Eigengrau

Eigengrau, German for "intrinsic gray" or "own gray," is the term used in vision science to describe the uniform gray background color that is perceived in the complete absence of light. It is not truly black, but rather a dark gray that varies slightly from person to person. This sensation is not due to any external stimulus but arises from within the visual system itself.

The perception of eigengrau is thought to be a result of spontaneous neural activity within the retina, specifically from photoreceptor cells and other neurons in the visual pathway. These cells are never completely "off" and can generate a baseline level of firing even without light input. This internal noise is interpreted by the brain as a faint visual signal, which we experience as eigengrau.

Eigengrau plays a role in how we perceive contrast and brightness. Because it represents the visual system's "zero point" in darkness, it serves as a reference. When light is introduced, the visual system can detect the difference between this baseline and the incoming photons, allowing us to perceive objects and their relative brightness. The phenomenon demonstrates that vision is an active process of interpretation by the brain, rather than a passive reception of light.