Guyton and Hall Physiology Chapter 52 - The Eye III. PDF

CHAPTER 52 The Eye: III. Central Neurophysiology of UNIT X Vision VISUAL PATHWAYS...

CHAPTER 52 The Eye: III. Central Neurophysiology of UNIT X Vision VISUAL PATHWAYS light reflex; (3) into the superior colliculus to control rapid Figure 52-1 shows the principal visual pathways from the directional movements of the two eyes; and (4) into the two retinas to the visual cortex. The visual nerve signals ventral lateral geniculate nucleus of the thalamus and sur- leave the retinas through the optic nerves. At the optic rounding basal regions of the brain, presumably to help chiasm, the optic nerve fibers from the nasal halves of control some of the body’s behavioral functions. the retinas cross to the opposite sides, where they join Thus, the visual pathways can be divided roughly into the fibers from the opposite temporal retinas to form the an old system to the midbrain and base of the forebrain optic tracts. The fibers of each optic tract then synapse and a new system for direct transmission of visual sig- in the dorsal lateral geniculate nucleus of the thalamus nals into the visual cortex located in the occipital lobes. and, from there, geniculocalcarine fibers pass via the optic In humans, the new system is responsible for perception radiation (also called the geniculocalcarine tract) to the of virtually all aspects of visual form, colors, and other primary visual cortex in the calcarine fissure area of the conscious vision. In many primitive animals, however, medial occipital lobe. even visual form is detected by the older system, using Visual fibers also pass to several older areas of the brain: the superior colliculus in the same manner that the visual (1) from the optic tracts to the suprachiasmatic nucleus cortex is used in mammals. of the hypothalamus, presumably to control circadian rhythms that synchronize various physiological changes FUNCTION OF THE DORSAL LATERAL of the body with night and day; (2) into the pretectal nuclei GENICULATE NUCLEUS OF THE in the midbrain to elicit reflex movements of the eyes to THALAMUS focus on objects of importance and activate the pupillary The optic nerve fibers of the new visual system terminate in the dorsal lateral geniculate nucleus, located at the dor- sal end of the thalamus and also called the lateral genic- Lateral geniculate body ulate body, as shown in Figure 52-1. The dorsal lateral Optic tract geniculate nucleus serves two principal functions. First, it Optic radiation Optic chiasm relays visual information from the optic tract to the visual Optic nerve cortex by way of the optic radiation. This relay function is so accurate that there is exact point to point transmission Left eye with a high degree of spatial fidelity all the way from the retina to the visual cortex. After passing the optic chiasm, half the fibers in each optic tract are derived from one eye and half are derived from the other eye, representing corresponding points on Visual Superior cortex colliculus the two retinas. However, the signals from the two eyes are kept apart in the dorsal lateral geniculate nucleus. This nucleus is composed of six nuclear layers. Layers II, III, and V (from ventral to dorsal) receive signals from the lat- eral half of the ipsilateral retina, whereas layers I, IV, and VI receive signals from the medial half of the retina of the Right eye opposite eye. The respective retinal areas of the two eyes connect with neurons that are superimposed over one Figure 52-1. Principal visual pathways from the eyes to the visual another in the paired layers, and similar parallel transmis- cortex. sion is preserved all the way to the visual cortex. 653 UNIT X The Nervous System: B. The Special Senses The second major function of the dorsal lateral genicu- late nucleus is to “gate” the transmission of signals to the visual cortex—that is, to control how much of the signal is allowed to pass to the cortex. The nucleus receives gating control signals from two major sources: (1) corticofugal fibers returning in a backward direction from the pri- Secondary mary visual cortex to the lateral geniculate nucleus; and visual areas (2) reticular areas of the mesencephalon. Both of these sources are inhibitory and, when stimulated, can turn off Calcarine fissure transmission through selected portions of the dorsal lat- eral geniculate nucleus. Both of these gating circuits help highlight the visual information that is allowed to pass. Primary Finally, the dorsal lateral geniculate nucleus is divided visual cortex in another way: 1. Layers I and II are called magnocellular layers be- cause they contain large neurons. These neurons re- 90° 60° 20° Macula ceive their input almost entirely from the large type Figure 52-2. The visual cortex in the calcarine fissure area of the M retinal ganglion cells. This magnocellular system medial occipital cortex. provides a rapidly conducting pathway to the visual cortex. However, this system is color blind, trans- Motor cortex Somatosensory area I mitting only black-and-white information. Also, its Form, point to point transmission is poor because there 3D position, are not many M ganglion cells, and their dendrites motion spread widely in the retina. 