Biopsychology, Visual System, 11th Edition | PDF

Chapter 6 The Visual System How We See Indiapicture/Alamy Stock Photo Chapter Overview and Learning Objectives Light Enters the Eye and LO 6.1 Explain how the pupil and the lens can affect the i...

Chapter 6 The Visual System How We See Indiapicture/Alamy Stock Photo Chapter Overview and Learning Objectives Light Enters the Eye and LO 6.1 Explain how the pupil and the lens can affect the image that falls Reaches the Retina on the retina. LO 6.2 Explain why some vertebrates have one eye on each side of their head, whereas other vertebrates have their eyes mounted ­side-by-side on the front of their heads. Also, explain the ­importance of binocular disparity. The Retina and Translation LO 6.3 Describe the structure of the retina and name the cell types that of Light into Neural Signals make up the retina. LO 6.4 Describe the duplexity theory of vision and explain the ­differences between the photopic and scotopic systems. LO 6.5 Explain the difference between the photopic and scotopic spectral sensitivity curves and explain how that difference can account for the Purkinje effect. 151 M06_PINE1933_11_GE_C06.indd 151 22/01/2021 10:56  152 Chapter 6 LO 6.6 Describe the three types of involuntary fixational eye movements and explain what happens when all eye movements are blocked. LO 6.7 Describe the process of visual transduction. From Retina to Primary LO 6.8 Describe the components and layout of the retina-­geniculate- Visual Cortex striate system. LO 6.9 In the context of the retina-geniculate-striate system, explain what is meant by retinotopic. LO 6.10 Describe the M and P channels. Seeing Edges LO 6.11 Describe contrast enhancement. LO 6.12 Define the term receptive field and describe the methods used by David Hubel and Torsten Wiesel to map the receptive fields of visual system neurons. LO 6.13 Describe the work of Hubel & Wiesel that helped to ­characterize the receptive fields of retinal ganglion cells, lateral geniculate neurons, and striate neurons of lower layer IV. LO 6.14 Describe the work of Hubel & Wiesel that characterized the receptive fields of simple and complex cells in the primary visual cortex. LO 6.15 Describe the organization of the primary visual cortex. LO 6.16 Describe how views about the receptive fields of retinal ganglion cells and lateral geniculate neurons have recently changed. LO 6.17 Describe the changing view of visual system receptive fields. Seeing Color LO 6.18 Describe the component and opponent-process theories of color vision. LO 6.19 Describe Land’s demonstration of color constancy and explain his retinex theory. Cortical Mechanisms of LO 6.20 Describe the three classes of visual cortex and identify their Vision and Conscious l­ocations in the brain. Awareness LO 6.21 Explain what happens when an area of primary visual cortex is damaged. LO 6.22 Describe the areas of secondary visual cortex and association cortex involved in vision. LO 6.23 Explain the difference between the dorsal and ventral streams and the functions that have been attributed to each stream by different theories. LO 6.24 Describe the phenomenon of prosopagnosia and discuss the associated theoretical issues. LO 6.25 Describe the phenomenon of akinetopsia and discuss the ­associated theoretical issues. M06_PINE1933_11_GE_C06.indd 152 22/01/2021 10:56  The Visual System 153 This chapter is about your visual system. Most people think their visual system has evolved to respond as accurately as The Case of Mrs. Richards: possible to the patterns of light that enter their eyes. They Fortification Illusions and the recognize the obvious limitations in the accuracy of their Astronomer visual system, of course; and they appreciate those curious instances, termed visual illusions, in which it is “tricked” into Each fortification illusion began with a gray area of blindness seeing things the way they aren’t. But such shortcomings near the center of her visual field—see Figure 6.1. During the are generally regarded as minor imperfections in a system next few minutes, the gray area would begin to expand into a that responds as faithfully as possible to the external world. horseshoe shape, with a zigzag pattern of flickering lines at its But, despite the intuitive appeal of thinking about it advancing edge (this pattern reminded people of the plans for a fortification, hence the name of the illusions). in this way, this is not how the visual system works. The It normally took about 20 minutes for the lines and the trail- visual system does not produce an accurate internal copy ing area of blindness to reach the periphery of her visual field. At of the external world. It does much more. From the tiny, this point, her headache would usually begin. distorted, upside-down, two-dimensional retinal images Because the illusion expanded so slowly, Mrs. Richards projected on the visual receptors that line the backs of the was able to stare at a point on the center of a blank sheet eyes, the visual system creates an accurate, richly detailed, of paper and periodically trace on the sheet the details of her three-dimensional perception that is—and this is the really illusion. This method made it apparent that the lines became important part—in some respects even better than the exter- thicker and the expansion of the area of blindness occurred nal reality from which it was created. Our primary goal in faster as the illusion spread into the periphery. this chapter is to help you appreciate the inherent creativity Interestingly, Dr. Richards discovered that a similar set of of your own visual system. drawings was published in 1870 by the famous British astrono- You will learn in this chapter that understanding the mer George Biddell Airy. They were virtually identical to those done by Mrs. Richards. visual system requires the integration of two types of research: (1) research that probes the visual system with sophisticated neuroanatomical, neurochemical, and neuro­ Figure 6.1 The fortification illusions associated with physiological techniques; and (2) research that focuses on migraine headaches. the assessment of what we see. Both types of research receive substantial coverage in this chapter, but it is the second type that provides you with a unique educational opportunity: the opportunity to participate in the very research you are studying. Throughout this chapter, you will be encouraged to participate in a series of Check It Out demonstrations