Our Goal: To devise better ways to prevent and treat vision loss due to amblyopia and strabismus, and to advance medical science by understanding the human visual system.
In the Laboratory for Visual Neuroscience at the University of California, San Francisco, we are seeking to discover how visual perception occurs in the human brain. The function of the visual system is to guide our behavior by providing an efficient means for the rapid assimilation of information from the environment. As we navigate through our surroundings, a continuous stream of light images impinges on our eyes. In the back of each eye a light-sensitive tissue, the retina, converts patterns of light energy into electrical discharges known as action potentials. These signals are conveyed along the axons of retinal ganglion cells to the lateral geniculate body, a relay nucleus in the thalamus. Most of the output of the lateral geniculate body is relayed directly to the primary visual cortex (striate cortex, V1), and then to surrounding visual association areas. To understand the function of the visual pathways, our research is focused on 5 major themes:
Organization of Primary Visual Cortex: Within the primary visual cortex, visual information is processed by cells arranged within an elaborate system comprised of overlapping vertical columns and horizontal layers. Our goal is to elucidate the basic functional architecture of the visual cortex, to understand how groups of cells are organized in a modular fashion for information analysis. We are employing autoradiography, axon tracing, cytochrome oxidase histochemistry, functional gene expression, and electrophysiology to accomplish this aim. (View More Details)
Mapping of Extrastriate Visual Cortex: After initial processing by the primary visual cortex, signals are transferred to a multitude of nearby visual areas (known collectively as extrastriate visual cortex), where further image analysis takes place. It is unclear why the primate brain has so many different visual areas beyond the primary visual cortex. The most popular theory is that various cortical areas are specialized for processing different aspects of vision, e.g., motion, color, form, etc. Our objective is to map the boundaries, layout, topography, functional architecture, and projections of visual areas in extrastriate cortex, to determine how they contribute to perception. (View More Details)
Amblyopia and Visual Development: For centuries, philosophers have debated the relative contributions of innate factors (“nature”) versus environmental influences (“nurture”) in human development. A disease called amblyopia (“lazy eye”) can occur if a child is deprived of normal sight in one eye during infancy. We are seeking to define how visual deprivation affects the normal circuitry and function of the visual cortex. This research may lead to better treatments for amblyopia. (View More Details)
Strabismus and Visual Suppression: Binocular vision is such a natural experience that we take it for granted. In some children, however, the eyes fail to achieve normal alignment during development, leading to a condition called strabismus (crossed eyes). To avoid double vision, children with strabismus suppress perception through their deviated eye. This strategy eliminates the annoying experience of seeing two images, but it has the drawback of eliminating the incentive to fuse the eyes. We are studying how visual suppression occurs in the primate visual cortex. Understanding the mechanism of suppression may provide clues to the etiology of strabismus. (View More Details)
The Human Visual Cortex: Our ultimate goal is to understand the human visual cortex. Whenever possible, guided by results obtained from animal experiments, we perform anatomical studies in specimens of human brain obtained at autopsy from subjects with a history of visual loss, amblyopia, or strabismus. These studies have confirmed the remarkable similarity of the monkey and human visual systems. (View More Details)
ORGANIZATION OF PRIMARY VISUAL CORTEX
In 1782 a young medical student named Francesco Gennari described a thin white stripe of myelin running through the gray matter of the occipital lobe. This stria, visible to the naked eye in fresh or fixed specimens, prompted him to suggest that the cortex might be subdivided into discrete anatomical regions. This suggestion was revolutionary, because anatomists had previously assumed that the cortex was a uniform sheet of tissue, lacking any internal subdivisions. Today, of course, it is a well accepted notion that the cerebral cortex is partitioned into dozens of distinct areas controlling all aspects of human behavior. It was Gennari's discovery which opened the door for our modern view of cortical function. Ironically, in identifying the first cortical area in the brain, Gennari had no inkling that he had stumbled upon the primary visual cortex.
More than a century was to elapse before it was finally proven by Henschen, a Swedish neuropathologist, that the stria of Gennari is coextensive with the primary visual cortex. The primary visual cortex is often called the "striate cortex", recalling the prominent white stria found by Gennari.
The representation of vision in the cerebral cortex was explored early in this century by the clinical examination of soldiers wounded in battle. Inouye constructed the first detailed retinotopic map of primary visual cortex by matching visual field deficits with the trajectory of missiles penetrating the occipital lobes. We have prepared a revised version of these maps with the aid of magnetic resonance, a high-resolution imaging technique that allows direct correlation of occipital lobe lesions with visual field defects in patients:
Figure 1: Retinotopic map of the human striate cortex. Upper right shows left ocipital lobe, with most of striate cortex buried in the calcarine fissure. Upper left shows the fissure opened, with distance (eccentricity) from the fovea (center of gaze) marked in degrees. The horizontal meridian (HM) runs roughly along the base of the fissure. Lower left shows the map, removed from the calcarine fissure and flattened artificially. Dots depict occipital pole; the central 1o is located on the exposed lateral convexity, although this varies from person to person. Note the immense magnification of central vision. Dark oval = blind spot, stippled zone = monocular crescent. From: Horton & Hoyt, Arch Ophthal. 109:861, 1991.
