The experience of seeing flashes, sparks, or shimmering light when the eyes are closed or in total darkness is a common visual phenomenon known as phosphenes. These are perceived visual sensations that occur without any light actually entering the eye to stimulate the photoreceptors. Phosphenes demonstrate the complex relationship between the eye, the optic nerve, and the brain’s visual processing centers.
How Internal Stimuli Create Light
The mechanism behind phosphenes involves the non-photonic stimulation of the visual system. The visual pathway, which includes the retina and the visual cortex, is designed to interpret any electrical or mechanical signal it receives as light. When light enters the eye, it stimulates specialized cells in the retina, generating an electrical impulse that travels to the brain.
If that pathway is activated by something other than light, the brain still registers the impulse as a visual image. This stimulation can be mechanical, such as pressure applied to the eyeball, or electrical and chemical, originating from within the visual processing areas of the brain. The visual cortex does not distinguish the source of the stimulus; it simply translates the signal into the perception of light or pattern.
Common Causes That Are Not Serious
Many of the most frequent reasons people experience phosphenes are benign and transient. The simplest example is gently rubbing the eyes, which causes mechanical pressure phosphenes. This pressure physically stimulates the light-sensitive cells of the retina, causing them to fire an electrical signal. The perceived flash or swirl of color quickly fades once the pressure is removed.
Another common cause is a rapid change in blood flow or pressure, often referred to as “seeing stars.” This can happen when standing up too quickly, coughing forcefully, or sneezing. These actions temporarily alter the blood flow to the visual cortex or the retina, causing a brief stimulation of the neurons. These flashes are short-lived, symmetrical in both eyes, and resolve within seconds.
The visual system can also generate physiological “static” or “dark noise” when the eyes are closed. Even in complete darkness, the cells of the visual system continue to have a low level of spontaneous electrical activity. The brain interprets this random, background firing of neurons as faint, shimmering, unstructured light. These occurrences are harmless and reflect the normal, ongoing activity of the nervous system.
When Flashing Lights Indicate a Medical Issue
While many phosphenes are unalarming, flashing lights can sometimes be a sign of a medical issue. One neurological cause is a migraine aura, medically termed a scintillating scotoma. This visual disturbance is characterized by a luminous, shimmering, jagged, zig-zag pattern that slowly expands across the field of vision. The aura results from an abnormal wave of electrical activity that sweeps across the visual cortex.
A migraine aura usually lasts between five and sixty minutes. While it often precedes a headache, it can also occur without any pain, a condition known as an acephalgic migraine. Since this visual event originates in the brain, it affects the corresponding visual field in both eyes. This distinct, expanding, geometric pattern differentiates it from other causes of flashing lights.
More concerning causes of flashes originate in the eye, specifically involving the retina, the light-sensing tissue at the back of the eye. As a person ages, the clear, jelly-like substance that fills the eyeball, called the vitreous humor, naturally shrinks and pulls away from the retina. This common process is known as a Posterior Vitreous Detachment (PVD). When the vitreous gel pulls or tugs on the retina, the mechanical tension stimulates the tissue, causing the perception of lightning streaks or flashes, often seen in the peripheral vision.
If the vitreous gel pulls too strongly, it can cause a Retinal Tear or, in serious cases, a Retinal Detachment, which is a medical emergency. Red-flag symptoms requiring immediate medical evaluation include a sudden increase in the frequency or number of flashes, especially if they are persistent. The appearance of many new floaters, or a shadow or “curtain” coming across the peripheral vision, indicates a potential tear or detachment and necessitates urgent attention.