designed to illustrate the relevance of what you are learning in this text to life outside its pages. This chapter is composed of six modules. The first three take you on a journey from the external visual world to the visual receptors of the retina and from there over the major visual pathway to the primary visual cortex. The next two modules describe how the neurons of this visual pathway mediate the perception of two particularly important fea- tures of the visual world: edges and color. The final mod- ule deals with the flow of visual signals from the primary visual cortex to other parts of the cortex that participate in the complex process of vision. Before you begin the first module of the chapter, we’d like you to consider an interesting clinical case. Have you 1 An attack begins, often when reading, as a gray area of blindness near the 2 Over the next 20 minutes, the gray area assumes a horseshoe shape and expands ever wondered whether one person’s subjective experiences center of the visual field. into the periphery, at which point the headache begins. are like those of others? This case provides evidence that at least some of them are. It was reported by ­Whitman ­Richards (1971), and his participant was his wife. Mrs. R ­ ichards ­suffered from migraine headaches (see Goadsby, 2015), and like 20 percent of migraine sufferers, she often experienced We will return to fortification illusions after you have visual displays, called fortification illusions, prior to her learned a bit about the visual system. At that point, you will attacks (see Charles & Baca, 2013; Thissen et al., 2014). be better able to appreciate their significance. M06_PINE1933_11_GE_C06.indd 153 22/01/2021 10:56  154 Chapter 6 Figure 6.2 The electromagnetic spectrum and the colors that had been associated Light Enters the with wavelengths visible to humans. Eye and Reaches Wavelength (meters) the Retina 10214 10212 10210 1028 1026 1024 1022 1 102 104 106 108 Everybody knows that cats, owls, and other nocturnal animals can Gamma X-rays Ultra- Infrared Radar Broadcast bands AC see in the dark. Right? Wrong! Some rays violet rays Short wave circuits rays Radio animals have special adaptations Television that allow them to see under very dim illumination, but no animal can Blue Green Yellow see in complete darkness. The light Black Violet Blue green Green yellow orange Red reflected into your eyes from the objects around you is the basis for your ability to see them; if there is no light, there is no vision. Black You may recall from high- 400 500 600 700 school physics that light can be Wavelength (nanometers) thought of in two different ways: as discrete particles of energy, called photons, traveling through space at about 300,000 kilometers (186,000 miles) per second, or as waves of energy. Both theories are useful; in some ways, Pupil and Lens light behaves like particles; and in others, it behaves like LO 6.1 Explain how the pupil and the lens can affect waves. Physicists have learned to live with this nagging the image that falls on the retina. inconsistency, and we must do the same. Light is sometimes defined as waves of electromag- The amount of light reaching the retinas is regulated netic energy between 380 and 760 nanometers (billionths by the donut-shaped bands of contractile tissue, the of a meter) in length (see Figure 6.2). There is nothing irises, which give our eyes their characteristic color (see special about these wavelengths except that the human Figure 6.3). Light enters the eye through the pupil, the hole visual system responds to them. In fact, some animals in the iris. The adjustment of pupil size in response to can see wavelengths that we cannot (see Gehring, 2014). changes in illumination represents a compromise between For example, rattlesnakes can see infrared waves, which are too long for humans to see; as a result, they can see Figure 6.3 The human eye. Light enters the eye through warm-blooded prey in what for us would be complete the pupil, whose size is regulated by the iris. The iris gives darkness. So, if we were writing this text for rattlesnakes, the eye its characteristic color—blue, brown, or other. we would be forced to provide a different definition of light for them. Wavelength and intensity are two properties of light that are of particular interest—wavelength because it plays an important role in the perception of color, and intensity because it plays an important role in the percep- tion of brightness. In everyday language, the concepts of wavelength and color are often used interchangeably, as are intensity and brightness. For example, we commonly refer to an intense light with a wavelength of 700 nanometers as being a bright red light (see Figure 6.2), when in fact it is our perception of the light, not the light itself, that is bright and red. We know that these distinctions may seem trivial to you now, but by the end of the chapter you will appreci- ate their importance. tarapong srichaiyos/Shutterstock M06_PINE1933_11_GE_C06.indd 154 22/01/2021 10:56  The Visual System 155 s­ ensitivity (the ability to detect the presence of dimly lit Eye Position and Binocular Disparity objects) and acuity (the ability to see the details of objects). When the level of illumination is high and sensitivity is LO 6.2 Explain why some vertebrates have one eye thus not important, the visual system takes advantage of on each side of their head, whereas other the situation by constricting the pupils. When the pupils vertebrates have their eyes mounted side-by- are constricted, the image falling on each retina is sharper side on the front of their heads. Also, explain and there is a greater depth of focus; that is, a greater the importance of binocular disparity. range of depths is simultaneously kept in focus on the No description of the eyes of vertebrates would be complete retinas. However, when the level of illumination is too without a discussion of their most obvious feature: the fact low to adequately activate the receptors, the pupils dilate that they come in pairs. One reason vertebrates have two to let in more light, thereby sacrificing acuity and depth eyes is that vertebrates have two sides: left and right. By of focus. having one eye on each side, which is by far the most com- Behind each pupil is a lens, which focuses incoming light mon arrangement, vertebrates can see in almost every direc- on the retina (see Figure 6.4). When we direct our gaze at