Chapter 1 Structure and Function of the Eye

Vision is a complex phenomenon in which light emanating from objects in the environment is captured by the eye and focused onto the retinal photoreceptors (Figures 1-1 and 1-2). Electrical signals originating from these cells pass through a number of cell types in the retina and throughout the central nervous system (CNS) before arriving at the visual cortex, where the sensation of vision occurs. Numerous species variations exist on this basic theme, each allowing the animal to exploit a particular ecologic niche. The basic similarities among all vertebrate eyes and how they respond to insult allow the comparative ophthalmologist to confidently treat a wide range of ocular conditions in a diverse array of species.

Figure 1-1 Frontal view of the external structures of the canine eye.

Figure 1-2 Internal structures of the canine eye (A). Also shown are the standard reference planes (B).

FUNDAMENTALS OF VISION

The act of “seeing” is a complex process that depends on (1) light from the outside world falling onto the eye, (2) the eye efficiently transmitting and properly focusing the images of these objects on the retina, (3) the retina detecting these light rays, (4) transmission of this information via the visual pathways to the brain, and (5) the brain processing this information so as to make it useful. Differentiating between objects (e.g., a predator versus its surroundings) is one of the most critical aspects of vision, and because this distinction is so important for survival, normal animals can “see” an object if it differs sufficiently from its surroundings in any one of five different aspects: luminance (“brightness”), motion, texture, binocular disparity (depth), and color. In general, objects are differentiated on the basis of their motion, texture, depth, and luminance roughly equally well, but separations based on color are less easily made. Although the individual components of vision can be divided into the ability to detect light and motion, visual perspective, visual field of view, depth perception, visual acuity, and the perception of color and form, the complete visual experience is a synthesis of these parts into a unified perception of the world.

Sensitivity to Light

The visual system of most domestic mammals has evolved to improve performance under a wide range of lighting conditions so that they may exploit specific ecologic niches. Of domestic mammals, cats are probably the most efficiently adapted for nocturnal vision, with a minimum light detection threshold up to seven times lower than that in humans. Other adaptations that permit cats to function well in nocturnal conditions are a tapetum lucidum, which reflects 130 times more light than the human fundus; a vertical slit pupil, which produces a smaller aperture in bright light than what is possible with a circular pupil but also allows the pupil to dilate 6mm more than the human pupil; a large cornea, which permits more light to enter the eye; a relatively posteriorly located lens, which produces a smaller but brighter image on the fundus; and a retina rich in light-sensitive rod photoreceptors (Figure 1-3). Many of the other domestic mammals have similar but fewer extreme adaptations for vision in dim light, allowing them to exploit a photic environment that is not strictly diurnal or nocturnal.

Figure 1-3 External view of the eye of a normal cat. Nocturnal adaptations that allow more light to enter the eye include a large cornea, a deep anterior chamber, and a relatively posteriorly located lens.

(From Czederpiltz JMC, et al. [2005]: Putative aqueous humor misdirection syndrome as a cause of glaucoma in cats: 32 cases. J Am Vet Med Assoc 227:1476.)

The tapetum is cellular in dogs and cats and collagenous in horses and ruminants, suggesting that the visual advantages this structure offers are of sufficient magnitude that it has evolved separately at least twice in mammals (Figure 1-4). In both cases, the variety of tapetal colors seen during ophthalmoscopy results from the differential interaction of light with the tapetum’s physical structure rather than from the inherent spectral composition, or color, of its pigments. The dorsal location of the tapetum may enhance the view of the usually darker ground, and the ventrally located, usually darkly pigmented nontapetal region may reduce light scattering originating from the brighter sky. In cats, the tapetum may also absorb light in the shorter wavelengths and, via fluorescence, shift it to a longer wavelength that more closely approximates the maximal sensitivity of the photopigment, rhodopsin. This shift may brighten the appearance of a blue-black evening or night sky and enhance the contrast between other objects in the environment and the background sky.

Figure 1-4 Cellular tapetum of a dog (A) and fibrous tapetum of a horse (B).

(B from Gilger B [2005]: Equine Ophthalmology. Saunders, St. Louis. A and B courtesy Dr. Christopher J. Murphy.)

The rhodopsin photopigment of dogs and cats is tuned to a slightly different wavelength of light from that in humans and, as is typical of species adapted to function well in dim light, takes longer to completely regenerate after extensive exposure to bright light. The ranges of wavelengths to which rhodopsin in dogs, cats, and humans is sensitive are similar, however, indicating that vision in dim light is not enhanced by expanding the range of detectable wavelengths. The slight wavelength shifts in the maximal sensitivity of rhodopsin across species suggests that domestic mammals and humans do not perceive the world in exactly the same way.

