Taking a Closer Look at Canine Vision

Vision is a beautiful, fine-tuned sequence of events with amazing results and it all starts with the sun. The sun emits energy in packages of light called photons. Photons behave as a particle and as a wave. The wave of each photon has a length, adeptly called the wave length. There is a large spectrum of wavelengths ranging from very short (10 nm) to very long (100 km). Examples of the very short wavelengths of light are gamma rays, x-rays, and ultraviolet radiation. Our eyes cannot see wavelengths but our sunburns assure they exist. The wavelengths of light humans can see are between 400-700 nm in length. They are called the visible spectrum of light and are seen as a rainbow of colors, ranging from violet to red. The longer wavelengths of light are infrared, microwaves, and radio waves.

The wavelengths of light emitted from the sun in the 380-700 nm range bounce off of surfaces on onto our eyes. When the light contacts our eye it gets focused on the retina by the cornea and lens and funneled to our retina.The retina is a thin collection of nerves is in the back of our eyes with is an extension from the brain. The retina is divided into10 layers of cells. Some of the cells are sensitive to the wavelengths of light that are focused on them. These cells are called rods and cones. When light, with a wavelength of 350-700 nm, gets focused onto the rods and cones, the rods and cones are stimulated. The stimulated rods and cones translate the light’s energy (wavelength) into chemical and electrical signals that are sent through nerve fibers to our brain. Cells in our brain interpret the signals and we see … a doorway or a smile on our friend’s face, and we react to our brain’s interpretations of the information from the collection of wavelengths of light by skillfully walking through the doorway or by smiling back at our friend.

Due to the adaptations to the environment, different species see the world differently. As Jean Baptiste Lamarck, Alfred Russel Wallace, and Charles Darwin explained, species need to adapt to the pressures in their environment to survive and reproduce. Fish, birds, dogs, cats, horses, humans, snakes, etcetera, have all developed different visual capabilities based on the different pressures in their environment. While humans’ eyes have not adapted to perceive wavelengths of light outside the 380-700 nm spectrum, bees perceive and respond to ultraviolet light (10-380 nm). Boa constrictors and pit vipers have infrared (780 nm to 1 mm) sensing organs in their eyes to form heat sensitive images of their prey.

What is the world like from a dog’s visual perspective versus a human’s perspective? For comparisons sake, vision can be divided into seven components. The components are sensitivity to light, sensitivity to linear motion, visual field-of-view, depth perception, visual acuity, accommodation, and color vision.

Dogs have a layer of tissue in the back of their eye called the tapetum. Humans do not have a tapetum. The tapetum allows the eye to capture more photon energy in low light settings. Dogs also have a higher ratio of rods to cones in their retina and rods are more sensitive than cones in low light conditions. These two features, the tapetum and a higher rod:cone ratio, mean dogs see better in dimmed light compared to humans.

A specific example of motion detection by the eye is linear motion. Linear motion describes a linear movement within the field of view such as a train moving on the horizon or a gazelle running along a field. By their design, rods are more sensitive to linear motion than cones. Just like dogs higher rod:cone ratio compared to humans provides increased sensitivity to light, their higher rod:cone ratio also makes them more sensitive to linear motion than humans. It has been shown that dogs can discriminate an object in linear motion at 900 meters away, whereas humans are only able to discriminate when the same object was in linear motion 580 meters, or less, away.

Dogs typically have a wider set of eyes than humans. The average field of view for a dog is 240 degrees. The average field of view for a human is 180 degrees, so dogs have a larger field of view of the horizon than humans.

To compare visual acuity, we need to understand the definition of the standard for human vision set at 20/20 vision. The term 20/20 vision is used to express normal visual acuity (the clarity or sharpness of vision), in humans, measured at 20 feet. This means a human with 20/20 vision can clearly see an object that is 20 feet away. Using this definition, the visual acuity of dogs is estimated at 20/75. When you compare a human with 20/20 vision to a dog, an object that a human can see clearly from 75 feet away, a dog can only see with the same clarity/sharpness when the dog is 20 feet away from the same object.

Accommodation describes the ability of the eye to shift focus from something far away to something close up. The ability to accommodate depends largely on the degree that the lens can be compressed. The lens is a structure within the eye that helps to focus light, similar to the lens in a pair of glasses. Unlike the rigid lens in a pair of glasses, the lens in the eye can be compressed to allow our area of focus to change from close-up to far-away depending on how much the lens is being compressed or decompressed by muscles within the eye.As all eyes age, the lens becomes harder and more difficult to compress. The consequence of increased lens hardness with time is loss of the ability to focus on things close up. When this occurs in humans, we purchase a pair of glasses for reading. Overall, due to differences in the size of the lens, location of the lens within the eye, and degree that the lens can be compressed, dogs have less of an ability to accommodate than humans so they do not see close up to the same degree that humans do. Like humans, their lenses get harder as a dog gets older. So dogs lose their already decreased ability to focus on things close up as they age.

Cones are used for color vision and dogs have cones in their retina. So, contrary to some common beliefs, dogs can see color. Specifically, based on studies of the types of photopigments in their cones, dogs can see the colors blue and yellow well. The rest of their color spectrum (violet to red) is seen as shades of grey. Dogs see more shades of grey when compared to humans, which affords them better contrast vision.

In summary, humans and dogs have different visual capabilities across the seven components of vision that include sensitivity to light, sensitivity to linear motion, visual field-of-view, depth perception, visual acuity, accommodation, and color vision. One system is not better than the other. Vision evolved differently based on the environmental pressures that impacted their species’ survival.

Everyday I am graced with working with many different patients and the people that love them and I wonder if a dog’s decreased ability to focus on things in the distance and their increased ability to see more shades of grey helps to explain their most inspiring attributes of living in the moment and unconditional love.

Dr. Nuhsbaum is a board-certified, veterinary ophthalmologist. She works to improve the ocular comfort, vision, and quality of life of animals. Dr. Nuhsbaum is part of the team of experts at Apex Veterinary Ophthalmology Specialists.