The upper and lower visual quadrants are represented in the lower and upper calcarine banks respectively, separated by the horizontal meridian along the base of the calcarine fissure. The fovea, a specialized region within the retina specialized for best acuity, is represented at the occipital pole where primary visual cortex usually extends about one centimeter onto the lateral convexity of the hemisphere. The vertical meridian corresponds to the perimeter of primary visual cortex located along the exposed medial surface of the occipital lobe. Most of primary visual cortex is actually buried within the depth of the calcarine fissure. The clearest view of the visual field map can be obtained by schematically unfolding and flattening visual cortex to create an artificial planar surface. The primary visual cortex contains a topographic but highly distorted representation of the contralateral hemifield of vision.
The most striking feature of the visual field map is the enormous fraction of visual cortex assigned to the representation of central vision. About 55% of the surface area of primary visual cortex is devoted to the representation of the central 10o of vision. The cortical "magnification factor" -- the millimeters of cortex representing one degree of visual field -- has a ratio of more than 40:1 between the fovea (0o eccentricity) and the periphery (60o eccentricity). The temporal crescent representation (stippled area, Figure 1, lower panels constitutes only about 5% of the total surface area of primary visual cortex. The representation of central vision is highly magnified compared with peripheral vision, so that the cortical area devoted to the central 1o of visual field roughly equals the cortical area allotted to the entire monocular temporal crescent.
Recently, we compiled a map of the visual field representation in striate cortex using a novel approach. We discovered that the shadows cast by retinal blood vessels (known as angioscotomas) are represented in striate cortex of the squirrel monkey. We systematically identified corresponding points between retinal vessels and their cortical representations. We then warped the retina and its blood vessels directly onto the cortex, using these corresponding points as a guide. Analysis of the resulting map showed that the macula (the central 10 degrees of the retina) is over-represented in striate cortex, even if one allows for its high concentration of ganglion cells. For further details, see: Adams & Horton, A precise retinotopic map of primate striate cortex generated from the representation of angioscotomas, J. Neurosci, 23:3771-3789, 2003.
The relatively magnified representation of the macula in primary visual cortex furnishes an important clue to how the cerebral cortex analyzes sensory information. In the retina, 250 µm of tissue equals about 1 degree for all points in the visual field. This must remain nearly constant because the eye is engaged in processing an optical image of the visual environment. The steep gradient in visual acuity, from 20/20 centrally to 20/400 peripherally, is achieved by variation in the density of cells in various layers. For example, in central retina the ganglion cells are stacked 6-8 cells deep, declining to a broken monolayer in peripheral retina. Free of any optical constraints, the cerebral cortex handles the richer flow of visual information emanating from the central retina in a different fashion. The cortical sheet is essentially uniform in thickness throughout primary visual cortex but far more tissue is allocated for the analysis of central vision.
In the visual cortex, the magnification factor -- rather than the cell density -- varies with eccentricity in the visual field representation. A similar strategy is employed by the somatosensory cortex to represent the most densely innervated regions of the body surface, as exemplified by the exaggerated size of the face, tongue, and hands of the homunculus.
As discussed in the preceding paragraph, the central degree of vision has a hugely magnified representation in striate cortex. If the same amount of tissue were devoted to the representation of every single degree of the visual field, an enormous amount of cerebral cortex would be required. In fact, in such an imaginary brain, the entire surface of the cerebral cortex would be occupied with the processing of visual information (as matters stand already, more than a third of the cerebral cortex is already devoted to vision). Thus, one can view the evolution of a circumscribed high acuity foveal zone as an economy strategy by nature -- both at the retinal and the cortical level.
Our constant visual goal in life is to project and maintain targets of interest upon the fovea. Foveation requires six extraocular muscles to move each eye, systems for smooth pursuit, systems to abruptly shift gaze (saccades), and systems to stabilize gaze during head and body movements (the vestibulo-ocular and opto-kinetic reflexes). These systems are governed by an elaborate circuitry housed to a large extent at the level of the brainstem. Familiarity with these systems and their basic anatomy is of immense clinical importance because a wide variety of disease processes attack the brainstem or the cranial nerves innervating the eye muscles.
Ocular Dominance Columns
Axon terminals of cells within individual laminae of the lateral geniculate body terminate predominately in layer IVc, the main input layer to striate cortex. Axon terminals serving the right eye and left eye are not randomly distributed, but rather, they are segregated into a system of alternating parallel stripes called ocular dominance columns. In monkeys, the ocular dominance columns have been revealed by injecting one eye with tritiated proline, a radioactive tracer (Figure 2C). They can also be labeled by removing one eye and then staining the cortex for cytochrome oxidase, a mitochrondrial enzyme (Figure 2B). This method exploits the fact that levels of metabolic activity diminish in columns formerly driven by the missing eye. In flattened sections from subjects with a history of monocular enucleation, a mosaic of alternating light and dark stripes emerges in layer IVc.
