The experience of closing your eyes and still seeing a shifting landscape of color, swirling patterns, or a faint, grainy static is a common visual phenomenon. This occurs because the visual system does not simply turn off when external light is removed. Even in total darkness, the eyes and brain remain active, generating internal signals that are perceived as light and color.
Phosphenes: The Internal Noise of the Eye
The scientific term for these internally generated light sensations is phosphenes. When external light is blocked, the background color perceived is not true black but a specific dark gray known as Eigengrau, or “intrinsic gray.” This Eigengrau represents the baseline activity level of the visual system, acting as constant background noise.
Phosphenes arise from the spontaneous, low-level electrical activity generated by the photoreceptor cells and neurons in the retina. These light-sensitive cells constantly fire electrical impulses, even without stimulation from photons. The visual system is never truly quiet, similar to the faint static heard when a radio is tuned between stations. This spontaneous firing is relayed to the brain, which automatically interprets the electrical signals as light.
Mechanical and External Triggers
Phosphenes can also be deliberately triggered through mechanical stimulation, moving beyond the spontaneous noise of the eye. Pressure phosphenes are the most common induced type, occurring when pressure is applied to the side of the closed eye, such as by rubbing the eyelids. This mechanical force physically deforms the retinal tissue at the back of the eye, causing the cells to fire an electrical signal identical to one caused by actual light.
The brain cannot distinguish between a signal caused by mechanical pressure and one caused by light, resulting in the perception of a flash or pattern of color. Other physical triggers include a sharp, non-penetrating blow to the head, which causes the sensation of “seeing stars.” Flashes can also be generated by the movement of the vitreous gel inside the eye, which may tug on the retina during rapid eye movements.
How the Visual Cortex Interprets Non-Light Signals
The perception of phosphenes as light and color is managed by the visual cortex, the brain’s processing center located in the occipital lobe. The visual cortex is a passive interpreter of incoming electrical signals. It is designed to process signals transmitted via the optic nerve and translate them into a visual image, regardless of the signal’s origin.
When the retina fires due to pressure or spontaneous noise, the electrical impulse travels the same pathway as a signal generated by external light. The brain receives this signal and automatically assumes it represents a visual input, manifesting it as colors, spots, or geometric shapes. The specific patterns often seen, such as checkerboards or spirals, are thought to be a result of the brain trying to organize this random electrical firing, corresponding to the retinotopic organization of the neurons within the primary visual cortex.
When Internal Light Patterns Signal a Problem
While most flashes of light behind closed eyes are normal phosphenes, a sudden change in these patterns can signal a medical concern. The distinction lies in the nature and persistence of the flashes: normal phosphenes are usually fleeting and dim, appearing when the eye is stimulated or closed in darkness.
Flashes that are sudden, bright, persistent, or resemble jagged lightning streaks (known as photopsias) warrant prompt examination by an eye care professional. These symptoms can be a sign of posterior vitreous detachment, where the vitreous humor separates from the retina. If the vitreous tugs too strongly on the retinal tissue, it can cause a retinal tear or a full retinal detachment. These conditions require immediate treatment to prevent permanent vision loss. A sudden increase in eye floaters accompanying new flashes is also a significant warning sign.