2. Layers III through VI are called parvocellular lay- ers because they contain large numbers of small 18 to medium-sized neurons. These neurons receive 17 their input almost entirely from the type P retinal ganglion cells that transmit color and convey accu- rate point to point spatial information, but at only a Primary moderate velocity of conduction rather than at high visual velocity. cortex Secondary Visual detail, visual color cortex ORGANIZATION AND FUNCTION OF THE Figure 52-3. Transmission of visual signals from the primary visual VISUAL CORTEX cortex into secondary visual areas on the lateral surfaces of the oc- Figures 52-2 and 52-3 show the visual cortex, which is cipital and parietal cortices. Note that the signals representing form, third-dimensional (3D) position, and motion are transmitted mainly located primarily on the medial aspect of the occipital into the superior portions of the occipital lobe and posterior portions lobes. Like the cortical representations of the other sen- of the parietal lobe. By contrast, the signals for visual detail and color sory systems, the visual cortex is divided into a primary are transmitted mainly into the anteroventral portion of the occipital visual cortex and secondary visual areas. lobe and the ventral portion of the posterior temporal lobe. Primary Visual Cortex. The primary visual cortex (see degree of visual acuity. Based on retinal area, the fovea Figure 52-2) lies in the calcarine fissure area, extending has several hundred times as much representation in the forward from the occipital pole on the medial aspect of primary visual cortex as do the most peripheral portions each occipital cortex. This area is the terminus of direct of the retina. visual signals from the eyes. Signals from the macular area The primary visual cortex is also called visual area I or of the retina terminate near the occipital pole, as shown the striate cortex because this area has a grossly striated in Figure 52-2, whereas signals from the more peripheral appearance. retina terminate at or in concentric half-circles anterior to the pole but still along the calcarine fissure on the medial Secondary Visual Areas of the Cortex. The secondary occipital lobe. The upper portion of the retina is repre- visual areas, also called visual association areas, lie lateral, sented superiorly, and the lower portion is represented anterior, superior, and inferior to the primary visual cor- inferiorly. tex. Most of these areas also fold outward over the lateral Note in the figure the large area that represents the surfaces of the occipital and parietal cortex, as shown in macula. It is to this region that the retinal fovea trans- Figure 52-3. Secondary signals are transmitted to these mits its signals. The fovea is responsible for the highest areas for analysis of visual meanings. For example, on all 654  CHAPTER 52 The Eye: III. Central Neurophysiology of Vision terminate in layer IV, but at points different from the M I signals. They terminate in layers IVa and IVcβ, the shal- lowest and deepest portions of layer IV, shown to the II right in Figure 52-4. From there, these signals are trans- mitted vertically both toward the surface of the cortex III and to deeper layers. It is these P ganglion pathways that UNIT X (a) transmit the accurate point to point type of vision, as well as color vision. (b) Color IV “blobs” Vertical Neuronal Columns in the Visual Cortex. The (c␣) visual cortex is organized structurally into several million (c␤) vertical columns of neuronal cells, with each column hav- ing a diameter of 30 to 50 micrometers. The same vertical V columnar organization is found throughout the cerebral cortex for the other senses as well (and also in the motor VI and analytical cortical regions). Each column represents a functional unit. One can roughly calculate that each of the visual vertical columns has perhaps 1000 or more neu- LGN LGN rons. (magnocellular) (parvocellular) After the optic signals terminate in layer IV, they are further processed as they spread outward and inward Retinal Retinal along each vertical column unit. This processing is "M" "P" believed to decipher separate bits of visual information ganglion ganglion at successive stations along the pathway. The signals that pass outward to layers I, II, and III eventually transmit sig- Fast, Black and White Very Accurate, Color nals for short distances laterally in the cortex. The signals Figure 52-4. Six layers of the primary visual cortex. The connections that pass inward to layers V and VI excite neurons that shown on the left side of the figure originate in the magnocellular transmit signals over much greater distances. layers of the lateral geniculate nucleus (LGN) and transmit rapidly changing black and white visual signals. The pathways to the right originate in the parvocellular layers (layers III–VI) of the LGN; they “Color Blobs” in the Visual Cortex. Interspersed among transmit signals that depict accurate spatial detail, as well as color. the primary visual columns, as well as among the columns Note especially the areas of the visual cortex called color blobs, which of some of the secondary visual areas, are special column- are necessary for detection of color. like areas called color blobs. They receive lateral signals from adjacent visual columns and are activated specifi- sides of the primary visual cortex is Brodmann’s area 18 cally by color signals. Therefore, these blobs are presum- (see Figure 52-3), which is where virtually all signals from ably the primary areas for deciphering color. the primary visual cortex pass next. Therefore, Brod- mann’s