Clean Run - January 2009 - 62

Field of View Field of view varies greatly among different species and, within dogs, among different breeds. Placement of the eyes determines the “panorama” or width of the visual field and the size of the forward field that is seen by both eyes simultaneously (binocular field). Binocular vision assists in judging distances (depth perception). We humans have a 200° visual field of which 140° is binocular. In dogs, the eyes are usually placed more to the sides, giving them a wider visual field with a smaller portion that is binocular. Most dogs have a 240° to 270° visual field of which 30° to 60° is binocular (see Figure 2). The very near, lower, central visual field in most dogs is blocked by the nose. In some breeds, long hair and ears can further reduce the field of vision. Brachycephalic breeds (Pug, Boston Terrier) have a shorter nose, but more widely spaced eyes; thus they have a smaller binocular field. The visual perspective is also affected by the height of the dog (and the eyes) (see Figure 3). Figure 3: Comparison of vision between an 8” dog (left photos), 22” dog (center photos), and human (right photos). These photos simulate the more muted colors seen by dogs as well as the variation in height. pupil size and the eyes’ accommodative ability. Much like a camera, a smaller pupil (aperture) results in a greater depth of field. Dogs have relatively large pupils enhancing their dim-light vision at the expense of their depth perception. Accommodation is the ability of the eye to alter its focal point, generally by changing the shape of the lens, which is discussed in the section below on acuity. Dogs also have a specialized, reflective layer behind the retina called the tapetum. The color of the tapetum in dogs ranges from blue-green to yellow and results in the characteristic “eye shine” when their eyes are seen in car headlights. Although the tapetum greatly enhances light sensitivity, it also results in a scattering of light that may reduce visual acuity. The photoreceptor cells in the retina consist of rods and cones. In general terms, cones are responsible for bright light and color vision; rods are sensitive to low intensity, dim light. Rods are the predominant type of photoreceptor in dogs and these rods are more sensitive to low light levels than human rod cells. Depth Perception The ability to judge distances is critical for performance dogs. They must accurately assess jump height and length, and instantly translate this assessment into stride length, speed, and height. Depth perception requires a mental comparison of the image created by each eye. Thus, an object must be visible within their relatively small binocular field and in accurate focus for each eye simultaneously. The range over which an object is in focus is dependent on Light Dogs have greater dim-light sensitivity than humans do for several reasons. Their eyes are larger and can dilate more widely for maximal light capturing ability. It is interesting to note that the size of the globe of the eye as a whole varies remarkably little over the large range of sizes of adult dogs. The eyes of a Chihuahua and a Great Dane are nearly the same size. Motion Dogs appear to be neurologically sensitized to preferentially react to moving objects, as would be expected of a predator species. Owners often tell me that their dog (with normal vision) does not appear to recognize them until they move or speak. Many dogs ignore stationary objects but reflexively chase them if they move.Their large peripheral field of vision, in combination with their rod-rich retina, makes dogs well suited to detection of movement (prey) in dim light. Color ©C RA IG LIZ OT TE © CLEAN RUN Figure 2: The width of the visual field and the amount of binocular overlap is determined by the placement of the eyes within the head and varies between breeds. Dogs have a much smaller binocular field than humans do. Although the common belief is that dogs see only in black and white, evidence suggests that they have some degree of useful color vision. The perception of color is determined by the presence of cone photoreceptors within the retina. Humans have a regional area, called the macula, consisting entirely of cone photoreceptors. This is the area Clean Run | January 09 62

Clean Run - January 2009

Table of Contents for the Digital Edition of Clean Run - January 2009