Chapter 2: The Beginning of the Perceptual Process PDF

CHAPTER 2 The Beginning of the Perceptual Process CHAPTER CONTENTS Starting at the Beginning Adapting to the Dark...

CHAPTER 2 The Beginning of the Perceptual Process CHAPTER CONTENTS Starting at the Beginning Adapting to the Dark Convergence Causes the Rods to Be Spectral Sensitivity More Sensitive Than the Cones Light, the Eye, and the Visual Receptors Electrical Signals in Neurons Lack of Convergence Causes the Cones to Have Better Acuity Than the Rods Light: The Stimulus for Vision Recording Electrical Signals in The Eye Neurons SOMETHING TO CONSIDER: Early Events Basic Properties of Action Potentials Are Powerful Focusing Light Onto the Receptors Chemical Basis of Action Potentials DEVELOPMENTAL DIMENSION: Infant Receptors and Perception Transmitting Information Across a Gap Visual Acuity Transforming Light Energy Into Electrical Energy Neural Convergence and THINK ABOUT IT Perception Some Questions We Will Consider: in the air enters his ears. In both cases, stimuli trigger a process that ends up with perception occurring as a result of activity How does the focusing system at the front of our eye affect our in the brain. Similar events occur for feeling the texture of the perception? (p. 25) tree’s bark, smelling its blossoms, and tasting its fruit. By the How do chemicals in the eye called visual pigments affect our time you "nish this book, you will see that although there are perception? (p. 30) numerous differences between the senses, they all operate ac- How can the way neurons are “wired up” affect perception? cording to similar principles. (p.!39) H ow does a tree become a perception of a tree? One way to answer this question is to refer back to the percep- tual process shown in Figure 1.1: Information about the tree (distal stimulus) is carried in light re!ected from the tree and into the eye. When this light reaches the receptors in the Starting at the Beginning The idea that perception starts at the beginning of the percep- tual process may sound obvious. But as we will see, there is enough going on right at the beginning of the perceptual pro- retina, creating the proximal stimulus, it becomes transformed cess to "ll a whole chapter and more, and most of what goes on into electrical signals that contain information about the tree, can affect perception. So, the "rst step in understanding per- which are transmitted to the brain, where eventually these elec- ception is to take a close look at the processes that begin, in the trical signals become transformed into a perception of the tree. case of vision, with light re!ected from an object into the eye. In this chapter we will focus on the beginning of the per- Figure 2.1 shows the "rst four steps of the visual process, ceptual process. Although we will use visual examples to de- which starts on the right and moves to the left to match the scribe the initial processes of the perceptual process, many of perceptual process in Figure 1.1. Following the sequence of the principles we will be describing hold for the other senses the physical events in the process, shown in black along the as well. Just as the person from Chapter 1 sees the tree because bottom of the "gure, we begin with Step 1, the distal stimulus light is re!ected from it into his eyes, he hears the rustle of its (the tree); then move to Step 2, in which light is re!ected from branches because sound energy in the form of pressure changes the tree and enters the eye to create the proximal stimulus on 21 Copyright 2017 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.  Rod Cone Seeing fine Seeing in Seeing in details dim light focus STEP 4 STEP 3 STEP 2 STEP 1 Neural processing: Receptor processes: Light is reflected Distal stimulus: Signals travel in a Receptors transform and focused to The tree network of neurons. light into electricity. create an image of the tree on the retina. Figure 2.1 Chapter preview. This chapter will describe the !rst three steps of the perceptual process for vision and will introduce Step 4. Physical processes are indicated in black; the perceptual outcomes of these processes are indicated in blue. the visual receptors; then to Step 3, in which receptors trans- Visible light, the energy within the electromagnetic form light into electrical signals; and "nally to Step 4, in which spectrum that humans can perceive, has wavelengths rang- electrical signals are “processed” as they travel through a net- ing from about 400 to 700 nanometers (nm), where 1 nano- work of neurons. Our goal in this chapter is to show how these meter 5 1029 meters, which means that the longest visible physical events in!uence the following aspects of perception, wavelengths are slightly less than one-thousandth of a shown in blue in Figure 2.1: (1) seeing in focus, (2) seeing in millimeter long. For humans and some other animals, the dim light, and (3) seeing "ne details. We begin by describing wavelength of visible light is associated with the different light, the eye, and the receptors in the retina that line the back colors of the spectrum, with short wavelengths appearing of the eye. blue, middle wavelengths green, and long wavelengths yel- low, orange, and red. Light, the Eye, and the The Eye Visual Receptors The eyes contain the receptors for vision. The "rst eyes, which appeared back in the Cambrian period (570–500 million The ability to see a tree, or any other object, depends on light years ago), were eyespots on primitive animals such as !at- being re!ected from that object into the#eye. worms that could distinguish light from dark but couldn’t detect features of the environment. Detecting an object’s details didn’t become possible until more sophisticated eyes