Seeing shapes, colors, or flickering lights when your eyelids are closed is a common visual phenomenon. This internal light show occurs because your visual system, composed of the eyes, optic nerve, and brain, does not simply turn off in the absence of external light. The activity you perceive results from the inherent sensitivity of your optical biology.
Understanding Phosphenes and Eigengrau
The colors and patterns seen when the eyes are closed fall into two distinct categories of visual sensation. Phosphenes are the flashes, streaks, or geometric patterns of light that appear to be generated from within the eye itself. They represent the perception of light without an external source.
The uniform, dark-gray background against which phosphenes appear is known as Eigengrau, a German term meaning “intrinsic gray.” Eigengrau is the brain’s default visual perception in complete darkness, not true black. This sensation is caused by the spontaneous, low-level electrical noise constantly generated by the retinal cells and visual pathways.
Common Triggers That Cause Visual Noise
While Eigengrau is the visual system’s baseline, certain actions can increase the intensity of phosphenes. The most common cause is mechanical stimulation, often called pressure phosphenes. Rubbing closed eyes or applying gentle pressure physically stimulates the retinal photoreceptor cells, causing them to fire signals as if they had detected light.
Flashes can also be triggered by sudden changes in blood flow or pressure inside the head. “Seeing stars” after a forceful sneeze, a deep cough, or standing up too quickly are examples of temporary shifts in vascular pressure exciting the optic nerve or retina. Furthermore, the visual system sometimes generates random, spontaneous neural signals, which the brain interprets as brief flickers of light or color.
The Physiology Behind Seeing Green
The reason the perceived internal light often takes on a green or blue-green hue is rooted in the specific biology of your photoreceptors. The human eye relies on two main types of light-sensing cells: cones for bright light and color vision, and rods for dim light vision. When your eyes are closed, light levels are extremely low, causing vision to shift from cone-dominant to rod-dominant.
This phenomenon is known as the Purkinje effect, which describes how the eye’s maximum sensitivity shifts in low light. Rod cells contain the photopigment rhodopsin, which is most efficiently activated by light at a wavelength of approximately 507 nanometers. This wavelength corresponds to the blue-green part of the spectrum. Because rods are most sensitive to green-blue light, their signals are the most prominent when the visual system is minimally stimulated by internal noise.
When Visual Phenomena Require Medical Attention
While occasional, fleeting flashes or static colors are normal, certain visual phenomena signal an underlying medical issue. A sudden onset of new flashes of light, especially if intense, constant, or occurring in peripheral vision, should be evaluated promptly by an eye professional. This symptom, known as photopsia, can indicate a vitreous detachment or a more serious retinal tear.
A dark curtain, shade, or shadow moving across the visual field is also a warning sign. Another element is a sudden, significant increase in floaters, which are small specks or strands that drift in vision. These changes, particularly when paired with flashes, are associated with a retinal detachment. Seeking an urgent eye examination is important for preserving vision, as early intervention can address these conditions effectively.