I walk slowly toward the yard without saying a word. A dog peers out. I am spotted. A cautious bark alerts the other dogs, and now the yard is filled with 22 eyes staring straight at me. I continue to approach, still silent. The dogs sniff the air, crane their necks, stare and begin to pace and bark nervously. It is apparent they’re not sure who I am. How close must I get before they recognize me, their breeder, the person who shares their daily life and puts the food bowls in front of them? As it turns out, very close. I finally speak, and the relief is palpable as they swamp me with enthusiastic greetings. The scary part is, these are Salukis -- sighthounds!
Sighthounds are the group of Greyhound-like dogs that have been selectively bred for thousands of years for their ability to chase swift game by sight. They are one of several groups of dogs that rely extensively on vision to perform the tasks for which they were bred. Retrievers depend upon visually tracking falling birds to mark their place; herders depend upon detecting slight movements of the stock as well as their master; and police and military dogs make extensive use of their visual sense in carrying out their duties. Perhaps most important of all, guide dogs must act as the eyes for their visually impaired handlers. Dogs may be famous for their sense of smell, but for at least some dogs, good vision is every bit as essential.
Selective breeding has achieved the unsurpassed diversity in physical and behavioral characteristics that is the hallmark of the domestic dog. If selection can achieve such remarkable results, can it also act on sensory abilities? Do sighthounds and other breeds in which vision is so vital to function have superior vision? And if so, can we further select for visual excellence to produce, for example, guide dogs with the best eyes possible? Before these questions can be addressed, the visual capabilities of dogs in general must be investigated.
A Dog’s Eye View
The visual sense, like the eye itself, is made up of a number of related components. The most basic function is the ability to perceive light, but this ability has been fine-tuned in many ways, so slight differences between intensities or wavelengths of light can be perceived. No eye can do it all, and in every species evolution has acted to fine-tune the abilities essential to that animal’s lifestyle. The dog is no exception.
The human and canine eye are built upon the same basic design, but each has modifications that enable it to perform optimally according to that spe-cies’ lifestyle. Humans evolved as a diurnal (active in the daytime) species while dogs evolved as a nocturnal or crepuscular (active at dawn and dusk) species. As a result, human eyes do not see well in the dark but have great acuity, color perception and depth perception. The capabilities of canine eyes are less well documented, but they are clearly very different from those of humans.
Anyone who has walked a dog at night can attest to the dog’s apparent well-developed night vision. Could my dog's equally apparent inability to recognize me be the price they pay for an increased ability to see in the dark? Just how well can dogs see fine details? Many factors affect an animal’s acuity, including pupil size, optics of the eye and retinal design.
The eye often is compared to a camera because it has an aperture (pupil), lens (cornea and lens) and receptive surface or film (retina). As with the camera, these features can be adjusted or modified to cope with different lighting conditions. Like the camera, the eye continually makes compromises between sensitivity at low levels of light and sensitivity to fine detail.
Light enters the pupil of the eye, which is the aperture controlled by the iris. The wider the pupil or aperture, the more light that can enter it. Large pupils are characteristic of animals active in dim light such as dogs. But there is a trade-off: With a larger aperture, the depth of field (or distance over which objects can be put into clear focus) decreases. Thus, in order to achieve focus over a large range the pupil must be constricted.
After passing through the pupil, the light passes through the lens. Camera lenses are rated for their light gathering ability; more expensive lenses gather more light and can be used with smaller apertures, thus combating the depth of field loss otherwise inherent in dim light. The same is true for eyes. Larger lenses have greater light-gathering ability and usually are found in animals active in dim light. Dog lenses are much larger than human lenses. Actually, unlike the camera, eyes have two lenses, because the b outer clear surface of the eye, the cornea, acts like a strong lens as well. An animal with a large pupil must have a concomitantly large cornea, and larger corneas usually are found in animals requiring good night vision. Notice how much larger your dog’s corneas are than yours. (See the graphic A Comparison of the Human and Canine Eye.)
Besides gathering light, a lens bends light rays as they pass through it. This ability to bend, or "refract" light is an essential feature of a lens. The more a lens can bend light, the more powerful the lens is said to be. In the ideal eye or camera, the power of the lens would be such that the entire scene would be focused perfectly upon the light-sensitive surface (either the retina or film). This ideal state only can he achieved with a pinhole aperture, however, so the lens must be fine-tuned in order to bring objects at different distances into focus. In the camera this fine-tuning is achieved by moving the lens back and forth. In the eye, fine-tuning is achieved by changing the curvature of the lens, a process known as accommodation.