Light: The Stimulus for Vision evolved to include optical systems that could produce im- Vision is based on visible light, which is a band of energy within ages and therefore provide information about shapes and the electromagnetic spectrum. The electromagnetic spectrum details of objects and the arrangement of objects within scenes is a continuum of electromagnetic energy that is produced (Fernald, 2006). by electric charges and is radiated as waves (see Figure# 1.21, Light re!ected from objects in the environment enters the page# 18). The energy in this spectrum can be described by eye through the pupil and is focused by the cornea and lens its wavelength—the distance between the peaks of the elec- to form sharp images of the objects on the retina, the network tromagnetic waves. The wavelengths in the electromagnetic of neurons that covers the back of the eye and that contains spectrum range from extremely short-wavelength gamma the receptors for vision (Figure 2.2a). There are two types of rays (wavelength 5 about 10212 meters, or one ten-billionth visual receptors, rods and cones, so called because of the rod- of a meter) to long-wavelength radio waves (wavelength 5 and cone-shaped outer segments (Figure 2.3). The outer seg- about 104 meters, or 10,000 meters). ments are the part of the receptor that contains light-sensitive 22 CHAPTER 2 The Beginning of the Perceptual Process Copyright 2017 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.  Receptor cells (rods and cones) Optic nerve fibers Back of eye Light Rod Pupil Cone Fovea (point of central focus) Retina Cornea Optic nerve Pigment Lens epithelium Retina (a) (b) Figure 2.2 An image of the tree is focused on the retina, which lines the back of the eye. The close-up of the retina on the right shows the receptors and other neurons that make up the retina. chemicals called visual pigments that react to light and trigger The fovea is so small (about the size of this “o”) that it electrical signals. Signals from the receptors !ow through the contains only about 1 percent, or 50,000, of the 6 mil- network of neurons that make up the retina (Figure 2.2b) and lion cones in the retina (Tyler, 1997a, 1997b). emerge from the back of the eye in the optic nerve, which con- 3. The peripheral retina contains many more rods than tains a million optic nerve "bers that conduct signals toward cones because there are about 120 million rods and only the brain. 6 million cones in the retina. The rod and cone receptors not only have different shapes, One way to appreciate the fact that the rods and cones they are also distributed differently across the retina. From are distributed differently in the retina is by considering Figure 2.4, which indicates the rod and cone distributions, we what happens when functioning receptors are missing from can conclude the following: one area of the retina. A condition called macular degenera- 1. One small area, the fovea, contains only cones. When we tion, which is most common in older people, destroys the look directly at an object, the object’s image falls on the cone-rich fovea and a small area that surrounds it. (Macula fovea. is a term usually associated with medical practice that in- 2. The peripheral retina, which includes all of the retina cludes the fovea plus a small area surrounding the fovea.) outside of the fovea, contains both rods and cones. It This creates a blind region in central vision, so when a per- is important to note that although the fovea has only son looks directly at something, he or she loses sight of it cones, there are also many cones in the peripheral retina. (Figure 2.5a). Rod Figure 2.3 (a) Scanning electromicrograph of the rod and cone receptors in the retina, showing the rod-shaped Cone and cone-shaped receptor outer segments. (b) Rod and Outer cone receptors, showing the inner and outer segments. segment The outer"segments contain the light-sensitive visual pigment. (From Lewis et al., 1969) Inner segment Rod Cone (a) (b) Light, the Eye, and the Visual Receptors 23 Copyright 2017 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.  Blind spot Cones Fovea (no receptors) Rods 180,000 160,000 140,000 per square millimeter Number of receptors 808 808 120,000 608 608 100,000 Blind 80,000 408 spot 408 60,000 208 208 08 40,000 20,000 Optic nerve 0 Fovea 708 608 508 408 308 208 108 08 108 208 308 408 508 608 708 808 Angle (degree) Figure 2.4 The distribution of rods and cones in the retina. The eye on the left indicates locations in degrees relative to the fovea. These locations are repeated along the bottom of the chart on the right. The vertical brown bar near 20 degrees indicates the place on the retina where there are no receptors because this is where the ganglion cells leave the eye to form the optic nerve. (Adapted from Lindsay & Norman, 1977) © Bruce Goldstein (a) (b) Figure 2.5 (a) In a condition called macular degeneration, the fovea and surrounding area degenerate, so the person cannot see whatever he or she is looking at. (b) In retinitis pigmentosa, the peripheral retina initially degenerates and causes loss of vision in the periphery. The resulting condition is sometimes called “tunnel vision.” Another condition, called retinitis pigmentosa, is a of the blind spot, you can become aware of it by doing the degeneration of the retina that is passed from one genera- following demonstration. tion to the next (although not always affecting everyone in a family). This condition first attacks the peripheral rod DEMONSTRATION Becoming Aware of the Blind Spot receptors and results in poor vision in the peripheral vi- Place the book (or your electronic device if you are reading the sual field (Figure 2.5b). Eventually, in severe cases, the fo- ebook) on your desk. Close your right eye, and position yourself veal cone receptors are also attacked, resulting in complete above the book/device so that the cross in Figure 2.7 is aligned blindness. with your left eye. Be sure the book page is "at and, while look- Before leaving the rod–cone distribution shown in ing at the cross, slowly move closer. As you move closer, be Figure# 2.4, note that there is one area in the retina, indi- sure not to move your eye from the cross, but at the same time cated by the vertical brown bar, where there are no receptors. keep noticing the circle off to the side. At some point, around Figure! 