Vision

P E R SPEC T I VE

Contents

CON TEN TS .

DOG VISION / CONE CELLS 4-7

WIDE EYED / LENS FACTS 8-13

PERSPECTIVE 12-15

Dog Vision

DOG VISION Main Story

It is a mistaken notion that dogs see only in black and white. If, however, what you mean by colorblind is that dogs see only a portion of the visible spectrum as compared with what humans see, then yes, dogs are colorblind. And there are a couple of methods that scientists use to determine this. But first, it is important to understand how dogs see.

Dogs have two types of color photoreception, or cone cells, on their retinas that recognize short and medium-to-long wavelengths of light, corresponding to bluish hues (short wavelength) and red-yellow ones (long wavelengths). People, on the other hand, have three types of cone cells that enable us to see the full range of colors that make up the visible spectrum. Since dogs have only two types of cone cells, the colors they can distinguish are almost identical to the colors a human who has red-green color blindness would see. Of course, colorblind humans still see many different colors, and scientists think dogs see this range of colors as well.

How do scientists know? One way is to shine beams of colored lights into dogs’ eyes and analyze the spectrum, or pattern, of light that is reflected back. The results are then compared with the pattern produced when the same lights are shined into human eyes.

Another way to study canine vision is to have the dogs “tell� scientists what they see. In one experiment, dogs are shown a series of three lights; in each case, two of the three lights are the same color. With a minimal amount of training, the dogs select with their noses the colored light that is different from the others. By varying the colors of the lights and repeating the process, scientists have determined that dogs see the world in black, white, and shades of gray, with long wavelength (red-yellow) and short wavelength (blue) colors thrown in.

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Dog Vision

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Cone Cells

CONE CELLS The Facts

Cone cells contain pigments that perceive specific wavelengths of color. Human vision is trichromatic - we have three types of cones that recognize different portions of the color spectrum. These cones allow us to see a range of colors that are a mix of red, blue, and green pigments. Dogs have only two types of cones - their dichromatic color vision is silar to that of a human with red-green color-blindness. In addition, a dog’s retina contains a much smaller ratio of cones to rods than ours does.

Two types of photoreceptor cells in the retina -- rods and cones -- respond to light and transmit electric impulses to the optic nerve through a series of chemical reactions.

Before you start feeling sorry for your dog though, keep in mind that although he may not have the color range and visual acuity (focus) that you depend on, his night vision is far superior. Thanks to a reflective structure behind their retina called the tapetum lucidum, dogs see objects in the dark as if lit by an eerie glow.

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Wide Eyed

WIDE EYED Sub Story

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.

“The human and canine eye are built upon the same basic design, but each has modifications that enable it to perform optimally”

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Wide Eyed

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.

“ The more a lens can bend light, the more powerful the lens is said to be.”

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 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.

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Perspective

IT’S ALL IN PERSPECTIVE The 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.

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“ The light-sensitive receptor cells are about the same size in all mammals. 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.

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.

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.

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D A N H A R R I S

UNIVERSITY OF DERBY GRAPHIC DESIGN 2013