In humans, this accommodative ability decreases with age because the lens gradually hardens and the muscles that control the lens shape gradually weaken. The result is increasing difficulty in focusing on objects at close range. In a sense, dogs don’t have this aging problem -- but only because they essentially are born with the accommodative ability of a person 50 to 60 years of age! The dog’s accommodative ability only is one-fifteenth of a young person’s.
In A Blur
When optimally focused, light forms a sharp image exactly at the plane of the light-sensitive receptors (the camera’s film or the eye’s retina, located at the rear of the globe). If the refractive powers of the cornea and lens are too powerful for the distance to the retina, the light will come to a focus prema-turely and will go on to become unfocused by the time it falls on the retina. This condition, called nearsightedness or myopia, results in difficulty focusing distant objects, If the cornea and lens have too little refractive power for the distance to the retina, the light still will be unfocussed when it falls on the retina. This condition, called farsightedness or hyperopia, results in difficulty focusing on close objects. Only when the eye’s refractive power is in perfect accordance with its retinal distance will light rays be brought into sharp focus on the retina. This is the desirable refractive state known as emmetropia.
It’s not difficult to estimate the refractive state in a cooperative dog by use of a retinoscope, an instrument for observing the focal point of a beam of light that has passed through all of the refracting surfaces of the eye. The results of early research seemed to suggest most dogs should be wearing glasses. As far back as 1901, researchers reported that domestic dogs were myopic, but wild-caught canids (wolves, jackals and dingos) were emmetropic or slightly hyperopic. (1) Deviations from emmetropia are measured in diopters (D), the power of a lens necessary to bring the image into focus. Powers of plus or minus D would indicate mild degrees of hyperopia or myopia, respectively; powers of plus or minus 3D would indicate strong glasses would be necessary! These early researchers reported an average of -3D, indicating strong myopia. (2) In the 1920s, a more extensive study of more than 100 dogs found great variability among dogs, ranging from -4.5 to +2.OD. (3) Yet modern studies of refractive states in dogs have suggested most dogs are within 0.5D of emmetropia. 4,5
In a recent study of 240 dogs, most dogs were found to be nearly emmetropic. (6) These researchers com-pared the results from breeds in which they examined five or more representatives (German Shepherd Dog, Chesa-peake Bay Retriever, Cocker Spaniel, Golden Retriever, Labrador Retriever, Poodle, Rottweiler, Miniature Schnauzer, Chinese Shar-Pei, Springer Spaniel, terriers as a group and mixed-breeds as a group). More than half of the German Shepherds, Rottweilers and Schnauzers were myopic, significantly more than found in the other groups. Rottweilers were the most severely af-fected, with myopic Rottweilers averaging almost -3D. Interestingly, in all these breeds the myopia tended to occur within the same families. A group of German Shepherd guide dogs had significantly lower prevalence of myopia (34 per cent) compared with nonguide German Shepherds. The retriever breeds tended to be more hyperopic than the other breeds in the study, but the degree of hyperopia was not great (averages from +0.4 to +0.8D depending on breed). These results, especially when combined with an earlier (but unsubstantiated) report that Grey-hounds were usually from +0.5 to + 1 .5D hyperopic, (7) are consistent with the idea that refractive state may have a hereditary component.
Besides providing some tantalizing evidence that breed differences may exist in refractive states, this and previous studies found a greater tendency to-ward myopia with increasing age in dogs. In a longitudinal study of ca-nine refractive states, 6-month-old Beagles averaged +0.4D, and two years later the same dogs averaged -0.5D. (8) Understanding the refractive states of dogs, and especially older dogs, has led to some practical implications. This is be-cause many dogs develop cataracts or other lens problems that necessitate removing the lens so vision can be saved or restored. When this operation was first performed on dogs, it was customary simply to remove the lens without trying to restore the eye’s preoperative refractive state. Yet without a lens, dogs are terribly hyperopic, averaging about +14D. With the advent of intraocular prosthetic lenses, it was hoped that dogs could be restored to near emmetropic refractive states. Optical models of the canine eye sug-gested that a prosthetic lens would have to be much stronger than the prosthetic lenses used for humans. (9), (10) This reflects the larger lens of the dog, as well as its more rearward placement in the dog’s eye, compared with the human. Now dogs cannot only profit from having cataracts removed, but they can expect sharp vision following lens replacement with the appropriate intraocular prosthesis.
