When Seeing Isn't Seeing It was cold in the bathroom, so the young woman turned on a small heater before she got in the shower. She didn't know that the heater was malfunctioning, filling the room with deadly, odorless carbon monoxide gas. Her husband found her unconscious on the floor and called for an ambulance to rush her to the emergency room. When she regained consciousness, "D.F." seemed to have gotten off lightly, avoiding what could have been a fatal accident. She could understand the doctors' questions and reply sensibly, move all her limbs, and perceive touch on her skin. But something was wrong with her sight. D.F. had lost the ability to identify things that she viewed. Even the faces of family members had become unfamiliar. More than
a decade later, D.F. still could not recognize commonplace objects, yet she was not entirely blind. If you showed her a flashlight, she could tell you that it was made of shiny aluminum with some red plastic, but she didn't recognize it ("Is it a kitchen utensil?"). Without telling her what it was, if you asked her to pick it up, her hand moved directly to grasp the flashlight exactly as one normally does. Shown a slot in a piece of plastic, D.F. could not tell you whether the slot was oriented vertically, horizontally, or diagonally; but if you handed her a disk and asked her to put it through the slot, she invariably turned the disk so that it went smoothly through (Ganel and Goodale, 2019). Could D.F. see or not?
Many species rely on vision to find food and mates, avoid predators, and locate shelter. However, the sheer volume of visual information poses a serious problem. Viewing the surrounding world has been compared to drinking from a waterfall. How does the visual system avoid being overwhelmed by the flood of information entering the eyes? One answer is that each species evolved visual capabilities that are tailored to that species' particular lifestyle. Most nocturnal species have better night vision than do animals that are active during the day, like us. Most rodent species, such as rats and mice, which live in tunnels and close quarters, have poor vision for distant objects, while daytime hunters like hawks have incredibly keen distance vision. Birds and bees can detect ultraviolet light, allowing them to see patterns in flowers that we cannot. But even within our limits of sight, we humans process a remarkable amount of visual information, which keeps about one-third of our cerebral cortex busy analyzing it.
7.1The Vision Pathway Extends from the Eye to the Brain
View Animation 7.2: Brain Explorer View Activity 7.1: The Structure of the Eye retina The receptive surface inside the eye that contains photoreceptors and other neurons. transduction The conversion of one form of energy to another, such as from light to neuronal activity. cornea The transparent outer layer of the eye, whose curvature is fixed. The cornea bends light rays and is primarily responsible for forming the image on the retina. refraction The bending of light rays by a change in the density of a medium, such as the cornea and the lens of the eyes. lens A structure in the eye that helps focus an image on the retina. ciliary muscle One of the muscles that control the shape of the lens inside the eye, focusing an image on the retina. accommodation The process by which the ciliary muscles adjust the lens to bring nearby objects into focus.
To begin this chapter, you'll learn how light entering the eye affects the firing of neurons and how that visual information reaches the brain. By the end of this section, you should be able to: 7.1.1 Describe how a visual scene is projected onto the back of the eyes. 7.1.2 Identify the major types of neurons there, which detect and analyze light. 7.1.3 Explain how we are able to detect visual images over a very broad range of illumination. 7.1.4 Describe the orderly mapping of information from a visual scene projecting into the brain. The eye is an elaborate structure that captures light at the front and projects detailed images of the external world onto a layer of neurons at the back. That layer of neurons, called the retina, turns the light into neural signals in a process called transduction. So, good vision requires an accurate optical image focused on the retina. In other words, light from a point on a target object must end up as a point of light--rather than a blur--on the retina. To produce this sharply focused optical image, the eye has many of the features of a camera, starting with the transparent outer layer of the eye, called the cornea (FIGURE 7.1). Light travels in a straight line until it encounters a change in the density of the medium, such as when it moves from air into water, which causes light rays to bend. This bending of light rays, called refraction, is the basis of such instruments as eyeglasses, telescopes, and microscopes. The curvature of the cornea, which does not change shape, refracts light rays and is primarily responsible for focusing on the retina. Light passing through the cornea is further refracted by the lens, which changes its shape to fine-tune that image on the retina. The change in the shape of the lens is controlled by the ciliary muscles inside the eye. Contraction of the ciliary muscles alters the focal distance of the eye, causing nearer images to come into focus on the retina; this process is called accommodation.
Here the right eye is viewed in cross section from above.
FIGURE 7.1 Structures of the Human Eye
The visual image focused on the retina is inverted top to bottom and reversed right to left. Retina Fovea Optic nerve The gap in the retina where the optic nerve leaves the eyeball is called the optic disc.
As mammals age, their lenses become less elastic and (A) Normal vision
therefore less able to bring nearby objects into focus
(we call this farsightedness). Aging humans correct this
problem either by holding books and menus farther
away from their eyes, or by wearing reading glasses.
In contrast, the most common vision problem in young
people is myopia (nearsightedness), which is difficulty
seeing distant objects. Myopia develops if the eyeball is too long, causing the cornea and lens to focus imag-
es in front of the retina rather than on it (FIGURE 7.2).
Distance vision can be restored in such cases by lenses
that correct refraction of the visual image so that it is
If you've ever played around with a magnifying
glass, you've probably noticed that if you hold the lens at arm's length, you can see a clearly focused image of
a distant scene through the glass but that scene is up-
side down and reversed. Like a magnifying glass, the
biconvex (bulging on both sides) shape of the lens of
the eye causes the visual scene that is focused on the
retina to be upside down and reversed compared with
Movement of the eyes is controlled by the extra-
ocular muscles, three pairs of muscles that extend from the outside of the eyeball
to the bony socket of the eye. Fixing your gaze on still or moving targets requires
delicate control of these muscles to anchor the visual image on the retina. Let's talk
about how that sharply focused visual image is processed in the retina.
Visual processing begins in the retina The first stages of visual information processing occur in the retina, the receptive surface inside the back of the eye. The retina is only 200-300 micrometers thick--as thick as 2-3 sheets of paper--but it contains several types of cells in distinct layers (FIGURE 7.3A). Sensory neurons that detect light are called photoreceptors. There are two types of photoreceptors in the retina, called rods and cones, reflecting their respective shapes (FIGURE 7.3B). Cones come in several different varieties, which respond differently to light of varying wavelengths, providing us with color vision (as described later in the chapter). Rods respond to visible light of almost any wavelength. Both rod and cone photoreceptors release neurotransmitter molecules into synapses on the bipolar cells, controlling their activity. The bipolar cells, in turn, connect with ganglion cells. The axons of the ganglion cells form the optic nerve, which carries information to the brain. Two additional types of cells--horizontal cells and amacrine cells--are especially significant in interactions within the retina. The horizontal cells make contacts among the receptor cells and bipolar cells; the amacrine cells contact both the bipolar cells and the ganglion cells. Interestingly, the rods, cones, bipolar cells, and horizontal cells generate only graded, local potentials; they do not produce action potentials. Unlike most neurons, these cells affect each other through the graded release of neurotransmitters in response to graded changes in electrical potentials. The ganglion cells, on the other hand, conduct action potentials in the same way that most other neurons do. From the receptor cells to the ganglion cells, enormous amounts of data converge and are compressed; the human eye contains about 100 million rods and 4 milWlioantsocno/nBerese,dblouvtethere are only 1 million ganglion cells to transmit all that information TtohethMeinbdr'asiMn.aTchhinues, a great deal Foundations of Brain and Behavior 4e of information processing is done inside the eye, as the input of over 100 million photoreceptors is compressed into the action potentials of 1MmMill4ieo_n07g.0a2ngl0i3o/n23c/e2ll0axons.
The cornea and lens refract light to focus a sharp image of the outside world on the retina. In myopia, the eyeball is too long, so images from distant objects are in focus in front of the retina. In this case, the image that actually reaches the retina is blurred. Eyeglasses refract the light before it reaches the cornea to bring the image into sharp focus on the retina.
FIGURE 7.2 Focusing Images on the Retina
myopia Nearsightedness; the inability to focus the retinal image of objects that are far away. extraocular muscle One of the muscles attached to the eyeball that controls its position and movements. photoreceptor A neural cell in the retina that responds to light. rod A photoreceptor cell in the retina that is most active at low levels of light. cone Any of several classes of photoreceptor cells in the retina that are responsible for color vision. bipolar cell An interneuron in the retina that receives information from rods and cones and passes the information to retinal ganglion cells. ganglion cell Any of a class of cells in the retina whose axons form the optic nerve. optic nerve Cranial nerve II; the collection of ganglion cell axons that extends from the retina to the brain. horizontal cell A specialized retinal cell that contacts both photoreceptors and bipolar cells. amacrine cell A specialized retinal cell that contacts both bipolar cells and ganglion cells and is especially significant in inhibitory interactions within the retina.
214CHAPTER7 (A) Cross section of retina Optic nerve bers Ganglion cell layer
Bipolar cell layer Rod and cone cell layer Pigmented epithelium
Both photoreceptors release neurotransmitter onto bipolar neurons. (D) Outer segments of rod
FIGURE 7.3 Anatomy of the Retina
The photoreceptor cells (rods and cones) are in the back of the retina.