tion without moving their heads. But then why do some something near, the tension on the ligaments holding each vertebrates, including humans, have their eyes mounted lens in place is adjusted by the ciliary muscles, and the lens ­side-by-side on the front of their heads? (See the first Check It assumes its natural cylindrical shape. This increases the ability Out demonstration on the next page.) This arrangement sac- of the lens to refract (bend) light and thus brings close objects rifices the ability to see behind so that what is in front can be into sharp focus. When we focus on a distant object, the lens viewed through both eyes simultaneously—an arrangement is flattened. The process of adjusting the configuration of that is an important basis for our visual system’s ability to the lenses to bring images into focus on the retina is called create three-dimensional perceptions (to see depth) from two- accommodation. dimensional retinal images (see Baden, Euler, & Berens, 2020). Figure 6.4 The human eye, a product of approximately 600 million years of evolution. Eye muscle Ligament Iris Fovea Pupil Blind spot Lens Cornea Optic Ciliary nerve muscle Sclera (the white of the eye) Retina Based on Lamb, T. D., Collin, S. P., & Pugh, E. N. (2007). Evolution of the vertebrate eye: Opsins, photoreceptors, retina and eye cup. Nature Reviews ­Neuroscience, 8, 960–975. M06_PINE1933_11_GE_C06.indd 155 22/01/2021 10:56  156 Chapter 6 greatest when you are inspecting things that are close. Journal Prompt 6.1 But the positions of the images on your two retinas can Why do you think the two-eyes-on-the-front arrangement never correspond exactly because your two eyes do not has evolved in some species but not in others? (After view the world from exactly the same position. ­Binocular you’ve written your answer, see the first Check It Out demonstration below for more on this issue.) ­d isparity—the difference in the position of the same image on the two retinas—is greater for close objects than for distant objects; therefore, your visual system can use The movements of your eyes are coordinated so that the degree of binocular disparity to construct one three- each point in your visual world is projected to correspond- dimensional perception from two two-dimensional retinal ing points on your two retinas. To accomplish this, your images (see Lappin, 2014). (Look at the second Check It eyes must converge (turn slightly inward); convergence is Out ­demonstration below.) Check It Out The Position of Eyes Here you see three animals whose eyes are on the front of their heads (a human, an owl, and a lion) and three whose eyes are on the sides of their heads (an antelope, a canary, and a squirrel). Why do a few verte- brate species have their eyes side-by-side on the front of the head while most species have one eye on each side? In general, predators tend to have the front-facing eyes because this ena- bles them to accurately perceive how far away prey animals are; prey animals tend to have side-facing eyes because this gives them a larger field of vision and the ability Top row from left: Guiziou Franck/Hemis/Alamy Stock Photo; Matthew Cuda/Alamy Stock Photo; to see predators approaching from most C.K. Lorenz/Science Source Bottom row from left: Naomi Engela Le Roux/123RF; Vasiliy Vishnevskiy/123RF; Colin Varndell/ directions. Nature Picture Library Check It Out Binocular Disparity and the Mysterious Cocktail Sausage If you compare the views from each eye (by quickly closing arm’s length in front of you—with the backs of your fingers one eye and then the other) of objects at various distances away from you (unless you prefer sausages with fingernails). in front of you—for example, your finger held at different Now, with both eyes open, look through the notch between distances—you will notice that the disparity between the your touching fingertips, but focus on the wall. Do you see two views is greater for the cocktail sausage closer objects. Now try between your finger- the mysterious demon- tips? Where did it come stration of the cocktail from? To prove to your- sausage. Face the far- self that the sausage is thest wall in the room a product of binocular- (or some other distant ity, make it disappear object) and bring the by shutting one eye. tips of your two point- Warning: Do not eat this ing fingers together at sausage. M06_PINE1933_11_GE_C06.indd 156 22/01/2021 10:56  The Visual System 157 cells (see Baden et al., 2016). Notice that the amacrine The Retina and Translation cells and the horizontal cells are specialized for lateral ­c ommunication (communication across the major chan- of Light into Neural Signals nels of sensory input). Retinal neurons communicate both chemically via synapses and electrically via gap junctions After light passes through the pupil and the lens, it (see Pereda, 2014). reaches the retina. The retina converts light to neural sig- Also notice in Figure 6.5 that the retina is in a sense nals, conducts them toward the CNS, and participates inside-out: Light reaches the receptor layer only after in the processing of the signals (Hoon et al., 2014; Seung passing through the other layers. Then, once the recep- & Sümbül, 2014). tors have been activated, the neural message is trans- mitted back out through the retinal layers to the retinal Structure of the Retina ganglion cells, whose axons project across the outside of the retina before gathering together in a bundle and LO 6.3 Describe the structure of the retina and name exiting the eyeball. This inside-out arrangement creates the cell types that make up the retina. two visual problems: One is that the incoming light is Figure 6.5 illustrates the fundamental cellular structure distorted by the retinal tissue through which it must of the retina. The retina is composed of five different pass before reaching the receptors. The other is that types of neurons: receptors, horizontal cells, bipolar for the bundle of retinal ganglion cell axons to leave the cells, amacrine cells, and retinal ganglion cells. Each of eye, there must be a gap in the receptor layer; this gap is these five types of retinal neurons comes in a variety of called the blind spot. subtypes: More than 60 different kinds of retinal neurons The first of these two problems is minimized by the have been identified (see Cepko, 2015; Seung & Süm- fovea (see Figure 6.6). The fovea is an indentation, about bül, 2014), including about 30 different retinal ganglion 