2.6 shows a close-up of the place where this occurs, 3!to 9 inches from the book/device, the circle should disappear. which is where the nerve "bers that make up the optic nerve When this happens, the image of the circle is falling on your leave the eye. Because of the absence of receptors, this place is blind spot. called the blind spot. Although you are not normally aware 24 CHAPTER 2 The Beginning of the Perceptual Process Copyright 2017 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.  Receptors Blind spot Optic nerve Figure 2.8 View the pattern as described in the text, and observe what happens when the center of the wheel falls on your blind spot. (Adapted from Ramachandran, 1992) Figure 2.6 There are no receptors at the place where the optic nerve leaves the eye. This enables the receptor’s ganglion cell !bers to #ow into the optic nerve. The absence of receptors in this area creates the blind spot. Focusing Light Onto the!Receptors Figure 2.7 Blind spot demonstration. Light re!ected from an object into the eye is focused onto the# retina by a two-element optical system: the cornea and Why aren’t we usually aware of the blind spot? One reason the lens. The cornea, the transparent covering of the front of is that the blind spot is located off to the side of our visual the eye, accounts for about 80 percent of the eye’s focusing "eld, where objects are not in sharp focus. Because of this and power, but like the lenses in eyeglasses, it is "xed in place so it because we don’t know exactly where to look for it (as opposed can’t adjust its focus. The lens, which supplies the remaining to the demonstration, in which we are focusing our attention 20#percent of the eye’s focusing power, can change its shape to on the circle), the blind spot is hard to detect. adjust the eye’s focus for objects located at different distances. But the most important reason that we don’t see the blind This change in shape is achieved by the action of ciliary muscles, spot is that some mechanism in the brain “"lls in” the place which increase the focusing power of the lens (its ability to where the image disappears (Churchland & Ramachandran, bend light) by increasing its curvature (compare Figure 2.9b 1996). The next demonstration illustrates an important prop- and Figure 2.9c). erty of this "lling-in process. We can understand why the eye needs to adjust its focus by "rst considering what happens when the eye is relaxed and DEMONSTRATION Filling In the Blind Spot a person with normal (20/20) vision views a small object that is far away. If the object is located more than about 20 feet Close your right eye and, with the cross in Figure 2.8 lined up away, the light rays that reach the eye are essentially parallel with your left eye, move toward the “wheel”. When the center of (Figure!2.9a), and the cornea–lens combination brings these the wheel falls on your blind spot, notice how the spokes of the parallel rays to a focus on the retina at point A. But if the ob- wheel #ll in the hole (Ramachandran, 1992). ject moves closer to the eye, the light rays re!ected from this object enter the eye at more of an angle, and this pushes the These demonstrations show that the brain does not "ll in focus point back so if the back of the eye weren’t there, light the area served by the blind spot with “nothing”; rather, it cre- would be focused at point B (Figure 2.9b). Because the light ates a perception that matches the surrounding pattern—the is stopped by the back of the eye before it reaches point B, the white page in the "rst demonstration, and the spokes of the image on the retina is out of focus. If things remained in this wheel in the second one. This “"lling in” is a preview of one of state, the person would see the object as blurred. the themes of the book: how the brain creates a coherent per- The adjustable lens, which controls a process called ception of our world. For now, however, we return to the begin- accommodation, comes to the rescue to help prevent blurring. ning of the perceptual process, as light re!ected from objects Accommodation is the change in the lens’s shape that oc- in the environment is focused onto the receptors. curs when the ciliary muscles at the front of the eye tighten Focusing Light Onto the!Receptors 25 Copyright 2017 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. and increase the curvature of the lens so that it gets thicker an object that is at least 20 feet away. As you stay focused on (Figure!2.9c). This increased curvature increases the bending the faraway object, notice the pencil point without actually look- of the light rays passing through the lens so the focus point ing at it (be sure to stay focused on the far object). The point will is pulled back to A to create a sharp image on the retina. This probably appear slightly blurred. means that as you look around at different objects, your eye Then slowly move the pencil toward you while still looking is constantly adjusting its focus by accommodating, especially at the far object. Notice that as the pencil moves closer, the point for nearby objects. The