area 18 is called visual area II, or simply V-2. The Interaction of Visual Signals From the Two Separate other, more distant secondary visual areas have specific Eyes. Recall that visual signals from the two separate designations—V-3, V-4, and so forth—up to more than a eyes are relayed through separate neuronal layers in the dozen areas. The importance of all these areas is that vari- lateral geniculate nucleus. These signals remain separat- ous aspects of the visual image are progressively dissected ed from each other when they arrive in layer IV of the and analyzed. primary visual cortex. In fact, layer IV is interlaced with stripes of neuronal columns, with each stripe about 0.5 THE PRIMARY VISUAL CORTEX HAS SIX millimeter wide; the signals from one eye enter the col- MAJOR LAYERS umns of every other stripe, alternating with signals from Like almost all other portions of the cerebral cortex, the the second eye. This cortical area deciphers whether the primary visual cortex has six distinct layers, as shown in respective areas of the two visual images from the two Figure 52-4. Also, as is true for the other sensory systems, separate eyes are “in register” with each other—that is, the geniculocalcarine fibers terminate mainly in layer IV, whether corresponding points from the two retinas fit but this layer is also organized into subdivisions. The rap- with each other. In turn, the deciphered information is idly conducted signals from the M retinal ganglion cells used to adjust the directional gaze of the separate eyes so terminate in layer IVcα, and from there they are relayed that they will fuse with each other (i.e., be brought into vertically, both outward toward the cortical surface and “register”). The information observed about degree of inward toward deeper levels. register of images from the two eyes also allows a person The visual signals from the medium-sized optic nerve to distinguish the distance of objects by the mechanism fibers, derived from the P ganglion cells in the retina, also of stereopsis. 655 UNIT X The Nervous System: B. The Special Senses Two Major Pathways for Analysis of Visual Informa- tion: (1) The Fast “Position” and “Motion” Pathway and (2) the Accurate Color Pathway. Figure 52-3 shows that after leaving the primary visual cortex, the visual information is analyzed in two major pathways in the secondary visual areas. 1. Analysis of Third-Dimensional Position, Gross Form, and Motion of Objects. One of the analytical pathways, demonstrated in Figure 52-3 by the black Retinal image Cortical stimulation arrows, analyzes the third-dimensional positions of Figure 52-5. Pattern of excitation that occurs in the visual cortex in visual objects in the space around the body. This path- response to a retinal image of a dark cross. way also analyzes the gross physical form of the visual scene, as well as motion in the scene. This pathway stimulated adjacent retinal receptors mutually inhibit one reveals where every object is during each instant and another. However, at any border in the visual scene where whether it is moving. After leaving the primary visual there is a change from dark to light or light to dark, mu- cortex, the signals flow generally into the posterior tual inhibition does not occur, and the intensity of stimu- midtemporal area and upward into the broad occip- lation of most neurons is proportional to the gradient of itoparietal cortex. At the anterior border of the pari- contrast—that is, the greater the sharpness of contrast etal cortex, the signals overlap with signals from the and the greater the intensity difference between light and posterior somatic association areas that analyze three- dark areas, the greater the degree of stimulation. dimensional aspects of somatosensory signals. The signals transmitted in this position-form-motion path- Visual Cortex Also Detects Orientation of Lines and way are mainly from the large M optic nerve fibers of Borders—“Simple” Cells. The visual cortex detects not the retinal M ganglion cells, transmitting rapid signals only the existence of lines and borders in the different ar- but depicting only black and white with no color. eas of the retinal image but also the direction of orienta- 2. Analysis of Visual Detail and Color. The red arrows tion of each line or border—that is, whether it is vertical in Figure 52-3, passing from the primary visual cortex or horizontal or lies at some degree of inclination. This into secondary visual areas of the inferior, ventral, and capability is believed to result from linear organizations medial regions of the occipital and temporal cortex, of mutually inhibiting cells that excite second-order neu- show the principal pathway for analysis of visual detail. rons when inhibition occurs all along a line of cells where Separate portions of this pathway specifically dissect there is a contrast edge. Thus, for each such orientation out color as well. Therefore, this pathway is concerned of a line, specific neuronal cells are stimulated. A line with such visual feats as recognizing letters, reading, oriented in a different direction excites a different set of determining the texture of surfaces, determining de- cells. These neuronal cells are called simple cells. They are tailed colors of objects, and deciphering from all this found mainly in layer IV of the primary visual cortex. information what the object is and what it means. “Complex” Cells