Rod cell Rods and cones differ in structure, but both contain stacks of discs that absorb light, triggering changes in membrane potential.
scotopic system A system in the retina that operates at low levels of light and involves the rods. convergence The phenomenon of neural connections in which many cells send signals to a single cell. photopic system A system in the retina that operates at high levels of light, shows sensitivity to color, and involves the cones. rhodopsin The photopigment in rods that responds to light. Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e MM4e_07.03 03/23/20
The two different populations of photoreceptors (rods and cones) provide input to two different functional systems in the retina. A rod-based system, called the scotopic system (from the Greek skotos, "darkness," and ops, "eye"), is very sensitive and thus works especially well in low light--we use rod vision to detect objects in dim light-- but it is insensitive to color. That's why in the darkness of night, when only our rods can detect light, we can't tell colors apart. There is a lot of convergence in the scotopic system because the information from many rods converges onto each ganglion cell. The other system uses cones, which are less sensitive than rods (i.e., they have a higher threshold before they respond), and therefore it requires more light to function. This photopic system (which, like the term photon, gets its name from the Greek phos, "light") shows differential sensitivity to wavelengths, enabling our color vision. Compared with the scotopic system, the photopic system has less convergence, with some ganglion cells reporting information from only a single cone. At moderate levels of illumination, both the rods and the cones function, and some ganglion cells receive input from both types of receptors. TABLE 7.1 summarizes the characteristics of the photopic and scotopic systems. Photoreceptors respond to light by releasing less neurotransmitter Rods and cones owe their extraordinary sensitivity to their unusual structure and biochemistry (FIGURE 7.3B-D). Each of these cells contains a stack of discs, which is where light particles are detected. Because light is reflected in many directions by the various parts of the eye, only a fraction of the light that strikes the cornea actually reaches the retina. The stacking of the discs increases the probability that one of them will capture the light particles that make it to the retina. The light particles, called quanta or photons, that strike the discs are captured by special photopigment receptor molecules. In the rods this photopigment is rhodopsin (from the Greek rhodon, "rose," and opsis, "vision"). Cones use similar photopigments, as we'll see later. Curiously enough, photoreceptors in the dark continually release neurotransmitter onto bipolar cells. When light hits photopigment in the photoreceptor, it triggers a cascade of chemical reactions that hyperpolarize the cell, causing the
Table 7.1 Properties of the Human Photopic and Scotopic Visual Systems
Receptors Approximate number of receptors per eye Photopigments Sensitivity Location in retina Receptive-field size and visual acuity Response time
Three classes of cone opsins; the basis of color vision Low; needs relatively strong stimulation; used for day vision Concentrated in and near fovea; present less densely throughout the retina Small in fovea, so acuity is high; larger outside fovea Relatively rapid
Rhodopsin High; can be stimulated by weak light intensity; used for night vision Outside fovea Larger, so acuity is lower Slow
cell to release less neurotransmitter onto bipolar cells (FIGURE 7.4). You can learn the
details of this process in A STEP FURTHER 7.1, on the website. It may seem puz-
zling that light causes photoreceptors to release less neurotransmitter, but remember
that the visual system responds to changes in light. Either an increase or a decrease in
the intensity of light can stimulate the visual system, and hyperpolarization is just as
much a neural signal as depolarization is.
This change of potential in photoreceptors is the initial electrical signal in the visual
pathway. Stimulation of rhodopsin by light hyperpolarizes the rods, just as light stimu-
lation of the cone pigments hyperpolarizes
them. For both rods and cones, the size of (A) Rod photoreceptor
tial determines how much less transmitter will be released (see Figure 7.4). Another
that their sensitivity to light is constantly changing, as discussed next.
are hyperpolarized when stimulated by light.
pupil The opening, formed by the iris, that allows light to enter the eye. iris The circular structure of the eye that provides an opening to form the pupil.
(B) Stimulation hyperpolarizes receptor Light ash
Different mechanisms enable the eyes to work over a wide range of light intensities Our visual system must respond to stimuli of vastly different intensities: a very bright light is about 10 billion times as intense as the weakest lights we can see. One way the visual system deals with this large range of intensities is by adjusting the size of the pupil, which is an opening in the colorful disc called the iris (see Figure 7.1). In Chapter 1 we mentioned that dilation (opening) of the pupils is controlled by the sympathetic division of the autonomic system and that constriction is triggered by the parasympathetic division. Because usually both divisions are active, pupil size reflects a balance of influences. Drugs that block acetylcholine
Ampli er Microelectrode Rod cell inner segment
FIGURE 7.4 Light Hyperpolarizes Photoreceptors
(B) Dark While the size of the pupil can change rapidly, it affects light entry by only about 16-fold. So it cannot possibly account for our ability to see over a billion-fold range of illumination.
range fractionation The means by which sensory systems cover a wide range of intensity values, as each sensory receptor cell specializes in just one part of the overall range of intensities. photoreceptor adaptation The tendency of rods and cones to adjust their light sensitivity to match current levels of illumination.
FIGURE 7.5 The Iris Controls the Size of the Pupil Opening
transmission in the parasympathetic synapses onto muscles controlling the iris relax them, opening the pupil widely. One drug that has this effect--belladonna--got its name (Italian for "beautiful woman") because it was thought to make a woman more beautiful by giving her the wide-open pupils of an attentive person. Other drugs, such as morphine, constrict the pupils. In bright light, the pupil contracts quickly to admit only about one-sixteenth as much light as when illumination is dim (FIGURE 7.5). Although rapid, the 16-fold difference in light controlled by the pupil doesn't come close to accounting for the billion-fold range of visual sensitivity (FIGURE 7.6). Another mechanism for handling different light intensities is range fractionation, the handling of different intensities by different receptors--some with low thresholds (rods) and others with high thresholds (cones) (see Figure 7.5). But the main reason we can see over such a vast range of light is photoreceptor adaptation: each photoreceptor constantly adjusts its sensitivity to match the average level of ambient illumination, over a tremendous range. Thus, the visual system is concerned with differences, or changes, in brightness--not with the absolute level of illumination. At any given time, a photoreceptor operates over a range of intensities of about a hundred-fold; that is, it is completely depolarized by a stimulus about one-tenth the ambient level of illumination, and a light 10 times more intense than the ambient level will completely hyperpolarize it. The receptors constantly shift their whole range of response to work around the prevailing level of illumination. Further adaptation, controlled by neural circuits, occurs in the brain.
Intensity of light re ected from objects (lamberts)
Outdoors under a tree on a sunny day Watson/Breedlove
Even within the photopic range, where cones are active, we can see over a million-fold range of brightness.
10-5 Threshold for perception of color; bright moonlight
In low light, we rely on the scotopic system activated by rods, so color discrimination is weak or absent.
Rose photo © Serhii Brovko/Shutterstock.com
FIGURE 7.6 The Wide Range of Sensitivity to Light Intensity
Acuity is best in foveal vision The whole area that you can see without moving your head or eyes is called your visual field. Visual acuity, commonly known as the sharpness of vision, is especially fine in the center of the visual field and falls off rapidly toward the periphery. That's why when we want to look at something closely, we center our gaze on the object of interest. The fine structure of the retina explains why our acuity is best in the center of the visual field, called the fovea (FIGURE 7.7A). Notice how much more densely packed cones are in the fovea, where acuity is highest (FIGURE 7.7B), than in other parts of the retina. The fovea has an especially dense concentration of cones, absorbing so much light that the region looks dark in the photo (Figure 7.7A). That is one reason visual acuity is so high in this region. People differ in their concentrations of cones (Legras et al., 2018), and this variation may be related to individual differences in visual acuity. Species differences in visual acuity also reflect the density of cones in the fovea. For example, hawks, whose acuity is much greater than that of humans, have much narrower and more densely packed cones in the fovea than we do. Acuity is reduced in the periphery of the retina in part because both rods and cones are larger there. The rods show a different distribution from the cones: they are absent in the fovea but more numerous than cones in the periphery of the retina (see Figure 7.7A). This is
Vision217 visual field The whole area that you can see without moving your head or eyes. visual acuity Sharpness of vision. fovea The central portion of the retina, which is packed with the highest density of photoreceptors and is the center of our gaze.
(A) Distributions of rods and cones across the retina Fovea
FIGURE 7.7 Densities of Retinal Receptors and Visual Acuity (After G. Osterberg, 1935. Acta Ophthalmol. Suppl. 13: 1.)
Receptors/mm2 Percentage of acuity (relative to foveal acuity)
Rods Cone (B) Variation of visual acuity across the retina 100 The variation of visual acuity across the retina re ects the distribution of cones. 50 Optic disc (blind spot)
218CHAPTER7 Ganglion cells Bipolar cells Photoreceptor cells
In the fovea, light reaches the cones without having to pass through blood vessels and other layers of cells. Fovea
FIGURE 7.8 An Unobstructed View
why, if you want to see a dim star, you do best to search for it a little off to the side of
your center of gaze. Not only are rods more sensitive than cones to dim light, but as
we mentioned earlier, input from many rods converges on each ganglion cell in the
optic disc The region of the retina that is devoid of photoreceptors because ganglion cell axons and blood vessels
scotopic system, further increasing the system's sensitivity to weak stimuli. But that greater convergence of rods comes at the cost of diminished acuity compared with the fovea. Rods provide high sensitivity with limited acuity; cones provide high acuity
with limited sensitivity. Thus, really fine vision requires good lighting.
blind spot The portion of the visual field
In addition to the tight packing of cones in the fovea, another reason acuity is
from which light falls on the optic disc.
greater there than elsewhere on the retina is that in this region light reaches the
If you close your left eye and focus on the F, you'll
those upper layers without reaching the photoreceptors. This is why the surface of the retina is depressed at
notice that when the book is about 10 inches away, the red dot in the
the fovea (see Figure 7.1A), giving the structure its name ( fovea means "pit" in Latin).
center seems to disappear, as its image falls on the blind spot (see
The optic disc, to the nasal side of the fovea, is where blood vessels and ganglion cell axons leave the eye (see
Figure 7.7).
Figure 7.7A). There are no photore-
Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e (B) MM4e_07.08 03/23/20 Here, when you focus your right
blind spot here that we normally do not notice. You can locate your blind spot, and experience firsthand some of its interesting features, with the help of FIGURE 7.9. The blind spot is much bigger than we usually appreciate; it is
eye on the F and hold the book the right distance away, the red
about 10 times larger than the image of a full moon, yet we typically don't even notice it! Again, brain systems "fill in"
FIGURE 7.9 Experiencing the Blind Spot
the missing information so that we perceive an uninterrupted visual scene.
Before we consider how information is processed at different levels of the visual system, we need to describe the pathway from the eye to the cortex, which we'll discuss next.
