When you perceive a flash, streak, or flickering light that has no external source, you are experiencing a spontaneous visual sensation. These unprompted lights can be alarming because they mimic the appearance of actual external light. This experience is essentially an internal signal—a misfire in the visual system that the brain interprets as true light. Understanding the physiological reasons behind these signals, which range from harmless fatigue to serious eye conditions, is the first step toward clarifying this phenomenon.
The Science of Seeing Phantom Lights (Phosphenes)
The term for seeing light without a light source is phosphene, a phenomenon where the retina or visual cortex is stimulated by non-photonic means. Vision relies on photoreceptor cells converting light into electrical signals that travel to the brain’s visual cortex. The brain interprets any strong electrical impulse arriving via the optic nerve as visual input, meaning it cannot distinguish between a signal caused by actual light and one caused by internal irritation. Phosphenes can be induced by physical, chemical, or mechanical stimulation anywhere along the visual pathway. For instance, mechanical pressure on the eyeball activates the photoreceptors directly, creating the appearance of stars, dots, or swirls of light.
Benign and Temporary Ocular Triggers
Many common, non-threatening occurrences can generate phosphenes by temporarily stressing the visual system. Rubbing your eyes generates pressure phosphenes by mechanically stimulating the retinal cells, causing brief flashes or patterns to appear. Similarly, standing up too quickly (orthostatic hypotension) can momentarily decrease blood flow and oxygen to the brain’s visual centers, resulting in the perception of “seeing stars.”
Prolonged screen use, often called digital eye strain, can also lead to temporary flickering. Many digital displays use Pulse Width Modulation (PWM) to control brightness, creating rapid, often imperceptible, light-dark cycles. This constant oscillation stresses the eyes and visual processing centers, contributing to fatigue and transient flickering. These disturbances resolve quickly once the stressor is removed.
Visual Disturbances Linked to Migraine and Aura
Flickering lights are a hallmark of a migraine aura, a temporary neurological disturbance that usually precedes a headache. The most distinct visual symptom is a scintillating scotoma, characterized by a shimmering, expanding area of visual loss bordered by a zig-zag line. This pattern is often referred to as a “fortification spectrum.”
The visual aura is caused by a wave of abnormal electrical activity across the visual cortex. This event typically lasts between five and sixty minutes before disappearing. A visual aura affects the visual field of both eyes simultaneously, which distinguishes it from an ocular problem affecting only one eye. These events can also occur without a subsequent headache, known as an acephalgic or silent migraine.
Serious Ocular Conditions Requiring Immediate Attention
While many light flashes are benign, the sudden onset of new or increased flashes can indicate a serious structural problem inside the eye. The vitreous is a clear, gel-like substance that fills the eyeball and is attached to the retina. As a person ages, the vitreous naturally shrinks and liquefies, leading to a posterior vitreous detachment (PVD), where the gel pulls away from the retina.
As the vitreous pulls away, it can tug on the retina, mechanically stimulating it and causing flashes of light, particularly in the peripheral vision. PVD is a common, age-related occurrence that is usually harmless. However, forceful pulling can create a retinal tear, which allows fluid to pass behind the retina, leading to retinal detachment.
Flashes caused by a tear or detachment are often accompanied by a sudden shower of new floaters, appearing as dark specks or cobwebs. The most concerning symptom is a dark shadow or curtain moving across the field of vision, indicating the retina has detached. Retinal detachment is a medical emergency requiring immediate intervention to prevent permanent vision loss.
When to Consult a Healthcare Professional
Any new or sudden onset of flashes or flickering lights warrants a professional eye examination to rule out a serious condition. If you experience a sudden increase in flashes or floaters, or if a dark shadow or curtain covers part of your vision, seek immediate medical attention from an ophthalmologist or emergency room. These symptoms suggest a retinal tear or detachment, which requires urgent treatment.
Flickering lights accompanied by other neurological symptoms, such as weakness on one side of the body, slurred speech, or confusion, also require immediate evaluation, as they could indicate a cerebrovascular event. For chronic or recurring flickering that does not involve these urgent symptoms, consultation with an eye care professional is still recommended. A thorough eye exam can confirm if the cause is a benign PVD, a migraine aura, or another systemic issue requiring management, such as uncontrolled blood pressure or a medication side effect.