0.33 centimeter in diameter, at the center of the retina; Figure 6.5 The cellular structure of the mammalian retina. Retinal Amacrine Horizontal Cone Rod ganglion cells cells receptors receptors Bipolar cells cells Light Back of eyeball To optic nerve and blind spot M06_PINE1933_11_GE_C06.indd 157 22/01/2021 10:56  158 Chapter 6 and another straight bar leaving the other side, it fills in Figure 6.6 A section of the retina. The fovea is the i­ndentation at the center of the retina; it is specialized the missing bit for you; and what you see is a continuous for high-acuity vision. straight bar, regardless of what is actually there. The com- pletion phenomenon is one of the most compelling demon- Axons Cell bodies of retinal of retinal strations that the visual system does much more than make ganglion ganglion Back of a faithful copy of the external world. cells cells Receptors eyeball It is a mistake to think that completion is merely a response to blind spots. Indeed, completion plays an impor- tant role in normal vision (see Murray & Herrmann, 2013; Weil & Rees, 2011). When you look at an object, your visual system does not conduct an image of that object from your retina to your cortex. Instead, it extracts key information about the object—primarily information about its edges and their location—and conducts that information to the cortex, where a perception of the entire object is created from that partial information. For example, the color and brightness of large unpatterned surfaces are not perceived directly but are filled in (completed) by a completion pro- cess called surface interpolation (the process by which we perceive surfaces; the visual system extracts information about edges and from it infers the appearance of large sur- Light Fovea faces). The central role of surface interpolation in vision is an extremely important but counterintuitive concept. We suggest you read this paragraph again and think about it. Are your creative thinking skills developed enough to feel comfortable with this new way of thinking about your own visual system? Journal Prompt 6.2 Try to give a specific example of a situation where ­surface interpolation would occur. Cone and Rod Vision LO 6.4 Describe the duplexity theory of vision and explain the differences between the photopic and scotopic systems. Retina Ralph C. Eagle, Jr./Science Source You likely noticed in Figure 6.5 that there are two different types of receptors in the human retina: cone-shaped recep- it is the area of the retina that is specialized for high- tors called cones and rod-shaped receptors called rods (see acuity vision (for seeing fine details). The thinning of the Figure 6.7). The existence of these two types of receptors retinal ganglion cell layer at the fovea reduces the dis- puzzled researchers until 1866, when it was first noticed tortion of incoming light. The blind spot, the second of that species active only in the day tend to have cone- the two visual problems created by the inside-out struc- only retinas, and species active only at night tend to h­ ave ture of the retina, requires a more creative ­s olution— ­rod-only retinas. which is illustrated in the accompanying Check It Out From this observation emerged the duplexity ­theory demonstration. of vision—the theory that cones and rods mediate dif- In the Check It Out demonstration, you will experience ferent kinds of vision. Photopic vision (cone-mediated completion (or filling in). The visual system uses informa- vision) predominates in good lighting and provides tion provided by the receptors around the blind spot to fill high-acuity (finely detailed) colored perceptions of the in the gaps in your retinal images. When the visual system world. In dim illumination, there is not enough light detects a straight bar going into one side of the blind spot to reliably excite the cones, and the more sensitive M06_PINE1933_11_GE_C06.indd 158 22/01/2021 10:56  The Visual System 159 Check It Out Your Blind Spot and Completion First, prove to yourself that you do have areas of blindness that correspond to your retinal blind spots. Close your left eye and stare directly at the A below, trying as hard as you can to not shift your gaze. While keeping the gaze of your right eye fixed on the A, hold the text at different distances from you until the black dot to the right of the A becomes focused on your blind spot and disappears at about 13 centimeters (5 inches). A If each eye has a blind spot, why is there not a black hole in your perception of the world when you look at it with one eye? You will discover the answer by focusing on B with your right eye while holding the text at the same distance as before. Suddenly, the broken line to the right of B will become whole. Now focus on C at the same distance with your right eye. What do you see? B C scotopic vision (rod-mediated vision) predominates. Figure 6.7 Cones and rods. The red colored cells are cones; the blue colored cells are rods. How­ever, the s­ ensitivity of scotopic vision is not achieved without cost: Scotopic vision lacks both the detail and the color of photopic vision. The differences between photopic and scotopic vision result in part from a difference in the way the two systems are “wired.” As Figure 6.8 illustrates, there is a large dif- ference in convergence between the two systems. In the scotopic system, the output of several hundred rods con- verges on a single retinal ganglion cell, whereas in the photopic system, only a few cones converge on each reti- nal ganglion cell. As a result, when dim light stimulates many rods simultaneously, the outputs of this stimula- tion converge and summate (add) on the retinal ganglion cell. On the other hand, the effects of the same dim light applied to a sheet of cones cannot summate to the same degree, and the retinal ganglion cells may not respond at all to the light. The convergent scotopic system pays for its high degree of sensitivity with a low level of acuity. When a retinal ganglion cell that receives input from hundreds of rods changes its firing, the brain has no way of know- ing which portion of the rods contributed to the change. Although a more intense light is required to change the firing of a retinal ganglion cell that receives signals from cones, when such a retinal ganglion cell does react, there is less ambiguity about the location of the stimulus that triggered the reaction. Ralph C. Eagle, Jr./Science Source M06_PINE1933_11_GE_C06.indd 