Sensitivity to Motion

Although little work has been done on the motion-detecting abilities of most domestic animals, it is probable that the perception of movement is a critical aspect of their vision and that they, like people, are much more sensitive to moving objects than stationary ones. Rod photoreceptors, which dominate the retinas of domestic mammals, are particularly well suited for detecting motion and shapes, and it follows that the motion-detecting abilities of domestic mammals—especially in dim light—would be well developed. In a study of the visual performance of police dogs, the most sensitive dogs could recognize a moving object up to 900m away but could recognize the same object, when stationary, at only 585m or less. Because of the superior visual acuity of the human fovea, the minimum threshold for motion detection in bright light for cats is approximately 10 to 12 times greater than that for humans. Although humans may be better equipped to detect motion when directly viewing an object in bright light, it is possible that the vision of domestic mammals may be superior in dim light, when an object is viewed peripherally, or if it is moving at a certain speed to which the retina is particularly attuned.

The ability to detect motion may help explain certain behavior—much of the very large peripheral visual field of the horse probably supports only the detection of brightness and motion. When combined with a “prey mentality,” this may cause the horse to treat every moving object in its peripheral field of view as dangerous and to be avoided. Similarly, many dogs and cats ignore static objects, but when these objects move, chase behavior is elicited, suggesting that the visual system has preferences for objects moving at certain speeds.

Sensitivity to Flickering Lights

Although not related to motion detection, the point at which rapidly flickering light fuses into a constantly illuminated light (flicker fusion) provides insight into the functional characteristics of rod and cone photoreceptors. The flicker frequency at which fusion occurs varies with the intensity and wavelength of the stimulating light. Because dogs can detect flicker at 70 to more than 80Hz, a television program in which the screen is updated 60 times/sec and appears to people as a fluidly moving story line may appear to dogs as rapidly flickering.

Visual Field of View

The extent of the visual field (i.e., the area that can be seen by an eye when it is fixed on one point) and the height of the eyes above the ground may vary greatly among breeds and species and has a major impact on the perception an animal has of its environment (Figure 1-5). For example, when the visual fields of its two eyes are combined, the horse has a total horizontal visual field of up to 350 degrees, with 55 to 65 degrees of binocular overlap and a virtually complete sphere of vision around its body (Figure 1-6). The length of the horse’s nose interferes with binocular vision, and so a horse views an object binocularly until it is about 1m away, at which point the horse must turn its head and observe with only one eye. In comparison, humans have a visual field of approximately 180 degrees (140 degrees of overlap), cats have a 200-degree field of view (140-degree overlap), and depending on breed, dogs have 250 degrees (30 to 60 degrees of binocular overlap) (Figure 1-7). The horse has only a few minor “blind spots,” which are located superior and perpendicular to the forehead, directly below the nose, in a small oval region in the superior visual field where light strikes the optic nerve itself, and the width of the head directly behind. Clearly, this extensive visual field makes it very difficult for a person or potential predator to “sneak up” on a horse.

Figure 1-5 The effect of visual perspective on vision. The same scene as viewed by a small dog with eyes located 8 inches above the ground (A), a tall dog with eyes 34 inches above the ground (B), and a person with eyes 66 inches above the ground (C).

(From Miller PE, Murphy CJ [1995]: Vision in dogs. J Am Vet Med Assoc 207:1623.)

Figure 1-6 The visual field of the horse showing a binocular field (65 degrees) comparable to that of a dog but with much larger panoramic monocular fields (146 degrees), and a very small blind area (3 degrees).

Figure 1-7 A, Visual field of a cat showing a large binocular field (140 degrees) with a relatively small monocular field (30 degrees) and a relatively large posterior blind area (160 degrees). B, Monocular and binocular visual fields in a typical mesocephalic dog. The dog has a modest binocular visual field (60 degrees) with relatively large monocular visual fields (90 degrees) and a posterior blind area of approximately 120 degrees.

Depth Perception

Depth perception is enhanced in those regions in which the visual fields of the two eyes overlap. Merely viewing an object with both eyes simultaneously does not guarantee improved perception of depth. Stereopsis (binocular depth perception) results when the two eyes view the object from slightly different vantage points and the resulting image is blended or fused into a single image. If the two images are not fused, double vision may result. (Such an alteration in vision may occur in animals with orbital diseases.) Although binocular depth perception is superior if the images can be blended into one, monocular depth perception is also possible. Horses make distance judgments on the basis of static monocular clues; these clues include relative brightness, contour, areas of light and shadows, object overlay, linear and aerial perspective, and density of optical texture. In addition, movement of the head results in an apparent change in the relative positions of the objects viewed (a phenomenon known as parallax) and produces the sensation that objects are moving at different speeds, allowing depth to be estimated (Figure 1-8).