following demonstration shows that becomes more blurred. When the pencil is about 12 inches away, this is necessary because everything is not in focus at once. shift your focus to the pencil point. This shift in focus causes the pencil point to appear sharp, but the far object is now out of focus. DEMONSTRATION Becoming Aware of What Is in Focus Accommodation occurs unconsciously, so you are usually un- When you changed focus from far away to the nearby pencil aware that the lens is constantly changing its focusing power point during this demonstration, you were changing your ac- to let you see clearly at different distances. This unconscious commodation. Either near objects or far objects can be in focus, focusing process works so ef#ciently that most people assume but not both at the same time. Accommodation, therefore, makes that everything, near and far, is always in focus. You can demon- it possible to adjust vision for different distances. However, as strate that this is not so by holding a pen or a pencil, point up, people get older, their ability to accommodate decreases due to at arm’s length, closing one eye, and looking past the pencil at hardening of the lens and weakening of the ciliary muscles, and Lens Retina Cornea A (a) Object far— Focus on retina (d) Myopia— Focus in front of retina eye relaxed eye relaxed Moving object closer pushes focus point back B (b) Object near— Focus behind retina eye relaxed Accommodation brings focus Corrective lens point forward (e) Correction of myopia A (c) Object near— Focus on retina accommodation Figure 2.9 Focusing of light rays by the eye. (a) Rays of light coming from a small light source that is more than 20 feet away are approximately parallel. The focus point for parallel light is at A on the retina. (b) Moving an object closer to the relaxed eye pushes the focus point back. Here the focus point is at B, but light is stopped by the back of the eye, so the image on the retina is out of focus. (c) Accommodation of the eye (indicated by the fatter lens) increases the focusing power of the lens and brings the focus point for a near object back to A on the retina, so it is in focus. This accommodation is caused by the action of the ciliary muscles, which are not shown. (d) In the myopic (nearsighted) eye, parallel rays from a distant spot of light are brought to a focus in front of the retina, so distant objects appear blurred. (e) A corrective lens bends light so it is focused on the retina. 26 CHAPTER 2 The Beginning of the Perceptual Process Copyright 2017 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. so they become unable to accommodate enough to see objects, electrical signals that eventually signal the properties of the or read, at close range. This loss of the ability to accommodate, distal stimulus to the brain. But the importance of these visual called presbyopia (for “old eye”), can be dealt with by wearing pigments extends beyond triggering electrical signals. Visual reading glasses, which brings near objects into focus by replacing pigments also shape our perceptions by determining our abil- the focusing power that can no longer be provided by the lens. ity to see dim lights and our ability to see light in different Another problem that can be solved by a corrective lens is parts of the visual spectrum. In this section, we "rst describe myopia, or nearsightedness, an inability to see distant objects transduction, and then how the receptors shape perception. clearly. The reason for this dif"culty, which affects more than 70 million Americans, is illustrated in Figure 2.9d. Myopia oc- Transforming Light Energy Into curs when the optical system brings parallel rays of light into Electrical!Energy focus at a point in front of the retina, so the image that reaches the retina is blurred. This problem can be caused by either of Transduction is the transformation of one form of energy into two factors: (1) refractive myopia, in which the cornea and/ another form of energy (see Chapter 1, page 7). Visual trans- or the lens bends the light too much, or (2) axial myopia, in duction occurs in the rod and cone receptors, which transform which the eyeball is too long. Either way, images of faraway light into electricity. The starting point for understanding how objects are not focused sharply, so objects look blurred. Cor- the rods and cones create electricity are the millions of mol- rective lenses can solve this problem, as shown in Figure 2.9e. ecules of a light-sensitive visual pigment that are contained in Finally, people with hyperopia, or farsightedness, can see the outer segments of the receptors (Figure 2.3). Visual pig- distant objects clearly but have trouble seeing nearby objects ments have two parts: a long protein called opsin and a much because the focus point for parallel rays of light is located be- smaller light-sensitive component called retinal. Figure 2.10a hind the retina, usually because the eyeball is too short. Young shows a model of a retinal molecule attached to opsin (Wald, people can bring the image forward onto the retina by accom- 1968). Note that only a small part of the opsin is shown here; it modating. However, older people, who have dif"culty accom- is actually hundreds of times longer than the retinal. modating, often use corrective lenses that bring the focus Despite its small size compared to the opsin, retinal is the point forward onto the retina. crucial part of the visual pigment molecule, because when the ret- Focusing an image clearly onto the retina is the initial step inal and opsin are combined, the resulting molecule absorbs vis- in the process of vision, but although a sharp image on the ret- ible light. When the retinal part of the visual pigment