1. Describe how structures of the eye refract light to focus an image on the retina. 2. How do the photopic and scotopic visual systems differ? 3. How are we able to discriminate differences in light over such a wide range of illumination? 4. Why is our vision so much more acute at the fovea than it is elsewhere? Neural signals travel from the retina to several brain regions The ganglion cells in each eye produce action potentials that are conducted along their axons to send visual information to the brain. These axons make up the optic nerve (also known as cranial nerve II), which brings visual information into the brain, eventually reaching the occipital cortex at the back of the brain. In vertebrates, many of the axons of each optic nerve cross to the opposite cerebral hemisphere. The optic nerves cross the midline at the optic chiasm (named for the Greek letter [chi] because of its crossover shape). Proportionally more axons cross the midline in prey animals, such as rabbits, that have laterally placed eyes with little overlap in their fields of vision (FIGURE 7.10). This arrangement gives a prey animal an especially wide field of view (good for spotting threats) at the cost of poor depth perception (which predators gain by comparing the overlapping visual fields of their front-facing eyes). In humans, axons from the half of the retina toward your nose (the nasal hemiretina) cross over to the opposite side of the brain. The half of the retina toward your temple (the temporal hemiretina) projects its axons to its own side of the brain. The result of these projections is that the right hemisphere of the brain "sees" the left side of the visual field, and the left hemisphere "sees" the right side of the visual field
occipital cortex Also called visual cortex. The cortex of the occipital lobe of the brain, corresponding to the visual area of the cortex. optic chiasm The point at which parts of the two optic nerves cross the midline.
In general, prey species such as rabbits (top) have wider elds of vision and a greater proportion of retinal bers crossing the midline than do predatory species such as cats (bottom). In any case, information from the left visual eld reaches the right brain, and vice versa.
FIGURE 7.10 Visual Fields
220CHAPTER7 3 At the optic chiasm, axons from the temporal halves of each retina continue into the optic tract on the same side. Axons from the nasal halves cross to the optic tracts on the opposite side. 2 The axons of retinal ganglion cells form the optic nerves. 1 The retinal image is inverted and reversed right to left compared to the visual eld.
4a Some axons in the optic tract terminate in the superior colliculus. 4b Most axons in the optic tract terminate in the lateral geniculate nucleus (LGN) of the thalamus.
5 Axons project back and forth between the LGN and the striate cortex via the optic radiations.
6a Most of the primary visual cortex is on the medial surface of the human brain.
6b An especially large proportion of primary visual cortex represents the foveal region.
7 The left primary visual cortex gets input from both eyes, but only from the right visual eld.
FIGURE 7.11 Visual Pathways in the Human Brain The right visual field, which falls on parts of both retinas, projects to the left cerebral hemisphere. Similarly, the left visual field projects to both eyes and then to the right cerebral hemisphere.
View Animation 7.3: Visual Pathways in the Human Brain optic tract The axons of retinal ganglion cells after they have passed the optic chiasm. Most of these axons terminate in the lateral geniculate nucleus. lateral geniculate nucleus (LGN) The part of the thalamus that receives information from the optic tract and sends it to visual areas in the occipital cortex. optic radiation Axons from the lateral geniculate nucleus that terminate in the primary visual areas of the occipital cortex. Wpartismona/rByreveidsluoavlecortex (V1) Also called TshteriMatiendc'osrMtexacohrinaerea 17. The region of FothuendoacticonipsitoaflBcroairnteaxndwBheehraevimoro4set visual information first arrives. MM4e_07.11 03/23/20 binocular Referring to two-eyed processes. extrastriate cortex Visual cortex outside of the primary visual (striate) cortex.
(FIGURE 7.11). After the axons of the retinal ganglion cells pass the optic chiasm, they are known collectively as the optic tract. A minority of retinal ganglion cells send their optic tract axons to the superior colliculus in the midbrain (see Figure 7.11, step 4a), which coordinates rapid movements of the eyes toward a target and controls the pupil's response to light levels. But most axons of the optic tract terminate on cells in the lateral geniculate nucleus (LGN) (step 4b), which is the visual part of the thalamus. Axons of the LGN neurons form the optic radiations (step 5), which terminate in the primary visual cortex (V1) of the occipital cortex at the back of the brain (step 6). The primary visual cortex is sometimes called striate cortex because cross sections of brain tissue from this region feature a prominent stripe, or striation, corresponding to convergent binocular ("two-eyed") inputs. This binocular input to layer IV of the primary visual cortex is important for depth perception. As Figure 7.11 shows, the visual cortex in the right cerebral hemisphere receives its input from the left half of the visual field, and the visual cortex in the left hemisphere receives its input from the right half of the visual field. In addition to the primary visual cortex (V1), numerous surrounding regions of the cortex are largely visual in function. These visual cortical areas outside the striate cortex are sometimes called extrastriate cortex. Working in parallel, these cortical regions process different aspects of visual perception, such as form, color, location, and movement, as we'll discuss later in this chapter. The striate cortex, as well as most extrastriate regions, contains a topographic projection of the retinas, which means there's a topographic projection of the visual field, discussed next.
The retina projects to the brain in a topographic fashion The retina represents a two-dimensional map of the visual field. As this information courses through the brain, the point-to-point correspondence between neighboring parts of visual space is maintained, forming a maplike projection (see Figure 7.11). Much of this topographic projection of visual space is devoted to the foveal region (FIGURE 7.12A). Human V1 is located mainly on the medial surface of the cortex (FIGURE 7.12B; see also Figure 7.11). About half of the human V1 is devoted to the fovea and the retinal region just around the fovea, even though this represents a tiny fraction of the total retina. This disproportionate representation does not mean that our spatial perception is distorted. Rather, this representation makes possible the great acuity in the central part of the visual field. In other words, another reason why our vision is so much more acute in the foveal region is that we devote proportionally more brain regions to analyzing information from that region. Because of the orderly mapping of the visual field (known as retinotopic mapping) at the various levels of the visual system, damage to parts of the visual system can be diagnosed from perceptual defects within the visual field. And if we know the site of injury in the visual pathway, we can predict the location of such a perceptual gap, or scotoma, in the visual field. Although the word scotoma comes from the Greek skotos, meaning "darkness," a scotoma is not perceived as a dark patch in the visual field; rather, it is a spot where nothing can be perceived, and usually rigorous testing is required to demonstrate its existence. As with the blind spots we all have, people may not be aware of scotomas that arise.
Vision221 topographic projection A mapping that preserves the point-to-point correspondence between neighboring parts of space. For example, a topographic projection extends from the retina to the cortex. scotoma A region of blindness within the visual fields, caused by injury to the visual pathway or brain.
A pattern of ickering lights was shown in a monkey's visual field...
...and a map of the visual eld was revealed by autoradiography in a attened portion of the primary visual cortex.
From R. B. H. Tootell et al., 1998. Proc. Natl. Acad. Sci. U.S.A. 95: 811. © National Academy of Sciences, U.S.A. From R. B. Tootell et al., 1982. Science 218: 902, courtesy of Roger Tootell
1 cm The small central region of the visual eld projects to a large part of primary visual cortex. (B) Human Maps of human visual cortex from fMRI show primary visual cortex as the innermost yellow region in each of these medial views.
FIGURE 7.12 Location of the Primary Visual Cortex
Within a scotoma, a person cannot consciously perceive visual cues, but some visual discrimination in this region may still be possible; this paradoxical phenomenon has been called blindsight. People with blindsight say they cannot see, but when asked to guess whether a stimulus is present, they're correct more often than could be expected by chance alone, or they may walk down a corridor strewn with objects without running into them (De Gelder et al., 2008). 1. Describe the path of information from the left visual field to the right side of the brain. 2. Name the structures that carry information from the eye to the brain. 3. Why is the proportion of primary visual cortex devoted to the fovea so large compared with other parts of the retina?
7.2Neurons at Different Levels of the Visual System Have Very Different Receptive Fields
blindsight The paradoxical phenomenon whereby, within a scotoma, a person cannot consciously perceive visual cues but may still be able to make some visual discrimination. receptive field The stimulus region and features that affect the activity of a cell in a sensory system. on-center bipolar cell A retinal bipolar cell that is excited by light in the center of its receptive field. off-center bipolar cell A retinal bipolar cell that is inhibited by light in the center of its receptive field. on-center ganglion cell A retinal ganglion cell that is activated when light is presented to the center, rather than the periphery, of the cell's receptive field. off-center ganglion cell A retinal ganglion cell that is activated when light is presented to the periphery, rather than the center, of the cell's receptive field.
This next section describes how neurons in the retina and brain respond to light that enters the eye. Reading this section should enable you to: 7.2.1 Describe the kinds of light stimuli that best excite or inhibit neurons in the retina, LGN, and striate and extrastriate cortex. 7.2.2 Understand why our perception of light and dark is not a simple function of how much light strikes the eye. 7.2.3 Explain how simple receptive fields of the retina can be combined to produce more-complex receptive fields in V1. 7.2.4 Contrast hierarchical models of visual processing with a spatial-frequency model. 7.2.5 Identify extrastriate brain regions specialized to detect complex forms and motion. As we noted in Chapter 5, the receptive field of a sensory cell consists of the stimulus features that excite or inhibit the cell. Understanding the receptive fields of cells in the visual system begins with the response of photoreceptors. At rest, both rod and cone photoreceptors steadily release the synaptic neurotransmitter glutamate. Light always hyperpolarizes the photoreceptors, causing them to release less glutamate. But the responses of the bipolar cells that receive this glutamate differ, depending on the type of glutamate receptor they possess. One group of bipolar cells consists of on-center bipolar cells. Glutamate is inhibitory to this type of cell, so light on the on-center bipolar cell's receptive field (which would cause the photoreceptor to release less glutamate) would excite this bipolar cell (think of taking the brakes off a system) (FIGURE 7.13A). The second group consists of off-center bipolar cells. Glutamate is excitatory to off-center bipolar cells, so shining light on this cell's receptive field (which causes the photoreceptor to release less glutamate) would inhibit this bipolar cell. It's called an off-center bipolar cell because turning off a light in the center of its receptive field excites it (FIGURE 7.13B). Bipolar cells also release glutamate, which always depolarizes ganglion cells. Therefore, when light is turned on, on-center bipolar cells depolarize (excite) on-center ganglion cells; when light is turned off, off-center bipolar cells depolarize (excite) off-center ganglion cells (see Figure 7.13). The stimulated on-center and
When light hits a cone, it is hyperpolarized, resulting in...
When the cone is in the dark, it is depolarized, resulting in...
