The experience of seeing lights, colors, or swirling patterns when your eyes are closed is a common phenomenon. These perceptions of light without an external source fall under the scientific term phosphenes. The word originates from the Greek words phos (light) and phainein (to show), describing a self-generated light show. This visual experience is a normal function of a constantly active sensory system. The visual apparatus, from the retina to the brain, does not simply switch off in darkness. The perception of these shapes and flashes is evidence that the visual pathway is still producing and transmitting signals.
The Baseline: Why Darkness Isn’t Truly Black
When you close your eyes in absolute darkness, the field you perceive is not pure black, but a uniform, dark gray color. This phenomenon is known scientifically as Eigengrau, or “intrinsic gray.” This baseline color is the result of the visual system’s minimal, spontaneous activity. Even without light hitting the retina, nerve cells and photoreceptors continue to fire occasional signals. The brain interprets these low-level, random electrical impulses as a dim, cloudy field rather than complete nothingness. This intrinsic activity establishes the lowest limit of visual perception, meaning the visual system is never truly at rest.
Phosphenes Caused by Physical Stimulation
One of the most direct ways to generate phosphenes is through mechanical pressure, commonly experienced when rubbing your eyes. When physical force is applied to the eyeball, it temporarily deforms the retina at the back of the eye. This mechanical deformation directly stimulates the photoreceptor cells, which are designed to detect light photons. The force causes these cells to fire electrical impulses, which are identical to the signals produced when actual light is present. The brain interprets these impulses as light flashes, often described as “seeing stars” or bright, swirling geometric shapes. This type of phosphene is also why a sudden blow to the head or intense activities like coughing or sneezing can briefly cause a flash of light. These actions momentarily increase pressure inside the eye, which is transmitted to the sensitive retinal tissue.
Spontaneous Visual Noise and Cortical Activity
Phosphenes can also occur without any external physical stimulus, such as when lying in a dark room or drifting off to sleep. This happens due to spontaneous activity, often referred to as “dark noise,” originating in the retina and the visual cortex of the brain. In the retina, the light-sensitive pigment rhodopsin can spontaneously change its shape due to thermal energy, causing the photoreceptor cell to fire an impulse. This random thermal event is indistinguishable from an actual photon of light hitting the cell, generating a fleeting spark of light.
At the level of the brain, the visual cortex remains highly active even when external visual input is blocked. When the eyes are closed, brain activity in the occipital lobe, the primary visual processing area, shifts into a pattern characterized by increased alpha-wave power. This heightened alpha rhythm is associated with a state of internally focused processing. The result is the perception of organized, shifting patterns, colors, and geometric shapes, a phenomenon sometimes categorized as Closed-Eye Visualizations.
Recognizing Abnormal Visual Symptoms
While most phosphenes are harmless manifestations of normal physiological processes, certain visual symptoms require immediate medical attention. Normal phosphenes are transient and often linked to a clear cause, like eye rubbing or a sudden movement. The flashes that signal a potential problem are typically new, persistent, or occur when the eyes are open. A sudden, significant increase in “floaters”—small specks or squiggly lines that drift across your vision—accompanied by persistent flashes of light should be evaluated promptly. These symptoms, known as photopsia, can indicate that the vitreous gel inside the eye is pulling on the retina. A persistent flash, especially one accompanied by a shadow or a “curtain” moving across the field of vision, is a warning sign. These changes may signal a retinal tear or a retinal detachment, a serious condition requiring urgent care to prevent permanent vision loss.