molecule ina is essential for clear vision, we do not see the image on the absorbs light, the retinal changes its shape, from being bent, as retina. Vision occurs not in the retina but in the brain. Before shown in Figure 2.10a, to straight, as shown in Figure 2.10b. This the brain can create vision, the light on the retina must activate change of shape, called isomerization, creates a chemical chain the visual receptors in the retina. reaction, illustrated in Figure 2.11, that activates thousands of charged molecules to create electrical signals in receptors. What is important about the chain reaction that follows Receptors and Perception isomerization is that it ampli"es the effect of isomerization. Isomerizing one visual pigment molecule triggers a chain of Light entering visual receptors triggers electrical signals when chemical reactions that releases as many as a million charged the light is absorbed by light-sensitive visual pigment molecules molecules, which leads to activation of the receptor (Baylor, in the receptors. This step is crucial for vision because it creates 1992; Hamer et al., 2005). Molecule in dark Retinal isomerized by light Retinal © Bruce Goldstein Opsin (a) (b) Figure 2.10 Model of a visual pigment molecule. The horizontal part of the model shows a tiny portion of the huge opsin molecule near where the retinal is attached. The smaller molecule on top of the opsin is the light-sensitive retinal. (a) The retinal molecule’s shape before it absorbs light. (b) The retinal molecule’s shape after it absorbs light. This change in shape, which is called isomerization, triggers a sequence of reactions that culminates in generation of an electrical response in the receptor. Receptors and Perception 27 Copyright 2017 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.  METHOD Measuring the Dark Adaptation Curve The #rst step in measuring a dark adaption curve is to have the subject look at a small #xation point while paying attention to a "ashing test light that is off to the side (Figure 2.12). Because the subject is looking directly at the #xation point, its image falls on the fovea, so the image of the test light falls on the pe- ripheral retina, which contains both rods and cones. While still in the light, the subject turns a knob that adjusts the intensity of One visual pigment molecule the "ashing light until it can just barely be seen. This threshold for seeing the light, the minimum amount of energy necessary to just barely see the light, is then converted to sensitivity. Be- cause sensitivity 5 1/threshold, this means that a high threshold corresponds to low sensitivity. The sensitivity measured in the light is called the light-adapted sensitivity, because it is mea- sured while the eyes are adapted to the light. Because the room (or adapting) lights are on, the intensity of the "ashing test light has to be high to be seen. At the beginning of the experiment, then, the threshold is high and the sensitivity is low. Figure 2.11 This sequence symbolizes the chain reaction that is triggered when a single visual pigment molecule is isomerized Once the light-adapted sensitivity to the "ashing test light by absorption of a single photon of light. In the actual sequence is determined, the adapting light is extinguished so the subject of events, each visual pigment molecule activates hundreds is in the dark. The subject continues adjusting the intensity of more molecules, which, in turn, each activate about a thousand the "ashing light so he or she can just barely see it, tracking the molecules. Isomerization of just one visual pigment molecule increase in sensitivity that occurs in the dark. As the subject be- activates about a million other molecules, which activates the comes more sensitive to the light, he or she must decrease the receptor. light’s intensity to keep it just barely visible. The result, shown as the red curve in Figure 2.13, is a dark adaptation curve. Visual pigments not only create electrical signals in the receptors, they also shape speci"c aspects of our perceptions. The dark adaptation curve shows that as adaptation pro- Next, we will demonstrate how properties of the pigments in- ceeds, the subject becomes more sensitive to the light. Note that !uence perception. We do this by comparing the perceptions higher sensitivity is at the bottom of this graph, so movement of caused by the rod and cone receptors. As we will see, the the dark adaptation curve downward means that the subject’s visual pigments in these two types of receptors in!uence two sensitivity is increasing. The red dark adaptation curve indicates aspects of visual perception: (1) how we adjust to darkness, and that the subject’s sensitivity increases in two phases. It increases (2) how well we see light in different parts of the spectrum. rapidly for the "rst 3 to 4 minutes after the light is extinguished and then levels off. At about 7 to 10 minutes, it begins increasing again and continues to do so until the subject has been in the Adapting to the Dark dark for about 20 or 30 minutes (Figure 2.13). The sensitivity at the end of dark adaptation, labeled dark-adapted sensitivity, is When we discussed measuring perception in Chapter 1, we about 100,000 times greater than the light-adapted sensitivity noted that when a person goes from a lighted environment to a measured before dark adaptation began. dark place, it may be dif"cult to see at "rst, but that after some Dark adaptation was involved in a 2007 episode of the Myth- time in the dark, the person becomes able to make out lights busters program on the Discovery Channel, which was devoted and objects that were invisible before (Figure 1.16, page #15). This process of increasing sensitivity in the