioritizes incoming Can we predict when the mind will information. However, we can use bottom-up or top-down processing? sometimes be so focused that we miss There are no hard and fast rules. Ob- important information. An astonishing viously, we always use bottom-up pro- 20 out of 24 expert radiologists skyhawk x/Shutterstock.com cessing, or the information would not be completely missed the image of a perceived. It is possible that bottom-up gorilla superimposed on scans of lungs processing alone allows us to respond while scanning for signs of cancer. appropriately to simple stimuli, like in- Source: T. Drew et al. (2013). “The dicating whether you saw a flash of light, Invisible Gorilla Strikes Again: or stimuli that are unfamiliar to us. As Sustained Inattentional Blindness stimuli become more complicated or familiar, like reading a sentence or recognizing a in Expert Observers,” Psychological friend in a crowd, we are more likely to engage in top-down processing in addition to Science, 24(9), 1848–1853, doi: bottom-up processing. 10.1177/0956797613479386 Measuring Perception Gustav Fechner (1801–1887) developed methods, which he called psychophysics, for studying the relationships between stimuli (the physics part) and perception of those stimuli (the psyche or mind part) (see Figure 5.2). Fechner’s careful psychophysics The study of relationships methods not only contributed to the establishment of psychology as a true science but are still between the physical qualities of stimuli used in research today. and the subjective responses they produce. The methods of psychophysics allow us to establish the limits of awareness, or thresholds, absolute threshold The smallest for each of our sensory systems. The smallest possible stimulus that can be detected at least amount of stimulus that can be detected. 50% of the time is known as the absolute threshold. Under ideal circumstances, our senses difference threshold The smallest de- tectable difference between two stimuli. are surprisingly sensitive (see Figure 5.3). For example, you can see the equivalent of a candle flame 30 miles (about 48 kilometers) away on a moonless night. We can also establish a difference threshold, or the smallest difference between two stimuli that can be detected at least 50% of the time. The amount of difference that can be detected depends on the size of the stimuli being compared. As stimuli get larger, differences must also become larger to be detected by an observer. Signal Detection Many perceptions involve some uncertainty. Perhaps you think you recognize the person coming toward you on the sidewalk. Do you wave right away? Or do you wait until the person is close enough that you know for sure it’s your friend? How do your personal feelings about making mistakes affect your decision? How embarrassed would you be if you waved at a stranger? FIGURE 5.2 This type of decision making can have serious implications, such as in the case of deci- sions made by radiologists examining the results of mammograms for signs of cancer or by Connecting the Physical intelligence officers assessing the possibility of an attack. Is there reason for concern or not? World and the Mind. “Golden” rectangles, named for their proportions rather than color, appear in art and architecture dating back to ancient Greece, but why are they attractive? Gustav Fechner (1801–1887) made 40 many attempts to link physical 35 Percent of choices realities with human psychological 30 responses. He asked people to 25 Photo Researchers, Inc/Alamy Stock Photo choose which rectangles are most 20 15 pleasing or least pleasing. His 10 results indicated that the most 5 pleasing rectangle was fourth 0 from the right. This rectangle is the closest to having golden Most pleasing The proportions (1:1.618). Its sides have Least pleasing Golden Rectangle a ratio of 13:21. HOW DOES SENSATION LEAD TO PERCEPTION? 153 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. FIGURE 5.3 Absolute Sensory Thresholds. An absolute threshold is the smallest amount of sensation that can be processed by our sensory systems under ideal conditions. Moving from left to right in this image, we see that the absolute threshold for touch is the equivalent of feeling the wing of a fly fall on your cheek from a distance of 0.4 inches (about 1 centimeter), the absolute threshold for olfaction is a drop of perfume in the air filling a six-room apartment, the absolute threshold for sweetness is the equivalent of one teaspoon (about 5 grams) of sugar in two gallons (about A drop of perfume diffused into the entire volume of air 7.5 liters) of water (the absolute threshold for bitter The wing of a fly falling in a six-room apartment tastes is even more sensitive), the absolute threshold on you from a distance of for hearing is the equivalent