Detect Line Orientation When a Line Is Displaced Laterally or Vertically in the Visual NEURONAL PATTERNS OF Field. As the visual signal progresses farther away from STIMULATION DURING ANALYSIS OF layer IV, some neurons respond to lines that are oriented VISUAL IMAGES in the same direction but are not position-specific. That is, even if a line is displaced moderate distances laterally Analysis of Contrasts in Visual Images. If a person or vertically in the field, the same few neurons will still be looks at a blank wall, only a few neurons in the primary stimulated if the line has the same direction. These cells visual cortex will be stimulated, regardless of whether the are called complex cells. illumination of the wall is bright or weak. Therefore, what does the primary visual cortex detect? To answer this Detection of Lines of Specific Lengths, Angles, or question, let us now place on the wall a large solid cross, Other Shapes. Some neurons in the outer layers of the as shown to the left in Figure 52-5. To the right is shown primary visual columns, as well as neurons in some sec- the spatial pattern of the most excited neurons in the ondary visual areas, are stimulated only by lines or bor- visual cortex. Note that the areas of maximum excitation ders of specific lengths, by specific angulated shapes, or occur along the sharp borders of the visual pattern. Thus, by images that have other characteristics. That is, these the visual signal in the primary visual cortex is concerned neurons detect still higher orders of information from the mainly with contrasts in the visual scene, rather than with visual scene. Thus, as one goes farther into the analytical noncontrasting areas. We noted in Chapter 51 that this is pathway of the visual cortex, progressively more charac- also true of most of the retinal ganglion because equally teristics of each visual scene are deciphered. 656  CHAPTER 52 The Eye: III. Central Neurophysiology of Vision 105 90 75 DETECTION OF COLOR Left 80 Right 120 60 Color is detected in much the same way that lines are 70 135 60 45 detected—by means of color contrast. For example, a red 50 area is often contrasted against a green area, a blue area 150 40 30 against a red area, or a green area against a yellow area. UNIT X 30 All these colors can also be contrasted against a white area 165 20 15 within the visual scene. In fact, this contrasting against 10 white is believed to be mainly responsible for the phenom- 180 80 70 60 50 40 30 20 10 20 30 40 50 60 70 80 0 enon called “color constancy”—that is, when the color Optic 10 of an illuminating light changes, the color of the “white” disc 20 345 195 changes with the light, and appropriate computation in the 30 brain allows red to be interpreted as red, even though the 40 330 210 illuminating light has changed the color entering the eyes. 50 The mechanism of color contrast analysis depends on 60 315 225 the fact that contrasting colors, called “opponent colors,” 70 240 80 300 excite specific neuronal cells. It is presumed that the ini- 255 285 270 tial details of color contrast are detected by simple cells, whereas more complex contrasts are detected by complex Figure 52-6. Perimetry chart showing the field of vision for the left eye. The red circle shows the blind spot. and hypercomplex cells. Another condition that can be diagnosed by perimetry Effect of Removing the Primary Visual Cortex is retinitis pigmentosa. In this disease, portions of the retina degenerate, and excessive melanin pigment is deposited in Removal of the primary visual cortex in the human being the degenerated areas. Retinitis pigmentosa usually causes causes loss of conscious vision—that is, blindness. How- blindness in the peripheral field of vision first and then ever, psychological studies demonstrate that such “blind” gradually encroaches on the central areas. people can still, at times, react subconsciously to changes in light intensity, to movement in the visual scene or, rarely, even to some gross patterns of vision. These reactions in- EYE MOVEMENTS AND THEIR CONTROL clude turning the eyes, turning the head, and avoidance. This vision is believed to be subserved by neuronal path- To make full use of the visual abilities of the eyes, almost ways that pass from the optic tracts mainly into the supe- equally as important as interpretation of the visual signals rior colliculi and other portions of the older visual system. from the eyes is the cerebral control system for directing the eyes toward the object to be viewed. Fields of Vision; Perimetry The field of vision is the visual area seen by an eye at a given Muscular Control of Eye Movements. The eye move- instant. The area seen to the nasal side is called the nasal ments are controlled by three pairs of muscles, shown in field of vision, and the area seen to the lateral side is called Figure 52-7: (1) the medial and lateral recti; (2) the su- the temporal field of vision. perior and inferior recti; and (3) the superior and inferior To diagnose blindness in specific portions of the retina, one charts the field of vision for each eye by a process called obliques. The medial and lateral recti contract to move the perimetry. This charting is performed by having the subject eyes from side to side. The superior and inferior recti con- look with one eye toward a central spot directly in front tract to move the eyes upward or downward. The oblique of the eye; the