Clean Run - January 2009 - 63

of the retina used for nearly all visual tasks. Dogs lack a true macula but do have a slightly greater concentration of cone photoreceptors centrally and in a horizontal band. In this “area centralis” cones still comprise only 10-20% of the photoreceptors. Yet, dogs may be better able to differentiate between subtle shades of gray than people can because of the dogs’ larger number of more sensitive rod photoreceptors. The limited numbers of cones that dogs have are of only two types with sensitivity to violet (429 nm) and yellow-green (555 nm). Thus, dogs have dichromatic color vision (similar to color-blind humans) compared with normal human trichromatic vision. Wavelengths ranging from 500 to 620 nm (seen as green, yellow, and red by humans) would all appear as yellow to dogs. Light that appears blue-green to people probably appears as blue-gray to dogs. Behavioral tests in dogs suggest that they can distinguish red (which appears yellow) from blue but often confuse red and green. Thus, it is likely that guide dogs for the blind interpret traffic lights based on the position of the light rather than by differentiating red from green. Since red, green, and yellow all appear similar to dogs, blue is the best color to contrast with the yellow agility contact zone (see Figure 4). distance in feet from which an individual can distinguish two objects, compared to the denominator or lower number, the distance at which an individual with normal acuity (20/20) could distinguish two objects.The fraction 20/100 is used to describe an individual who can distinguish two objects at 20’ that can be seen by a normal individual at 100’. Refractive power is measured in diopters (D), the reciprocal of the focal length in meters. For example, a 3 D lens would bring parallel rays into focus at 1/3 meter. Convex lenses are measured in positive diopters and are used to treat hyperopia (farsightedness). Concave lenses are measured in negative diopters and are used to treat myopia (nearsightedness). The refractive power of the eye as a whole represents a combination of the length of the globe, curvature of the cornea, and position and refractive power of the lens. In an ideal situation, the image falls accurately onto the photoreceptors (rods and cones) of the retina. The refractive state of the eye can be measured using retinoscopy (described below). There are several terms used to describe various refractive states. Emmetropia: being able to focus clearly, in a relaxed state on an object at a distance (>20’). Myopia: near objects are in focus and distance objects are blurred. This is most often the result of an eye that is longer (axial length = front to back) than normal. Hyperopia: close objects are blurred; usually caused by a globe with short axial length. Aniosmetropia: the two eyes do not focus equally. This will affect binocular vision and depth perception. Astigmatism: the horizontal and vertical planes of focus are different; not commonly seen in dogs. Presbyopia: reduced ability of the lens to change in shape; occurs with increasing age and density of the lens. Surveys of the refractive state of dogs reveal that most are near emmetropia. One survey of 240 “normal” dogs revealed that some breeds (German Shepherd and Rottweiler) had a higher incidence of myopia; however, German Shepherd guide dogs were likely to be significantly less myopic.This leads to speculation that screening of dogs for use as guides may have eliminated those for whom visiondependent behavior was considered unsatisfactory. Besides the ability of the optical features of the eye to cause an image to fall accurately on the retina, acuity is also determined by the density of the photoreceptors and the proportion of cells that are sensitive to small amounts of light (rods) versus those that are sensitive to color and require larger amounts of light (cones). Continuing with the camera/film analogy, the rod photoreceptors are similar to high-speed film; the image produced in low light conditions tends to be grainy with lower resolution (acuity). Information from the photoreceptors is transmitted in a series of cellular connections; first to the bipolar cells, then to the ganglion cells, the axons of which form the optic nerve. The number of cells that converge in this chain affects acuity. In the human macula, there is a nearly 1:1 ratio of cone photoreceptors to ganglion cells, meaning that information from each small area in the retina is represented by a corresponding communication in the brain. In non-primate mammals, including the dog, many photoreceptors transmit to a single bipolar cell and then many bipolar cells to a single ganglion cell.This situation creates a more “dilute” visual image in the brain. When considering acuity, it is important to recognize the role of accommodation, where contraction of the lens changes its refractive state to adjust for near vision. Muscle within the ciliary body Acuity Visual acuity is defined as the “clarity of vision”: the ability to distinguish two separate objects. It represents the sensitivity of the eyes (ability to bring an object into precise focus on the retina) and the interpretative faculty of the brain (the number of visual receptor cells that converge to form an image in the brain). We commonly hear our own visual acuity expressed as a Snellen fraction 20/x. The numerator or upper number in this designation represents the A B C D Figure 4: Comparison of the view of the same scene by a human (A, C) and a dog (B, D); for the dog yellow, green, and red appear similar. January 09 | Clean Run 63

Clean Run - January 2009

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