The light columns correspond to the ocular dominance columns of the enucleated eye:
Figure 2: Flattened left striate cortex from a normal macaque monkey. A) Drawing of the ocular dominance columns. BS = blind spot representation, MC = monocular crescent representation. B) Montage of sections processed for cytochrome oxidase after enucleation of the right eye, showing alternating dark and pale stripes, corresponding to the ocular dominance columns in layer IVc. C) Montage compiled from alternate sections processed for autoradiography after tritiated proline injection into the vitreous of the left eye. Note the perfect match between the dark columns labeled by their metabolic activity in (B) and the light columns revealed by transneuronal axon tracing in (C).
Although ocular dominance columns are present in all macaques and humans tested so far, their expression in some species is inconsistent. For example, in the squirrel monkey, ocular dominance columns are present in some animals and not in others. Even more surprisingly, in some animals they are present in only part of the cortex. These observations have confounded efforts to determine the function of ocular dominance columns. For more details see: Adams & Horton, Capricious expression of cortical columns in the primate brain, Nature Neuroscience, 6:113-114, 2003.
Cytochrome Oxidase Patches
Striate cortex contains a regular array of patches containing increased activity of the metabolic enzyme, cytochrome oxidase. This array is most obvious in sections cut parallel to the pial surface (Figure 3) (Horton JC, Cytochrome oxidase patches: a new cytoarchitectonic feature of monkey visual cortex, Phil. Trans. R. Soc. (Lond.) B, 304:199-253, 1984). In three dimensions, these patches form cylinders that extend through the cortex from the white matter to the brain surface (except in layers IVc and IVa, where they are absent).
Figure 3: Tangential section from a normal macaque monkey, cut through layers 2/3, showing patches of increased cytochrome oxidase activity , each about 250 µm by 150 µm, in striate cortex. The patches are organized into rows that are in register with the ocular dominance columns in layer 4c. In the right drawing, the patches end at the V1/V2 border (dashed line).
Cytochrome oxidase patches may constitute the basic modules of striate cortex. In macaques and humans, they form rows in register with the ocular dominance columns. Curiously, this special relationship between cytochrome oxidase patches and ocular dominance columns is absent in the squirrel monkey (Horton & Hocking, Anatomical demonstration of ocular dominance columns in striate cortex of the squirrel monkey. J. Neurosci, 16:5510-5522, 1996).
Cytochrome oxidase patches in layers 2/3 receive direct input from the lateral geniculate body (Horton JC, Phil. Trans. R. Soc. (Lond.) B, 304:199-253, 1984), organized into separate projections serving the left eye and the right eye (Horton & Hocking, An adult-like pattern of ocular dominance columns in striate cortex of newborn monkeys prior to visual experience. J. Neuroscience, 16:1789-1805, 1996). This input originates from a recently discovered class of cells in the lateral geniculate body called the konio cells.
MAPPING OF EXTRASTRIATE VISUAL CORTEX
The cerebral cortex is a rather disappointing tissue at first glance. Its surface appears as a convoluted, gray sheet of tissue with no recognizable boundaries. In cross-section, architectonic borders can be identified with stains for cells and myelin, but only between a few areas. In a classic work, Korbinian Brodmann compiled a complete histological survey of the cerebral cortex, dividing it into 47 distinct areas. Sadly, most of the boundaries he drew have not stood the test of time.
For example, in the visual cortex Brodmann recognized three regions: areas 17, 18, and 19. Area 17, the primary visual cortex (V1), is easy to identify because it contains a prominent myelin band, the stria of Gennari, and a precise representation of the contralateral field of vision. However, the boundary between area 18 and 19 is invisible, and it is clear that there are far more than 3 visual areas in the primate brain. After processing in area 17, visual information is distributed for further analysis to at least a dozen surrounding visual areas. These areas are known collectively as “extrastriate visual cortex”. Mapping the boundaries, organization, and projections of these visual areas is a difficult challenge, in part because the primate cerebral cortex is highly folded into sulci and gyri. To facilitate anatomical studies of the macaque cortex, we have recently developed a method for flattening it prior to preparation of histological sections.
The flatmount technique allows one to obtain single sections of the entire cortex, greatly facilitating the analysis of anatomical data. Figure 4 shows an example of a flattened hemisphere from a macaque monkey:
Figure 4: Flattened right hemisphere from a macaque monkey. A full description of the flatmount technique is provided in: Sincich LC, Adams DL, and Horton JC, Complete flatmounting of the macaque cerebral cortex, Visual Neuroscience, 20:663-686, 2003).
Function of Extrastriate Cortex
Why does the brain contain so many cortical areas for vision? A generation ago, visual neuroscience was in the throes of a debate about how information is disseminated from the primary visual cortex (V1) to higher visual areas in the primate brain. Traditionalists believed that the visual image is first digested in V1, and then passed essentially intact through a series of higher cortical areas for further processing to extract perception. Revisionists proposed that images are broken down by V1 into various components, such as color, form and motion. According to this scheme, these separate components are then distributed via parallel projections to extrastriate areas specialized for their analysis. The matter appeared to be settled by a series of studies conducted by Livingstone and Hubel (1984, 1987). First, they identified three compartments in V1 corresponding to color, form and motion.
