Non-Visual Effect of light on Pets (Part 1)
Light can affect all animals and pets in various aspects. Many animals prefer to stay in the dark, while some others constantly seek sunlight. There are multiple types of lights, and each can influence pets differently, both beneficial and harmful. A large subset of pets, dogs, and cats included are far more sensitive to noise and light than humans, as they see the world differently. Recent advances in home technology likely have made things far worse for pets around the house. For instance, LED lights, becoming more popular in this decade, produce a flicker so fast that it's invisible to the human eye. To pets, LED lights, especially those made with lower quality parts, may have a strobe-like effect. Dogs, for example, are more than three times as sensitive to the type of flicker produced by LED lights than humans . So, it is essential to be familiar with rays' effects around pets, which affect them both with biological characters like their protoplasm, metabolism, pigmentation, movements, biological clocks, reproduction, and development, and with cognitive behaviors. This article comprises three parts. The first part discusses the fundamentals of vision in dogs, cats, and horses. The second part explains some light effects such as LED colored light and ultraviolet radiation on some pets and domestic animals. The final part points out some applications of light in veterinary.
2 Vision in Dogs
A large amount of studies has been conducted about the visual processing of dogs, mainly as a means of studying cognition, many of them have been a matter of controversy. To date, the most common approach to assessing dog cognition is via visual tasks. In fact, approximately 74% of dog cognition studies use visual tasks . Although little is known about vision in dogs, this article discusses the fundamentals of dogs' vision like sensitivity to light, color vision, and sensitivity to flickering lights.
2.1 Sensitivity to Light
Dogs are highly adapted to function in dim light. That is since their retina is mainly composed of rod photoreceptor cells . The rod photopigment, Rhodopsin, is a G-protein-coupled receptor that is highly sensitive to light and improves vision in dim light conditions. Dogs typically have a rhodopsin peak sensitivity to light wavelengths of 506– 510nm . In comparison, humans have peak sensitivity to slightly shorter wavelengths of 495 nm .
There is also another factor that increases dogs’ sensitivity in dim light conditions more than humans which is the reflective tapetum lucidum, a layer of tissue in the eye of many vertebrates.
It is located immediately behind the retina . As dogs are sensitive to a variety of light conditions, this can affect their recovery from exposure to bright light. To compare, when a dog and a human come inside after having been outside, the adaptation time to indoor light is twice as long in dogs as it is in humans. This should be considered when bringing dogs from bright outdoor directly into indoor environments .
2.2 Color Vision
One of the most controversial issues in the study of vision in dogs is distinguishing between different colors. To compare, humans have three types of cone photoreceptor cells (long-wave (red), medium-wave (green), and short-wave (blue), at spectral peaks of 558 nm, 531 nm, and 419nm, respectively). Dogs have only two, which almost identically correspond to short-wave and long-wave sensitivities (blue at a spectral peak of 555 nm and yellow at 429 nm)  as shown in Figure 1. As a result, it has been shown that dogs cannot distinguish between green, yellow, and red color cues. However, there is some even evidence that suggests dogs may be able to perceive these colors even without possessing the cone photoreceptor cells believed to be responsible for this ability . As mentioned, further studies are needed to identify the extent to which dogs perceive color.
Figure 1 Difference of spectrum perceived by dogs and humans .
In addition, it appears that dogs may have a capacity to perceive ultraviolet (UV) light. Dogs were identified to have lenses transmitting significant amounts of UV rays (at 335 nm); so even though dogs do not have a specific UV visual pigment, they may be sensitive to UV light .
Another study , suggests that dogs may have a magnetic sense associated with their visual system. The study observed the presence of Cryptochrome 1, which is responsible for sensitivity to blue light and is involved in responding to light-dependent magnetic orientation based on the earth’s magnetic field.
2.3 Sensitivity to Flickering Lights
The flicker fusion rate is the point at which rapidly flickering light appears to meld into constantly illuminated light. If the frequency is below the threshold of sensitivity, the flicker will be viewable. Electroretinography studies of anesthetized dogs suggested that they could detect flickering up to a maximum of approximately 20 Hz . Recent studies  on unanesthetized dogs suggest more sensitive flicker detection. They have observed flicker fusion frequencies to be 80 Hz in dogs compared to 60 Hz in humans. As a result, dogs are more sensitive to flicker than humans, and they can be affected by some screens currently in use.
