Grainy Sight and Clear Sight

Grainy Sight and Clear Sight

In addition to the focusing ability of the eye, a second factor affects visual acuity in  dogs: the type and arrangement of photoreceptors in the eye produce certain ef-  fects similar to those seen in photographic film. The film in a camera is covered with an emulsion containing certain photosensitive silver-based salts that react  chemically when they are hit by light. Film that is more sensitive in dim light has  larger grains of these salts to increase the likelihood that any one grain will capture  enough light to produce a chemical reaction. Unfortunately, the resulting image  also looks “grainy” (much like a poor-quality digital photo in which the pixels or  patches of color are larger), which means some of the smaller details may not be  clear. If the light levels are higher, one can use a less sensitive film, in which the  grains are tiny and tightly packed, so that even small details can be registered.

The grains in a film emulsion can be compared to the photoreceptors in the  retina. Rods (dim light receptors) increase their sensitivity by having a large clump  of them all connected to the same “ganglion cell,” which pools their information  before sending it out of the eye. In effect, the rods act like big grains of photosen-  sitive salt, since any light hitting any one of the rods will trigger that ganglion cell.  In contrast, only one or two cones (which operate only in brighter light) are con-  nected to any single ganglion cell that sends its information to the brain. So cones  operate much like small grains of photosensitive salt—they perform poorly in dim  light, but if the light level is high enough, they have a fine enough mosaic to dis-  criminate small details well.

Since animals cannot change their photoreceptor makeup the way a photog-  rapher can change the sensitivity of the film he uses in a camera to fit current light  conditions, eyes have evolved to maximize their efficiency for the animal’s behavior  and survival. In animals whose eyes contain both rods and cones, the photore-  ceptors are distributed differently in the different regions of the eye. In humans  there is a small region, right in the middle of your line of sight, that is called the  “fovea.” It contains only cones, which are densely packed to provide us with max-  imum detail vision in bright light. As we move away from the center of the fovea,  the number of cones drops off, which explains why our ability to see details in pe-  ripheral vision also declines. When you look at something, you are actually aiming  your fovea at it, meaning that you are mostly reading this page by pointing your  fovea at successive words in a sentence. Words that your fovea is not pointing at  appear blurred and become even less distinct the farther away they are from the  center of vision. Try this little experiment: Close one eye and let the other eye stare  directly at the center of the page. Notice that the page seems fairly clear. Now block  your central vision by holding up your finger in front of your eye. Keep the center of  your eye pointed at your finger but pay attention to the parts of the page that are  not blocked by your finger. The print on the page to the sides of your finger be-  comes blurry and may be unreadable.

For human beings the rods are found in increasing numbers as you move away  from the center of your field of vision. This means that your greatest light sensi-  tivity is outside of the fovea, in the periphery of the eye. That is why people who  have to observe dim lights at night often find that the light is more visible if they  look off to the side of the target, since the light then falls on a part of the retina that  contains a higher density of rods. In effect, then, we have two different “films” in

our eyes in different regions—a central area for fine detail vision in brighter light  and a surrounding area with greater light sensitivity but poor acuity for small de-  tails.  The dog’s eye also has separate areas of sensitivity, but the arrangement is dif-  ferent. The fovea of a dog’s eye is a bit larger and shaped like an oval lying on its  long side. As in humans, it is also a region of densely packed photoreceptive cells,  but it is not made up exclusively of cones, as in the human eye. It contains many  rods as well, although they are thinner and fewer of them connect to a single gan-  glion cell. All of this provides better acuity in this part of the eye. From the ends of  this oval of densely packed cells, a horizontal streak, which also contains many  cells tightly crammed together, extends across the eye. The cells crowded together  in the streak give the dog better visual acuity in that region of the eye as well, and  would have helped his hunting ancestors search the horizon for prey. This same  arrangement is also found in other fast-moving animals that live on the open  plains, such as horses and antelope, and is considered an adaptation that evolved  to help these animals scan for predators.

The swift-running greyhound, which hunts by sight, has the most pronounced  visual streak. Dog breeds that rely more heavily on their noses than their eyes when  hunting, such as the beagle, have a less distinct horizontal stripe. Thus our selec-  tive breeding for particular behaviors in dogs has caused significant and unex-  pected changes in the basic physiology and neurology of each breed. Not only do  dogs see the world differently than humans, but a greyhound sees the world differ-  ently than a beagle.