of the sound of a mosquito 0.4 inches (about 1 centimeter) 10 feet (about 3 meters) away, and the absolute Photos, left to right: Gladskikh Tatiana/Shutterstock.com; Kuttelvaserova Stuchelova/Shutterstock.com; Christopher Elwell/Shutterstock.com; AlexRoz/Shutterstock.com. threshold for vision is seeing a candle flame 30 miles (about 48 kilometers) away on a dark, clear night. This situation is different from the thresholds described earlier because it adds the cognitive signal detection The analysis of sensory process of decision making to the process of sensation. In other words, signal detection is a and decision-making processes in the de- tection of faint, uncertain stimuli. two-step process involving (a) the actual intensity of the stimulus, which influences the ob- server’s belief that the stimulus did occur, and (b) the individual observer’s criteria for decid- ing whether the stimulus occurred. Experiments on signal detection provide insight into this type Another example of signal detection is a jury’s of decision making. In these experiments, trials with a single, faint decision about whether a person is guilty. Based stimulus and trials with no stimulus are presented randomly. The on frequently uncertain and conflicting evidence, participant states whether a stimulus was present on each trial. The pos- jurors must weigh their concerns about convicting sible outcomes of this experiment are shown in Table 5.1. In the case of reading mammograms, we can use such experiments to help us an innocent person (false alarm) or letting a real understand why two people might respond differently, even if they criminal go (miss). were sensing the same information. Ideally, a radiologist would identify 100% of all tumors without any false alarms, but mammograms are not that easy to evalu- ate. A radiologist afraid of missing a tumor might identify anything that looks remotely like a tumor as the basis for more testing. Few cases of cancer would be missed (high hit rate), but many healthy patients would go through unnec- essary procedures (high false alarm rate). In con- trast, another radiologist According to Fechner’s work on the might need a higher difference threshold, British Olympian level of certainty about YURI CORTEZ/AFP/Getty Images/Newscom Zoe Smith would be more likely to the presence of a tumor notice the difference between 1 and before asking for fur- 2 kilogram (about 2- and 4-pound) ther tests. This would dumbbells than the difference reduce the number of between her new record of 121 false alarms, but it would kilograms (266.76 pounds) in the clean also run a higher risk of and jerk event and the former record overlooking tumors (high of 120 kilograms (264.56 pounds). miss rate). 154 Chapter 5 THE PERCEIVING MIND: SENSATION AND PERCEPTION Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The sound of a mosquito flying about 10 feet (about 3 meters) away A candle flame seen at 30 miles (about 48 kilometers) on a clear night Photos, left to right: fotomak/Shutterstock.com; seeyou/Shutterstock.com; taedong/Shutterstock.com; Karen H. Ilagan/Shutterstock.com. A teaspoon of sugar (about 5 grams) in two gallons (about 7.5 liters) of water Does this mammogram indicate the woman has cancer or not? Many decisions we make are based on ambiguous stimuli. Signal detection theory helps us understand how an individual doctor balances the risks of missing a cancer and those of alarming a healthy patient. BSIP/Contributor/Universal Images Group/Getty Images TABLE 5.1 Possible Outcomes in Signal Detection Participant Response Stimulus Present Stimulus Absent Yes Hit False alarm No Miss Correct rejection HOW DOES SENSATION LEAD TO PERCEPTION? 155 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. SUMMARY 5.1 Assessing Perception Concept Definition Example Absolute threshold The smallest amount of stimulation that is Seeing light from a candle flame detectable. 30 miles away on a dark night. Difference threshold The smallest difference between two Being able to detect the difference between stimuli that can be detected. two different weights. Signal detection Correctly identifying when a faint stimulus A radiologist correctly detecting cancer from is or is not present. a mammogram. Credits: Top row—Karen H. Ilagan/Shutterstock.com; Second row—YURI CORTEZ/AFP/Getty Images/Newscom; Bottom row—BSIP/Contributor/Universal Images Group/Getty Images. vision The sense that allows us to process reflected light. How Do We See? Vision, the processing of light reflected from objects, is one of the most important sensory systems in humans. Approximately 50% of our cerebral cortex processes visual information, in comparison to only 3% for hearing and 11% for touch and pain (Kandel & Wurtz, 2000; Sereno & Tootell, 2005). We will begin our exploration of vision with