Seeing visual “glitches” or phenomena—things that appear in the field of vision but do not physically exist—is a common concern. These visual disturbances are medically known as photopsia or entoptic phenomena. They can manifest as spots, lines, flashes, or shimmering patterns. These symptoms originate from various parts of the visual system, including the eye’s internal structure, the retina, or the brain’s visual processing center. Understanding the source helps determine if the symptom is a harmless, age-related change or requires urgent medical evaluation.
Glitches Caused by Vitreous Changes (Floaters and Spots)
Many people occasionally notice translucent specks, threads, or cobweb shapes drifting through their vision, commonly called floaters. These are physical shadows cast onto the retina by debris suspended within the eye’s main chamber. This chamber is filled with the vitreous humor, a gel-like substance primarily composed of water and a meshwork of collagen fibers.
With age, the vitreous undergoes a natural process called syneresis, where the gel liquefies and shrinks. This shrinkage causes the collagen fibers to collapse and clump together, forming the dense particles perceived as floaters. Floaters are most noticeable when looking at a plain, bright background, like a clear sky or a white wall, because the contrast makes their shadows more distinct.
A sudden increase in floaters often signals a Posterior Vitreous Detachment (PVD). PVD occurs when the liquefying vitreous separates from the retina’s surface. This is a normal aging event that occurs in most people, typically between the ages of 40 and 70. The separation can cause a large, ring-shaped floater or a sudden influx of smaller ones. The fibers pulling away from the retina can also briefly stimulate the light-sensitive tissue, leading to the temporary perception of flashes.
Glitches Caused by Brain Activity (Auras and Geometric Patterns)
Some visual disturbances originate not in the eye itself but in the brain’s visual cortex. The most common example is the migraine aura, medically known as scintillating scotoma, which can precede or occur without a headache. This visual event often begins as a small, flickering spot near the center of vision.
The spot typically expands into a shimmering arc, often with jagged, zigzag edges resembling a kaleidoscope. This phenomenon is caused by a wave of abnormal electrical activity, called Cortical Spreading Depression (CSD). CSD moves across the surface of the occipital lobe at the back of the brain. The visual disturbance usually affects both eyes because its origin is neurological, not ocular.
These geometric patterns may last anywhere from a few minutes up to an hour before resolving. When the visual aura occurs without a subsequent headache, it is referred to as an acephalgic or ocular migraine. These episodes are generally reversible and are not a sign of damage to the physical structure of the eye.
Glitches Indicating Retinal Stress (Flashes and Sudden Shadows)
Flashes of light, known as photopsia, signal mechanical stimulation of the retina. The retina is the tissue at the back of the eye that converts light into electrical impulses. Since the retina only registers light, any physical tugging or pressure is interpreted by the brain as a flash, often appearing as brief arcs or streaks in the peripheral vision. While flashes can accompany the benign process of PVD, persistent or increasing flashes can be a sign of retinal stress.
If the vitreous gel is abnormally adherent to the retina, its separation can exert excessive traction, leading to a Retinal Tear. A tear can cause a sudden, marked increase in floaters, sometimes described as a “shower of soot” if a retinal blood vessel is broken. The acute onset of flashes and floaters is the hallmark of a new tear.
A tear allows fluid to pass beneath the retina, which can lead to a Retinal Detachment. This is a condition where the retina separates from its underlying supportive tissue. A classic symptom of detachment is the perception of a gray curtain or shadow blocking a portion of the field of vision. This shadow corresponds to the area of the retina that has lost its blood supply and is no longer functioning.
When to Seek Immediate Medical Attention
Certain visual glitches represent a time-sensitive medical emergency requiring an immediate eye examination to prevent permanent vision loss. Any sudden onset of a large number of new floaters, often described as a burst or shower of spots, should prompt an urgent visit to an eye specialist.
The abrupt appearance of persistent or numerous flashes of light is also a warning sign. The most serious symptom is the sensation of a shadow, veil, or gray curtain moving across any part of the field of vision. If any of these symptoms occur, particularly following a head or eye trauma, a dilated eye exam must be performed immediately.