dark, called dark adaptation, is measured by determining a dark adaptation Peripheral retina Fixation point curve. In this section we will show how the rod and cone receptors control an important aspect of vision: the ability of the visual system to adjust to dim levels of illumination. We will describe how the dark adaptation curve is measured, and how the increase in sensitivity that occurs in the dark has been linked to properties of the rod and cone visual pigments. Fovea Test light Measuring the Dark Adaptation Curve The study of dark adaptation begins with measuring the dark adaptation Figure 2.12 Viewing conditions for a dark adaptation experiment. curve, which is the function relating sensitivity to light to time In this example, the image of the !xation point falls on the fovea, in the dark, beginning when the lights are extinguished. and the image of the test light falls on the peripheral retina. 28 CHAPTER 2 The Beginning of the Perceptual Process Copyright 2017 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.  Pure cone curve Figure 2.13 Three dark adaptation Pure rod curve curves. The red line is the two-stage dark Both rods and cones adaptation curve, with an initial cone branch and a later rod branch, which Rod light-adapted sensitivity occurs when the test light is in the peripheral retina, as shown in Figure 2.12. The green line is the cone adaptation Low curve, which occurs when the test light falls on the fovea. The purple curve is the rod adaptation curve measured in a rod monochromat. Note that the downward movement of these curves represents an Cone light-adapted sensitivity increase in sensitivity. The curves actually begin at the points indicating “light- Logarithm of sensitivity adapted sensitivity,” but there is a slight delay between the time the lights are turned off and when measurement of the curves begins. C Maximum cone sensitivity Rod–cone break Dark-adapted sensitivity R High Maximum rod sensitivity 10 20 Time in dark (min) to investigating myths about pirates. One of the myths was that additional dark adaptation experiments, one measuring adapta- pirates wore eye patches to preserve night vision in one eye so tion of the cones and another measuring adaptation of the rods. that when they went from the bright light outside to the dark- ness below decks, removing the patch would enable them to see. Measuring Cone Adaptation The reason the red curve To determine whether this would work, the Mythbusters car- in Figure 2.13 has two phases is that the !ashing test light fell ried out some tasks in a dark room just after both of their eyes on the peripheral retina, which contains both rods and cones. had been in the light and did some different tasks with an eye To measure dark adaptation of the cones alone, we have to en- that had previously been covered with a patch for 30#minutes. It sure that the image of the test light falls only on cones. We isn’t surprising that they completed the tasks much more rap- achieve this by having the subject look directly at the test light idly when using the eye that had been patched. Anyone who has so its image falls on the all-cone fovea, and by making the test taken a course on sensation and perception could have told the light small enough so that its entire image falls within the fo- Mythbusters that the eye patch would work because keeping an vea. The dark adaptation curve determined by this procedure eye in the dark triggers the process of dark adaptation, which is indicated by the green line in Figure 2.13. This curve, which causes the eye to increase its sensitivity in the dark. measures only the activity of the cones, matches the initial Whether pirates actually used patches to help them see phase of our original dark adaptation curve but does not in- below decks remains an unproven hypothesis. One argument clude the second phase. Does this mean that the second part against the idea that pirates wore eye patches to keep their sen- of the curve is due to the rods? We can show that the answer to sitivity high is that patching one eye causes a decrease in depth this question is “yes” by doing another experiment. perception, which might be a serious disadvantage when the pirate is working on deck. We will discuss why two eyes are Measuring Rod Adaptation We know that the green important for depth perception in Chapter 10. curve in Figure 2.13 is due only to cone adaptation because our Although the Mythbusters showed that dark adapting one test light was focused on the all-cone fovea. Because the cones eye made it easier to see with that eye in the dark, we have a more are more sensitive to light at the beginning of dark adaptation, speci"c goal. We are interested in showing that the "rst part of they control our vision during the early stages of adaptation, the dark adaptation curve is caused by the cones and the sec- so we can’t see what the rods are doing. In order to reveal how ond part is caused by the rods. We will do this by running two the sensitivity of the rods is changing at the very beginning of Receptors and Perception 29 Copyright 2017 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. dark adaptation, we need to measure dark adaptation in a per- "nally catches up to the cones’. The rods then become more son who has no cones. Such people, who have no cones because sensitive than the cones, and rod adaptation, indicated by the of a rare genetic defect, are called rod monochromats. Their second branch of the dark adaptation curve, becomes visible. all-rod retinas provide a way for us to study rod dark adap- The place where the rods begin to determine the dark adapta- tation without interference from the cones. (Students some- tion curve is called the rod–cone break. times