a description of the visual stimulus, and then we will follow the processing of that stimulus by the mind into a meaningful perception. The Visual Stimulus Visible light, or the energy within the electromagnetic spectrum to which our visual systems respond, is a type of radiation emitted by the Sun, other stars, and artificial sources such as a lightbulb. As shown in Figure 5.4, light energy moves in waves, like the waves in the ocean. Wavelength, or the distance between successive peaks of waves, is decoded by our visual FIGURE 5.4 system as color or shades of gray. The height, or amplitude, of the waves is translated by the visual system into brightness. Large-amplitude waves appear bright, and low-amplitude waves Light Travels in Waves. The distance appear dim. between two peaks in a light wave The human visual world involves only a small part of this light spectrum (review (wavelength) is decoded by the visual Figure 5.1). Gamma rays, x-rays, ultraviolet rays, infrared rays, microwaves, and radio waves system as color and the height, or lie outside the capacities of the human eye. amplitude, of the wave as brightness. Wavelength Amplitude Baseline 156 Chapter 5 THE PERCEIVING MIND: SENSATION AND PERCEPTION Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  The Biology of Vision Human vision begins with the eye. The eye is roughly sphere shaped and about the size of a ping-pong ball. Its hard outer covering helps the fluid-filled eyeball retain its shape. Toward the front of the eye, the outer covering becomes clear and forms the cornea. The cornea begins the process of bending light to form an image on the back of the eye. Traveling light next en- ters the pupil, which is actually an opening formed by the muscles of the iris (see Figure 5.5). The iris, which means “rainbow” in Greek, adjusts the opening of the pupil in response to the amount of light present in the environment and to signals from the autonomic nervous system, described in Chapter 4. Arousal is associated with dilated pupils, while relaxation is associated with more constricted pupils. Directly behind the pupil and iris is the main optical instrument of the eye, the lens. Muscles attached to the lens can change its shape, allowing us to accommodate, or adjust our focus to see near or distant objects. Behind the lens is the main chamber of the eye, and lo- cornea The clear surface at the front of the eye that begins the process of directing cated on the rear surface of this chamber is the retina, a thin but complex network of neurons light to the retina. specialized for the processing of light. pupil An opening formed by the iris. Located in the deepest layer of the retina are specialized receptors, the rods and cones, iris The brightly colored circular muscle that transduce the light information. However, before light reaches these receptors, it must surrounding the pupil of the eye. pass through layers of blood vessels and neurons. We normally do not see the blood vessels lens The clear structure behind the pupil and neural layers because of sensory adaptation. As we mentioned previously in this chapter, that bends light toward the retina. adaptation occurs when sensory systems tune out stimuli that never change. Because the retina Layers of visual processing cells in blood vessels and neural layers are always in the same place, we see them only under unusual the back of the eye. circumstances, such as during certain ophthalmology (eye) tests. Fovea Optic disk James P. Gilman, C.R.A./Medical Images.com (blind spot) Blood vessels Retina Fovea FIGURE 5.5 Optic disk Cornea The Human Eye. Light entering the eye travels through the cornea, the pupil, and the lens before reaching Pupil the retina. Among the landmarks on the retina are the fovea, which is Iris specialized for seeing fine detail, and Lens the optic disk, where blood vessels Blood vessels enter the eye and the optic nerve exits the eye. Source: Argosy Publishing, Inc. HOW DO WE SEE? 157 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. FIGURE 5.6 Now You See It—Now You Don’t. There are no photoreceptors in the optic disk, producing a blind spot in We can identify several landmarks on the surface of the retina. The blood vessels serv- each eye. We do not see our blind spots ing the eye and the axons that leave the retina to form the optic nerve exit at the optic disk. with both eyes open because our brain Because there are no rods and cones in the optic disk, each eye has a blind spot. Normally, we fills in the hole. You can demonstrate are unaware of our blind spots because perception fills in the missing details. However, if you your blind spot by holding your follow the directions in Figure 5.6, you should be able to experience your own blind spot. textbook at arm’s length, closing one Toward the middle