Figure 1-8 A number of cues allow depth to be perceived with one eye or in a two-dimensional photograph. These cues include apparent size (the left tower appears closer because it is larger than the right), looming (cars moving toward the viewer appear to become progressively larger), interposition (near objects such as the bridge overlay the more distant hills), aerial perspective (water vapor and dust in the air make the more distant hills less distinct and relatively color-desaturated), shading (shadows on the tower suggest depth), perspective (the parallel roadways appear to converge toward the horizon), relative velocity (the nearer cars appear to move faster than more distant ones), and motion parallax (if the eye is fixed on the center of the bridge, the images of near objects appear to move opposite to the direction the observer moves the head, whereas distant objects move in the same direction as the head).

(From Gilger B [2005]: Equine Ophthalmology. Saunders, St. Louis.)

Visual Acuity

Visual acuity refers to the ability to see details of an object separately and in focus. It depends on the optical properties of the eye (i.e., the ability of the eye to generate a precisely focused image), the retina’s ability to detect and process images, and the ability of higher visual pathways to interpret images sent to them. In general, visual acuity in most domestic mammals is limited by the retina and not by the optical properties of the eyes or by postretinal neural processing in the brain. The latter two factors can limit visual discrimination in a variety of disease states, such as when the lens is removed or when higher CNS visual pathways are impaired.

Optical Factors in Visual Acuity

The optical media of the eye, namely the cornea, aqueous humor, lens, and vitreous humor, are responsible for creating a properly focused image on the retina. The cornea and, to a lesser extent, the lens are the principal refracting surfaces of the eye, and their ability to bend (refract) light is determined by their radii of curvature and the differences between their refractive index and that of the adjacent air or fluid. If the focal length of the focusing structures of the eye does not equal the length of the eye, a refractive error is present. In a normally focused (emmetropic) eye, parallel rays of light (effectively anything 20 feet or more away from the eye) are accurately focused on the retina. If parallel rays of light are focused in front the retina, myopia (nearsightedness) results. If they are focused behind the retina, hyperopia (farsightedness) results (Figure 1-9). Such errors in refraction are usually expressed in units of optical power called diopters (D). The extent of the error can be expressed by the formula D = 1/f, where f equals the focal length (in meters) of either the lens or the optical system as a whole. Therefore if an eye is 2D myopic at rest, it is focused at a plane located 0.5m in front of the eye. Similarly, an eye that is emmetropic at rest but can accommodate (change focus) 3D is capable of clearly imaging objects on the retina that range from as far away as the visual horizon (infinity) to as near as 0.33m in front of the eye.

Figure 1-9 Top, The image is properly focused on the retina (emmetropia). Middle, The image is focused in front of the retina, making the eye nearsighted (myopia). Bottom, The image is in focus at a plane that is behind the retina, making the eye farsighted (hyperopia).

(Modified from Miller PE, Murphy CJ [1995]: Vision in dogs. J Am Vet Med Assoc 207:1623.)

The average resting refractive state of the dog is within 0.25 D of emmetropia. There are individuals, however, that are significantly myopic, and breed predispositions to myopia are found in German shepherds and Rottweilers. In one study, 53% of German shepherds were myopic by −0.5 D or more in a veteri-nary clinic population, but only 15% of German shepherds in a guide dog program were myopic, suggesting that dogs with visual disturbances such as nearsightedness do not perform as well as normally sighted dogs. It may be reasonable to screen dogs that will be expected to perform visually demanding tasks, or those on which human life relies for refractive errors, before embarking on extensive training programs. Although studies of the refractive errors of cats and horses are somewhat conflicting, it appears that the average refraction for these species approximates emmetropia, although deviations of 1 to 2D do regularly occur.

In addition to myopia and hyperopia, other optical aberrations (e.g., astigmatism) may result from imperfections in the refractive media such as the cornea or lens and lead to degradation of the image formed on the retina. Astigmatism occurs when different regions of the optical system (especially cornea or lens) do not focus light in a uniform fashion, resulting in warping of the image, an extreme example of which can be found in the irregular mirrors found at carnivals. Spontaneous astigmatism is generally uncommon in dogs but has been observed in a variety of breeds. Astigmatism commonly accompanies corneal diseases that result in scarring and distortion of the corneal curvature (Figure 1-10).

Figure 1-10 Corneal astigmatism after a corneal laceration with subsequent scarring. Note the irregular flash artifact on the cornea indicating that light is being unevenly focused on the retina.

