The experience of seeing colors, swirls, or patterns when your eyes are closed is common, and your curiosity about the purple you perceive is shared by many. This phenomenon demonstrates that the visual system does not simply shut off when external light is removed. Instead, the perception of light and color continues, driven by various internal activities within the eyes and the brain. Understanding these biological processes shows how our sense of sight works, even in apparent darkness.
The Scientific Explanation for Seeing Light
The general experience of perceiving light or color without light entering the eye is known as a phosphene. Derived from the Greek terms phos (light) and phainein (to show), phosphenes are visual sensations generated internally by stimulating the visual system with energy other than photons. They are not hallucinations.
The visual system—including the retina, optic nerve, and visual cortex—can be activated by mechanical, electrical, or spontaneous chemical activity. The brain interprets any signal from this pathway as a visual image. Because the visual pathway is wired to perceive light, any stimulus that causes nerve cells to fire is translated into a flash of light or color.
Colors Caused by Internal Brain Activity
The most frequent cause of seeing color in complete darkness is the intrinsic electrical activity of the visual system, known as Eigengrau, or “intrinsic gray.” This is the uniform, dark background color perceived in the absence of light, which is never truly black. The visual system is never completely silent; photoreceptor cells in the retina and neurons in the visual cortex generate random, spontaneous signals.
These electrical firings are interpreted by the brain as faint light, resulting in the perception of dark gray instead of a true void. This continuous, low-level activity is also known as “visual noise.” The purple or blue colors that often appear are part of this noise, caused by spontaneous events in the rod cells of the retina. These events, triggered by the thermal isomerization of the pigment rhodopsin, are indistinguishable from those caused by actual photons.
How Physical Pressure Creates Visual Effects
A different type of phosphene occurs when a mechanical force stimulates the visual system, known as a pressure phosphene. Rubbing your closed eyes or experiencing a sudden movement like a forceful sneeze can physically deform the retina. This mechanical distortion activates the photoreceptors, causing them to send signals to the brain that are interpreted as light.
The classic example is “seeing stars” after a sudden impact to the head, caused by the physical shock activating retinal cells. Pressure phosphenes typically present as bright, geometric shapes, rings of light, or a scintillating grid. These visual patterns are related to the location and type of pressure applied, as the force physically triggers the retinal ganglion cells to fire.
When This Phenomenon Signals a Problem
While most phosphenes are harmless, certain visual sensations can signal a problem requiring medical attention. Normal phosphenes, like those from pressure or visual noise, are usually transient or stable. However, the sudden appearance of new or persistent flashes of light, especially those resembling lightning streaks or camera flashes, can be a symptom of a serious issue.
These flashes often occur when the vitreous gel, the clear substance filling the eyeball, changes consistency and pulls against the retina. This tugging can cause a sensation of light and may indicate a posterior vitreous detachment, which is common with age. A sudden increase in “floaters”—dark specks or cobwebs—accompanied by flashes can indicate a retinal tear or detachment. If you notice a shadow or curtain covering part of your vision, or a sudden onset of repeated flashes, consult an eye care professional immediately to prevent permanent vision loss.