The color compartments were equated with the cytochrome oxidase patches in layers 2/3, the form compartments were designated as the interpatches in layers 2/3, and the motion compartment was assigned to layer 4b. Second, they demonstrated that each compartment in V1 sends a segregated projection to a distinct compartment in the next visual area (V2). Finally, they showed that the compartments in V2 are also divided by color, form, and motion. Contemporaneously, other investigators traced specific connections from V2 compartments to higher visual areas (V4, MT). Figure 5 summarizes this proposed pattern of connections:
Figure 5: Compartmentalization of visual information from the lateral geniculate body to extrastriate cortex, according the scheme proposed by Livingstone and Hubel (Science 1988).
Livingstone and Hubel’s work furnished powerful support for the idea that color, form, and motion can be assigned to distinct cortical compartments in V1 and V2. However, we have recently undertaken a re-examination of the projections from V1 to V2 and discovered a pattern of connections that conflicts with Livingstone and Hubel’s original description. The discrepancies highlighted by our new anatomical data cast doubt on their account of the projections from V1 to V2 and cast doubt on the notion that color, form, and motion processing are well segregated in the early visual cortex.
In area V2 (the second visual area), the cytochrome oxidase stain has revealed three parallel compartments, organized into a system of repeating thick-pale-thin-pale stripes that stretch from the V1 border to the V3 border:
Figure 6: Cytochrome oxidase section showing parallel stripes in macaque V2, organized into a repeating sequence of pale and dark stripes. The dark stripes alternate between thick (large arrows) and thin (small arrows). From: Horton & Hocking, An adult-like pattern of ocular dominance columns in striate cortex of newborn monkeys prior to visual experience. J. Neurosci, 16:1789-1805, 1996
Rather than arising from three compartments, we have found that projections from V1 to V2 originate from only two separate compartments: cytochrome oxidase patches and interpatches. They terminate in different compartments in V2. The patches send projections to thin stripes, whereas the interpatches send projections to thick stripes and pale stripes. The projections are not equal in strength: the strongest output from V1 terminates in pale stripes (Sincich & Horton, Pale cytochrome oxidase stripes in V2 receive the richest projection from macaque striate cortex. J. Comp. Neurology, 447:18-33, 2002).
The projections from V1 arise from layers 2/3, 4A, 4B, and 5/6. Only layers 1 and 4C do not project to V2. To determine if segregated (but physically intermingled) interpatch populations supply either pale stripes or thick stripes, we made paired injections of different retrograde tracers into V2. In three cases we succeeded in placing tracer injections into adjacent pale and thick stripes. Up to a third of the retrogradely filled cells in V1 were double-labeled, indicating that many interpatch cells project indiscriminately to both pale and thick stripes. The figure below shows our revised diagram of projections from V1 to extrastriate cortex.
Figure 7: Bipartite pattern of projections from V1 to V2. Cells in patches project to thin stripes, whereas cells in interpatches project to thick and pale stripes. Cells in layer 4B also project directly to area MT. (Sincich & Horton, Divided by cytochrome oxidase: a map of the projections from V1 to V2 in macaques. Science, 295:1734-1737, 2002).
Our studies have identified numerous projections from V1 to V2 that were previously unknown. They include outputs from layer 4B to pale stripes, layer 4B to thin stripes, and layers 2/3 to thick stripes. This new account of the V1 to V2 pathway is difficult to reconcile with the prior model (figure 5) based on the idea that only three projections exist from V1 to V2. The old model proposed that each projection carried information about color, form, and motion to thin, pale, and thick stripes respectively. Instead, we find that each V2 stripe type is richly supplied by all output layers of V1 (figure 7).
Previously, it was reported that layer 4B is a magno-dominated layer that projects exclusively to thick stripes. Although it is true that layer 4B receives strong input from magno-recipient layer 4Ca (Fitzpatrick et al., 1985), it also receives major parvo input (Yabuta et al., 2001). Moreover, our data show that layer 4B projects to all stripe types in V2. In light of these facts, it is inaccurate to construe layer 4B as simply a magno channel for conveying motion information to extrastriate cortex.
Movshon and Newsome (1996) used antidromic stimulation to characterize the physiological properties of layer 4B neurons that project directly to area MT. The units were oriented and highly direction-selective. Thus, it appears that at least one subset of neurons in layer 4B does convey information about motion direction to area MT. Do these same units project to area V2? To address this issue, we have injected MT and V2 with different tracers and examined the retrogradely filled cells in layer 4B (Sincich LC and Horton JC, Independent projection streams from macaque striate cortex to the second visual area and middle temporal area. J. Neurosci. 23:5684-5692, 2003). Only about 5% of cells were double-labeled.