other eye is closed. A small dot of light or muscles function mainly to rotate the eyeballs to keep the a small object is then moved back and forth in all areas of visual fields in the upright position. the field of vision, and the subject indicates when the spot of light or object can and cannot be seen. The field of vision Neural Pathways for Control of Eye Movements. for the left eye is plotted as shown in Figure 52-6. In all Figure 52-7 also shows brain stem nuclei for the third, perimetry charts, a blind spot caused by lack of rods and fourth, and sixth cranial nerves and their connections cones in the retina over the optic disc is found about 15 with the peripheral nerves to the ocular muscles. Also degrees lateral to the central point of vision, as shown in shown are interconnections among the brain stem nuclei the figure. via the nerve tract called the medial longitudinal fascicu- Abnormalities in the Fields of Vision. Occasionally, blind spots are found in portions of the field of vision oth- lus. Each of the three sets of muscles to each eye is recip- er than the optic disc area. Such blind spots, called scoto- rocally innervated so that one muscle of the pair relaxes mata, are frequently caused by damage to the optic nerve while the other contracts. resulting from glaucoma (too much fluid pressure in the Figure 52-8 illustrates cortical control of the oculo- eyeball), allergic reactions in the retina, or toxic condi- motor apparatus, showing spread of signals from visual tions such as lead poisoning or excessive use of tobacco. areas in the occipital cortex through occipitotectal and 657 UNIT X The Nervous System: B. The Special Senses occipitocollicular tracts to the pretectal and superior col- the oculomotor system, from the vestibular nuclei via the liculus areas of the brain stem. From both the pretectal medial longitudinal fasciculus. and the superior colliculus areas, the oculomotor con- trol signals pass to the brain stem nuclei of the oculomo- FIXATION MOVEMENTS OF THE EYES tor nerves. Strong signals are also transmitted from the Perhaps the most important movements of the eyes are body’s equilibrium control centers in the brain stem into those that cause the eyes to “fix” on a discrete portion of the field of vision. Fixation movements are controlled by two neuronal mechanisms. The first of these mecha- Superior Superior nisms, called the voluntary fixation mechanism, allows a rectus oblique person to move the eyes voluntarily to find the object on which he or she wants to fix the vision. The second is the involuntary fixation mechanism that holds the eyes firmly Lateral Medial on the object once it has been found. rectus rectus The voluntary fixation movements are controlled by a cortical field located bilaterally in the premotor corti- cal regions of the frontal lobes, as shown in Figure 52-8. Bilateral dysfunction or destruction of these areas makes Nuclei it difficult for a person to “unlock” the eyes from one point N. III of fixation and move them to another point. It is usually necessary to blink the eyes or put a hand over the eyes for a short time, which then allows the eyes to be moved. N. IV Conversely, the involuntary fixation mechanism that Inferior Inferior rectus oblique causes the eyes to “lock” on the object of attention once Medial longitudinal fasciculus it is found is controlled by secondary visual areas in the occipital cortex, located mainly anterior to the primary N. VI visual cortex. When this fixation area is destroyed bilater- ally in an animal, the animal has difficulty keeping its eyes Figure 52-7. Anterior view of the right eye showing extraocular directed toward a given fixation point or may become muscles of the eye and their innervation. N., Nerve. totally unable to do so. Voluntary fixation area Involuntary fixation area Visual association areas Primary visual cortex Occipitotectal and occipitocollicular tracts Frontotectal tract Pretectal nuclei Visceral nucleus III nerve Superior colliculus Oculomotor nucleus N. III Trochlear nucleus N. IV Abducens nucleus N. VI Vestibular nuclei Figure 52-8. Neural pathways for control of conjugate movement of Medial longitudinal fasciculus the eyes. N., Nerve. 658  CHAPTER 52 The Eye: III. Central Neurophysiology of Vision Saccadic Movement of the Eyes—A Mechanism of Successive Fixation Points. When a visual scene is mov- ing continually before the eyes, such as when a person is riding in a car, the eyes fix on one highlight after another in the visual field, jumping from one to the next at a rate of two to three jumps per second. The jumps are called sac- UNIT X cades, and the movements are called opticokinetic move- ments. The saccades occur so rapidly that no more than 10% of the total time is spent moving the eyes, with 90% Voluntary movement to of the time being allocated to the fixation sites. Also, the fixation site brain suppresses the visual image during saccades, so the person is not conscious of the movements from point to point. Figure 52-9. Movements of a spot of light on the fovea, showing sudden “flicking” eye movements that move the spot back toward Saccadic Movements During Reading. During the the center of the fovea whenever it drifts to the foveal edge. The dashed lines represent slow drifting movements, and the solid lines process of reading, a person usually makes several sac- represent sudden flicking movements. (Modified from Whitteridge D: