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How Do We See the World?

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How Do We See the World? p CHAPTER 8 How Do We See the World? Focus on Disorders: D. B., Karl Lashley, Location in the Visual World and Migraines Coding Location in the Retina Location in the Lateral Geniculate Nucleus and The Nature of Sensory Cortical Region V1 Experience The Visual Corpus Callosum Sensory Experience and Sensory Reality Analyzing Sensory Information Neural Activity Seeing Shape The Anatomy of the Seeing Color Visual System Neural Activity in the Dorsal Stream Light: The Stimulus for Vision The Eye The Visual Brain in Action Focus on Disorders: Optical Errors of Refraction Injury to the Visual Pathway Leading to the Cortex and Visual Illuminance Injury to the “What” Pathway The Photoreceptors Focus on Disorders: Carbon Monoxide Poisoning Retinal Neurons Injury to the “How” Pathway Visual Pathways The Dorsal and Ventral Streams MacDuff Everton/Image Bank Micrograph: Carolina Biological Supply/Phototake 276 I p orn in a small English town in 1940, D. B. had an sity, who made a remarkable discovery about D. B.’s blind- uneventful childhood medically until he began to ness. He found that, although D. B. could not identify ob- B experience headaches at about age 14. These jects in his blind area, he could very accurately “guess” if headaches were migraines, described in “D. B., Karl Lash- a light had blinked on there. He could even say where the ley, and Migraines” on page 278. Before each headache, light that he did not “see” was located. Apparently, even D. B. received a warning: the sensation of an oval-shaped though D. B. could not consciously perceive a light in his area containing a flashing light that appeared just to the blind region, his brain knew when a light had blinked and left of center in his field of vision. In the next few minutes, where it had appeared. This phenomenon is referred to as the oval enlarged, and, after about 15 minutes, the flash- blindsight. D. B.’s brain, in other words, knew more than ing light vanished and the region of the oval was blind. he was consciously aware of. D. B. described the oval as an opaque white area sur- You may be surprised to learn that what applies to rounded by a rim of color. A headache on the right side of D. B. also applies to you. You are consciously aware of his head followed. The headache could persist for as long only part of the visual information that your brain is pro- as 48 hours, but usually D. B. fell asleep before that much cessing. This is an important principle of how the human time elapsed. When he awakened, the headache was gone visual system works, which Weizkrantz was able to detect and his vision was normal again. because of D. B.’s injury. Vision is not unique in this D. B.’s attacks continued at intervals of about 6 weeks regard. We are also unaware of much of the processing for 10 years. After one attack, he did not totally regain his that takes place in other sensory pathways, such as those vision but was left with a small blind spot. During some of hearing and touch. But vision is the focus of this chap- attacks, he also began to experience occasional loss of ter. The ability to lose conscious visual perception while skin sensation along the left side of his body. Like his vi- retaining unconscious vision leads to this chapter’s major sual symptoms, these symptoms disappeared once the question: How do we “see” the world? headache was gone. The function of the visual system is to convert light When D. B. was 26 years old, a neurologist found a energy into neural activity that has meaning for us. In this collection of abnormal blood vessels at the back of his chapter, we explore how this happens. We begin with a right occipital lobe. These blood vessels were causing the description of what it really means to experience sensory migraine attacks. (In case you suffer from migraines, we information transmitted by our environment. Then in an hasten to point out that this cause is unusual.) By the time overview of the visual system’s anatomy, we consider the D. B. was 30, the migraines became more severe and be- anatomical structure of the eyes, the connections between gan to interfere with his family and social life, as well as the eyes and the brain, and the sections of the brain that his job. Because no drug treatment was effective, D. B. process visual information. Next we focus on how neu- had the malformed blood vessels surgically removed in rons respond to visual input, enabling the brain to distin- 1973. The operation relieved his pain and generally im- guish different features, such as color and shape. Finally, proved his life, but a part of his right occipital lobe was we explore the culmination of the visual system—to un- deprived of blood and died. As a result, D. B. became derstand what we see. How do we process light energy in blind in the left half of his field of vision. In other words, such a way as to grasp the meaning of written words or to as he looks at the world through either eye, he is unable to see the beauty in a painting by Monet? see anything to the left of the midline. D. B. came to the attention of Lawrence Weizkrantz, a world-renowned visual neuroscientist at Oxford Univer- I 277 p 278 I CHAPTER 8 D. B., Karl Lashley, and Migraines Focus on Disorders D. B. and Karl Lashley have several things in common. One area; and, as that area enlarged, he could detect lines of dif- is that they are both well known among neuroscientists. ferent orientations in it. The results of blood-flow studies D. B. is a well-studied patient in visual neuroscience, and have shown that, during such an aura, there is a reduction of Karl Lashley was a pioneer of research in this field. Both blood flow in the posterior occipital cortex, and this reduc- D. B. and Lashley also suffered severe migraines. The term tion spreads at a rate of about 2 millimeters per minute. The migraine (which derives from a Greek word meaning “half aura may gradually expand to fill an entire side of the per- of the skull”) refers to recurrent attacks of headache son’s field of vision, but rarely, if ever, does it cross that are usually localized to one side of the head; over to the opposite side. When a visual aura is at that vary in severity, frequency, and duration; and its maximum, the person can see nothing on that that are often accompanied by nausea and vomit- side of the world. Vision then returns, although ing. Migraine is perhaps the most common of all most people feel dizzy and often nauseated for a neurological disorders, afflicting some 5 to 20 per- while. Although D. B. and Lashley had visual cent of the population at some time in their lives. Karl Lashley auras, auras may also be auditory or tactile, and, in Both D. B. and Karl Lashley suffered from classic mi- some cases, they may result in an inability to move or to graine, which is preceded by an aura that usually lasts for 20 talk. After the aura passes, most people suffer a severe to 40 minutes. Lashley carefully described his visual aura, headache that results from a dilation of cerebral blood ves- which turned out to be common to many migraine sufferers. sels. The headache is usually on one side of the head, just as The aura began as a spot of flashing light and then slowly the aura is on one side of the field of vision. Left untreated, enlarged. Lashley had no sight in the flashing (scintillating) migraine headaches may last for hours or even days. XX XX X Novastock/Stock Connection/PictureQuest X = Fixation point The development of a migraine scotoma as described by Karl Lashley. A person looking at the small “x” first sees a small patch of lines, as shown in the photo- graph at the far left. Information in the world is not visible at that location. The striped area continues to grow outward, leaving a white area where the stripes had been. Within 15 to 20 minutes, the visual field is almost completely blocked by the scotoma. Normal vision returns shortly thereafter. THE NATURE OF SENSORY EXPERIENCE As we look at the world, we naturally assume that what we see is what is really “out there.” Indeed, cameras and videos reinforce this impression by seeming to re-create the very same visual world that we experience first hand. But there is a problem. Our version of the world, whether we see it directly or reproduced on film, is always a cre- ation of the brain. What we see is not a faithful reproduction of what is “out there” but rather a construction of reality that the brain manufactures. p HOW DO WE SEE THE WORLD? I 279 Sensory Experience and Sensory Reality Dogs provide a good example of the difference between the world that is “out there” and our perception of it. Dogs have very limited capacity to distinguish colors; they likely see very little color. Yet dogs have an olfactory system that smells in “Techni- color” compared with our simple “black and white” version of smell. Which system offers a correct analysis of the world? Neither. The brain of a human and that of a dog each create a set of sensory experiences that is merely one of many different versions of “reality.” These sensory experiences are not genuine reproductions of the world; rather, they exist only in the mind of the perceiver.
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