wonder why we can’t simply present the test !ash to the Why do the rods take about 20 to 30 minutes to reach peripheral retina, which contains mostly rods. The answer is their maximum sensitivity (point R on the curve) compared that there are enough cones in the periphery to in!uence the to only 3 to 4 minutes for the cones (point C)? The answer to beginning of the dark adaptation curve.) this question involves a process called visual pigment regenera- Because the rod monochromat has no cones, the light- tion, which occurs more rapidly in the cones than in the rods. adapted sensitivity we measure just before we turn off the lights is determined by the rods. The sensitivity we determine, which Visual Pigment Regeneration From our descrip- is labeled “rod light-adapted sensitivity” in Figure 2.13, indi- tion of transduction earlier in the chapter, we know that light cates that the rods are much less sensitive than the cone light- causes the retinal part of the visual pigment molecule, which adapted sensitivity we measured in our original experiment. We is initially bent as shown in Figure 2.10a, to change its shape can also see that once dark adaptation begins, the rods increase as in Figure 2.10b. This change from bent to straight is shown their sensitivity, as indicated by the purple curve, and reach their in the upper panels of Figure 2.14, which also shows how the "nal dark-adapted level in about 25 minutes (Rushton, 1961). retinal eventually separates from the opsin part of the molecule. The end of this rod adaptation measured in our monochromat This change in shape and separation from the opsin causes matches the second part of the two-stage dark adaptation curve. the molecule to become lighter in color, a process called visual Based on the results of our dark adaptation experiments, pigment bleaching. This bleaching is shown in the lower pan- we can summarize the process of dark adaptation. As soon as els of Figure 2.14. Figure 2.14a is a picture of a frog retina that the light is extinguished, the sensitivity of both the cones and was taken moments after it was illuminated with light. The the rods begins increasing. However, because the cones are red color is the visual pigment. As the light remains on, more much more sensitive than the rods at the beginning of dark ad- and more of the pigment’s retinal is isomerized and breaks aptation, we see with our cones right after the lights are turned away from the opsin, so the retina’s color changes as shown in out. One way to think about this is that the cones have “cen- Figures 2.14b and 2.14c. ter stage” at the beginning of dark adaptation, while the rods When the pigments are in their lighter bleached state, are working “behind the scenes.” However, after about 3 to 5 they are no longer useful for vision. In order to do their job of minutes in the dark, the cones have reached their maximum changing light energy into electrical energy, the retinal needs sensitivity, as indicated by the leveling off of the dark adapta- to return to its bent shape and become reattached to the opsin. tion curve. Meanwhile, the rods are still adapting, behind the This process of reforming the visual pigment molecule is called scenes, and by about 7 minutes in the dark, the rods’ sensitivity visual pigment regeneration. Figure 2.14 A frog retina was dissected from the eye in the dark and then exposed to light. The top row shows how the relationship Retinal between retinal and opsin changes after the retinal absorbs light. Only a small part of the opsin molecule Opsin Opsin Opsin is shown. The photographs in the bottom row show how the color of the retina changes after it is exposed to light. (a) This picture of the retina was taken just after the light was turned on. The dark red color is caused by the high concentration of visual pigment in the receptors that are still in the unbleached state. (b,"c) After the retinal isomerizes, the"retinal and opsin break apart, and the retina becomes bleached, as © Bruce Goldstein indicated by the lighter color. (a) (b) (c) 30 CHAPTER 2 The Beginning of the Perceptual Process Copyright 2017 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.  When you are in the light, as you are now as you read this measured by determining the spectral sensitivity curve—the book, some of your visual pigment molecules are isomerizing relationship between wavelength and sensitivity. and bleaching, as shown in Figure 2.14, while at the same time, others are regenerating. This means that in most normal light Spectral Sensitivity Curves The following is the psy- levels, your eye always contains some bleached visual pigment chophysical method used to measure a spectral sensitivity and some intact visual pigment. When you turn out the lights, curve. the bleached visual pigment continues to regenerate, but there is no more isomerization, so eventually the concentration of METHOD Measuring a Spectral Sensitivity Curve regenerated pigment builds up so your retina contains only in- tact visual pigment molecules. To measure sensitivity to light at each wavelength across the This increase in visual pigment concentration that occurs spectrum, we present one wavelength at a time and measure as the pigment regenerates in the dark is responsible for the in- the subject’s sensitivity to each wavelength. Light of a single crease in sensitivity we measure during dark adaptation. This wavelength, called monochromatic light, can be created by relationship between pigment concentration and sensitivity using special #lters or a device called a spectrometer. To deter- was demonstrated by William Rushton (1961), who devised a mine a person’s spectral sensitivity, we determine the person’s procedure to measure the regeneration of