This result means that distinct populations of cells in layer 4B supply areas V2 and MT, underscoring the specificity of intercortical connections.
The pattern of projections that we report from V1 to V2 vitiates a sharp division of form, color, and motion/stereopsis by CO stripe class. Localization of form, color, and motion perception certainly occurs in extrastriate cortex, at least in some primates. Achromatopsia in humans, for example, provides compelling evidence that color is localized to the fusiform and lingual gyri. Our point is that form, color, and motion should not be equated with pale, thin, and thick V2 stripes respectively. Moreover, it is misleading to associate parvo and konio with the “ventral” pathway and magno with the “dorsal” pathway. It is likely, given that interpatches project to both pale stripes and thick stripes, that parvo and magno inputs are distributed to both V4 and MT.
Bypassing V1: A Direct Geniculate Input to Area MT
Historically, the extrastriate cortical regions adjacent to the primary visual cortex were defined as “higher” because they were not thought to receive direct geniculate input. In humans, loss of V1 devastates eyesight by cutting off the flow of visual information from the LGN to extrastriate visual cortex. Curiously, patients affected by such lesions manifest some residual visual perception. This perception is optimal for moving stimuli, which occurs either consciously (Riddoch syndrome) or unconsciously (blindsight). The preservation of some vision following loss of the primary visual cortex has engendered considerable controversy because it defies conventional ideas about the organization of the visual system.
The simplest explanation for blindsight phenomena is that a visual pathway exists that bypasses V1 to reach area MT, a region in the superior temporal sulcus implicated in motion perception. We have recently discovered a direct projection from the lateral geniculate nucleus to the motion-selective area MT, a cortical area not previously considered “primary”. (Sincich, L.C., Park, K.F., Wohlgemuth, M.J., Horton, J.C., Bypassing V1: a direct geniculate input to area MT, Nature Neuroscience, Oct 2004). The constituent neurons are mostly koniocellular, send virtually no collateral axons to primary visual cortex (V1), and equal about 10% of the V1 population innervating MT. This pathway could explain the persistence of motion sensitivity in subjects following injury to V1.
AMBLYOPIA AND VISUAL DEVELOPMENT
Amblyopia is defined as a disease caused by abnormal visual experience during early childhood, which results in a decrease in acuity that can not be explained by pathology within the eye itself. For example, if a child is born with a dense cataract in one eye, the retina will be deprived of normal visual stimulation (figure 8). When the child grows up, the visual acuity in the affected eye will be poor -- even if the cataract is removed later by surgery and replaced by a clear lens of the appropriate refractive power. Without an ocular explanation for the permanent loss of vision caused by visual deprivation, investigators have long suspected that amblyopia is caused by anomalous wiring of the eye's central connections in the brain. This view has been confirmed by experiments suturing closed the lids of one eye in kittens or monkeys at an early age to simulate cataract (Wiesel and Hubel 1963, 1965; von Noorden et al. 1970).
It is important to note that amblyopia develops only in the young, when the visual system is still immature, and hence vulnerable to the effects of deleterious environmental manipulations. This brief interval of early vulnerability is called the “critical period”.
Figure 8: A child with amblyopia in the left eye due to a congenital cataract. Prolonged deprivation from the cataract can cause rewiring of connections in the visual cortex, leading to permanent visual loss despite later removal of the cataract.
In primates, inputs to the primary visual cortex driven by the eyes are segregated in layer 4c into alternating bands of input known as ocular dominance columns. To determine if the ocular dominance columns require visual experience to develop, we examined striate cortex in newborn monkeys delivered by Caesarean section. We found that columns were present, although they were not quite as well segregated as in adults:
Figure 9: Autoradiographic montage of layer 4c in a macaque monkey at birth, showing ocular dominance columns arranged in an adult-like pattern. (From Horton and Hocking, J. Neurosci, 16:1789-1805, 1996).
These studies indicate that formation of ocular dominance columns is programmed innately and not contingent on visual experience. The same is true for the cytochrome oxidase patches in V1 and the stripes in V2. From the location of the blind spot representation and the border of the monocular crescent in the cortex shown in Figure 9, one can also infer that the retinotopic map develops without visual experience.
Although the ocular dominance columns do not require visual experience for their formation, they can be altered drastically by post-natal visual deprivation occurring during the critical period. In their classic studies, Hubel and Wiesel showed that visual deprivation causes shrinkage of the eye’s ocular dominance columns in the cortex. This phenomenon is presumed to contribute to the occurrence of amblyopia. Shrinkage of the ocular dominance columns reflects a loss of input from laminae of the lateral geniculate body conveying information from the deprived eye. This loss of input means that fewer cells are available in the cortex to handle information emanating from the deprived eye.
We have shown in monkeys that shrinkage of ocular dominance columns is most severe when deprivation begins right after birth:
Figure 10:Figure 10: Autoradiographic montage (left) and drawing (right) of ocular dominance columns in layer 4c after deprivation of a macaque starting at age 1 week. The proline label was injected into the normal eye; the ocular dominance columns of the deprived eye appear as unlabeled, shrunken fragments of columns.