159 22/01/2021 10:56  160 Chapter 6 Cones and rods differ in their distri- Figure 6.8 Schematic representations of the convergence of cones or bution on the retina. As Figure 6.9 illus- rods on a retinal ganglion cell. There is a low degree of convergence in ­ one-fed pathways and a high degree of convergence in rod-fed pathways. c trates, there are no rods at all in the fovea, only cones. At the boundaries of the foveal Low Convergence in Cone-Fed Circuits indentation, the proportion of cones declines markedly, and there is an increase in the number of rods. The density of rods reaches a maximum at 20 degrees from the cen- Retinal Bipolar Cone ter of the fovea. Notice that there are more ganglion cell rods in the nasal hemiretina (the half of each cell retina next to the nose) than in the temporal hemiretina (the half of each retina next to the temples). High Convergence in Rod-Fed Circuits Spectral Sensitivity LO 6.5  xplain the difference between E the photopic and scotopic spectral sensitivity curves and explain how that difference can account for the Purkinje effect. Generally speaking, more intense lights appear brighter. However, wavelength also Retinal Bipolar Rod has a substantial effect on the perception ganglion cell cell of brightness. Because our visual systems are not equally sensitive to all wavelengths in the visible spectrum, lights of the same intensity but of different wavelengths can Figure 6.9 The distribution of cones and rods over the human retina. The figure illustrates the number of cones and rods per square millimeter as a differ markedly in brightness. A graph of function of distance from the center of the fovea. the relative brightness of lights of the same intensity presented at different wavelengths Left Right is called a spectral sensitivity curve. By far the most important thing to remember about spectral sensitivity curves 808 808 is that humans and other animals with both 608 608 cones and rods have two of them: a ­photopic 408 408 ­spectral sensitivity curve and a scotopic 208 208 08 ­spectral sensitivity curve. The photopic spec- tral sensitivity of humans can be determined by having subjects judge the relative bright- per Square Millimeter Number of Receptors 160,000 ness of different wavelengths of light shone on the fovea. Their scotopic spectral sensi- 120,000 tivity can be determined by asking subjects 80,000 to judge the relative brightness of different Rods wavelengths of light shone on the periphery 40,000 of the retina at an intensity too low to activate Cones the few peripheral cones located there. 0 808 608 408 208 08 208 408 608 808 The photopic and scotopic spectral Temporal hemiretina Nasal hemiretina ­sensitivity curves of human subjects are plot- ted in Figure 6.10. Under photopic conditions, Center of Fovea notice that the visual system is maximally sensitive to wavelengths of about 560 nano- Based on Lindsay, P. H., & Norman, D. A. (1977). Human Information Processing (2nd ed.). New York, NY: Academic Press. meters; thus, under photopic conditions, a M06_PINE1933_11_GE_C06.indd 160 22/01/2021 10:56  The Visual System 161 light at 500 nanometers would have Figure 6.10 Human photopic (cone) and scotopic (rod) spectral sensitivity to be much more intense than one at curves. The peak of each curve has been arbitrarily set at 100 percent. 560 nanometers to be seen as equally bright. In contrast, under scotopic conditions, the visual system is maxi- 100% mally sensitive to wavelengths of about 500 ­nanometers; thus, under scotopic Relative Spectral Sensitivity 80 conditions, a light of 560 nanometers Scotopic Photopic would have to be much more intense than one at 500 nanometers to be seen 60 as equally bright. Because of the difference in phot- 40 opic and scotopic spectral sensitiv- ity, an interesting visual effect can be observed during the transition from 20 photopic to scotopic vision. In 1825, Jan Purkinje described the following occurrence, which has become known 400 500 600 700 as the Purkinje effect (­pronounced “pur-KIN-jee”). One evening, just before Wavelength (nanometers) dusk, while ­Purkinje was walking in his garden, he noticed how his yellow and red flowers appeared brighter in rela- tion to his blue ones. What amazed him was that just a few task when most of them are crammed into the fovea? (See minutes later, as the sun went down, the relative brightness Figure 6.9.) Look around you. What you see is not a few of his flowers had somehow been reversed; the entire scene, colored details at the center of a grayish scene. You seem when viewed at night, appeared completely in shades of gray, to see an expansive, richly detailed, lavishly colored but most of the blue flowers appeared as brighter shades of world. How can such a perception be the product of a gray than the yellow and red ones. Can you explain this shift photopic system that, for the most part, is restricted to in relative brightness by referring to the photopic and scotopic a few degrees in the center of your visual field (the entire spectral sensitivity curves in Figure 6.10? area that you can see at a particular moment)? The Check It Out demonstration provides a clue. It shows that what Eye Movement we see is determined not just by what is projected on the retina at that instant. Although we are not aware of it, LO 6.6 Describe the three types of involuntary the eyes continually scan the visual field, and our visual fixational eye movements and explain what perception at any instant is a summation of recent visual happens when all eye movements are blocked. information. It is because of this temporal integration If cones are responsible for mediating high-acuity color vision that the world does not vanish momentarily each time under photopic conditions, how can they accomplish their we blink. Check It Out Periphery of Your Retina Does Not Mediate the Perception of Detail or Color Close your left eye, and with your right eye stare at the fixation detail and color at 20 degrees or more from the fixation point point (+) at a distance of about 13 centimeters (5 inches) from because there are so few cones there. Now look at the page the page. Be very careful that your gaze does not shift. You will again with your right eye, but this time without fixing your gaze. notice when your gaze is totally fixed that it is difficult to see Notice the difference that eye movement makes to your vision. W F D M E A 508 408 308 208 108 58 08 M06_PINE1933_11_GE_C06.indd 