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"Can my dog watch TV?"
Answers to Questions about Companion Animal Vision – Part 2
by Noelle La Croix, DVM, Dip. ACVO
In part 1 of this article, we reviewed color vision, light sensitivity and visual acuity in the dog. This article will review canine depth perception, field of view, and motion detection. I will also describe "flicker fusion" and how it relates to dogs watching television.
Depth perception (sensation)
To successfully interact with a 3-dimensional world, a mammalian visual system determines depth from visual cues within a 2-dimensional retinal image. In humans, this is commonly described as "depth perception." However, "perception" implies the cognitive ability to relate one's experiences to others. For a dog, "depth sensation" is more accurate terminology that describes the ability to determine the precise location of visualized objects. All animals with complex locomotive ability require accurate depth sensation to move effectively within their enviroment. Binocular vision is an important evolutionary adaptation for depth sensation.
To maintain a singular binocular image, the eyes of mammals can simultaneously move in opposing directions (vergence). The viewing of a close object promotes ocular convergence, whereas a distant object promotes ocular divergence. Ultimately the viewed object is focused upon the visual streak of both canine eyes, and this state is described as fixation. However, during fixation there are usually differences in the viewing angle to objects in the visual field of each eye. This results in subtle differences in the fields of view and corresponding retinal images of each eye. It is mainly this binocular disparity that is interpreted by the brain as depth sensation. Increased binocular overlap promotes depth sensation. The brain is able to make more comparisons between each retinal image, resulting in a greater regard for their differences.
In the dog there is typically 30-60 degrees of overlap between the visual fields of each eye. Human binocular overlap is much greater at about 180 degrees. However, the small canine binocular overlap is apparently sufficient for their lifestyle. Many dogs can catch fast moving objects in the air with ease. By contrast, a dog with monocular vision is often fearful of descending a staircase. Depth sensation is clearly an important component of canine vision.
Field of view
A "field of view" describes the part of the world seen by an organism at a particular moment in time within a visual plane. Predatory animals have generally narrow fields of view, relying instead upon binocular overlap (depth sensation) to visually isolate targeted prey. In contrast, prey animals generally have wider fields of view to scan their world for predators surrounding them. Dogs evolved as predatory animals with a maximal field of view corresponding to about 240 degrees. As an example of a prey animal, horses evolved to a field of view of 357 degrees!
In a world of prey and predators, motion detection evolved. The cone-rich human fovea is highly sensitive to movement in bright light. The rod-rich canine retina is highly sensitive to motion in dim lighting conditions. The peripheral retina is particularly involved in motion detection. There are retinal areas that are sensitive to specific movements within each 3-dimensional plane. Dogs are generally visually stimulated by moving objects, but ignore stationary objects. In a 1936 study, police dogs could detect moving objects up to 900 meters away, while ignoring similarly placed stationary objects. So a dog may only "see" a distant owner if that owner is moving.
Can dogs watch TV?
The traditional tube-based American television produces a complete image 60 times per second (60 Hz "refresh rate"), with each image being a duplicate of a filmed image or "frame." The refresh rate of a more modern high-definition television (HDTV) is similar, but with many more projected lines per image.
Intermittent televised images appear stationary to a human observer since their projection rate exceeds the human "flicker fusion" rate. This rate roughly corresponds to the speed at which a retina “updates” an image to the brain. The flicker fusion rate can vary with a multitude of other factors including image brightness and observer fatigue. Ultimately the rate-limiting step of retinal image updating is the response of photoreceptors. Cones are slower to update than rods. The canine retina is predominately rods that can detect flicker within images projected at a rate below 70 to 80 Hz. Therefore most televisions produce images that dogs perceive as flickering, without fluid (realistic) motion. Some newer HDTV’s operate in excess of 120 Hz, and can therefore project images that appear fluidly to a dog. Getting a dog interested in watching commercial programming is another matter entirely (Figure 1).
I hope these articles have helped explaining some aspects of the vision of human's best friend.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: A 12-year-old female spayed Weimaraner that prefers to ignore the visualized flickering of a fluorescing cathode ray tube.