Chapter 1 Structure and Function of the Eye

VASCULAR ANATOMY AND PERIPHERAL NEUROANATOMY

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

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.