One of the most commonly circulated pieces of pop dog-knowledge is that they are colour blind. Like most pseudo-scientific, inadequately substantiated and cliched snippets of information, this one too falls short of being entirely true, in the sense that it’s believed and proliferated. Interestingly though, it does hold a smidgen of truth, as dogs aren’t quite capable of seeing the world exactly as we do.
A dog’s vision isn’t just restricted to perceiving shades of grey, black and white. A dog’s retina, much like a human’s, contains two main kinds of photoreceptors- rods and cones. Let’s get into cones, which are responsible for colour perception, first- Dogs have only 2 types of cones in their retinas, as opposed to humans who have 3, which means a dog’s vision can be safely equated to that of someone with red-green colourblindness. This was established by the work of Jay Neitz, a colour vision scientist at the University of Washington.
Like people with red-green colourblindness, dogs perceive colours differently than humans with normal colour vision. For dogs, what most people see as red most likely appears to be dark brown, while green, yellow and orange all look “yellowish.” Something that looks blue-green to humans — say, the ocean or a pool of water — looks grey to a dog, and purple objects just look blue.
Neitz’s research indicates that dogs, like colourblind people, may use certain cues to distinguish one colour from another.A lot of the time there are good cues to help them figure it out; for example, red objects tend to be darker than green objects. So, if it’s a dark apple, a red-green colour-blind person would know that it’s probably a red one, and if it’s a lighter apple, it may be a Granny Smith.
(The glowing, blue green film that’s visible in the picture, is the tapetum!)
However, it is essential to understand that although dogs don’t perceive colour as well as we do, it doesn’t mean we have superlative eyesight in comparison. Rods, the second type of photoreceptor cells present in both human and dog retinas, are responsible for light sensitivity and motion detection. Dogs’ retinas actually contain a higher concentration of these, which allows them to have a higher acuity of vision in darker environs. It makes them much better than us at picking up subtle movements in very dim light. Furthermore, they have a layer behind their retinas called a “tapetum” which reflects light that wasn’t captured by the rods and cones, back onto the retina. We humans don’t have these. This facilitates dogs to be great at nocturnal hunting even if it’s pitch dark, as it allows the smallest amounts of light/movements to be captured by the eye.