(From Gilger B [2005]: Equine Ophthalmology. Saunders, Philadelphia. Courtesy Dr. Ellison Bentley.)

Although visual acuity requires that optical portions of the eye be transparent and that optical blur from refractive errors or astigmatism be limited, an adjustable focusing (accommodative) mechanism is needed if objects at different distances are to be seen with equal clarity. Accommodation in dogs and cats may be brought about by altering the curvature of the lens surface or, more likely, by moving the lens anteriorly (Figure 1-11). The accommodative range for most domestic animals is quite limited and does not generally exceed 2 to 3 D for dogs, 4 D for cats, and less than 2 D for horses. This finding suggests that dogs are capable of accurately imaging objects on the retina that are within 50 to 33cm of their eyes but that objects nearer than this will be blurred. Hence, dogs use other senses, such as smell or taste, to augment vision in the investigation of very near objects. For comparison, young children are capable of accommodating approximately 14 D, or to about 7 cm.

Figure 1-11 Classic accommodation in primates. Left, Distant vision. Relaxation of the ciliary muscle increases tension on the lens zonules, which flattens the lens and brings distant objects into focus. Right, Near vision. Contraction of the ciliary muscle reduces tension on the zonules, which allows the elastic lens capsule to assume a more spheric shape. The resulting increase in lens power allows near objects to be brought into focus on the retina. The importance of this mechanism of accommodation in most domestic mammals is debated.

(Modified From Getty R [1975]: Sisson and Grossman’s The Anatomy of the Domestic Animals, 5th ed. Saunders, Philadelphia.)

Loss of the lens, as occurs after cataract surgery, results in severe hyperopia (farsightedness), with objects being focused approximately 14 D behind infinity, and a reduction in visual acuity to 20/800 or worse. This means that aphakic eyes are unable to image any object clearly, whether near or far away, and are unable to accommodate. Although the aphakic dog is extremely “farsighted,” it must be kept in mind that, for objects of similar size, objects that are closer to the dog will create a much larger image on the retina than objects that are located far away. Therefore the aphakic dog may be able to better visually orient to near objects despite being “farsighted.” Surprisingly, although this degree of hyperopia is markedly debilitating to some dogs, most dogs are still able to visually orient adequately in their environment without correction.

Retinal Factors in Visual Acuity

The retina may be the limiting factor in visual acuity for normal domestic animals, and its architecture may provide clues to the potential visual abilities of the eye. Enhanced vision in dim light as occurs in dogs typically requires that a greater number of photoreceptors (primarily rods) synaptically converge on a single ganglion cell. This results in reduced visual acuity, just as high-speed photographic film produces a “grainy” image in bright daylight. Additionally, the tapetum also scatters light and further degrades visual acuity in bright light. Retinas with excellent resolving power have a high ratio of ganglion cells to photoreceptors, a large number of ganglion cells and optic nerve fibers, and a high density of photoreceptors and usually lack a tapetum. In primates, the fovea has one ganglion cell per cone, whereas in cats, the peak ratio is one ganglion cell for every four cones. In all species, there are fewer ganglion cells in the periphery of the retina than in the center, and the ratio may decline to 1:16 in primates and 1:20 in cats, explaining the reduced visual acuity of their peripheral visual fields.

Domestic mammals lack the highly developed primate fovea but, instead, have a generally oval visual streak that contains the greatest density of photoreceptors, ganglion cells, and rhodopsin and thereby affords the greatest visual acuity. The visual streak, located in the tapetal region slightly superior and temporal to the optic nerve, has approximately linear, short temporal and longer nasal extensions (Figure 1-12). The oval temporal part of the visual streak is relatively free of blood vessels larger than capillaries, and nerve fibers take a curved course to the optic disc dorsal and ventral to the visual streak, presumably to avoid interfering with light reaching the photoreceptors. The temporal, oval portion of the streak may facilitate binocular vision, whereas the nasal, linear portion may be used to scan the horizon and better use the wider field of view available to the domestic mammals.

Figure 1-12 Diagram of retinal ganglion cell densities from the right retinas of a German shepherd with a very pronounced wolflike visual streak (A) and a beagle with a moderately pronounced visual streak (B). Retinas were cut radially to flatten them and are displayed at the same magnification. The intensity of the dots reflects varying ganglion cell densities. The irregular shape in the center of each retina is the region of the optic nerve head. Ganglion cells could not be seen in this area because of thick, overlying nerve fiber layer.

(From Miller PE, Murphy CJ [1995]: Vision in dogs. J Am Vet Med Assoc 207:1623.)

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