Almost all of us believe that when we close our eyes in complete darkness, the color we see is black. However, we actually see a "unique" shade of dark gray called eigengrau. Although this term is not commonly used in scientific papers, it sheds some light, so to speak, on how our eyes perceive the world around us. The German term eigengrau roughly translates to "intrinsic gray," but it is now more commonly referred to as "brain gray." Although the history of the word is uncertain, it first appeared in the 19th century, and the term is credited to Gustav Fechner, a pioneer in experimental psychology. Fechner spent most of his career researching the relationship between sensations and the stimuli that produce them. In the case of eigengrau, he observed that even when people were in complete darkness, whether in a pitch-black room or with their eyes closed, they still perceived hints of gray.
Where did this "new color" come from? The eye is a remarkably complex organ, but when it comes to transmitting what we see to the brain, it relies on two types of cells: rod cells and cone cells. In its simplest form, cone cells see colors, while rod cells focus on light and contrast, primarily in black and white. In low light, the brain relies more on rod cells than on cone cells, which continue to capture as many photons (essentially particles of light) as possible. It seems that when we close our eyes, these rod cells do not just stop working; they continue sending signals to the brain, creating the illusion of seeing a subtle hint of light, and therefore perceiving gray, not deep black, or eigengrau.
Naturally, this is not the only thing you can see with your eyes closed. You may have noticed flashes, swirls, or spots of light after rubbing your eyes, known as phosphenes. Similar to eigengrau, these are produced by the eye sending signals to the brain, creating the illusion of light, in this case, due to pressure on the eyeball. Speaking of sensations that are not actually present, scientists discovered earlier this year that humans can "hear" silence, settling what seemed to be a debate that had lasted for centuries about this theory.