161 22/01/2021 10:56  162 Chapter 6 Our eyes continuously move even when we try to Figure 6.11 illustrates the relationship between the keep them still (i.e., fixated). Involuntary fixational eye ­absorption spectrum of rhodopsin and the human scotopic movements are of three kinds: tremor, drifts, and saccades spectral sensitivity curve. The fact that the two curves are (small jerky movements, or flicks; pronounced “sah- nearly identical leaves little doubt that, in dim light, our KAHDS”). Although we are normally unaware of fixational sensitivity to various wavelengths is a direct consequence eye movements, they have a critical visual function (see of rhodopsin’s ability to absorb them. Ibbotson & Krekelberg, 2011; Spering & Carrasco, 2015; Rhodopsin is a G-protein–coupled receptor that Zirnsak & Moore, 2014). When eye movements or their responds to light rather than to neurotransmitter mole­ main effect (movement of images on the retina) are blocked, cules (see Krishnan & Schiöth, 2015; Manglik & Kobilka, visual objects begin to fade and disappear. This happens 2014). Rhodopsin receptors, like other G-protein–coupled because most visual neurons respond only to changing receptors, initiate a cascade of intracellular chemical events images; if retinal images are artificially stabilized (kept when they are activated (see Figure 6.12). When rods are from moving on the retina), the images start to disappear in darkness, their sodium channels are partially open, and reappear. Thus, eye movements enable us to see during thus keeping the rods slightly depolarized and allowing a fixation by keeping the images moving on the retina. steady flow of excitatory glutamate neurotransmitter mole­ cules to emanate from them. However, when rhodopsin receptors are bleached by light, the resulting cascade of Visual Transduction: The Conversion intracellular chemical events closes the sodium channels, of Light to Neural Signals hyperpolarizes the rods, and reduces the release of gluta- mate (see Oesch, Kothmann, & Diamond, 2011). The trans- LO 6.7 Describe the process of visual transduction. duction of light by rods exemplifies an important point: Transduction is the conversion of one form of Signals are often transmitted through neural systems by energy to another. Visual transduction is the conver- decreases in activity. sion of light to neural signals by the visual receptors. A breakthrough in the study of visual transduction came in 1876 when a red pigment (a pigment is any substance Figure 6.11 The absorption spectrum of rhodopsin compared with the human that absorbs light) was extracted from scotopic spectral sensitivity curve. rods. This pigment had a curious prop- erty. When the pigment—which became known as rhodopsin—was exposed to The sensitivity of human The ability of rhodopsin to continuous intense light, it was bleached vision to different absorb different wavelengths wavelengths under scotopic of light under scotopic (lost its color) and lost its ability to conditions conditions absorb light, but when it was returned to the dark, it regained both its redness 100% Absorption Spectrum of Rhodopsin and its light-absorbing capacity. Scotopic Spectral Sensitivity It is now clear that rhodopsin’s absorption of light (and the accom- 80 panying bleaching) is the first step in rod-mediated vision. Evidence comes 60 from demonstrations that the degree to which rhodopsin absorbs light in vari- 40 ous situations predicts how humans see under the very same conditions. For example, it has been shown that 20 the degree to which rhodopsin absorbs lights of different wavelengths is related to the ability of humans and 400 500 600 700 other animals with rods to detect the Wavelength (nanometers) presence of different wavelengths of light under scotopic conditions. M06_PINE1933_11_GE_C06.indd 162 22/01/2021 10:56  The Visual System 163 conduct signals from each retina to Figure 6.12 The inhibitory response of rods to light. When light bleaches r­ hodopsin molecules, the rods’ sodium channels close; as a result, the rods the primary visual cortex (also known become hyperpolarized and release less glutamate. as striate cortex or V1) via the lateral geniculate nuclei of the thalamus. In the DARK In the LIGHT Retina-Geniculate- 1 Rhodopsin molecules are inactive. 1 Light bleaches rhodopsin molecules. Striate System LO 6.8 Describe the components 2 Sodium channels are kept open. 2 As a result, sodium channels close. and layout of the ­retina- geniculate-striate system. About 90 percent of axons of reti- nal ganglion cells become part of the 3 Sodium ions flow into the rods, partially 3 Sodium ions cannot enter rods, and, as a ­retina-geniculate-striate pathways (see Tong, 2003). No other sensory system depolarizing them. result, the rods become has such a predominant pair (left and hyperpolarized. right) of pathways to the cortex. The organization of these visual pathways is illustrated in Figure 6.13. Examine it carefully. The main idea to take away from Figure 6.13 is that all signals from the left visual field reach the right primary visual cortex, either ipsilat- erally from the temporal hemiretina of the right eye or contralaterally (via the optic chiasm) from the nasal hemiretina of the left eye—and that the opposite is true of all signals from the right visual field. Each lat- eral geniculate nucleus has six layers, and each layer receives input from all parts of the contralateral visual field of one eye. In other words, each lateral geniculate nucleus receives visual input only from the contralat- eral visual field; three layers receive 4 Glutamate release input from one eye, and three receive 4 Rods continuously release glutamate. is reduced. input from the other. Most of the lat- eral geniculate neurons that project to the primary visual cortex terminate in the lower part of cortical layer IV (see Muckli & Petro, 2013), producing a characteristic stripe, or striation, when viewed in cross From Retina to Primary section—hence, primary visual cortex is often referred to Visual Cortex as striate cortex. Note: Figure 6.13 depicts only the axo- nal ­projections from the lateral geniculate nuclei to the Many pathways in the brain carry visual information. By primary visual cortex, but there are just as many pro- far the largest and most thoroughly studied visual path- jections from the primary visual cortex to the lateral ways are the retina-geniculate-striate pathways, which ­geniculate