The experience of seeing light when no external source is present, often occurring when the eyes are closed or in darkness, is common. This perception is generally divided into two categories: phosphenes and photopsia. Phosphenes are harmless, fleeting flashes or patterns caused by mechanical or electrical stimulation of the visual system. Photopsia refers to flashes frequently associated with underlying changes or conditions within the eye itself. While phosphenes are typically benign, photopsia warrants closer attention from an eye care professional.
Understanding Harmless Light Perception
The most frequent reason for seeing flashes or shapes in the dark is mechanical stimulation of the retina, known as pressure phosphenes. When physical force is applied to the closed eyelid, such as rubbing the eyes, this pressure stimulates the photoreceptor cells. This mechanical activation triggers the same electrical signals as real light, causing the brain to interpret the signal as a brief burst of light or color. These flashes often appear as swirling colors, dots, or geometric patterns that quickly fade once the pressure is released.
Another common source of light perception in darkness relates to the natural activity of the visual system when deprived of light input. This is sometimes called “visual noise” or “dark noise,” resulting from the retina’s hypersensitivity after dark adaptation. When the eyes are closed for an extended period, light-sensitive rod cells become highly responsive. Even in total darkness, these cells can spontaneously activate due to random thermal or chemical events.
This spontaneous activation sends weak, random signals to the visual cortex, interpreted by the brain as faint, static-like shimmering or shifting patterns. This low-level activity is normally masked by external light, only becoming noticeable in pitch-black environments. This visual noise is a normal physiological function of the retina and is distinct from the more dramatic, sudden flashes that might indicate a problem. Small opacities in the vitreous humor, commonly called “eye floaters,” are separate from phosphenes, as they are shadows cast onto the retina rather than direct stimulation of photoreceptor cells.
Flashes Caused by Changes Inside the Eye
Flashes that appear as sudden, dramatic streaks of light, especially in the peripheral vision, often signal a change in the internal structure of the eye. The most frequent cause is Posterior Vitreous Detachment (PVD). The vitreous is a clear, gel-like substance that fills the main cavity of the eyeball and is loosely attached to the retina. As a person ages, the vitreous gel naturally liquefies and shrinks, causing it to pull away from the retina.
When the shrinking vitreous tugs on the delicate retinal tissue, it mechanically stimulates the photoreceptor cells. The brain interprets this as a flash of light, often described as a lightning streak or a camera flash. These flashes are typically more noticeable in low light conditions or when moving the eyes. PVD is a normal age-related process that affects most people eventually.
In some cases, the vitreous gel may be abnormally attached, and the pulling force during separation can create a retinal tear or hole. This is a severe complication because fluid from the vitreous cavity can pass through the tear, collecting behind the retina and causing it to lift away, which is known as retinal detachment. Flashes associated with a tear or detachment are often sudden in onset, increase in frequency, and may be accompanied by a significant increase in floaters. A shadow or a curtain-like obstruction in the peripheral vision requires immediate medical attention to prevent permanent vision loss.
Brain and Circulatory Triggers
Flashes of light that originate outside the eyeball itself are often due to neurological or systemic changes, primarily involving the visual processing centers of the brain. A common neurological cause is the visual aura associated with a migraine, which can occur with or without a subsequent headache. The aura is thought to be caused by a wave of altered electrical activity, known as cortical spreading depression, that slowly moves across the visual cortex.
Migraine aura flashes are distinct from ocular flashes, typically appearing as complex, structured visual disturbances, such as shimmering, zigzag lines or arcs of flickering light. These visual effects characteristically begin near the center of vision, expand outward, and usually affect both eyes simultaneously, lasting anywhere from five minutes to an hour. The bilateral nature and geometric complexity of the disturbance help differentiate it from the unilateral, brief flashes caused by retinal traction.
Systemic changes, such as sudden drops in blood pressure, can also trigger transient visual phenomena. When a person stands up too quickly, orthostatic hypotension can occur, leading to a brief reduction in blood flow to the brain and visual cortex. This temporary lack of oxygen can cause a momentary “graying out” of vision or the perception of brief, generalized white flashes, often accompanied by lightheadedness or dizziness. These flashes are short-lived and resolve once normal circulation is restored.