What colors does my cat actually see?

Are you talking about physics with us today?

Yes, this week was the launch of the 150th anniversary year of the French Physical Society, SFP. So the SFP is what we call a learned society, in this case a society of physicists. I’ll have a chance to talk to you about this again, because we’ll be celebrating it throughout the year, and as a result, I’ll be regularly devoting columns to physics. And today we will start talking about colors. Imagine that today I am giving a lecture on colors in Muba, Tourcoing, and I went to visit their exhibition called Kaleidoscope. The exhibition is excellent and revolves around colors. I took a few pictures of the works on display and showed them to my cat when I got home. As you can imagine, he didn’t react, which left him completely unfazed.

What does it have to do with physics?

Exactly, I wondered what he really saw, my cat. How a cat perceives colors!

It’s more anatomy, isn’t it?

Yes, but to understand what is happening, it is important to understand the nature of color and how we perceive it. In terms of physics, there are colors, infinity. They vary from red to violet, passing through orange, yellow, green and blue. These are the colors of the rainbow, and these colors are pure, as we used to say in the early 20th century, so I’m going to talk more about physical colors.

Because there are others?

Yes. Often, light contains many different colors. I mean, a light source, even if it’s very small, emits many colors. Each point of an object emits many colors.

Are you saying that this microphone that I see as blue actually emits multiple colors?

Exactly. And we actually only see one. More precisely, our eyes see three of them thanks to three types of sensors in our retina called blue, green and red cones. Almost all colors are detected by at least two cones, often 3, but in a different way for each cone. Suddenly, the brain receives different signals for each physical color, and thanks to this we can distinguish all these colors. But when the object we are looking at emits several colors, everything is collected in the eye, and the brain sees a kind of average of all the colors emitted by the object. Sometimes this average corresponds to the physical color, and sometimes it does not: in this case, the brain invents a color that does not exist physically, for example, white.

okay What about cats?

Well, cats, like almost all mammals, have only two types of sensors in their eyes, blue and red cones, but not green. Therefore, they have a perception very close to the deuteranopic color blind, who cannot distinguish between red and green. But for cats, this is emphasized by the fact that the perception zones of the two sensors are further apart than for humans. In fact, when looking at a single physical color, the cat’s brain has only two colors, blue and red. Obviously, a cat sees only two colors in the rainbow with almost no gradient.

Surprise! Wow. So a cat can only see two colors!

No, I didn’t say that. There remain mixtures of several physical colors that can be perceived with different intensities by the two types of cones. For example, the rest of us similarly see turquoise blue, which is composed of one physical color—hence turquoise blue—and turquoise blue, which is a mixture of blue and green. A cat will see gray physical turquoise as blue because it corresponds to the sensitivity hole between its two cones, but it will see violet composite turquoise, which is a mixture of the two colors it perceives, as blue. In short, not only does he see fewer colors than we do, he doesn’t even see the same colors. So I can’t imagine how he saw the photos of the paintings I showed him. I understand that maybe he didn’t like it as much as I did!

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