Figure 11: Autoradiographic montage and drawing of the columns in a macaque deprived starting at age 5 weeks. The columns serving the deprived eye are much less shrunken, compared with deprivation commencing at age 1 week, as shown above. From: Horton .C and Hocking DR, Timing of the critical period for plasticity of ocular dominance columns in macaque striate cortex. J. Neurosci, 17:3684-3709, 1997.
These anatomical findings offer further rationale for removal of dense, unilateral cataracts at the earliest possible age. It is clear from our experiments in the macaque that shrinkage of ocular dominance columns induced by monocular occlusion begins to accrue soon after birth. The most severe fragmentation of ocular dominance columns was seen in those animals that experienced the earliest deprivation. Therefore, the anatomical effects of deprivation are cumulative, and early intervention is warranted to minimize the disruption of the columnar architecture of the cortex.
STRABISMUS AND VISUAL SUPPRESSION
Strabismus is a disease that develops in children, characterized by misalignment of the eyes. The crucial point is that the ocular misalignment occurs without any weakness of the eye muscles, abnormality of the cranial nerves, or intrinsic disorder of the eyes. The primary culprit is a failure of the neural mechanisms responsible for maintaining binocular fusion. We are seeking to elucidate the defects in brain function responsible for strabismus. We are motivated by a conviction that understanding the neural basis of strabismus is essential to finding better methods of preventing and treating the disease.
The majority of children with strabismus develops inward crossing of the eyes, known as esotropia. In some children the eyes drift in an outward direction, giving rise to exotropia. Strabismus usually appears early in life, between birth and age 3. It affects about 2% of the children in the United States, and around the world. Most children who are affected are otherwise normal, and have no risk factors for neurological disease. However, studies have demonstrated a higher incidence of strabismus in premature or low birth weight children, and in children with developmental impairment.
In strabismus, normal stereovision is impossible, because the eyes are unable to fuse together on a target. As a result, children with strabismus have poor depth perception, impairing their ability later in life to perform certain jobs, to compete in sports, and to enjoy a normal three-dimensional view of their environment. In addition, many children with strabismus eventually develop amblyopia, depriving them of sight in one eye. If anything happens to their good eye, they face a grave predicament. There are also intangible, but significant, psychological aspects to strabismus. Because direct eye contact is so important in social interactions, children with strabismus often face difficulty in developing a positive self-image and later suffer discrimination when seeking employment.
Historically, research on strabismus has been carried out predominantly by clinicians, who have concentrated, quite appropriately, upon empirical approaches. Important progress has come from devising better methods to screen pediatric populations for strabismus, using the corneal light reflex, random-dot stereograms, and photoscreening devices which optically detect eye misaligment. As a result, children with strabismus are being identified in greater numbers than ever before. In some cases, strabismus can be treated by prescription of eyeglasses because misalignment of the eyes is driven by an uncorrected refractive error. In most children, however, strabismus eventually requires surgery on the eye muscles. Surgery can improve a child’s appearance, but unfortunately, seldom results in perfect eye re-alignment with restoration of binocular function. The fundamental problem is that the neural mechanism for ocular fusion has been lost.
Muscle surgery can re-position the eyes, but without a neural drive to fuse, the eyes never again lock onto a common target.
To make progress against this disease, we must employ invasive experimental techniques that cannot be used in children. As a substitute, we are privileged to be able to use the macaque monkey as an experimental model. This species is extremely similar to the human in terms of its visual function. Extensive testing has shown that, like the human, it has 20/20 visual acuity, trichromatic color vision, and high level stereoscopic depth perception. Baby macaques develop strabismus spontaneously, at about the same rate as human infants.
During the first few weeks of life, the eyes wander and cross quite freely, until an extraordinary event takes place. The brain learns to fuse the stream of images coming from each eye, to create a single view of the world. Each retina contains a specialized region called the fovea, capable of highest visual acuity (20/20). The challenge for each newborn child is to align and move the eyes together, so that visual targets are projected accurately onto each fovea. Once this goal is achieved, stereoscopic depth perception begins to emerge and maturation of visuomotor behavior accelerates rapidly.
In a small number of children, the eyes drift out of alignment again, resulting in strabismus. It is not clear why, although uncorrected refractive error plays an important role in some children. These children attempt to overcome their blurred vision by accommodating. Unfortunately, accommodation is linked in the brainstem to convergence, inducing the eyes to cross. Prescription of eyeglasses ameliorates this problem by reducing the urge to accommodate. In many strabismic children, however, no instigating factor can be identified. These children do not respond to prescription of eyeglasses, and in many respects, are the most difficult to treat successfully.
When strabismus develops, few children complain of diplopia (“double vision”). In part, this reflects the fact that many are too young to speak. Another critical factor is that children learn within a few days to suppress the vision from the deviated eye, in order to get rid of double vision. Thus, suppression plays a key role in strabismus, because it eliminates the “error signal” that would normally induce children to re-adjust muscle tone to bring their eyes back into alignment. A major objective of our research is to explain how visual suppression occurs.