cadic movements of the eyes for each line. In this case, the Central control of the eye movements. In: Field J, Magoun HW, Hall visual scene is not moving past the eyes, but the eyes are VE [eds]: Handbook of Physiology, vol. 2, sec. 1. Washington, DC: trained to move by means of several successive saccades American Physiological Society, 1960.) across the visual scene to extract the important informa- tion. Similar saccades occur when a person observes a To summarize, posterior “involuntary” occipital corti- painting, except that the saccades occur in upward, side- cal eye fields automatically “lock” the eyes on a given spot ways, downward, and angulated directions one after an- of the visual field and thereby prevent movement of the other from one highlight of the painting to another, and image across the retinas. To unlock this visual fixation, so forth. voluntary signals must be transmitted from cortical “vol- untary” eye fields located in the frontal cortices. Fixation on Moving Objects—“Pursuit Movement.” The eyes can also remain fixed on a moving object, which Mechanism of Involuntary Locking Fixation—Role is called pursuit movement. A highly developed cortical of the Superior Colliculi. The involuntary locking mechanism automatically detects the course of move- type of fixation discussed in the previous section re- ment of an object and then rapidly develops a similar sults from a negative feedback mechanism that pre- course of movement for the eyes. For example, if an ob- vents the object of attention from leaving the foveal ject is moving up and down in a wavelike form at a rate of portion of the retina. The eyes normally have three several times per second, the eyes at first may be unable to types of continuous but almost imperceptible move- fixate on it. However, after a second or so, the eyes begin ments: (1) a continuous tremor at a rate of 30 to 80 cy- to jump by means of saccades in approximately the same cles/sec caused by successive contractions of the motor wavelike pattern of movement as that of the object. Then, units in the ocular muscles; (2) a slow drift of the eye- after another few seconds, the eyes develop progressively balls in one direction or another; and (3) sudden flick- smoother movements and finally follow the wave move- ing movements that are controlled by the involuntary ment almost exactly. This represents a high degree of au- fixation mechanism. tomatic subconscious computational ability by the pursuit When a spot of light becomes fixed on the foveal system for controlling eye movements. region of the retina, the tremulous movements cause the spot to move back and forth at a rapid rate across the Superior Colliculi Are Mainly Responsible for Turn- cones, and the drifting movements cause the spot to drift ing the Eyes and Head Toward a Visual Disturbance. slowly across the cones. Each time the spot drifts as far Even after the visual cortex has been destroyed, a sudden as the edge of the fovea, a sudden reflex reaction occurs, visual disturbance in a lateral area of the visual field often producing a flicking movement that moves the spot away causes immediate turning of the eyes in that direction. from this edge back toward the center of the fovea. Thus, This turning does not occur if the superior colliculi have an automatic response moves the image back toward the also been destroyed. To support this function, the various central point of vision. points of the retina are represented topographically in the These drifting and flicking motions are demonstrated superior colliculi in the same way as in the primary visual in Figure 52-9. The dashed lines show the slow drifting cortex, although with less accuracy. Even so, the principal across the fovea, and the solid lines show the flicks that direction of a flash of light in a peripheral retinal field is keep the image from leaving the foveal region. This invol- mapped by the colliculi, and secondary signals are trans- untary fixation capability is mostly lost when the superior mitted to the oculomotor nuclei to turn the eyes. To help colliculi are destroyed. in this directional movement of the eyes, the superior 659 UNIT X The Nervous System: B. The Special Senses colliculi also have topological maps of somatic sensations side of the central pathway. Therefore, some optic path- from the body and acoustic signals from the ears. ways from the two eyes are exactly in register for objects 2 The optic nerve fibers from the eyes to the colliculi, meters away; still another set of pathways is in register for which are responsible for these rapid turning movements, objects 25 meters away. Thus, the distance is determined are branches from the rapidly conducting M fibers, with by which set or sets of pathways are excited by nonregis- one branch going to the visual cortex and the other going ter or register. This phenomenon is called depth percep- to the superior colliculi. In addition to causing the eyes to tion, another name for stereopsis. turn toward a visual disturbance, signals are relayed from the superior colliculi through the medial longitudinal fas- Strabismus—Lack of Fusion of the Eyes ciculus to other levels of the brain stem to cause turning Strabismus, also called squint or cross-eye, means lack of of the whole head and even of the whole body toward fusion of the eyes in one or more of the visual coordinates: the direction of the disturbance. Other types of