Even when your eyes are closed, they’re still working (Picture: Getty/Image Source)

What colour do you see when you close your eyes?

Here’s a hint – it isn’t black.

What we’re actually seeing is a unique colour, which has its own specific name – eigengrau.

The German word technically translates into ‘intrinsic grey’, but is more commonly now referred to as ‘brain grey’.

While the history of the word isn’t certain, it first appeared in the 19th century. Gustav Fechner, a pioneer in experimental psychology, is most commonly credited with coining the term. 

Fechner spent much of his career researching the relationship between sensations and the stimuli producing them.

In the case of eigengrau, he noticed that even when people were in complete darkness – whether in a pitch black room or with their eyes closed – they were still seeing hints of grey.

So where was this new colour coming from?

From inside our own eyes.

Eyes are amazing organs, and very complex, but when it comes to transmitting what we see outside into the brain, it relies on two types of cells – rods and cones. 

Rod and cone cells are found in the retina, at the back of the eyeball (Picture: Getty/iStockphoto)

At their most basic, cones cells see colour, rod cells focus on light and contrast – essentially black and white.

In low light, the brain relies more on rod cells than cone cells, which continue to catch as many photons (essentially light particles, but like all things in physics, so much more) as possible.

And, it seems, when we close our eyes these rods cells don’t just switch off. They continue sending signals to the brain, which creates the illusion of seeing the tiniest hint of light – hence seeing grey, not pitch black.

Or eigengrau.

Gustav Fechner, who studied eigengrau (Picture: Fine Art Images/Heritage Images/Getty)

Of course, that isn’t the only thing you can see with your eyes closed.

You may have noticed flashes, swirls or spots of light after rubbing your eyes. These are known as phosphenes, and like eigengrau, are caused by the eye sending signals to the brain creating the illusion of light – in this case due to pressure on the eyeball.

What do you see when you rub your eyes? (Picture: Getty/iStockphoto)

And speaking of sensations that aren’t really there, earlier this year scientists discovered that humans can ‘hear’ silence, settling what was apparently a centuries-old debate.

So now you know.

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It’s German.