Glutamate ...decreased transmitter release. On-center bipolar cell (inhibited by glutamate) Depolarized Increased transmitter release On-center ganglion cell Increased ring rate
The glutamate has opposite effects on the two kinds of bipolar cells because they possess different glutamate receptors. Off-center bipolar cell (excited by glutamate) Hyperpolarized Decreased transmitter release Off-center ganglion cell Decreased ring rate
Glutamate ...a steady release of the transmitter glutamate. On-center bipolar cell (inhibited by glutamate) Hyperpolarized Decreased transmitter release On-center ganglion cell Decreased ring rate
The glutamate has opposite effects on the two kinds of bipolar cells because they possess different glutamate receptors. Off-center bipolar cell (excited by glutamate) Depolarized Increased transmitter release Off-center ganglion cell Increased ring rate
Light falling on the same patch of retina increases the ring of on-center ganglion cells (left) while decreasing the ring of off-center gangion cells (right).
The dark patch of retina decreases the ring rate of on-center ganglion cells (left) while increasing the ring of off-center ganglion cells (right).
off-center ganglion cells then fire nerve impulses and report "light" or "dark" to higher visual centers. Neurons in the retina and the LGN have concentric receptive fields Recordings from single ganglion cells show that in addition to the on- or off-center portion we've just discussed, their receptive fields also include a ring around that center, which is called a surround because it surrounds the central patch. Thus the entire receptive field of a bipolar cell is concentric, consisting of a roughly circular central area and the ringlike area surrounding it. Through various retinal connections, the photoreceptors in the central area and those in the ring surrounding it tend to have opposite effects on the next cells in the circuit. Thus, both bipolar cells and ganglion cells have two basic types of retinal receptive fields: on-center/off-surround and off-center/ on-surround. These antagonistic effects of the center and its surround explain why uniform illumination of the entire receptive field has little effect on ganglion cell activity, compared with a well-placed small spot of light within the cell's receptive field. Neurons in the LGN, which are stimulated by retinal ganglion cells, also have these Wcaotsnocne/nBtrreiecdolonv-ecenter/off-surround or off-center/on-surround receptive fields, as shown The Mind's Machine in FIGURE 7.14. Foundations of Brain and Behavior 4e MM4e_07.13 08/31/20
FIGURE 7.13 Connections of Cones to Bipolar Cells (After D. Purves et al., 2001. Neuroscience [2nd ed.]. Oxford University Press/Sinauer. Sunderland, MA.)
on-center/off-surround Referring to a concentric receptive field in which stimulation of the center excites the cell of interest while stimulation of the surround inhibits it. off-center/on-surround Referring to a concentric receptive field in which stimulation of the center inhibits the cell of interest while stimulation of the surround excites it.
Each retinal bipolar cell and ganglion cell has a concentric receptive eld, with antagonistic center and surround. Bipolar cells respond with changes in local membrane potentials, while ganglion cells respond with action potentials.
(A) An on-center/off-surround cell Receptive eld
An on-center/off-surround cell is excited by illumination in the center of its receptive eld and inhibited by illumination in the surround.
Bipolar cell responses: changes in polarization Light
Ganglion cell responses: action potentials Light
(B) An off-center/on-surround cell Receptive eld
Changes in illumination have the opposite effects on an off-center/on-surround cell.
Bipolar cell responses: changes in polarization Light
Ganglion cell responses: action potentials Light
Illumination of center and surround has little effect. FIGURE 7.14 Receptive Fields of Retinal Cells Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e MM4e_07.14 03/24/20 lateral inhibition The phenomenon by which interconnected neurons inhibit their neighbors, producing contrast at the edges of regions.
To understand why the effect of light falling on the surround of a firing ganglion cell is opposite to the effect of light falling in the center, we need to understand the concept of lateral inhibition, in which sensory receptor cells inhibit the reporting of information from neighboring receptor cells. As illustrated in FIGURE 7.15, the bipolar cells that relay information from photoreceptors to ganglion cells also inhibit one another. So when one bipolar cell is active, it inhibits its neighbors. Because of this lateral inhibition, the ganglion cells stimulated by the right-hand edge of each dark band in FIGURE 7.16A are inhibited by the neighboring photoreceptors stimulated by the lighter band next door. Thus, ganglion cells stimulated by the right edge of each bar report receiving less light than they actually do (i.e., that edge looks darker to us). Conversely, the left edge of each bar appears lighter than the rest of the bar. Again, in FIGURE 7.16B two indicated patches, which clearly differ in the brightness we perceive, reflect the same amount of light. If you use your pinkie to cover the edge where the two tiles meet, you'll see that the two patches are the same shade of gray. How are such puzzling effects produced? Although the contrast effect in Figure 7.16A is determined, at least in part, by lateral inhibition among adjacent retinal cells, the entire areas indicated in Figure 7.16B, not just the edges, appear different, so the effect must be produced higher in the visual system. One explanation is that we are accustomed to light sources coming from overhead (such as the sun, or a room light), so our brain assumes that the upper patch must actually be darker than the lower patch, because the upper one should be receiving more light than the lower one. The important point is that our visual experience is not a simple reporting of the physical properties of light. Rather, our perception of light versus dark is created by the brain in response to many factors, including surrounding stimuli. For example, if you read this book in bright sunlight, the black ink reflects far more light to your eyes than the blank parts of the page do indoors. Yet, whether you're in sunlight or indoors, you perceive the ink as black and the blank parts as white. Later in the chapter we'll find
B from D. Purves et al., 1999. J. Neurosci. 19: 8543. © 1999 Society for Neuroscience + + + +
FIGURE 7.15 Lateral Inhibition in the Retina
This is a simpli ed view; in fact, each bipolar cell inhibits many of its neighbors and not just those directly adjacent.
Because of lateral inhibitory connections, the two central receptor cells differ more in their rates of ring than do adjacent cells.
that our experience of color is also created by the visual system, not a simple reporting of the wavelengths of light, and is very sensitive to nearby stimuli.
1. Given that all photoreceptors are hyperpolarized by light, how can the same photoreceptor excite some bipolar cells while inhibiting others? 2. What is a receptive field, and what two kinds of receptive fields are displayed by retinal ganglion cells? 3. Describe lateral inhibition in the retina and how it can sharpen our vision yet make us susceptible to the optical illusion we experience in Figure 7.16A.
View Animation 7.4: Receptive Fields in the Retina
Each strip is uniform, yet they all look lighter on the left edge and darker on the right edge.
Of the two indicated patches, the upper one looks darker, even though they are in fact the same shade of gray. If you don't believe this, use your nger to cover up the line where they meet. See?
FIGURE 7.16 The Effect of Context on the Perception of Brightness
226CHAPTER7 simple cortical cell Also called bar detector or edge detector. A cell in the visual cortex that responds best to an edge or a bar that has a particular width, as well as a particular orientation and location in the visual field. complex cortical cell A cell in the visual cortex that responds best to a bar of a particular size and orientation anywhere within a particular area of the visual field and that needs movement to make it respond actively.
RESEARCHERS AT WORK Neurons in the visual cortex have varied receptive fields Neurons from the LGN send their axons to cells in the primary visual cortex (V1), but the spots of light that are effective stimuli for LGN cells (FIGURE 7.17A; see also Figure 7.14A) are not very effective for cortical cells. In 1959, David Hubel and Torsten Wiesel reported that visual cortical cells require more-specific, elongated stimuli than those that activate LGN cells and ganglion cells. Hubel and Wiesel categorized cortical cells according to the types of stimuli that produced maximum responses. So-called simple cortical cells respond best to an edge or a bar that has a particular width and a particular orientation and location in the visual field (FIGURE 7.17B). These cells are therefore sometimes called bar detectors or edge detectors. Like the simple cells, complex cortical cells have elongated receptive fields, but they also require movement of the stimulus to make them respond actively. For some of these cells, any movement in their field is sufficient; others are more demanding, requiring motion in a specific direction (FIGURE 7.17C).
FIGURE 7.17 Receptive Fields of Cells at Various Levels in the Cat Visual System
Hypothesis Cells at higher levels of the visual system respond to progressively more complex stimuli.
Test Compare receptive fields of neurons at each level, and see how they relate to one another.
Examples of receptive fields of brain cells:
(A) Lateral geniculate cell with concentric field; on-center/off-surround.
1. Response to light in center of cell's field
2. Response to light in periphery of cell's field
(B) A simple cortical cell is sensitive to orientation. This particular cell responds strongly only when the stimulus is a vertical stripe.
(C) A complex cortical cell is also sensitive to motion. This particular cell responds strongly only when
(B) A simple cortical cell is sensitive to orientation. This particular cell responds strongly only when the stimulus is a vertical stripe. RESEARCHERS AT WORK(continued)
(C) A complex cortical cell is also sensitive to motion. This particular cell responds strongly only when the stimulus moves down. It responds weakly to upward motion and does not respond at all to sideways motion.
Result (A) Visual cells in the LGN have concentric receptive fields. (B) Visual cells in the cerebral cortex may show orientation specificity or respond only to motion, or... (C) ...they may respond only to motion in a particular direction.
Conclusion Neurons at each level of the visual system combine input from neurons at lower levels to make progressively more complex receptive fields. Thus, retinal and LGN neurons respond best to spots of light on the retina, while cortical cells respond best to lines of particular orientation, or lines that move in a particular direction. Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e Spatial-frequency analysis is unintuitive but efficient MM4e_07.17 08/26/20 Hubel and Wiesel's theoretical model of visual analysis can be described as hierarchical; that is, more-complex receptive fields are built up from inputs of simpler ones. For example, a simple cortical cell can be thought of as receiving input from a row of LGN cells (FIGURE 7.18A), and a complex cortical cell can be thought of as receiving input from a row of simple cortical cells. (FIGURE 7.18B)
On-center retinal ganglion cells Light on retina
- + + - + + - - - - ++ - + + - - ++ - - ++ -
This simple cortical cell receives input from a row of on-center ganglion cells, and so responds better to a bar of light than to any single spot of light.
(B) Bar of light moving across the retina
FIGURE 7.18 Simple Receptive Fields Can Combine to Make Complex Receptive Fields (After D. H. Hubel and T. N. Wiesel, 1962. J. Physiol. 160: 106.)
