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  • Writer's pictureDennis Hammett

Why doesn't he recognize me?

Human and canine eyes are built around the same general design, but they have significant differences that affect how each species perceives the world. We use a greater portion of the brain and is probably the most important of our senses. This is not true with our canine friends and hunting partners. In many ways the dog’s visual abilities are poorer than humans, although there are some aspects that are better than ours.

Let’s start with a superior ability: Canines are more comfortable than humans operating in dim light. The type of eye needed for twilight and nighttime activity require sensitivity to low brightness, but the perception of color is really nit very important. In the construction of the dog’s eye a sacrifice of a certain amount of detail-resolving ability has been made in order to function at low light levels. For example, a dog’s pupils are much larger than those in humans that lets in more light but results in a loss of depth of field (which is the range of near-to-far distance in which objects are in clear focus). Like humans a dog can enlarge or contract it’s pupils but cannot make their pupils small enough to give them the dame depth of field that humans have.

When light passes through the pupil is caught and registered by special neural cells called “photoreceptors” of two types, either rods or cones. The rods are specialized to work under dim light conditions and dogs have a much lager proportion of rods in their eyes than humans. In addition behind the retina is the “reflecting” tapetum that bounces back at the retina not caught by the photosensitive cells (hence the green or yellow glow in the night when a beam of light is shined in their face at night). And there is a cost for the extra light gathering because the light that hits the reflective surface comes from various directionsit does not return along exactly the same path as it entered but bounces off at an angle. Because the incoming direction of the light and the reflected direction are different, lines in the image on the retina are smeared and the ability to clearly see fine details are compromised.

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