2023-10-17 04:00:03
By Langis Michaud, University of Montreal
Samuel is a young 6-year-old patient whom I have been treating for several months for myopia which is starting to develop. He’s a very alert boy for his age. He often asks me questions about the tests, about what I see in his eye. But the last one rather surprised me. He knows that some people don’t see colors well, like his father. But what about his little poodle, Scotch?
Unlike humans, dogs’ eyes are located more on the side of the skull. They therefore have a wider visual field.
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I am not a veterinarian and do not want to infringe on their skills. But as an optometrist, I may have some information in my pocket that can answer Samuel’s question.
Cones and sticks
Ambient light is composed of particles (photons), which line up in rays. Rays of light travel and strike objects. Certain rays are absorbed, and others are reflected according to the characteristics of their surfaces and the composition of their matter (Matter is the substance which makes up any body having a tangible reality. Its…). The wavelengths of the reflected rays determine the color of the object perceived by the eye.
Like everything that affects human vision, the perception of this color is complex. The retina, the sensitive part that lines the back of the eye, contains two types of photon receptors: cones and rods. The cones, in the center of the retina (fovea), perceive bright light and are therefore responsible for color perception.
There are three types of cones. Each type contains a particular photopigment, called opsin, which therefore defines its nature. This opsin is produced under the influence of certain specific genes. The shortest opsin (“S Cone” for short) reacts especially when blue light (420 nm) is present. The longest one (“L Cone”) is more sensitive to orange-red (560 nm), and the one between the two (“M Cone”, for middle) activates in the presence of green (530 nm).
That being said, each cone reacts to each of the rays that enter the eye. For example, a red ball will cause a weak response from the S cone (3/10), a slightly stronger response from the M cone (5/10) and a large reaction of the L cone (8/10).
The brain recombines the signals emitted by each of these cones to form the color to be perceived. So, in the previous example, the perceived color would have the code 3-5-8, which corresponds to what the person knows to be red. A pink might have a code of 4-6-6, and a blue of 8-6-3. Each of these combinations of 3 cone signals is therefore unique and allows all the shades and their variations to be appreciated.
At least, as long as the genetic code is intact.
The genes associated with color vision can be mutated or defective: the person will then present a partial or complete deficiency. Several abnormalities of this type exist, the best known being color blindness (red-green deficiency).
Color blindness is associated with difficulty perceiving red and green.
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And the animals in all this?
Color vision, in humans and animals, developed by evolution. It therefore results from the needs of each species (In the life sciences, the species (from the Latin species, “type”…) according to the environment (The environment is everything that surrounds us. It is the set of natural elements and…) in which it lives, depending on the prey to hunt or the threats to avoid.
For example, birds have a 4th opsin that allows them to see ultraviolet (UV) light. For their part, humans cannot perceive it, because our crystalline lens (internal lens) filters UV rays. UV rays influence the behavioral decisions of birds, particularly when searching for food and choosing a partner.
Bird color vision is therefore more complex. As such, the pigeon, which can perceive a myriad of colors, is the one that wins the prize of all species!
Insects also perceive UV rays. This function is essential for them to spot pollen, although their color vision is very modest. Their eyes are composed of multiple lenses (ommatidia) which perceive more movements than color More practical in fast flight!
Most mammals that live in forests have only two opsins. They lost, during evolution, that which is associated with orange-red. This explains why they do not notice the hunters’ orange bibs, unlike us.
Snakes are more sensitive to red and infrared, thanks to thermal receptors. An advantage for spotting prey to hunt, because they can distinguish their heat even at night.
Unsurprisingly, it is the monkey that comes closest to humans, with its three opsins. It is said to be trichromatic.
Dogs only perceive green yellow and blue violet. Colors are perceived as paler, like pastels.
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Back to Scotch
The dogs’ vision – and therefore that of Scotch – is very different (In mathematics, the different is defined in the algebraic theory of…). Unlike humans, their eyes are located more on the side of the skull. Consequence: the dog has a larger visual field (250 to 280 degrees), but less simultaneous vision.
His vision of movements is well developed throughout his visual field. But its central vision is much weaker than ours, six times worse. This is the equivalent of the vision of a very myopic person, without glasses. For what ? Because the dog’s retina does not contain a fovea, and therefore fewer cones.
Fewer cones, but more rods. And as a bonus, an additional layer of the retina, the tapetum lucidum – or carpet. All these ingredients combined make it better seen in dim light and at night. This layer receives the light and reflects it again on the retina for a 2nd exposure. This is why you feel like your dog’s eyes are glowing at night.
When it comes to colors, dogs are dichromatic. They only perceive green yellow and blue violet. Colors are perceived as paler, like pastels. And certain colors do not create contrast: this is why a red ball on green grass will appear to them as pale yellow on a gray background, with little contrast.
It is therefore possible, depending on the color of the ball, that Scotch does not see it… and that he looks at Samuel with a lost look. As for infrared, he perceives heat through his nose, not through his eyes.
Cats, on the other hand, are also dichromatic. Their vision is therefore similar, but their color palette is different – it is oriented towards purple and green. No perception of red-green, so they are colorblind! Their clear vision is limited to a few meters in front of them. They are very short-sighted too.
Their evolution has meant that their other senses compensate. Among other things, although they only perceive certain contrasts, they are formidable at perceiving movement. A mouse moves quickly!
All species adapt to their environment and humans are no exception. Who knows what our color vision will be in 500 years, after being exposed to more and more electronic devices and artificial colors?
It’s up to Samuel to answer them when he’s older!
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