This complex cortical cell receives input from a row of simple cortical cells, and so responds better to a bar of light moving across the retina than to any stationary bar of light. The bar must have a particular orientation as it moves across the retina to best stimulate this neuron.
228CHAPTER7 spatial-frequency model A model of visual perception that emphasizes the analysis of the different spatial frequencies present in various orientations and in various parts of a visual scene. View Animation 7.5: Spatial Frequencies
Other theorists extrapolated from this hierarchical model, suggesting that higher- order circuits of cells could detect any possible form. Thus it was suggested that, by integration of enough successive levels of analysis, a neuron might respond only to a person's grandmother, and such hypothetical "grandmother cells" were frequently mentioned in the literature. According to this view, whenever such a cell was excited, up would pop a mental picture of one's grandmother. This hypothesis was given as a possible explanation for facial recognition. Critics soon pointed out both theoretical and empirical problems with the hierarchical model. For one thing, a hierarchical system like this would require a vast number of cells--perhaps more neurons than the cortex possesses--in order to account for all the visual objects we might ever encounter. Although some neurons are indeed activated by the sight of very specific faces (e.g., "Halle Berry neurons" were found, which were activated by photos of that actress) in both humans (Pedreira et al., 2010) and monkeys (Freiwald et al., 2009), these neurons do not respond to specific features of the face, as we would expect if they were built up from feature detectors. Rather, they respond only when the whole face or most of the face is presented (Freiwald et al., 2009). Confronted with the inadequacies of the hierarchical model, scientists proposed an alternative account of vision, known as the spatial-frequency model (F. W. Campbell and Robson, 1968), that is more powerful but less intuitive. This model proposes that the visual system analyzes the number of cycles of light-dark (or color) patches in any stimulus. Some cycles are narrow, others broad. Some cycles of light-dark are oriented vertically, others horizontally, and others somewhere in between. If cortical neurons are indeed optimized to detect light-dark cycles, then they should respond to repeating bars of light, as in the examples shown in FIGURE 7.19, even better than to a single bar of light. And that's precisely what researchers found (R. L. De Valois and De Valois, 1988). The idea that the visual system processes spatial-frequency information was revolutionary because it led to entirely different conceptions of how the visual system might work. The idea suggests that, rather than specifically detecting such seminaturalistic features as bars and edges, the system is breaking down complex stimuli into their individual spatial-frequency components (Kauffmann et al., 2015). In such a system, we might require a view of the whole face, which includes the low-frequency components, for recognition. This could explain why "Halle Berry neurons" do not respond to small portions of a face, because such snippets contain only high-frequency components. The spatial-frequency approach has proven useful in the analysis of many aspects of human pattern vision, and it provides the basis of high-definition television (HDTV). If you would like to know more about how the spatial-frequency model works, see A STEP FURTHER 7.2, on the website. Cortical cells respond even better to these repeating patterns of light than to single bars of light.
FIGURE 7.19 Examples of Spatial Frequencies in Vision The brain combines information about all these spatial frequencies to give us our perception of the scene.
Each cortical cell res best to such patterns in a particular orientation--vertical...
...or somewhere in between-- with a particular frequency (narrow or wide bars), and in a particular part of the visual eld.
Neurons in the visual cortex beyond area V1 have complex receptive fields and help identify forms Area V1 represents only a small fraction of the total amount of cortex that is devoted to vision. From area V1, axons extend to cortical areas involved in the perception of form: V2, V4, and the inferior temporal area (FIGURE 7.20A-C). The receptive fields of the cells in many of these extrastriate visual areas are even more complex than those of area V1. The visual areas of the human brain (FIGURE 7.20D) have been less thoroughly mapped than those of the monkey brain, and mainly by neuroimaging (the spatial resolution of which is not as fine as that of the electrophysiological recording used in the monkey brain), but the general layout appears similar in the two species, especially for V1 (Tootell et al., 2003). An astonishing proportion of primate cortex analyzes visual information. The areas that are largely or entirely visual in function occupy over half of the surface of the macaque cortex (Van Essen et al., 2001) and about 30% of human cortex (Tootell et al., 2003). We will discuss only a few of the main visual cortical areas and their functions. Area V2 is adjacent to V1, and many of its cells have receptive fields similar to those of V1 cells. Many V2 cells can respond to illusory contours, such as the boundaries of
FIGURE 7.20 Main Visual Areas in Monkey and Human Brains (Parts A-C after D. J. Felleman and D. C. Van Essen, 1991. Cereb. Cortex 1: 1.)
Macaque visual areas in occipital and temporal cortex are shown in pink.
Frontal eye elds Lateral prefrontal Orbitofrontal
The known visual areas in the macaque are indicated in color here on a attened cortex. V2 V1
Here, the occipital regions of human brain shown in Figure 7.12B are " attened" virtually, to reveal that V1 is larger than secondary visual cortex regions (V2, V3, etc.).
Representation of the center of the fovea
From R. B. H. Tootell et al., 1998. Proc. Natl. Acad. Sci. U.S.A. 95: 811. © National Academy of Sciences, U.S.A.
230CHAPTER7 Some cells in V2 respond to illusory contours such as those of the upright triangle shown here. FIGURE 7.21 A Geometric Figure with "Illusory" or "Subjective" Contours
the upright triangle in FIGURE 7.21. Clearly, such cells respond to complex relations among the parts of their receptive fields. Area V4 cells generally give their strongest responses to the frequency gratings that we discussed earlier (see Figure 7.19). However, some V4 cells give even stronger responses to concentric and radial stimuli, such as those in FIGURE 7.22 (Roe et al., 2012). Area V4 also has many cells that respond most strongly to color differences, as we will see later when we discuss color vision. The prefrontal cortex also contains a restricted region of neurons that are activated by faces (Dinh et al., 2018). These findings indicate that a pathway mediating visual recognition extends from V1 through temporal cortex to the prefrontal cortex. Later we'll see that this pathway was damaged in D.F., the woman described at the start of the chapter. Visual perception of motion is analyzed by a special system that includes cortical area V5 In area V5 (also called the medial temporal area or area MT; see Figure 7.20C) of monkeys, neurons respond to moving visual stimuli, indicating that they are specialized for the perception of motion and its direction. Imaging studies show that moving stimuli also evoke responses in human area V5. Experimental lesions of area V5 in monkeys trained to report the direction of perceived motion impaired their performance, at least temporarily. Conversely, electrically stimulating an area of V5 that normally responds to stimuli moving up caused monkeys to report that dots on the screen were moving up even when they were actually moving to the right. In other words, the electrical stimulation appeared to alter the monkeys' experience of visual motion (Zeki, 2015). One striking report described a woman who had lost the ability to perceive motion after a stroke damaged her area V5 (Zihl and Heywood, 2015). The woman was unable to perceive continuous motion and saw only separate, successive still images. This impairment led to many problems in daily life. She had difficulty crossing streets, because she could not follow the positions of automobiles in motion: "When I'm looking at the car at first, it seems far away. But then when I want to cross the road, suddenly the car is very near." She also had difficulty following conversations, because she could not see the movements of speakers' lips. Except for her inability to perceive motion, her visual perception appeared normal.
From J. L. Gallant et al., 1993. Science 259: 100, courtesy of Jack Gallant
03/24/20 These concentric and radial stimuli evoke maximal responses (red) from some cells in visual cortical area V4. The two most effective stimuli are highlighted with white arrows. FIGURE 7.22 Complex Stimuli Evoke Strong Responses in Visual Cortex
1. How could information from LGN neurons with simple concentric receptive fields be combined in a cortical cell such that it would respond best to a line of light? 2. How could information from simple cortical cells be combined in another cortical cell so that it would respond best to a moving line of light? 3. Describe the spatial-frequency hypothesis of vision. 4. What are some examples of visual receptive fields outside of V1 that respond to very complex stimuli? 5. What kind of stimuli affect the firing of neurons in V5 (also called area MT)?
7.3Color Vision Depends on Integrating Information from the Retinal Cones
For most people, color is a striking aspect of vision. In this portion of the chapter, you'll learn how our brain uses input from three different types of cones to construct our experience of color. By the end of this section, you should: 7.3.1 Know the physical properties of light that make objects appear colored. 7.3.2 Realize that our perception of color does not simply reflect those properties of light. 7.3.3 Be able to discuss the two major theories of how the visual system can detect colors. 7.3.4 Understand how comparing the activity of two or more types of cones informs us about color. 7.3.5 Understand why men are more likely than women to have difficulty distinguishing colors.
For most of us, the visible world has several distinguishable hues: blue, green, yellow, red, and their intermediates. These hues appear different because every light particle, or photon, vibrates as it travels across space, behaving like a sinusoidal wave. Photons vary in the frequency of vibration and therefore the wavelength (the distance between two adjacent peaks of the wave) of the light, and we can detect some of these differences, perceiving faster-vibrating (thus shorter-wavelength) photons as blue and green, and slower-vibrating (longer-wavelength) photons as more orange and red. The human visual system responds only to particles whose wavelengths lie within a very narrow section of the total electromagnetic range, from about 400 to 700 nanometers (nm) (FIGURE 7.23). If particles have shorter or longer wavelengths than this narrow range, we no longer call them photons but give them names like X-rays or radio waves. The color of an object depends on which wavelengths of light it absorbs versus which wavelengths it reflects. Our eyes detect the reflected wavelengths to distinguish different colors, as illustrated in FIGURE 7.24.
wavelength The length between two peaks in a repeated stimulus such as a wave, light, or sound.
Only a narrow range of the entire electromagnetic spectrum is visible, and we refer to particles in that range as photons or light particles.
FIGURE 7.23 The Wavelengths of Light
Sunlight consists of photons of all wavelengths.
This patch looks "blue" because it absorbs/ subtracts most of the long wavelengths and some of the medium wavelengths. The short- and medium-wavelength light that is re ected to the eye appears blue.
This patch looks "yellow" because it re ects best in the middle range of wavelengths and absorbs the other wavelengths.
Mix the two pigments together, and what you have left when each has absorbed its wavelengths are some remaining medium wavelengths that look "green."