An Eye For Details
After an image is focused onto the retina, retinal anatomy imposes the next limiting factor in perceiving fine details. Returning to our camera analogy, the eye’s retina is like the camera’s film. Any photographer knows film comes in different sizes and speeds.
Anyone who has tried to enlarge a photo from tiny 110 camera film knows how poor the end result is. The film is simply too small to record fine details. For best results, a large area of film needs to be covered so there is plenty of room for details to be recorded. The same is true for eyes. The light-sensitive receptor cells are about the same size in all mammals. Obviously, more can be packed onto a larger retina, and the size of the image on the retina can be greater if that retina is big.
The retina and film also both depend upon "sampling grain" to ensure good acuity. Film captures images because it is coated with an emulsion contain-ing silver grains that undergo a chemical reaction when exposed to light. In very dim light, the chances of a silver grain being hit by sufficient light to cause a reaction can be increased simply by making the silver grain larger. The result is film that is very sensitive in low light levels but that creates a "grainy" image lacking fine detail. In bright light, it’s better to select a film coated with tiny grains of silver, which can create an image of exquisite detail.
So, how do animals (and dogs in particular) avoid a grainy image -- or do they?
The Rod/Cone Connection
Animals can’t select different film or retinal speeds according to lighting conditions, but they have evolved several ways of coping. Like film, retinal receptors contain chemicals that react when exposed to light. One way is to use both large and small "grains," or receptor types, in the same retina. The two types of specialized receptors are the rods and cones. The rods are analogous to the large grains; not only is each rod very sensitive to light, but the responses from groups of rods are pooled and analyzed by higher-level processing cells. This response pooling increases the area over which light is caught (in essence, creating a larger "grain"), thus increasing sensitivity at the expense of acuity. In contrast, the cones are like the small silver grains; they won’t detect very dim light, but if the light is sufficiently bright their fine mosaic can result in the ability to discriminate fine details.
Rods predominate in the retinas of nocturnal animals, and cones predominate in the retinas of diurnal animals. Most mammals have both. If the cones were distributed evenly among the rods, the increased acuity due to their fine mosaic would be somewhat negated. To combat this, the rods and cones are distributed unevenly, with more rods toward the periphery and more cones toward the center of the retina. In some animals in which fine vision especially is critical (such as humans), a small pure cone area, called the fovea, is placed directly in the line of vision. In fact, the fovea is the part of your eye you are using to read this text. It covers only a very small area of your vision, however. Try reading with your finger blocking your central vision and notice how difficult it is. This can give you some idea of how it must be like for an animal without a fovea to make out fine details.
Do dogs have a fovea? In 1902 a researcher claimed to have found a fovea in sighthounds but not in other dog breeds. (11) A more extensive investigation using 50 Greyhounds in the 1950s found no such area, however. (12) It now generally is agreed dogs don’t have a pure cone area, although they do have an area of increased cone density toward the center of the retina. Some recent evidence has revived the possibility that breed differences may exist in cone distribution. This evidence comes from examinations not of cones but of retinal ganglion cells.
Signals from the rods and cones reach the brain by means of intermediate processing cells known as ganglion cells. The important thing to know about these cells is that their density roughly mirrors both the number of cones and the resulting acuity in different parts of the retina, and they are easier to count than either rods or cones. Most species have an oval area in the mid-dle of the retina in which ganglion cell density is greatest. Other species have a horizontal streak across the retina in which ganglion cell density is greatest. Species with more highly developed streaks tend to be fast animals that live on the open plains, such as gazelles and horses. The streak is believed to aid animals in scanning the horizon. The dog has both: a central oval area is superimposed upon a horizontal streak, although there is great variation in the extent to which either predominates. Of the carnivores so far investigated, the cheetah and the Greyhound have the most highly developed streaks, with less developed streaks in the dingo, fox and other breeds of dog. (13)
In a comparison of ganglion cell distribution in wolves, German Shepherds and Beagles, wolves had higher overall ganglion cell densities and more pronounced streaks than domestic dogs. (14) But unexpectedly, dogs from one family of Beagles had more pronounced streaks and higher ganglion cell densities than those from another family of Beagles, suggesting a possible genetic component. Although this evidence suggests individual differences also might exist in visual acuity among dogs, surprisingly little is known about visual acuity in dogs in general.