Some children alternate fixation between the two eyes, looking for a few moments with one eye, and then switching abruptly to the other. In such cases, each eye maintains normal visual acuity, but the child loses all binocular function. In other children, one eye becomes dominant, and the other remains constantly deviated. Because this eye is always suppressed, it frequently develops amblyopia. These children not only lose stereovision, but face the threat of blindness in one eye. Amblyopia is often referred to as “lazy eye”, an inaccurate term used to soften bad news for parents by implying that nothing too serious is wrong, except that the eye is mischievously not working hard enough. In fact, the problem in amblyopia lies not with the eye, but the visual cortex, where inputs are woven into ocular dominance columns.
In macaques and humans, the ocular dominance columns can be labeled using cytochrome oxidase after monocular enucleation. They become visible because enzyme activity is lost in the columns serving the missing eye. This phenomenon illustrates that levels of cytochrome oxidase fluctuate dynamically with neuronal activity. We have also tested how strabismus affects cytochrome oxidase activity. In normal subjects, with good binocular vision, no pattern of cytochrome oxidase stripes is seen in the cortex. In strabismic animals, we have found a pattern of light and dark stripes in the primary visual cortex. This abnormal pattern of enzyme activity suggests that strabismus may affect the metabolism of one eye’s columns:
Figure 12: Right visual cortex from an exotropic monkey that preferred to fixate with the left eye. Cytochrome oxidase staining shows dark and light stripes in layer 4c. The boxed region is shown at higher power in Figure 13.
To correlate this metabolic pattern of stripes with the ocular dominance columns, we injected a tracer, [3H]proline, into the left eye to label its ocular dominance columns. Figure 13 compares the labels at higher magnification:
Figure 13: (Left) Boxed region from Fig. 12, showing 4 dark stripes marked by arrows. Note that the dark stripes are slightly narrower than the light stripes. (Right) Same region, showing the ocular dominance column, labeled by light bands of proline after injection of tracer into the left eye. Note match between the patterns, indicating that stronger cytochrome oxidase activity is present in the left eye’s columns, the very ones which represent the eye used by this monkey to fixate.
Comparison showed that metabolic activity, reflected by cytochrome oxidase, was greater in the ocular dominance columns of the left eye, which the animal preferred to use for fixation. Therefore, we deduced that suppression of vision in the right eye was accompanied by a loss of activity in its ocular dominance columns. This was the first demonstration of an anatomical change in the visual cortex that might explain suppression in strabismus.
The dark cytochrome oxidase columns (Figure 13) induced by strabismus were slightly thinner than the ocular dominance columns. This difference reflects the presence of binocular border stripes and monocular core zones in striate cortex:
Figure 14: (left) Schematic diagram showing subdivision of ocular dominance columns into monocular core zones and binocular border stripes. In normal animals, these regions contain equal cytochrome oxidase activity, so no pattern of enzyme activity is visible in layer 4c (for the sake of illustration only, the border strips are lighter in this drawing). For further details, see Horton & Hocking, Monocular core zones and binocular border strips in primate striate cortex revealed by the contrasting effects of enucleation, eyelid suture, and retinal laser lesions upon cytochrome oxidase activity. J. Neurosci, 18:5433-5455, 1998. (right) Striate cortex from a strabismic macaque that had no eye preference.
It alternated fixation freely between the eyes. As a result, enzyme activity was equal in the eyes’ core zones, but reduced within the border stripes (arrows).
In strabismus, metabolic activity is lost in the border stripes, because these regions are rich in binocular cells, which require ocular fusion to maintain normal levels of metabolic activity. In addition, in subjects with a strong fixation preference, activity is also lost in the core zones of the deviated eye’s ocular dominance columns. This gives rise to a pattern of thin dark columns alternating with wide, pale columns, shown in Figure 13.
Some subjects with strabismus show no eye preference. Instead, they alternate fixation freely between the eyes. In these subjects, reduced metabolic activity appears in the border stripes, but staining remains equal within core zones. This pattern is illustrated in Figure 14, right. The diagram below summarizes these two patterns of cytochrome oxidase activity associated with strabismus:
Figure 15: (A) Pattern of pale border stripes, associated with alternating suppression of one eye. (B) Pattern of thin, dark columns alternating with wide, pale columns, seen in strabismic subjects who strongly favor fixation with one eye.
Frequently, both patterns of metabolic activity diagrammed in Figure 15 are visible in the cortex of subjects with strabismus. Figure 16 shows the patterns of cytochrome oxidase activity present in a macaque monkey with an alternating exotropia, and no fixation preference. In the central (foveal) visual field representation, a pattern of pale border stripes was present, suggesting that each eye’s fovea was used for perception about half the time. In peripheral cortex, however, metabolic activity was stronger in the contralateral eye’s columns because the temporal retina of each eye was suppressed, as in humans, to avoid diplopia. This result illustrates that activity in the cortex can vary, depending on the fixation behavior and suppression pattern of the subject.