FIGURE 7.24 Colored Objects Reflect Different Wavelengths of Light
Watson/Breedlove The Mind's Machine Fboruigndhattnioensssof BrOainneaonfd tBherheaevibora4seic dimensions of light perception, varying from MdaMrk4teo_0li7g.2h4t. 03/24/20 hue One of three basic dimensions of light perception, varying through the spectrum from violet to red. saturation One of three basic dimensions of light perception, varying from rich to pale. trichromatic hypothesis A hypothesis of color perception stating that there are three different types of cones, each excited by a different region of the spectrum and each having a separate pathway to the brain.
There are three dimensions of color perception: 1. Brightness, which varies from dark to light 2. Hue, which varies continuously through blue, green, yellow, orange, and red (and is what most people mean when they use the term color) 3. Saturation, which varies from rich, full colors to gray; for example, rich red through pink to gray as saturation decreases It is important to understand that the perception of a particular hue is not a simple function of the wavelength of light. For example, a patch reflecting light of a particular wavelength is perceived as various different hues, depending on several factors, including the intensity of illumination, prior exposure to a different stimulus, and the surrounding field. As illumination fades, the blues in a painting or a rug appear more prominent and the reds appear duller, even though the wavelength distribution in the light reflecting off those objects has not changed. In addition, the hue perceived at a particular point is strongly affected by the pattern of wavelengths and intensities in other parts of the visual field, as FIGURE 7.25 illustrates. To understand how the visual system creates our experience of color, we must understand how cone photoreceptors work. Color perception requires receptor cells that differ in their sensitivities to different wavelengths The first stage of color detection is accomplished by different types of cone photodetectors. On the basis of observations of mixing pigments and lights, scientists at the start of the nineteenth century predicted that there would be three separate kinds of receptors in the retina. Endorsed in 1852 by the great physiologist-physicist-psychologist Hermann von Helmholtz, this trichromatic hypothesis (from the Greek tri, "three," and chroma, "color") became the dominant account. Helmholtz predicted that blue-sensitive, green-sensitive, and red-sensitive receptors would be found, that each would be sharply tuned to its part of the spectrum, and that each type would have a separate path to the brain. The color of an object would be recognized, then, on the basis of which color receptor(s) were activated. This system would be like the mechanisms for discriminating touch and temperature on the basis of which skin receptors and labeled neural lines are activated (see Chapter 5). Later in the nineteenth century, physiologist Ewald Hering proposed a different explanation. He argued, on the basis of visual experience, that there are four unique
From R. B. Lotto and D. Purves, 2000. Proc. Natl. Acad. Sci. U.S.A. 97: 12834. © National Academy of Sciences, U.S.A. Dress photo © Cecilia Bleasdale Courtesy of Akiyoshi Kitaoka
The woman's two eyes are the same shade of gray in both pictures. But when a red screen is placed over one eye, we perceive the gray eye as blue. A blue screen causes us to perceive that same gray as red.
The dotted squares re ect the same wavelength of light, but most of us perceive one as green and the other as yellow.
The dress that broke the internet. People perceive this dress as either white and gold or as blue and black, depending on how they interpret the lighting of the scene. If you cut a rectangle 2 cm x 1 cm out of a sheet of paper and place the opening over the dress, you'll see it's the same color in the two panels.
FIGURE 7.25 Color Perception
hues (blue, green, yellow, red) and three opposed pairs of colors--blue versus yellow, green versus red, and black versus white--and that three physiological processes with opposed positive and negative values must therefore be the basis of color vision. As Wwatseown/iBllreseedel,obveoth this opponent-process hypothesis and the trichromatic hypothesis The Mind's Machine Foaurnedaetinoncsoomf Bpraaisnsaendd iBnehcavuirorre4ne t color vision theory, but neither of the old hypotheses is sufficient by itself. MM4eM_0e7a.2s5ure0m3/e2n4/ts20of photopigments in cones have borne out the trichromatic hypothesis in part. Each cone of the human retina has one of three classes of pigments (each pigment has a name, but we'll just refer to them as opsins). The response of the cone depends on which wavelength of light its pigment absorbs to start the process depicted in Figure 7.4. These pigments do not, however, have the narrow spectral distributions that Helmholtz predicted. Despite what you may have heard in other classes (or even read in other textbooks!), the human visual system does not have receptors that are sensitive to only narrow parts of the visible spectrum, such as "red" cones and "green" cones; instead, the receptor pigments exhibit broad sensitivities that substantially overlap. In fact, two of the three retinal cone pigments show some response to light of almost any wavelength. The pigments have different peaks of sensitivity, but even the peaks are not as far apart as Helmholtz predicted, and those peaks don't always correspond to a
opponent-process hypothesis A hypothesis of color perception stating that different systems produce opposite responses to light of different wavelengths.
FIGURE 7.26 Spectral Sensitivities of Human Photopigments
Each pigment has a peak sensitivity but responds to a wide range of wavelengths.
S, short-wavelength M, medium-wavelength L, long-wavelength
Knowing only that an M cone is active, you cannot tell whether it was stimulated by weak light at 530 nm ("green"), or by strong light anywhere from 450 nm ("blue") to 620 nm ("red"). Only by comparing responses of different cones can the brain extract color information.
particular color. As FIGURE 7.26 shows, the cone pigment peaks occur at about 420 nm (in the part of the spectrum where we usually see violet under daylight conditions), about 530 nm (where most of us see green), and about 560 nm (where most of us see yellow-green). Despite Helmholtz's prediction, none of the curves peak in the long-wavelength part of the spectrum, where most of us see red (about 630 nm). Under ordinary conditions, almost any object, no matter what color it is, stimulates at least two kinds of cones, thus ensuring high visual acuity and good perception of form. It is the subsequent processing performed by the nervous system, comparing the differences in activation across cones, that extracts color information about the light falling on the retina. Thus, certain ganglion cells and certain neurons at higher stations in the visual system are color-specific, even though the photoreceptors are not. In a similar manner, photoreceptors are not form-specific (they respond to single points of light), but form is detected later in the system, by comparison of the outputs Watson/Breedlove of different receptors. The Mind's Machine Because the cones are not color detectors, the most appropriate brief names for Foundations of Brain atnhdeBmehacvaionr 4bee taken from their peak areas of wavelength sensitivity: short (S) for the MM4e_07.26 03/2r4e/c2e0ptor with peak sensitivity at about 420 nm, medium (M) for 530 nm, and long (L) for 560 nm (see Figure 7.26). There are typically twice as many L as M receptors, but the ratio varies across individuals (FIGURE 7.27A,B). There are far fewer S cones, which explains why acuity is much lower with short-wavelength illumination (blue light) than in the other parts of the visible spectrum. In some insects, including bees, the short-wavelength receptors respond to ultraviolet wavelengths that we humans cannot see. This ability permits bees to see color patterns in flowers that are invisible to us (FIGURE 7.28). Most birds have not three but four different types of cones, and they can also detect ultraviolet light (Osorio and Vorobyev, 2008). The genes for wavelength-sensitive pigments in the retina have been analyzed, and the similarities in structure of the three genes suggest that they are all derived from a common ancestral gene (Carvalho et al., 2017). In addition, the genes for the medium- and long-wavelength pigments occupy adjacent positions on the X chromosome and are much more similar to each other than either is to the gene for the short-wavelength pigment on chromosome 7. Probably our primate ancestors had only one photopigment gene on the X chromosome, which became duplicated. Then
FIGURE 7.27 Distributions of
People with normal color vision have more L receptors than M receptors, but the exact proportions can vary from one person to another. There are fewer S cones than any other kind.
A spot of blue light in this region would strongly stimulate an S cone, and so would probably be perceived as blue even though most of the photoreceptors being stimulated are in fact L cones.
This retina is from a person who cannot distinguish between red and green. The dark spots are probably sites where M cones would have been if he could have produced them. The yellow dots are cones that could not be classi ed with con dence.
mutations caused the two genes to become more and more different, until their responses to various wavelengths of light were no longer the same. Thus, our ancestors went from having only two cone pigments (one on the X chromosome and the S pigment on chromosome 7) to three, with an associated improvement in color vision. This evolution of a third photopigment may have happened recently (in evolutionary terms); for example, most South American monkeys have only a single longer-wavelength pigment. The fact that the genes for the M and L pigments are on the X chromosome also explains why defects of red-green color vision are much more frequent in human males (about 8%) than in human females (about 0.5%). Because males have only one X chromosome, mutations in the genes for the M and L pigments can impair color vision (FIGURE 7.27C). But if a female has defective photopigment genes on one of her two X chromosomes, normal copies of the genes on her other X chromosome can compensate. Even though the term color blindness is commonly used, most people with impaired color vision are able to distinguish some hues. Complete color blindness can be caused by brain lesions or by the congenital absence of any cones, but in humans it is extremely rare. Likewise, even those mammalian species with weak color vision can discriminate some colors, as we discuss in Signs & Symptoms next.
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When we humans look at owers, we cannot see the re ected ultraviolet light...
FIGURE 7.28 How Flowers Look to the Birds and the Bees
...which may reveal patterns visible only to animals, like birds and bees, that detect light in that range.
Most Mammalian Species Have Some Color Vision
Animals exhibit different degrees of color vision. Many species of birds, fishes, and insects have excellent color vision. Humans and Old World monkeys also have an excellent ability to discriminate wavelengths. Many other mammals (e.g., cats) cannot discriminate wavelengths very well, but most mammals have at least some degree of color vision. Although only certain primates have good trichromatic color vision (vision based on three classes of cone photopigments), most mammalian species have at least dichromatic color vision (based on two classes of cone pigments). Most so-called color-blind (actually color-deficient) people have dichromatic vision and can distinguish short-wavelength stimuli (blue) from long-wavelength stimuli (not blue) (FIGURE 7.29). When a gene carrying a third photopigment was introduced into photoreceptors of adult male squirrel monkeys with such dichromatic vision, they soon displayed excellent trichromatic vision (Mancuso et al., 2009). Likewise, introducing photopigment genes in mice enabled them to
discriminate colors they normally cannot see (G. H. Jacobs et al., 2007), so it may be possible to correct dichromatic vision in humans. There is a continuum of color vision capabilities, including at least four categories among mammalian species: 1. Excellent trichromatic color vision is found in diurnal primates such as humans and rhesus monkeys. 2. Robust dichromatic color vision is found in species that have two kinds of cone photopigments and a reasonably large population of cones, for example the dog and the pig. 3. Feeble dichromatic color vision occurs in species that have two kinds of cone pigments but very few cones, such as the cat and the coati. 4. Minimal color vision is possessed by species that have only a single kind of cone pigment and must compare activity of rods and cones to discriminate wavelength, such as the owl monkey and raccoon.