of the retina is the fovea, which is specialized for seeing fine detail. When eye, focusing your other eye on the dot, we stare directly at an object, the image of that object is projected onto the fovea. The fovea and moving the book toward you until is responsible for central vision, as opposed to peripheral vision, which is the ability to see the stack of money disappears. objects off to the side while looking straight ahead. The image projected on the retina is upside down and reversed relative to the actual FIGURE 5.7 orientation of the object being viewed (see Figure 5.7). You can duplicate this process by looking at both sides of a shiny spoon. In the convex (or outwardly curving) side, you see your What the Retina “Sees.” The image image normally. In the concave (or inwardly curving) side, you see your image as your retina projected on the retina is upside down sees it. Fortunately, the visual system easily decodes this image and provides realistic percep- and reversed, but the brain is able to tions of the actual orientations of objects. interpret the image to perceive the correct orientation of an object. Photoreceptors The human retina contains three types of photoreceptors: rods, cones, Source: Argosy Publishing, Inc. and a group of specialized ganglion cells that respond to brightness. Rods and cones are named after their shapes. Each human eye contains over Retina 110 million rods and about 6 million cones. There are only Light about 50,000 of the specialized ganglion cells. These photoreceptors are responsible for different as- pects of vision. Rods are more sensitive to light than cones, and they excel at seeing dim light. As we observed previ- ously, under ideal circumstances, the absolute threshold Ariwasabi/Shutterstock.com for human vision is the equivalent of a single candle flame from a distance of 30 miles (about 48 kilometers; see Hecht et al., 1942). Rods become more common as we move from the fovea to the periphery of the retina, so your peripheral vision does a better job of viewing dim light than your central vision does (see Figure 5.8). Before the develop- Cornea Optic nerve ment of night goggles, soldiers patrolling in the dark were Lens trained to look to the side of a suspected enemy position fovea An area of the retina that is special- ized for highly detailed vision. rather than directly at their target. rod A photoreceptor specialized to detect dim light. Rods cone A photoreceptor in the retina that processes color and fine detail. FIGURE 5.8 Distribution of Rods and Cones Across the Retina. In humans, cones, indicated by red, blue, and green dots, become less frequent as you move from the fovea to the periphery of the retina. The colors of the dots representing cones indicate the colors to which each shows a maximum response (see Figure 5.11). Rods (light brown dots) and cones are named according to their shapes. Cones Source: Argosy Publishing, Inc. 158 Chapter 5 THE PERCEIVING MIND: SENSATION AND PERCEPTION Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  This extraordinary sensitivity of rods has costs. Rods do Primary visual cortex (occipital lobe) not provide information about color, nor do they provide Midbrain Thalamus clear, sharp images. Under starlight, normal human vision is 20/200 rather than the normal daylight 20/20. In other words, an object seen at night from a distance of 20 feet would have the same clarity as an object seen in bright Optic chiasm sunlight from a distance of 200 feet. Cones function best under bright light and provide the ability to see both sharp images and color. The observation that animals lacking rods and cones Retina still showed some visual responses led to the search for additional photoreceptors in the retina. A special class of ganglion cells in the retina turned out to be important for Optic tract setting our biological clocks (see Chapter 6), maintaining Optic nerve pupil reflexes, and assessing the brightness of a visual stimulus (Yamakawa et al., 2019). Visual Pathways When rods and cones absorb light, they trigger responses in four additional layers of neurons within the retina. Axons from the final layer of cells, the ganglion cells, leave the back of the eye to form the optic nerve. The optic nerves cross the midline at the optic chiasm (named after its X shape, or the Greek letter chi). At the optic chiasm, the FIGURE 5.9 axons closest to the nose cross over to the other hemisphere, Visual Pathways. Visual information while the axons to the outside from the retina travels to the thalamus proceed to the same hemisphere. and then to the primary visual cortex This partial crossing means in the occipital lobe. While looking that if you focus straight ahead, straight ahead, visual stimuli to the everything to the left of center in right of center are processed