nonvi- horizontal, vertical, or rotational. The basic types of strabis- sual disturbances, such as strong sounds or even stroking mus are shown in Figure 52-10: (1) horizontal strabismus; of the side of the body, cause similar turning of the eyes, (2) torsional strabismus; and (3) vertical strabismus. Com- binations of two or even all three of the different types of head, and body, but only if the superior colliculi are intact. strabismus often occur. Therefore, the superior colliculi play a global role in ori- Strabismus is often caused by abnormal “set” of the fu- enting the eyes, head, and body with respect to external sion mechanism of the visual system. That is, in a young disturbances, whether they are visual, auditory, or somatic. child’s early efforts to fixate the two eyes on the same ob- ject, one of the eyes fixates satisfactorily while the other “FUSION” OF THE VISUAL IMAGES FROM fails to do so, or they both fixate satisfactorily but never THE TWO EYES simultaneously. Soon the patterns of conjugate movements of the eyes become abnormally “set” in the neuronal control To make the visual perceptions more meaningful, the pathways themselves, so the eyes never fuse. visual images in the two eyes normally fuse with each Suppression of the Visual Image From a Repressed Eye. other on “corresponding points” of the two retinas. The In a few patients with strabismus, the eyes alternate in fix- visual cortex plays an important role in fusion. We previ- ing on the object of attention. In other patients, one eye ously discussed that corresponding points of the two reti- alone is used all the time, and the other eye becomes re- nas transmit visual signals to different neuronal layers of pressed and is never used for precise vision. The visual acu- the lateral geniculate body, and these signals, in turn, are ity of the repressed eye develops only slightly, sometimes relayed to parallel neurons in the visual cortex. Interactions remaining as 20/400 or less. If the dominant eye then be- occur between these cortical neurons to cause interference comes blinded, vision in the repressed eye can develop only excitation in specific neurons when the two visual images to a slight extent in adults but far more in young children. are not “in register”—that is, are not precisely “fused.” This This demonstrates that visual acuity is highly dependent excitation presumably provides the signal that is transmit- on proper development of central nervous system synaptic ted to the oculomotor apparatus to cause convergence or connections from the eyes. In fact, even anatomically, the numbers of neuronal connections diminish in the visual divergence or rotation of the eyes so that fusion can be cortex areas that would normally receive signals from the re-established. Once the corresponding points of the two repressed eye. retinas are in register, excitation of the specific “interfer- ence” neurons in the visual cortex disappears. AUTONOMIC CONTROL OF ACCOMMO- Neural Mechanism of Stereopsis for DATION AND PUPILLARY APERTURE Judging Distances of Visual Objects Because the two eyes are more than 2 inches apart, the AUTONOMIC NERVES TO THE EYES images on the two retinas are not exactly the same. That The eye is innervated by both parasympathetic and sym- is, the right eye sees a little more of the right-hand side of pathetic nerve fibers, as shown in Figure 52-11. The the object, and the left eye sees a little more of the left- parasympathetic preganglionic fibers arise in the Edinger- hand side; the closer the object, the greater the disparity. Westphal nucleus—the visceral nucleus portion of the Therefore, even when the two eyes are fused with each third cranial nerve—and then pass in the third nerve to other, it is still impossible for all corresponding points in the two visual images to be exactly in register at the same time. Furthermore, the nearer the object is to the eyes, the less the degree of register. This degree of nonregister pro- vides the neural mechanism for stereopsis, an important mechanism for judging the distances of visual objects up to about 200 feet (61 meters). The neuronal cellular mechanism for stereopsis is Horizontal Torsional Vertical strabismus strabismus strabismus based on the fact that some of the fiber pathways from the retinas to the visual cortex stray 1 to 2 degrees on each Figure 52-10. Basic types of strabismus. 660  CHAPTER 52 The Eye: III. Central Neurophysiology of Vision refractive power. How does a person adjust accommoda- tion to keep the eyes in focus all the time? Accommodation of the lens is regulated by a negative feedback mechanism that automatically adjusts the refrac- Edinger- tive power of the lens to achieve the highest degree of Pretectal Westphal Ciliary visual acuity. When the eyes have been focused on some far UNIT X region nucleus ganglion object and must then suddenly focus on a near object, the N. III lens usually accommodates for best acuity of vision within less than 1 second. Although the precise control mecha- N. II nism that causes this rapid and accurate focusing of the eye is not fully understood, the following features are known. Pons First, when the eyes suddenly change distance of the N. V fixation point, the lens changes its strength in the proper direction to achieve a new state of focus within a fraction of a second. Second, different types of clues help change Carotid plexus the lens strength in the proper direction, as