Among those species of South American monkeys that are generally dichromats, some females are actually trichromatic. Why? Because the gene encoding one photopigment is on the X chromosome. Since females have two X chromosomes, if the two chromosomes carry different genes for the photopigment, then the female possesses a total of three different kinds of cones and therefore has trichromatic vision. Interestingly, a woman may carry slightly different genes for the long-wavelength photopigment on her two X chromosomes (Jordan et al., 2010) and therefore have four different kinds of cones. Such "tetrachromats" tend to be very good at discriminating colors and very sensitive to clashing colors. It's interesting to speculate on whether such women have a different experience of, for example, green than those of us who are trichromats and dichromats. We will take up the question of our subjective experience of color again in Chapter 14.
Photo by David McIntyre; simulation created using software from Vischeck [www.vischeck.com]
FIGURE 7.29 Simulating Color Blindness (A) The photograph on the right has been adjusted to simulate the experience of the most common form of color blindness in humans, which is the absence of cones sensitive to medium-wavelength light (M cones). For such individuals, the world's colors consist of blue (detected by short-wavelength
photopigment encoded on the seventh chromosome) and not blue (detected by the long-wavelength photopigment encoded on the X chromosome). (B) In a typical test for color vision, dichromats may have a difficult time detecting the numerals displayed in figures like this.
spectrally opponent cell Also called color-opponent cell. A visual system neuron that has opposite firing responses to different regions of the spectrum.
Some retinal ganglion cells and LGN cells show spectral opponency Monkeys discriminate colors about as well as humans do. Recordings made from monkeys reveal that most ganglion cells and LGN cells are excited and fire in response to some wavelengths and are inhibited by other wavelengths. FIGURE 7.30A shows the response of one such LGN cell as a light centered on its receptive field changes from one wavelength to another. Firing is stimulated by wavelengths above 600 nm,
where the L cones are most sensitive; it is inhibited at shorter wavelengths, where the L cones are less sensitive than the M cones. A cell exhibiting this response pattern is therefore called a plus L/minus M cell (+L/-M). This is an example of a spectrally opponent cell (or color-opponent cell) because two regions of the spectrum have opposite effects on the cell's rate of firing. Figure 7.30 shows the responses of the four main kinds of spectrally opponent cells. Each spectrally opponent ganglion cell receives input from two or three different kinds of cones through bipolar cells. The connections from at least one type of cone are excitatory, and those from at least one other type are inhibitory. The spectrally opponent ganglion cells thus record the difference in stimulation of different types of cones. For example, a +M/-L cell responds to the difference in the excitation of M and L cones. The peaks of the sensitivity curves of the M and L cones are not very different (see Figure 7.26). However, whereas the M-minus-L difference curve (FIGURE 7.30B) shows a clear peak at about 500 nm (in the green part of the spectrum), the L-minus-M difference function (see Figure 7.28A) shows a peak at about 650 nm (in the red part of the spectrum). Thus, +M/-L and +L/-M cells yield distinctly different neural response curves. LGN cells that are excited by the L and M cells but inhibited by S cells--that is, +(L+M)/-S cells--peak in the red range (FIGURE 7.30C), while cells excited by S but inhibited by L and M--that is, +S/-(L+M) cells--peak in the blue-violet range (FIGURE 7.30D). Spectrally opponent neurons are the second stage in the system for color perception, but they still cannot be called color cells, because (1) they send their outputs into many higher circuits--for detection of form, depth, and movement, as well as hue; and (2) their peak wavelength sensitivities do not correspond precisely to the wavelengths that we see as the principal hues. The brightness detectors receive stimulation from both M and L cones (+M/+L); the darkness detectors are inhibited by those same cones (-M/-L). In the monkey LGN, 70-80% of the cells are spectrally opponent; in the cat, very few spectrally opponent cells are found--only about 1%. This difference explains why monkeys so easily distinguish between colors and it's so difficult to train cats to discriminate even large differences in color.
The four main types of spectrally opponent cells are: (A) +L/-M
Each type is excited by one band of wavelengths and inhibited by another.
Some visual cortical cells and regions appear to be specialized for color perception In the cortex, spectral information is used for various kinds of information processing. Forms are segregated from their background by differences in color or intensity (or both). The most important role that color plays in our perception is to denote which parts of a complex image belong to one object and which belong to another. Some animals use displays of brightly colored body parts to call attention to themselves, but color can also be used as camouflage. Some spectrally opponent cortical cells contribute to the perception of color, providing the third stage of the color vision
FIGURE 7.30 Responses by the Four Main Types of Spectrally Opponent Cells in Monkey LGN (After R. L. De Valois and K. K. De Valois, 1993. Vision Res. 33: 1053.)
Watson/Breedlove The Mind's Machine Foundations of Brain and Behavior 4e
238CHAPTER7 Dorsal pathway Ventral pathway FIGURE 7.31 Parallel Processing Pathways in the Visual System
system. These cells are not just responding to the differences between two types of cones, as retinal ganglion cells and LGN cells do. Rather, they are responding to differences in colors that we perceive; in other words, they are perceptually opponent, as predicted by Hering: red versus green, blue versus yellow, and black versus white (R. L. De Valois and De Valois, 1993). The spectral responses of these cells correspond to the wavelengths of the principal hues specified by human observers, and their characteristics also help explain other color phenomena. Visual cortical region V4 is particularly rich in color-sensitive cells; each of these cells truly responds best to a particular hue, including the four that Hering postulated (blue, green, yellow, red). V4 cells respond best if the color outside the receptive field is different from the color preferred inside the receptive field (Roe et al., 2012). 1. What two main hypotheses were developed to explain our ability to discriminate colors? Which aspects of the visual system appear to match each hypothesis? 2. Describe some examples in which our perception of color is not simply the detection of particular wavelengths of light. 3. Why do we label cones as S cones, M cones, and L cones rather than blue, green, and red cones? 4. Why are men more likely than women to have difficulty distinguishing some colors? 5. Why is it a good idea to make life rafts yellow if they are to be detected on a blue sea? 7.4What versus Where: Cortical Visual Areas Are Organized into Two Streams Vision is so crucial for us primates that we devote lots of brain space to analyzing visual stimuli and work hard to correct vision deficiencies. After reading this final section of the chapter, you should be able to: 7.4.1 Identify the major streams of visual processing that deal with what a stimulus is, and where it is. 7.4.2 Understand why D.F. can use vision to guide her movements but not to recognize objects. 7.4.3 Describe the underlying causes of nearsightedness and how it can be avoided. 7.4.4 Understand the role of visual experience in sharpening vision, especially in children. Mortimer Mishkin and Leslie Ungerleider (1982) proposed that primates have two main visual processing streams, both originating in primary visual cortex: a ventral processing stream responsible for visually identifying objects, and a dorsal stream responsible for appreciating the spatial location of objects and for visually guiding our movement toward them (FIGURE 7.31). They called these processing streams, respectively, the what and where streams. PET studies, as well as brain lesions in patients, indicate that the human brain possesses what and where visual processing streams similar to those that have been found in monkeys. The two streams are not completely separate, because there are normally many cross connections between them. In the ventral stream, including regions of the occipitotemporal, inferior temporal, and inferior frontal areas, information about faces becomes more specific as the stream proceeds farther forward. In Chapter 15, we'll
The brain region that was damaged in patient D.F. is seen from lateral views...
From T. W. James et al., 2003. Brain 126: 2463
(B) Areas activated in control participants while visually identifying objects
This same brain region is activated (yellow) when control participants look at intact pictures of various objects rather than at scrambled pictures.
FIGURE 7.32 Object Recognition Centers in the Brain
D.F.'s inability to recognize the objects she sees appears to be due to damage in this region on the border of the occipital and temporal cortex.
discuss a portion of the ventral stream, the fusiform gyrus, that is specialized to identify faces. Discovery of these separate visual cortical streams helps us understand the case of patient D.F., described at the start of this chapter. Recall that, as a result of carbon monoxide poisoning, D.F. lost the ability to perceive faces and objects but retained the ability to reach and grasp objects under visual control. D.F.'s visual ventral (what) stream appears to have been devastated, but her dorsal (where) stream seems unimpaired. An opposite kind of dissociation had already been reported: damage to the dorsal parietal cortex often results in optic ataxia, in which patients have difficulty using vision to reach for and grasp objects, yet some of these patients can still identify objects correctly (R. A. Andersen et al., 2014). We'll discuss optic ataxia again in Chapter 14. High-resolution MRI of D.F.'s brain (FIGURE 7.32A) reveals diffuse damage concentrated in the ventrolateral occipital cortex (T. W. James et al., 2003). Throughout the brain there is evidence of atrophy, indicated by shrunken gyri and enlarged sulci. WatsFoInG/UBRreEed7lo.3v2eB shows the area activated in fMRI recordings when healthy participants The Mind's Machine FounvdiaetiwonesdofpBircatinuarneds Boefhaovbiojer c4ets; it corresponds to D.F.'s lateral occipital lesion. When D.F. reached for and grasped objects, her fMRI activation in the parietal lobe was similar MMt4oe_t0h7a.3t2of c06o/n1t6r/o2l0participants, indicating that her dorsal stream is largely intact. D.F.'s intact dorsal pathway not only tells her where objects are, but also guides her movements to use these objects properly (Ganel and Goodale, 2019). It is still puzzling that one part of D.F. knows exactly how to grasp a pencil held in front of her, yet another part of her--the part that talks to you--has no idea whether the object she's holding is a pencil, a ruler, or a bouquet of flowers. This condition is reminiscent of the cortical damage that causes blindsight, mentioned earlier: people with such damage report being unable to see, but they show evidence that they can. In Chapter 14 we'll learn about other people who can see only one thing at a time, or
optic ataxia Spatial disorientation in which the patient is unable to accurately reach for objects using visual guidance.
