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Dogs are dichromates whereas humans are trichromates. That means that our eyes contain 3 different types of cone cells, each of which are responsible for detecting a unique colour (for a description of both rods and cones, click here). That’s why every colour you can think of is a combination of the 3 primary colours.
But dogs only have 2 types of colour-sensing cone cells, and instead of them sensing red, blue or yellow, they’re tuned to violet and yellow-green. This means that dogs have less sensitivity in their green, yellow and red detection than humans. But conversely, humans have less sensitivity in their blue and purple detection than dogs.
And cats? Well they are a bit of a mystery. We know that they have at least two types of cone cells, one tuned to violet and one to green. But there have been studies the showed evidence of a third type of cone cell, one sensitive to light at 500 nm (greenish-blue to us). Other studies have rejected this finding, and yet others have found evidence of a cone cell sensitive at 610 nm (red to us). Currently, it’s believed they have vision similar to rhesus monkeys, called photopic trichromatic vision. In essence, they likely see similar colours to us, but not quite in the same clarity or saturation.
There’s is growing evidence that dogs and cats can see into the ultraviolet range, something no human can do!
Cats’ and dogs’ vision systems evolved to help them hunt. They’re better at seeing movement than still objects; they see best in low light rather than bright; and they have larger visual fields (up to 270° compared to the 180° of humans!)
Cats and dogs also have evidence of their previous nictitating membranes, or third translucent eyelids, in the corners of their eyes. These would once have allowed them to maintain their sight on prey when hunting without their eyes drying out.
So when you throw a red ball into a green field, Rover sees a yellow ball being thrown into a white or gray field. Those colours just aren’t that different, so don’t be too surprised when he can’t find the ball that’s obvious to you!
