The experience of closing your eyes only to perceive a swirl of colors, flashes, or strange geometric patterns is a common occurrence that points to the continuous activity of the visual system. This phenomenon can happen whether you are in a brightly lit environment or in complete darkness. These internal light shows are usually a normal, harmless sign that the complex machinery responsible for sight never truly powers down. Understanding why this happens requires separating external light from the signals your brain creates internally.
Distinguishing Between Real Light and Generated Light
When you close your eyes in a room with the lights on, the glow you perceive is often due to external light physically penetrating your eyelids. Eyelid skin is thin, and while it blocks most visible light, some photons still reach the retina. The light that filters through the thin skin and underlying blood vessels is predominantly in the red and orange spectrum. Shorter, higher-energy wavelengths, such as blue and green, are largely absorbed or scattered by the tissue layers, leaving the longer, lower-energy red wavelengths to pass through.
The complex, dynamic patterns and flashes, however, are generated from within your visual system. These internal sensations occur even in pitch blackness, where no external light is available. These self-generated visual signals are known by a specific scientific term.
The Science of Phosphenes
The internally generated visual sensations are called phosphenes, defined as the perception of light without light entering the eye. Phosphenes arise from the mechanical or electrical stimulation of the retina or the visual cortex in the brain. The visual system converts light into electrical signals that the brain interprets as images.
The brain cannot distinguish between an electrical signal originating from a photon hitting the retina and one generated by pressure or random neural activity. It interprets any signal arriving via the optic nerve as light, meaning a non-light stimulus results in a visual perception.
Phosphenes represent “visual noise,” the spontaneous, low-level firing of cells in the visual pathway. Even in total darkness, photoreceptor cells and visual cortex neurons maintain a baseline level of activity. This constant, random electrical discharge is the biological equivalent of static, which the brain processes as rudimentary light or patterns.
Common Triggers for Phosphene Activity
The most common trigger for phosphenes is mechanical stimulation, often called pressure phosphenes. Gently rubbing closed eyes applies pressure to the eyeball, physically stimulating the photoreceptor cells in the retina. This mechanical force causes the cells to fire an electrical impulse, which the brain perceives as bright, temporary flashes or geometric shapes that shift with the pressure.
Other everyday actions can also induce these light sensations by temporarily changing the pressure on the eye or altering blood flow to the visual centers. Forceful actions like sneezing, intense coughing, or standing up too quickly can lead to a brief perception of “seeing stars.” These activities momentarily increase pressure within the head or cause a temporary drop in blood pressure, stimulating neurons in the retina or visual cortex.
Even rapid, spontaneous eye movements, called saccades, can sometimes generate faint phosphenes. When the eyes are closed and moving quickly, the slight tension on the retina can cause a brief, arc-shaped flash in the periphery of the visual field. These fleeting visual events are a routine function of the eye’s mechanics and continuous electrical activity.
Visual Symptoms Requiring Medical Attention
While most phosphenes are benign, a sudden change in their frequency, intensity, or association with other symptoms can signal a serious underlying issue. The sudden onset of persistent or frequent flashes of light, known as photopsia, can be a warning sign. Unlike fleeting phosphenes from eye rubbing, photopsia may occur with the eyes open or closed and can be accompanied by a sudden increase in floaters.
A significant increase in floaters (specks, threads, or cobwebs drifting across vision) along with persistent flashes, can be caused by the vitreous gel separating from the retina. This event, known as posterior vitreous detachment, can sometimes lead to a retinal tear. The mechanical tugging on the retina by the separating vitreous gel causes photoreceptor cells to fire, resulting in the perception of light flashes.
A medical emergency is indicated if flashes and floaters are accompanied by a shadow or a curtain-like obstruction covering any part of your vision. This symptom suggests a potential retinal detachment, where the light-sensing tissue pulls away from its underlying support layer. Immediate medical attention is necessary, as timely treatment is required to prevent permanent vision loss.