240CHAPTER7 amblyopia Reduced visual acuity that is not caused by optical or retinal impairments. Hey There, You with the Stars over Your Eye As a treatment for amblyopia, this girl is wearing a patch over her "good" eye--the one she has been relying on while ignoring information from her other, "weak" eye. Increased visual experience through the weak eye will strengthen its influence on the cortex.
Courtesy of Patch Pals, www.PatchPals.com
who can see faces but cannot identify to whom they belong. Imagining what such disjointed visual experience must be like helps us appreciate how effortlessly our brains usually bind together information with our marvelous sense of sight. Visual neuroscience can be applied to alleviate some visual deficiencies Vision is so important that many investigators have sought ways to prevent its impairment, to improve inadequate vision, and to restore sight to the blind. In the United States, half a million people are blind. Recent medical advances have reduced some causes of blindness but have increased blindness from other causes. For example, medical advances permit people with diabetes to live longer, but because we don't know how to prevent blindness associated with diabetes, there are more people alive today with diabetes-induced blindness. In the discussion that follows, we will first consider ways of avoiding the impairment of vision. Then we will take up ways of improving an impaired visual system. REDUCING VISUAL IMPAIRMENT Studies of the development of vision show that the incidence of myopia (nearsightedness) can be reduced. Myopia develops if the eyeball is too long, causing the eye to focus images in front of the retina rather than on the retina (see Figure 7.2). As a result, distant objects appear blurred. Considerable evidence suggests that the reason some children develop myopia is that certain environmental factors cause the eyeball to grow excessively. Previously it was thought that the modern habit of looking closely at nearby objects (books, computer screens, and so on) might be responsible for myopia, but mounting evidence suggests that indoor lighting may be to blame (Lagrèze and Schaeffel, 2017). Before civilization, most people spent the bulk of their time outdoors, looking at objects illuminated by sunlight. But with the advent of indoor lighting, we've come to spend a lot of time looking at things with light that, while containing many wavelengths, does not exactly match the composition of sunlight. Several studies found that children with myopia spend less time outdoors than do other children, but that correlation could be caused by genes that favor both myopia and indoor activities, like reading. Indeed, the advent of public schools in various nations is accompanied by increased rates of myopia. However, one of these studies focused on people of Chinese origin who lived in either Singapore, where crowded conditions mean that people spend little time outdoors, or Sydney, Australia. Even though these populations should be genetically similar, 30% of the Chinese children living in Singapore, who averaged only 30 minutes a day outdoors, were myopic, versus only 3% of those living in Sydney, who averaged 2 hours a day outdoors (Rose et al., 2008). What's more, in these populations myopia correlates much more strongly with time spent indoors than with time spent reading. Of course, too much sunlight can be a bad thing, especially for our skin. So almost all children in Australia wear hats to shield their faces when outdoors, yet they still benefit from being outdoors in terms of avoiding myopia. Likewise, there's no evidence that wearing sunglasses blocks the benefit of light from the sun. The next challenge will be to determine what it is about indoor lighting, as opposed to sunlight, that encourages the eyeball to grow excessively in children, leading to myopia. EXERCISING VISION The misalignment of the two eyes (lazy eye) can lead to a condition called amblyopia, in which acuity is poor in one eye, even though the eye and retina are normal. If the two eyes are not aligned properly during the first few years of life, the primary visual cortex of the child tends to suppress the information arriving from one eye, and that eye becomes functionally blind (see Figure 4.12B). Studies of the development of vision in children and other animals show that most cases of amblyopia are avoidable.
The balance of the eye muscles can be surgically adjusted to bring the two eyes into better alignment. Alternatively, if the weak eye is given regular practice, with the good eye covered, vision can be preserved in both eyes. Attempts to alleviate amblyopia by training alone, however, have produced mixed results. The optimal treatment appears to be a combination of both surgical correction and eye patches and visual exercises (Pediatric Eye Disease Investigator Group, 2005). SIGNS & SYMPTOMS Macular Degeneration Is the Leading Cause of Vision Loss as We Age Macular degeneration is a visual impairment caused by damage to the retina. The most common type, "dry" macular degeneration, is caused by atrophy of the retinal pigmented epithelium (see Figure 7.3), resulting in death of overlying photoreceptors. In the more severe, "wet" macular degeneration, abnormal growth of retinal capillaries leads to detachment of the retina and/or death of photoreceptors. The damage is mostly restricted to the fovea, but because visual acuity is poor for the rest of the retina, vision is quite impaired (FIGURE 7.33). An NIH-conducted trial (Age-Related Eye Disease Study [AREDS]) found that supplemental vitamins and antioxidants could slow the disease only slightly (Evans and Lawrenson, 2017). (A) Normal vision
Loss of photoreceptors in the fovea degrades vision at the center of the eld. The surviving photoreceptors in the periphery are too scarce and too large to provide much resolution.
FIGURE 7.33 Simulating Visual Experience with Macular Degeneration (After D. J. Marmor & M. F. Marmor, 2010. Arch. Ophthalmol. 128: 117.)
1. What are the two main streams of visual processing in the cortex, and what aspects of vision does each stream support? 2. Describe D.F.'s symptoms, and relate them to the brain damage revealed by MRIs. 3. What is the evidence that indoor lighting may cause myopia in children? 4. Why does degeneration of the fovea impair acuity in the whole visual field? Recommended Reading Gregory, R. L. (2015). Eye and Brain: The Psychology of Seeing (5th ed.). Princeton, NJ: Princeton University Press. Ings, S. (2008). A Natural History of Seeing: The Art and Science of Vision. New York, NY: Norton. Masland, R. (2020). We Know It When We See It: What the Neurobiology of Vision Tells Us about How We Think. New York, NY: Basic Books. Purves, D., and Lotto, R. B. (2011). Why We See What We Do Redux: A Wholly Empirical Theory of Vision. Sunderland, MA: Oxford University Press/Sinauer. Wolfe, J. M., Kluender, K. R., Levi, D. M., Bartoshuk, L. M., et al. (2021). Sensation & Perception (6th ed.). Sunderland, MA: Oxford University Press/Sinauer.
7 · VISUAL SUMMARY You should be able to relate each summary to the adjacent illustration, including structures and processes. The online version of this Visual Summary includes links to figures, animations, and activities that will help you consolidate the material.
1 Light is bent, or refracted, by the transparent outer layer of the eye, the cornea, focusing an image on the retina in the back of the eye. We vary the thickness of the lens to fine-tune the image. The refracted light forms an image on the retina that is upside down and reversed. Review Figures 7.1 and 7.2, Animation 7.2, Activity 7.1 3 The retina consists of layers of neurons, with the photoreceptors in the very back stimulating bipolar cells, which stimulate ganglion cells. The ganglion cells of the retina project their axons to the brain via the optic nerve. Amacrine cells and horizontal cells communicate across the retina, using processes such as lateral inhibition to analyze brightness. Review Figures 7.3, 7.14, and 7.15
Dazzling light; bright sun on snow Outdoors in full sunlight Outdoors under a tree on a sunny day Comfortable indoor illumination Threshold for perception of color Threshold when dark-adapted
2 The retina contains two different types of photoreceptors to detect light forming the focused image. Rods are very sensitive, working even in very low light, and they respond to light of any wavelength. Rods drive the scotopic system, which can work in dim light. Each of the three different types of cones responds better to some wavelengths of light than others, allowing us to detect colors. The cones provide information for the photopic system, which requires more light to function. Photoreceptors adapt to function across a wide range of light intensities. Review Figures 7.3-7.6, Table 7.1
4 The center of our visual field lands on the fovea, the portion of the retina with the greatest density of photoreceptors, an absence of overlying cell layers, and more direct synaptic connections to ganglion cells, providing us with our greatest visual acuity (sharpness of vision). Cones are concentrated in the fovea, and rods are concentrated in the peripheral retina, so our peripheral vision is best for seeing dim objects, but it provides no color information. Review Figures 7.7-7.9 5 The left visual field falls on the nasal hemiretina of the left eye and the temporal hemiretina of the right eye. Only nasal retinal ganglion cells of each eye send their axons across the midline, forming the optic chiasm, so the left visual field projects to the right hemisphere, and the right visual field projects to the left hemisphere. Review Figures 7.11 and 7.12, Animation 7.3 7 The receptive fields of bipolar cells and ganglion cells consist of a circular center and a surround that have opposing effects: either on-center/off-surround or off-center/ on-surround. Review Figures 7.13 and 7.14, Animation 7.4
6 Most ganglion cells of the retina synapse on neurons in the lateral geniculate nucleus (LGN) of the thalamus. The LGN neurons send axons to synapse on neurons in layer IV of the primary visual cortex (V1) in the occipital cortex. V1 sends information to many different cortical areas, called extrastriate cortex (nearly one-third of the human cortex), to further analyze visual information. Review Figures 7.11 and 7.20
8 Receptive fields of cells at successively higher levels in the visual cortex change in two main ways: (1) they become larger (occupy larger parts of the visual field), and (2) they require increasingly specific stimuli to evoke responses. For example, they respond best to a bar of light at a particular angle or to bars that move in a particular direction. Review Figures 7.17-7.22, Animation 7.5
9 Rods detect light using a pigment called rhodopsin. Our detection of hue (color) depends on the three different cone photopigments (opsins). Each cone responds to a wide range of wavelengths, not just a single color. Our perception of hue results from the relative activity of each type of cone. One way of assessing this relative activity is by retinal connections that yield spectrally opponent neurons. Review Figures 7.23-7.30
S, short-wavelength M, medium-wavelength L, long-wavelength
10 Visual cortical areas are organized into two main streams: a ventral what stream that serves in the recognition of faces and objects, and a dorsal where stream that serves in location and visuomotor skills. Review Figures 7.31 and 7.32
The Mind's Machine digital resources include additional videos, flashcards, and other study tools.