follows: 1. Chromatic aberration appears to be important. That is, red light rays focus slightly posteriorly to blue Superior cervical ganglion light rays because the lens bends blue rays more than red rays. The eyes appear to be able to detect Cervical which of these two types of rays is in better focus, sympathetic and this clue relays information to the accommoda- trunk tion mechanism with regard to whether to make the lens stronger or weaker. Upper thoracic segments of 2. When the eyes fixate on a near object, the eyes must spinal cord converge. The neural mechanisms for convergence Figure 52-11. Autonomic innervation of the eye, showing also the cause a simultaneous signal to strengthen the lens of reflex arc of the light reflex. N., Nerve. the eye. 3. Because the fovea lies in a hollowed-out depression the ciliary ganglion, which lies immediately behind the that is slightly deeper than the remainder of the ret- eye. There, the preganglionic fibers synapse with post- ina, the clarity of focus in the depth of the fovea is ganglionic parasympathetic neurons, which in turn send different from the clarity of focus on the edges. This fibers through ciliary nerves into the eyeball. These nerves difference may also give clues about which way the excite the following: (1) the ciliary muscle that controls strength of the lens needs to be changed. focusing of the eye lens; and (2) the sphincter of the iris 4. The degree of accommodation of the lens oscillates that constricts the pupil. slightly all the time at a frequency up to twice per The sympathetic innervation of the eye originates in second. The visual image becomes clearer when the the intermediolateral horn cells of the first thoracic seg- oscillation of the lens strength is changing in the ap- ment of the spinal cord. From there, sympathetic fibers propriate direction and becomes poorer when the enter the sympathetic chain and pass upward to the lens strength is changing in the wrong direction. This superior cervical ganglion, where they synapse with post- could give a rapid clue as to which way the strength of ganglionic neurons. Postganglionic sympathetic fibers the lens needs to change to provide appropriate focus. from these neurons then spread along the surfaces of the The brain cortical areas that control accommodation carotid artery and successively smaller arteries until they closely parallel those that control fixation movements of reach the eye. There, the sympathetic fibers innervate the the eyes. Analysis of the visual signals in Brodmann’s cor- radial fibers of the iris, which open the pupil, as well as tical areas 18 and 19 and transmission of motor signals to several extraocular muscles of the eye, discussed subse- the ciliary muscle occur through the pretectal area in the quently in relation to Horner’s syndrome. brain stem, then through the Edinger-Westphal nucleus, and finally via parasympathetic nerve fibers to the eyes. CONTROL OF ACCOMMODATION (FOCUSING THE EYES) CONTROL OF PUPILLARY DIAMETER The accommodation mechanism—that is, the mechanism Stimulation of the parasympathetic nerves also excites that focuses the lens system of the eye—is essential for a the pupillary sphincter muscle, thereby decreasing the high degree of visual acuity. Accommodation results from pupillary aperture; this process is called miosis. Con- contraction or relaxation of the eye ciliary muscle. Con- versely, stimulation of the sympathetic nerves excites the traction causes increased refractive power of the lens, as radial fibers of the iris and causes pupillary dilation, called explained in Chapter 50, and relaxation causes decreased mydriasis. 661 UNIT X The Nervous System: B. The Special Senses Pupillary Light Reflex. When light is shone into the eyes, clinical condition called Horner syndrome. This syndrome the pupils constrict, a reaction called the pupillary light consists of the following effects: reflex. The neuronal pathway for this reflex is illustrated 1. Because of interruption of sympathetic nerve fibers to by the upper two black arrows in Figure 52-11. When the pupillary dilator muscle, the pupil remains persis- tently constricted to a smaller diameter than the pupil light impinges on the retina, a few of the resulting impuls- of the opposite eye. es pass from the optic nerves to the pretectal nuclei. From 2. The superior eyelid droops because it is normally main- here, secondary impulses pass to the Edinger-Westphal tained in an open position during waking hours, partly nucleus and, finally, back through parasympathetic nerves by contraction of smooth muscle fibers embedded in to constrict the sphincter of the iris. Conversely, in dark- the superior eyelid and innervated by the sympathetics. ness, the reflex becomes inhibited, which results in dila- Therefore, destruction of the sympathetic nerves makes tion of the pupil. it impossible to open the superior eyelid as widely as The function of the light reflex is to help the eye normally. adapt extremely rapidly to changing light conditions, as 3. The blood vessels on the corresponding side of the face explained in Chapter 51. The limits of pupillary diameter and head become persistently dilated. are about 1.5 millimeters on the small side and 8 milli- 4. Sweating (which requires sympathetic nerve signals) cannot occur on the side of the face and head affected meters on the large side. 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