by Noelle La Croix, DVM, Dip. ACVO
“Does my dog see in black and white?” Answers to Questions about Companion Animal Vision – Part 1
The mammalian eye is a device for collecting and focusing light, for distinguishing between different wavelengths and intensities, and for converting that information into electrical impulses that are sent to the brain. Eyes evolved independently in various ecological niches creating species-specific morphologies. In general, an eye conferred a selective survival advantage to an individual, and therefore a reproductive advantage in a population.
Humans are the most visually orientated and dependent species of mammal. Humans evolved as an arboreal (tree-dwelling) species. In this environment, acurate color vision with a high degree of depth perception facilitated swinging from tree limbs, eating ripe fruit, and even interpreting subtle facial expressions within a social group.
Dogs evolved in an environment most closely related to that of modern wolves. Wolves travel in packs and hunt in both day and night conditions. Wolves primarily use olfaction, rather than vision, to find prey 1.5 to 4.5 miles distant. However, wolves mainly rely on vision to accurately chase and kill their fast-moving prey. Wolves also express a variety of facial gestures and body postures to establish pack hierarchy. The faces of wolves accentuate these gestures, reducing the detail an observer requires. Wolf vision evolved to detect fast-moving prey in light or dark conditions. Fine visual acuity, and even color vision, was not as strongly selected for as in humans.
Color vision in dogs
Dogs do see colors. However, canine cones account for only 10% of their fundic photoreceptors. Two distinct types of canine cones are known to exist. The first is maximally sensitive to violet light, and the second is maximally sensitive to yellow-green light. The color visual spectrum perceived by dogs is therefore divided into two hues, blue-violet and green-yellow. The remaining orange-red range of the visual spectrum is probably perceived as yellow. Dogs fail to differentiate green, yellow, orange or red objects. Dogs also fail to differentiate blue-green from gray objects. The color vision of dogs therefore strongly resembles human red-green color blindness.
Canine retinal sensitivity to light
The dog is not highly adapted for solely day or night vision. As in wolves, dogs evolved to hunt under all light conditions. This contrasts with the exceptional nocturnal vision of cats. The minimal light threshold for vision in felines is 6 times lower than that of humans. The canine threshold is somewhere between that of cats and humans. In both dogs and cats, a large number of rods (light sensitive photoreceptors) and a reflective tapetum account for this increased light sensitivity. Photons striking a tapetum are reflected back to the retina increasing the chance of visual stimulation. However, this secondary stimulation is accompanied by increased light scatter and decreased visual acuity.
Visual acuity of the canine
In general, canine visual acuity is thought to be limited by the retina, rather than by optical properties of the eye or post-retinal neural processing. Most dogs are neither nearsighted (myopic) nor farsighted (hyperopic), but are emmetropic. Emmetropia is the state of vision where a distant object appears in sharp focus with the lens in a neutral or relaxed state. Some myopic dog breeds include the Rottweiler, Collie, Miniature Schnauzer, and Toy Poodle. Hyperopic breeds include the Australian Shepherd, Alaskan Malamute, and Bouvier des Flandres.
The typical mammalian eye is focused to see distance objects in a relaxed state. This state evolved to detect predators and/or prey at a distance. Closer objects are brought into focus by an eye that can increase its focal power. This “accomodation” is typically brought about by ciliary muscles changing lens curvature, thereby changing focal power which is measured in diopters (D). The canine accommodative range is limited to 2 to 3 D. Dogs can not clearly visualize objects within 33 to 50 cm of their lenses. In contrast, human children can accommodate to 14D and clearly visualize objects 7 cm from their lenses.
The fovea is part of the human retina with the densest concentration of photoreceptors and ganglion cells. It is centrally located and its stimulation results in the sharpest images visualized (highest visual acuity). The canine retina lacks a fovea, but has an oval “visual streak” just above the optic nerve. The visual streak is oriented with its longer axis on a horizontal plane, and it is centered near the temporal fundus. The visual streak contains the highest concentration of canine photoreceptors and ganglion cells, and is thereby responsible for the greatest visual acuity. However, the canine visual streak has fewer cones and ganglion cells than the human fovea. Canine visual acuity is therefore lower than that of humans. For example, humans with perfect vision can see a small well-focused object 20 feet from their lenses (20/20 vision on the Snellen eye chart, see Figure 1). Typical dogs can focus on an object 20 feet away too, but that object would be much larger in comparison. That larger object would be in focus up to 75 feet from a typical human lens. Dogs therefore have about 20/75 vision in human terminology.
Canine depth perception, motion sensitivity, and their field of view will be discussed in my next article. Part 2 will also finally explain if a dog can in fact watch television!
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: A Snellen eye chart comparing the typical visual acuity of cats, dogs, horses, and humans. Labels indicate the smallest-sized text that could be “read” by each species relative to the others.