nuclei. M06_PINE1933_11_GE_C06.indd 163 22/01/2021 10:56  164 Chapter 6 A dramatic demonstration of the Figure 6.13 The retina-geniculate-striate system: the neural projections from the retinas through the lateral geniculate nuclei to the left and right primary visual retinotopic organization of the primary cortex (striate cortex). The colors indicate the flow of information from various visual cortex was provided by Dobelle, parts of the visual fields of each eye to various parts of the visual system. Mladejovsky, and Girvin (1974). They implanted an array of electrodes in the primary visual cortex of patients who were blind because of damage to their eyes. If electrical current was adminis- tered simultaneously through an array of electrodes forming a shape, such as a cross, on the surface of a patient’s cortex, the patient reported “seeing” a glowing image of that shape. This finding and recent research on retinal implants (see Temporal Roska & Sahel, 2018; Wood, 2018) could hemiretina be the basis for the development of visual Nasal prostheses that could benefit many blind hemiretinas Optic nerve people (see Shepherd et al., 2013). Temporal hemiretina Optic chiasm Journal Prompt 6.3 Optic tract How does the ­prosthesis developed by Dobelle et al. (1974) demonstrate the retinotopic Lateral ­organization of the primary visual geniculate cortex? nuclei The M and P Channels LO 6.10 Describe the M and P channels. Not apparent in Figure 6.13 is the fact that at least two parallel channels of Primary visual cortex communication flow through each lat- eral geniculate nucleus. One channel runs through the top four layers. These layers Based on Netter, F. H. (1962). The CIBA Collection of Medical Illustrations. Vol. 1, The Nervous System. are called the parvocellular layers (or New York, NY: CIBA. P layers) because they are composed of neurons with small cell bodies (parvo means “small”). The Retinotopic Organization other channel runs through the bottom two layers, which are called the magnocellular layers (or M layers) because LO 6.9 In the context of the retina-geniculate-striate they are composed of neurons with large cell bodies (magno system, explain what is meant by retinotopic. means “large”). The retina-geniculate-striate system is retinotopic; each The parvocellular neurons are particularly respon- level of the system is organized like a map of the retina. sive to color, fine pattern details, and stationary or slowly This means two stimuli presented to adjacent areas of the moving objects. In contrast, the magnocellular neurons are retina excite adjacent neurons at all levels of the system particularly responsive to movement. Cones provide the (see Kremkow et al., 2016). The retinotopic layout of the majority of the input to the P layers, whereas rods provide primary visual cortex has a disproportionately large rep- the majority of the input to the M layers. resentation of the fovea; although the fovea is only a small The parvocellular and magnocellular neurons project part of the retina, a relatively large proportion of the pri- to different areas in the lower part of layer IV of the striate mary visual cortex (about 25 percent) is dedicated to the cortex. In turn, these M and P areas of lower layer IV project analysis of its input. to different areas of visual cortex. M06_PINE1933_11_GE_C06.indd 164 22/01/2021 10:56  The Visual System 165 Scan Your Brain This is a good place to pause and scan your brain to check 5. The difference in the position of the same image on the your knowledge on the basics of the visual process before two retinas is called _______. you move on to studying how we perceive edges and color. 6. Nasal hemiretina of the right visual field project to the Fill in the following blanks with the most appropriate terms. _______ hemisphere, while temporal hemiretina project to The correct answers are provided at the end of the exercise. the _______ hemisphere. Before proceeding, review material related to your errors and 7. The location on the retina where a bundle of cell axons omissions. leaves the eye is called the _______. 8. The theory that the two types of retinal receptors, rods 1. Light reflected from objects enters the eye though the and cones, mediate different kinds of vision is called the _______. _______ theory. 2. Depending on how close or far away an object is, the 9. There are two _______ channels that run through the lens is adjusted using the _______ muscles. ­lateral geniculate nucleus called M and P channels. 3. About 25 percent of the primary visual cortex is ­dedicated to analyzing input from the _______. (7) blind spot, (8) duplexity, (9) parallel. 4. _______ is the process by which the lenses adjust their (4) Accommodation, (5) binocular disparity, (6) ipsilateral, contralateral, shape to bring images to focus on the retina. Scan Your Brain answers: (1) pupil, (2) ciliary, (3) fovea, Mach bands; they enhance the contrast at each edge and make the edge easier to see. Seeing Edges It is important to appreciate that contrast enhancement Edge perception (seeing edges) does not sound like a par- is not something that occurs just in books. Although we are ticularly important topic, but it is. Edges are the most infor- normally unaware of it, every edge we look at is highlighted mative features of any visual display because they define for us by the contrast-enhancing mechanisms of our ner- the extent and position of the various objects in it. Given vous systems. In effect, our perception of edges is better the importance of perceiving visual edges and the unrelent- than the real thing (as determined by measurements of the ing pressure of natural selection, it is not surprising that physical properties of the light entering our eyes). the visual systems of many species are particularly good at edge perception. Before considering the visual mechanisms underlying Figure 6.14 The illusory bands visible in this figure are often called Mach bands, although Mach used a different edge perception, it is important to appreciate exactly what ­figure to generate them in his studies (see Eagleman, 2001). a visual edge is. In a sense, a visual edge is nothing: It is merely the place where two different areas of a visual image meet. Accordingly, the perception of an edge is really the per- ception of a contrast between two adjacent areas of the visual field. This module reviews the perception of edges (the perception of contrast) between areas that