by the the visual field is processed by the left hemisphere, while visual stimuli to Visual field the left of center are processed by the right hemisphere, while everything of right eye to the right of center is processed by right hemisphere. the left hemisphere. This organization Source: Argosy Publishing, Inc. Visual field provides us with significant advantages of left eye when sensing depth, which we discuss later in the chapter. Beyond the optic chiasm, the visual pathways are known as optic tracts (see Figure 5.9). About 90% of the axons in the optic tracts synapse in the thalamus. The thalamus sends information about vision to the amygdala and the primary visual cortex in the occipital lobe. The amygdala uses visual information to make quick emotional judgments, especially about potentially harmful stimuli. The remaining optic tract fibers connect with the hypo- thalamus, where their input provides information about light needed to regulate sleep–wake cycles, discussed in Chapter 6, or with the superior colliculi of the midbrain, which manage a number of visually guided reflexes, such as changing the size of the pupil in response to light conditions. The primary visual cortex begins, but by no means finishes, the processing of visual input. The primary visual cortex responds to object shape, location, movement, and color (Hubel & Wiesel, 1959; Livingstone & Hubel, 1984; Hubel & Livingstone, 1987). Two major pathways optic nerve The nerve exiting the retina of the eye. radiating from the occipital cortex into the adjacent temporal and parietal lobes continue the analysis of visual input. The parietal pathway helps us process movement in the visual envi- optic tracts Nerve pathways traveling from the optic chiasm to the thalamus, hy- ronment. The temporal pathway responds to shape and color and contributes to our ability to pothalamus, and midbrain. recognize objects and faces. HOW DO WE SEE? 159 Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Visual Perception and Cognition To see something requires the brain to interpret the information gathered by the eyes. How do you know your sweater is red or green, based on the information sent from the retina to the brain? How do you recognize your grandmother at your front door? Color Vision Most of us think about colors in terms of the paints and crayons we used in elementary school. Any kindergartner can tell you that mixtures of red and yellow make orange, red and blue make purple, and yellow and blue make green. Mixing them all together produces a lovely muddy brown. Colored lights, however, work somewhat differently (see Figure 5.10). The primary colors of light are red, green, and blue, and mixing them together produces white light, like sunlight. If you have ever adjusted the color on your computer monitor or television, you know that these devices also use red, green, and blue as primary colors. Observations supporting the FIGURE 5.10 existence of three primary colors of light gave rise to a trichromatic theory of color vision. Trichromatic theory is consistent with the existence of three types of cones in the retina Mixing Colored Lights. The primary that respond best to short (blue), medium (green), or long (red) wavelengths. Our ultimate colors of paint might be red, yellow, experience of color comes not from the response of one type of cone but from comparisons and blue, but in the world of light, the among the responses of all three types of cones (see Figure 5.11). primary colors are red, green, and blue. Color vision deficiency occurs when a person has fewer than the typical three types of cones. Mixed together, they produce white. We no longer use the term colorblind, as this is not accurate. Most people with color vision defi- ciencies see color differently than someone with all three cone types. Very rarely, individuals have either one type of cone or none. To these people, the world appears to be black, white, and gray. trichromatic theory A theory of color vision based on the existence of different Trichromatic theory does a good job of explaining color vision deficiency, but it is less types of cones for the detection of short, successful in accounting for other color vision phenomena, such as color afterimages. For medium, and long wavelengths. example, if you stare at the yellow, green, and black flag in Figure 5.12 and then focus on the opponent process theory A theory dot within the white rectangle to the right, you will see an afterimage of the American flag in of color vision that suggests we have a its more traditional colors of red, white, and blue. red–green color channel and a blue–yellow An opponent process theory of color vision does a better job than the trichromatic color channel in which activation of one theory in explaining these color afterimages. This theory proposes the existence of color color in each pair inhibits the other color. 