Color and Acuity Differences
Most humans have the ability to see all the different colors of the electromagnetic spectrum, and consequently perceive all its' colors. [The color that is perceived is a result of the visual system, and not the spectrum itself.] Animals, such as the dog, are often thought of as being at a disadvantage by not being able to see all the hues of the spectrum. Evolutionarily however, the dog and the human each developed the visual system that worked best for them. Humans have depended on their diurnal ability and a sense of color throughout time to help them find food. Dogs on the other hand, were not originally diurnal animals, until humans domesticated them. Consequently, the ability to see at night was originally more important to the dog than color. After all, their prey is often camouflaged with the surroundings, so they are unable to rely on color vision cues as heavily as humans do to find food.
The retina of the eye is lined with both rods and cones in humans and dogs. The rods are much more prevalent in both species, but even more so in the dog than the human. The rods are adapted to work best in low light and are used for motion detection. The central retina of the canine eye contains about 20% cones, while humans have an area of 100% cones called the fovea. The cones work best in mid to high levels of light and have the ability to detect color.
Humans are believed to have three different cone types, a trichromat, while dogs have two, a dichromat. Each cone contains a photopigment that is maximally sensitive to a separate wavelength of light. These photopigments are what makes color vision possible. A human's three cone types are maximally sensitive at 445 nm, 535 nm, and 570 nm respectively. The canine cones are maximally sensitive at 429 nm and 555 nm.
The fact that dogs have two different types of cones does not mean that their brains have the ability to interpret the information that the rods and cones send. To determine what colors a dog can see, behavioral studies can be done. One such study conducted by Neitz, Geist and Jacobs involves three colored squares being placed in front of a dog. By training the dog to pick the odd colored square of the three, the researcher is able to guess what colors the dog can see. The question that then arose was if the dog was choosing the panel for it's color or due to it's brightness. It was found by use of different brightnesses for the squares that the dog was indeed choosing the square for the color and not brightness cues. Through these studies it has been suggested that an average dog sees similar to a human deuteranope, a person that is red-green colorblind. Consequently, the dog's world consists of yellows, blues, and grays. When a human perceives a red object it appears as yellow to the dog, while a green object appears as white, a shade of gray. This white region, also called the neutral point, occurs around 480 nm in visual spectrum. According to the electromagnetic spectrum, 480 nm would appear as a greenish-blue hue. All wavelengths longer than the neutral point are indistinguishable from one another to the dog and would all appear as yellow.
Figure 1 shows two electromagnetic spectrums (from Dr. Plonsky, 1998). The top spectrum is what the dog is able to perceive, and the bottom is what a human would perceive in the visible spectrum (wavelengths from 380 to 760).
The dog's ability to see detail, also termed acuity, is around six times poorer than an average human. Acuity, measured in cycles per degree, is how many lines that can be seen as distinct entities in the visual field. Humans have the ability to see about 30 cycles per degree while dogs can see about 12.
Figure 2 shows the differences in acuity between a human, top, and a dog, bottom (from East, 1998). To a dog, the top pattern would appear as a uniform gray blur. To see the individual lines, each line would have to be thicker. If it were possible to test a human and a dog using an eye chart, and the human had 20/20 vision, the dog would be between 20/50 and 20/100, making them nearsighted. This measurement of the dog's vision, and acuity, can be estimated by use of a retinoscope, which measures the refractive ability of the eye.
Acuity is affected by the size of the pupil, size of lens and cornea, and the arrangement of the rods and cones on the retina. The pupil, controlled by the iris muscle is able to expand and contract to let in different amounts of light. An animal that is active in dim light, dogs, tends to have a large pupil. The larger the pupil the more the field depth decreases. The optics of the eye for an animal found in dim light includes a larger thicker lens for greater light gathering ability and a larger cornea that refracts light. The arrangement of the rods and cones on the retina affects the acuity by the number and location of the rods and cones, and presence or absence of a fovea.
Acuity is poorer when only the rods are stimulated versus when only cones or a mix of the two are stimulated. This is so because each cone has its own intermediate neuron, while a group of rods share a single neuron to the brain. This meeting of the neurons for the rods into a single neuron results in lower acuity, but a higher sensitivity. The lower acuity is due to the stimulus information being grouped into a single neuron, consequently reducing the information that any single rod gives. On the other hand, when the cones are stimulated, they have their own line to the brain that delivers information to help distinguish the details of the object. This allows each cone to be independently stimulated increasing the ability to distinguish detail. The fovea, being made up of only cones, therefore is the area that is extremely significant to detailed vision. Since the dog has no such area of concentrated cones, it would make sense that they have a significantly lower visual acuity than a human would have.
Humans may have the ability to perceive a more colorful detailed world than the dog, however this does not mean that the dog is disadvantaged. Evolutionarily, the dog has not needed these surface characteristics to survive. Instead, they developed the ability to see in dimmer light and to detect motion that aids in their survival. Humans on the other hand, are visual creatures that heavily depend on both color and acuity to go about our everyday lives.