differ from one another in brightness (i.e., that show brightness contrast). Contrast Enhancement Mach Intensity LO 6.11 Describe contrast enhancement. bands Carefully examine the stripes in Figure 6.14. The inten- What is there sity graph in the figure indicates what is there—a series of homogeneous stripes of different intensity. But this is not exactly what you see, is it? What you see is indicated in the Mach bands Brightness brightness graph. Adjacent to each edge, the brighter stripe looks brighter than it really is and the darker stripe looks darker than it really is. The nonexistent stripes of bright- ness and darkness running adjacent to the edges are called What you see M06_PINE1933_11_GE_C06.indd 165 22/01/2021 10:56  166 Chapter 6 Receptive Fields of Visual Neurons: When Hubel and Wiesel compared the receptive fields recorded from retinal ganglion cells, lateral geniculate Hubel & Wiesel nuclei, and lower layer IV striate neurons, four commonal- LO 6.12 Define the term receptive field and describe ties were readily apparent: the methods used by David Hubel and Torsten At each level, the receptive fields in the foveal area of Wiesel to map the receptive fields of visual the retina were smaller than those at the periphery; this system neurons. is consistent with the fact that the fovea mediates fine- The Nobel Prize–winning research of David Hubel and grained (high-acuity) vision. ­Torsten Wiesel (see Hubel & Wiesel, 2004) is the fitting focus All the neurons (retinal ganglion cells, lateral genicu- of this discussion of seeing edges. Their research revealed late neurons, and lower layer IV neurons) had receptive much about the neural mechanisms of vision, and their fields that were circular. method has been adopted by subsequent generations of All the neurons were monocular; that is, each neuron sensory neurophysiologists. had a receptive field in one eye but not the other. Hubel and Wiesel’s influential method is a technique Many neurons at each of the three levels of the retina- for studying single neurons in the visual systems of labora- geniculate-striate system had receptive fields that tory animals—their research subjects were cats and mon- comprised an excitatory area and an inhibitory area keys. First, the tip of a microelectrode is positioned near a separated by a circular boundary. single neuron in the part of the visual system under investi- gation. During testing, eye movements are blocked by para- Let us explain this last point—it is important. When Hubel lyzing the eye muscles, and the images on a screen in front and Wiesel shone a spot of achromatic light onto the vari- of the subject are focused sharply on the retina by an adjust- ous parts of the receptive fields of a neuron in the retina- able lens. The next step in the procedure is to identify the geniculate-striate pathway, they discovered two different receptive field of the neuron. The receptive field of a visual responses. The neuron responded with either “on” firing neuron is the area of the visual field within which it is pos- or “off” firing, depending on the location of the spot of sible for a visual stimulus to influence the firing of that neu- light in the receptive field. That is, the neuron either dis- ron. The final step in the method is to record the responses played a burst of firing when the light was turned on (“on” of the neuron to various simple stimuli within its recep- ­firing), or it displayed an inhibition of firing when the light tive field in order to characterize the types of stimuli that was turned on and a burst of firing when it was turned off most influence its activity. Then the electrode is advanced (“off” firing). slightly, and the entire process of identifying and character- For most of the retinal ganglion cells, lateral geniculate izing the receptive field properties is repeated for another nuclei, and lower layer IV striate neurons, the reaction— neuron, and then for another, and another, and so on. The “on” firing or “off” firing—to a light in a particular part general strategy is to begin by studying neurons near the of the receptive field was quite predictable. It depended receptors and gradually work up through “higher” and on whether they were on-center cells or off-center cells, as “higher” levels of the system in an effort to understand the ­illustrated in Figure 6.15. increasing complexity of the neural responses at each level. On-center cells respond to lights shone in the central region of their receptive fields with “on” firing and to lights shone in the periphery of their receptive fields with inhibi- Receptive Fields of the tion, followed by “off” firing when the light is turned off. Retina-Geniculate-Striate System: Off-center cells display the opposite pattern: They respond Hubel & Wiesel with inhibition and “off” firing in response to lights in the center of their receptive fields and with “on” firing to lights LO 6.13 Describe the work of Hubel & Wiesel that in the periphery of their receptive fields. helped to characterize the receptive fields In effect, on-center and off-center cells respond best to of retinal ganglion cells, lateral geniculate contrast. Figure 6.16 illustrates this point. The most effec- neurons, and striate neurons of lower layer IV. tive way to influence the firing rate of an on-center or off- Hubel and Wiesel (1979) began their studies of visual sys- center cell is to maximize the contrast between the center tem neurons by recording from the three levels of the retina- and the periphery of its receptive field by illuminating geniculate-striate system: first from retinal ganglion cells, either the entire center or the entire surround (periphery) then from lateral geniculate neurons, and finally from the while leaving the other region completely dark. Diffusely striate neurons of lower layer IV. They tested the neurons illuminating the entire receptive field has little effect on fir- with stationary spots of achromatic (uncolored) light shone ing. Hubel and Wiesel thus concluded that one function of on the retina. They found little change in the receptive fields many of the neurons in the retina-geniculate-striate system as they worked through the levels. is to respond to the degree of brightness contrast between M06_PINE1933_11_GE_C06.indd 166 22/01/2021 10:56  The Visual System 167 the two areas of their receptive fields (see Figure 6.15 The receptive fields of an on-center cell and an off-center cell. ­Livingstone