100 Percentage of maximum response Rods Cones 75 Green FIGURE 5.11 50 Red Responses by Cones to Colored Light. Our perception of color results from a comparison of the responses of 25 Blue the red, green, and blue cones to light. A 580-nanometer light is perceived as yellow and produces a strong response in green cones, a moderate response in red cones, and little response in blue cones. 400 450 500 550 600 650 700 750 Wavelength (nm) FIGURE 5.12 Afterimages Demonstrate Opponent Process Theory. If you stare at the dot in the center of the yellow, green, and black flag for a minute and then shift your gaze to the dot in the white space on the right, you should see the flag in its traditional red, white, and blue. 160 Chapter 5 THE PERCEIVING MIND: SENSATION AND PERCEPTION Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  channels: a red–green channel and a blue–yellow channel. We cannot see a color like reddish green or bluish yellow because the two colors share the same channel. The channels are “opponent” or competing. Activity in one color group in a channel inhibits activity in the other color group. Returning to our green, yellow, and black flag, how can we use Dan Tuffs/Contributor/Getty Images Entertainment/Getty Images opponent process theory to explain our experience of the red, white, and blue afterimage? By staring at the flag, you fatigue some of your visual neurons. Because the color channels compete, reducing activ- ity in one color group in a channel, such as green, increases activity in the other group, which is red. Fatiguing green, black, and yellow causes a rebound effect in each color channel, and your afterimage looks red, white, and blue. (Black and white also share a channel.) If you stare at an image of a real red, white, and blue flag and then look at a white piece of paper, your afterimage looks like our green, black, and yellow illustration. Which of these two theories of color vision, trichromatic theory or opponent process theory, is correct? The trichromatic theory pro- vides a helpful framework for the functioning of the three types of Monty Roberts, known as the Horse Whisperer, attributes his cones in the retina. However, as we move from the retina to higher abilities to observe horse behavior to his complete lack of color levels of visual analysis, the opponent process theory seems to fit vision. This condition is quite rare, occurring in only 1 person out observed phenomena neatly. Both theories help us understand color of every 30,000. vision but at different levels of the visual system. PSYCHOLOGY AS A HUB SCIENCE Color and Accessible Web Design NOW THAT YOU HAVE AN UNDERSTANDING of color perception, we can consider one of the practical problems associated with individual differences in color vision. Between 7% and 10% of males and about 0.4% of females have a form of red–green color vision deficiency (see Figure 5.13). Males are more affected than females because the genes for the pigments used by red and green cones are located on the X chromosome, making red–green color de- ficiency a sex-linked condition (see Chapter 3). Smaller numbers of people lack blue cones (0.0011%) or cones altogether (0.00001%). Given the frequency of color vision deficiency, making visual materials accessible to people with all types of color vision is a serious concern. Color can be an effective tool for designing exciting and engaging websites, but many graphic web designers, who typically have excellent vision, fail to consider how the site might look to a person with a color vision deficiency. One clue for designing an accessible site can be found in other systems based on color, such as traffic lights. Although most of us rely on the color information from the red, yellow, or green lights, the lights also vary in location. In other words, color should never be the only basis for extracting FIGURE 5.13 meaning. A second major concern is contrast, which we discuss in the next section. The strong contrast Detecting Color Vision between black letters on the white pages makes text Deficiency. The Ishihara Color Test, PRISMA ARCHIVO/Alamy Stock Photo easy to read for most people. Colored text against a designed by Shinobu Ishihara in 1917, colored background might add interest, but it runs is a standard method for detecting the risk of being harder to read, especially when reds color vision deficiency. The test and greens are used. As shown in Figure 5.14, online is printed on special paper, so the resources simulate how a web page looks to a person recreated image here would not be with color vision deficiency, which helps designers considered a valid basis for diagnosing maximize accessibility. color deficiency. HOW DO WE SEE? 161 Copyright 202