The presentation of static images on a monitor may also be affected due to dogs’ greater sensitivity to flicker fusion rates. And also, the number of times in a second that a display renews an on-screen image might be perceived differently by dogs than by humans. For example, LCD monitors typically exhibit no-refresh rate-induced flicker and are commonly set to present at 60 Hz. Pixels on LCD monitors do not necessarily flash on/off between frames. Therefore, the flicker effect often observed on older screens may no longer be a potential confound as long as the flicker fusion rate is above the threshold observed in dogs. Due to the mentioned physiological differences between dogs and humans, it is essential to utilize proper technological tools .
3 Vision in Cats
3.1 Sensitivity to Lights
Everyone might have noticed the sparkle of cats' eyes in the night. Cats are called night hunters and it is suggested that cats’ nocturnal vision is six times better than humans. The main reason that the cats are nocturnal is that they have an extra layer in the back of their eye called a tapetum underlying the retina. The light which enters the eye and is not absorbed by the rods and cones in the retina contacts the tapetum and is reflected by the rods and cones again. In this way, the light that has not been absorbed has another chance to be absorbed. The tapetum functions like a reflector. There is another factor that helps cats to improve their nocturnal vision and that is their ability to dilate their pupils so widely (shown in figure 2). In this way, the maximum amount of light enters the eye in dim lighting, exactly like the function of the diaphragm in optical lenses in cameras .
Figure 2 Dilating of cats' pupil's size by changing the amount of light 
3.1 Sensitivity to Lights
Color vision in cats has not been recognized precisely and, further studies are needed to figure out how cats see the colors. Even the conclusions of studies are contrary to each other. In one study , it has been claimed that there are two types of cone cells in cats, one tuned to violet and one to green. Also, there have been studies  that claim there is a third type of cone cell, which is sensitive to light at 500 nm (greenish-blue to humans).
Other studies  have rejected this finding and have found evidence of a cone cell sensitive at 610 nm (red to humans). Currently, it is believed that cats have a vision similar to rhesus monkeys, called photopic trichromatic vision. In conclusion, they likely see similar colors to humans, but with different clarity or saturation .
4 Vision in Horses
4.1 Sensitivity to Lights
Horses have one of the largest eyes among mammals. They have a clear retinal surface that provides a relative magnification of the image that is 50% larger than the human eyes . See figure 3.
Horses have superior night vision. They also have better vision on slightly cloudy days than sunny days . The large eye of the horse improves achromatic tasks, particularly in dim conditions .
However, horses are less able than humans to adjust to sudden changes in light. For example, when moving from a bright day into a dark place it takes time to be compatible with the new condition. This may frighten a horse simply because it cannot be seen adequately. It is also essential in riding; quick movement from light to dark or vice versa will make it difficult for the horse to recognize objects.
Figure 3 horse's eye 
4.2 Color Vision
Studies on horses’ ability to discriminate specific wavelengths from grey have shown inconsistent results. Although all of these studies concluded that the horse could distinguish blue from grey, the results for red, green, and yellow were variable . One of the researches has obtained significant results regarding the discrimination of two colors, blue (462 nm) and red (700 nm), but not green (496 nm).
Other researches presented the ability to see more colors, blue (470, 474, 482 nm), yellow (579, 582, 583 nm) and green (532, 533, 545 nm), and a tendency for red (609, 611, 615 nm) . Another researcher provided that the horses in his study could discriminate yellow best, followed by green and then blue, but had some difficulty with red.
One other research group showed that the horses in this study successfully reached the criterion for learning (85% correct responses) with red and blue—one of the subjects performed at chance levels only for yellow and green . However, the other observation concluded that horses have the ability to distinguish all four colors (blue 470 nm, red 617 nm, green 538 nm, yellow 581 nm) successfully, but some individuals with partial color blindness have been reported.
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