visual pigment in threshold for seeing monochromatic lights across the spectrum humans by measuring the darkening of the retina that occurs using one of the psychophysical methods for measuring thresh- during dark adaptation. (Think of this as Figure 2.14 proceed- old described in Chapter 1 (p. 14) and Appendix A (p. 384). The ing from right to left.) threshold is usually not measured at every wavelength, but at Rushton’s measurements showed that cone pigment takes regular intervals. Thus, we might measure the threshold #rst 6 minutes to regenerate completely, whereas rod pigment takes at 400 nm, then at 410 nm, and so on. The result is the curve in more than 30 minutes. When he compared the course of pig- Figure 2.15a, which shows that the threshold is higher at short ment regeneration to the dark adaptation curve, he found that the rate of cone dark adaptation matched the rate of cone Threshold curve pigment regeneration and the rate of rod dark adaptation High matched the rate of rod pigment regeneration. These results demonstrated two important connections between perception and physiology: 1. Our sensitivity to light depends on the concentration of a Relative threshold chemical—the visual pigment. 2. The speed at which our sensitivity increases in the dark depends on a chemical reaction—the regeneration of the visual pigment. What happens to vision if something prevents visual pig- ments from regenerating? This is what occurs when a per- Low son’s retina becomes detached from the pigment epithelium (see 400 500 600 700 Figure#2.2b), a layer that contains enzymes necessary for pig- (a) Wavelength (nm) ment regeneration. This condition, called detached retina, can occur as a result of traumatic injuries of the eye or head, as Spectral sensitivity curve when a baseball player is hit in the eye by a line drive. When this High occurs, the bleached pigment’s separated retinal and opsin can no longer be recombined, and the person becomes blind in the Relative sensitivity area of the visual "eld served by the separated area of the retina. This condition is permanent unless the detached area of retina is reattached, which can be accomplished by laser surgery. Spectral Sensitivity Our discussion of rods and cones has emphasized how they control our vision as we adapt to darkness. Rods and cones also differ in the way they respond to light in different parts Low 400 500 600 700 of the visible spectrum (Figure 1.21, page 18). The differences in the rod and cone responses to the spectrum have been studied (b) Wavelength (nm) by measuring the spectral sensitivity of rod vision and cone Figure 2.15 (a) The threshold for seeing a light as a function of vision, where spectral sensitivity is the eye’s sensitivity to light wavelength. (b) Relative sensitivity as a function of wavelength—the as a function of the light’s wavelength. Spectral sensitivity is spectral sensitivity curve. (Adapted from Wald, 1964) Receptors and Perception 31 Copyright 2017 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. and long wavelengths and lower in the middle of the spectrum; that is, less light is needed to see wavelengths in the middle of the spectrum than to see wavelengths at either the short- or long-wavelength end of the spectrum. The ability to see wavelengths across the spectrum is often plotted not in terms of threshold versus wavelength, as in Figure!2.15a, but in terms of sensitivity versus wavelength. Us- ing the equation, sensitivity 5 1/threshold, we can convert the threshold curve in Figure 2.15a into the curve in Figure 2.15b, which is called the spectral sensitivity curve. We measure the cone spectral sensitivity curve by having a subject look directly at a test light so that it stimulates only the cones in the fovea. We measure the rod spectral sensitivity Figure 2.17 Flowers for demonstrating the Purkinje shift. See text for explanation. curve by measuring sensitivity after the eye is dark adapted (so the rods control vision because they are the most sensitive re- ceptors) and presenting test "ashes in the peripheral retina, off 5 to 10 minutes so it dark adapts, then switching back and to the side of the #xation point. forth between your eyes and noticing how the blue !ower in Figure 2.17 is brighter compared to the red !ower in your dark-adapted eye. The cone and rod spectral sensitivity curves in Figure 2.16 show that the rods are more sensitive to short-wavelength light Rod- and Cone-Pigment Absorption Spectra Just than are the cones, with the rods being most sensitive to light as we can trace the difference in the rate of rod and cone dark of 500 nm and the cones being most sensitive to light of 560 adaptation to a property of the visual pigments (the cone pig- nm. This difference in the sensitivity of cones and rods to dif- ment regenerates faster than the rod pigment), we can trace the ferent wavelengths means that as vision shifts from the cones difference in the rod and cone spectral sensitivity curves to the in the light-adapted eye to the rods after the eye has become rod and cone pigment absorption spectra. A pigment’s absorption dark adapted, our vision shifts to become relatively more sen- spectrum is a plot of the amount of light absorbed versus the sitive to short-wavelength light—that is, light nearer the blue wavelength of the light. The absorption spectra of the rod and and green end of the spectrum. cone pigments are shown in Figure 2.18. The rod pigment ab- You may have noticed an effect of this shift to short- sorbs best at 500#nm, the blue-green area of the spectrum. wavelength sensitivity if you have observed how green foliage There are three absorption spectra for the cones because seems to stand out more near dusk. This enhanced percep-