Figure 16: Pattern of cytochrome oxidase activity in an exotropic monkey. “Plus” signs indicate that dark CO columns coincided with the right eye’s ocular dominance columns; “minus” signs indicate that dark CO columns coincided with the left eye’s ocular dominance columns. Dots correspond to an intermediate pattern of staining. As one would expect in an animal without a fixation preference, neither eye dominated in the central field representation.
These experiments have provided the first direct evidence for the mechanism of visual suppression. Like a switch, inhibitory mechanisms functioning between ocular dominance columns may allow one eye to turn off neuronal activity in the other eye’s columns, leading to suppression of perception. However, this evidence is still preliminary. Our laboratory is currently engaged in further psychophysical and electrophysiological experiments in awake, trained macaques raised with strabismus. Our goal is to record the activity of single cells in the cortex, to learn if their firing rates correlate with the monkey’s report of which eye is actively perceiving.
THE HUMAN VISUAL CORTEX
Our research on the human visual system is based on clinical experience with patients suffering from diseases that produce eye movements abnormalities, double vision, visual field deficits, optic nerve and retinal dysfunction. Through neuro-ophthalmological examination, neuroradiological imaging, electrophysiology, pathology, and laboratory studies we characterize deficits in function to arrive at a diagnosis. This process allows us to serve patients, and to further elucidate the natural history, pathogenesis, and treatment of neuro-ophthalmological diseases.
In addition, we conduct studies of human brain tissues willed by patients to medical science. Through post-mortem examination of the occipital lobes we have been able to identify important functional elements of the human visual cortex. Figure 17 shows a montage of sections through layer 4c, processed for cytochrome oxidase, from a man who died several years after enucleation of one eye. A pattern of alternating light and dark columns are visible. The pale columns correspond to the ocular dominance columns of the missing eye:
Figure 17: Montage of cytochrome oxidase sections through layer 4c cut through striate cortex along the medial face of the right occipital lobe, showing ocular dominance columns. They appear visible because loss of physiological activity in the missing eye’s columns causes a reduction in the levels of metabolic enzymes. From: Horton & Hedley-Whyte, Phil. Trans. R. Soc. (Lond.) B, 304:255-272, 1984.
In the superficial layers of striate cortex, dark and light rows of cytochrome oxidase patches were visible, aligned with the ocular dominance columns in layer 4c, just as in the macaque monkey:
Figure 18: (Top) Rows of dark and light cytochrome oxidase patches from layers 2/3, in a region of cortex corresponding to the ocular dominance columns illustrated in Figure 17. (Bottom) Borders of ocular dominance columns from Figure 17 have been transferred onto the pattern of patches, showing that dark and light rows of patches are in register with dark and light ocular dominance columns.
In Figure 19 the pattern of ocular dominance columns along the medial face of the occipital lobe has been superimposed photographically on the brain surface:
Figure 19: Pattern of ocular dominance columns along the medial face of the right human occipital lobe. The dashed line corresponds to the V1/V2 border. The portion of the column mosaic shown in Figure 17 is at the lower right. Human ocular dominance columns are similar to those in macaques, but wider.
Most of human striate cortex is buried within the calcarine fissure, and therefore hidden from view in Figure 19. By flatmounting the entire striate cortex one can reconstruct the complete pattern of ocular dominance columns within V1:
Figure 20: Mosaic of ocular dominance columns in striate cortex revealed by processing the tissue for cytochrome oxidase in a patient who lost sight in one eye prior to his death. The image above shows the actual tissue montage; the image below is a sketch of the columns. Note the partial reconstruction of dark and pale cytochrome oxidase stripes in V2. BS = blind spot; MC = monocular crescent.
We have examined the ocular dominance columns in a patient with strabismic amblyopia and in a patient with anisometropic amblyopia (see: Horton JC & Stryker MP, Anisometropia induces amblyopia without shrinkage of ocular dominance columns in human striate cortex. Proc. Natl Acad. Sci, 90:5494-5498, 1993 and Horton JC & Hocking DR, Pattern of ocular dominance columns in human striate cortex in strabismic amblyopia. Visual Neuroscience, 13:787-795, 1996). Neither case showed evidence of shrinkage of the amblyopic eye’s ocular dominance columns. From these findings, we conclude that amblyopia is not always associated with reduction in the size of ocular dominance columns. It is possible that when amblyopia begins at a later age it is not accompanied by a change in the dimensions of ocular dominance columns. Presumably abnormalities in intracortical wiring, yet to be revealed, are responsible for amblyopia in these cases.
From our studies, it is apparent that many anatomical features of the macaque visual cortex are also present in the human visual cortex. Their similarity gives us confidence that our research findings in the macaque are applicable to the human. This is gratifying, because our ultimate goal is to understand how the human visual system functions. Continuing studies in the macaque, therefore, will allow us to make further advances in understanding the human visual system. In turn, human studies will continue to help shape our experiments in the macaque.