Stars And Stripes
The evidence so far indicates that even if dogs are emmetropic, the lower cone and ganglion cell density compared with that of humans suggest rather poor acuity. A more precise estimate of acuity can be calculated by considering the optics of the eye and the resulting size of an image falling on the retina, in conjunction with ganglion cell density. This theoretical resolving power, or Nyquist limit, usually is reported as the finest separation of a series of evenly spaced parallel lines that an animal should be able to differentiate as stripes rather than a uniform gray field.
Each black/white line pair is called a cycle, and the acuity thus calculated is reported in cycles per degree. Degrees are a way of describing acuity that is independent of viewing distance; briefly, a degree describes how large an area on the retina an image covers. For example, the full moon subtends an area of about 4 degrees on your retina. The Nyquist limit thus calculated for the dog results in values of 4.5 to 6.5 cycles per degree. A threshold value of five cycles per degree would indicate a dog just could discern five black/white stripes on a 1-degree spot, or 20 black/white stripes fit on an area roughly the size of the full moon. Humans would have no difficulty seeing this number of stripes in such a space. This is the theoretical visual acuity limit for the dog; it’s a little more difficult to find out if the dog really can make out this detail.
One way to see if an animal can discern fine detail is to train it to respond to large stripes, but not to a uniform gray field. As the stripes are made progressively finer, at some point the dog won’t be able to tell the difference between the fine stripes and the uniform gray field. Early studies provided a dismal view of canine acuity. A single Bull Terrier was unable to discriminate between two striped patterns of about 1 vs. 35 cycles per degree -- a blatant difference to human eyes (and for that matter, chickens and monkeys, which were tested in the same study). (15) In fact, people can discern stripes of up to about 30 to 50 cycles per degree. More recently, a single Poodle was able to discriminate between striped patterns as fine as approximately 6 cycles per degree, (16) in much closer agreement with the theoretical Nyquist limit. It also is roughly the same as the threshold obtained in cats. Thus, even in a dog with emmetropic vision, its retinal composition limits its acuity to a much lower level than humans enjoy.
If the dog’s retina is so poorly designed for acuity, does it really matter if the image that gets there is a little out of focus? Should we really worry about implanting the proper power lens in a dog that may not be able to appreciate it? A recent study found that mimicking a 2D myopia greatly reduced acuity; mimicking the refractive state resulting from lens removal resulted in acuity of less than I cycle per poor vision, indeed. Thus, a poor refractive state can change a dog’s acuity from bad to worse.
From years of study, then, researchers have been able to draw these conclusions about canine vision:
Most dogs are nearly emmetropic. Some breeds have a higher prevalence of myopia than others. Unconfirmed evidence exists of greater prevalence of hyperopia in Greyhounds. Myopia increases with age.
Existing evidence suggests dogs have very little ability to accommodate.
Dogs have a much lower ratio of cones to rods than humans do. They have no fovea.
Individual differences exist in the density and distribution of ganglion cells, which roughly reflect cone distribution.
Theoretical calculations and behavioral estimates of dog visual acuity suggest it is at least six times poorer than human acuity. Although anatomical evidence suggests the basis for individual and breed differences in acuity, this question has not been investigated.
Given this information, are my Salukis just pretending they don’t recognize me? Probably not entirely. Their ability to make out facial features from a distance probably is so poor that they rely upon other cues, such as the way I sound, smell or walk, to confirm my identification. In all eyes compromises must be made. The dog has given up the ability to perceive very fine detail, but what has it gotten in return? It turns out that the dog may have made a good trade after all. Next time this other major dimension of vision will be explored: the ability to detect light. In addition, we will explore the most controversial dog vision question of all -- do dogs see color?
D. Caroline Coile is an award-winning author who has written many books and articles about dogs. She holds a doctorate in the field of neuroscience and behavior, with special interests in canine sensory systems, genetics and behavior. Her "Baha" Salukis have been top-ranked in conformation, field and obedience competition with Best in Show, Best in Specialty and Best in Field awards to their credit. Dr. Coile is currently working on a book about Canine Senses. She can be reached by e-mail at firstname.lastname@example.org.