The occasional perception of flashes of light in the absence of an actual light source is a phenomenon medically known as photopsia. This visual disturbance often occurs when the eyes are closed, in dark environments, or when moving the head suddenly. Flashes can be a harmless, common experience, but they can also signal changes within the eye that require professional attention. Understanding the origin of these internal light sensations, whether mechanical, neurological, or structural, is helpful for determining their significance.
Phosphenes: How Pressure Creates Light
Phosphenes are visual sensations of light induced by stimuli other than photons, such as mechanical pressure, electrical current, or magnetic fields. The most common phosphene is a benign experience resulting from external pressure applied to the closed eye. When a person rubs their eyes, the mechanical force physically stimulates the cells of the retina, the light-sensitive tissue at the back of the eye.
The retina’s function is to convert light energy into electrical signals for the brain to interpret as vision. When the retinal cells are stimulated by pressure, they fire electrical impulses just as they would if real light had reached them. The brain’s visual processing center interprets both light-generated and pressure-generated signals as light. This can result in a momentary perception of various shapes, such as bright circles, diffuse colored patches, or a scintillating light grid.
Similar mechanical phosphenes can be experienced momentarily after a sudden sneeze, a heavy cough, or a blow to the head, sometimes described as “seeing stars.” These events cause a transient, non-harmful mechanical stimulation of the visual system. The resulting flashes or spots are typically short-lived and disappear as soon as the pressure or movement subsides.
Neurological Causes of Visual Flashes
Flashes of light can also originate within the brain’s visual processing center, separate from physical stimulation of the eye. The most frequent example is a visual aura associated with a migraine. These flashes are caused not by the retina but by a slow wave of electrical activity across the visual cortex, a process known as cortical spreading depression.
Migraine auras are characterized by positive visual symptoms, including bright flashing lights, shimmering spots, or distinct zigzag lines that expand across the field of vision. Unlike flashes caused by retinal issues, which are typically brief and peripheral, migraine auras often last between five to sixty minutes and affect vision in both eyes simultaneously. This pattern is distinct from flashes resulting from direct eye stimulation and may occur before, during, or sometimes without a headache.
Structural Issues Within the Eye
A more significant cause of photopsia involves changes within the structure of the eye, specifically the vitreous humor and its relationship with the retina. The vitreous is a clear, gel-like substance that fills the large central cavity of the eye and is normally attached to the retina. As a person ages, the vitreous gel naturally liquefies and shrinks, a process that eventually leads to a posterior vitreous detachment (PVD).
During a PVD, the shrinking vitreous begins to pull away from the retina. At points where the vitreous remains firmly attached, this separation creates traction, or a gentle tugging, on the retina. Because the retina is sensitive tissue that registers any stimulation as light, this mechanical tugging causes the perception of flashes. These flashes are often described as brief, localized lightning streaks or camera flashes, typically occurring in the peripheral vision.
Retinal Tears and Detachment
While PVD is a normal aging process that affects most people over the age of 70, the tractional force can sometimes be strong enough to cause a retinal tear. A retinal tear is a serious complication where the vitreous pulls a piece of the retina away from its underlying support tissue. If fluid seeps through this tear, it can lead to a retinal detachment, where the retina separates from the layer that provides it with oxygen and nourishment. The flashes in this situation are a direct result of the retina being physically pulled and irritated, and they can precede the detachment itself.
Recognizing Urgent Symptoms
While many instances of light flashes are benign, the sudden onset of new or increased photopsia can signal a time-sensitive event, such as a retinal tear or detachment. A person should seek emergency eye examination if they experience a sudden, dramatic increase in the frequency or intensity of flashes. Flashes accompanied by a sudden shower of new floaters, which appear as tiny specks, dots, or cobwebs drifting across the vision, are particularly concerning symptoms.
The appearance of a shadow or a dark curtain descending over a portion of the visual field is a sign that a retinal detachment may have progressed. Retinal detachment is a painless medical emergency that can lead to permanent vision loss if not addressed quickly. Early diagnosis of a retinal tear allows for a preventative procedure, such as laser treatment, which can help stop the tear from progressing to a full detachment.