The common experience of “seeing stars” is the temporary perception of light and shapes in the absence of any external light source. This phenomenon is scientifically known as phosphenes, which are visual sensations generated when the visual system is activated by non-light stimuli. Phosphenes can manifest as spots, swirls, flashes, or geometric patterns that appear in the visual field. The cells responsible for sight, located in the retina and the brain’s visual cortex, can be triggered by various types of energy, not just photons. The appearance of phosphenes is usually fleeting, indicating a temporary activation of these sensory pathways.
The Role of Physical Impact
A sudden external force is one of the most direct ways to induce phosphenes, leading to the popular phrase “seeing stars” after a blow to the head. This occurs because the retina, a thin layer of tissue at the back of the eye, is highly sensitive to mechanical stimulation. When a physical impact occurs, the force momentarily compresses the eyeball or jars the head, causing the delicate photoreceptor cells within the retina to be physically stimulated.
This mechanical pressure causes the photoreceptors to fire an electrical signal, which the brain interprets as light, since the visual processing centers cannot distinguish the source of the signal. Rubbing one’s eyes vigorously is a milder, self-inflicted example of this process, generating transient, colored patterns of light. Even a minor concussion can cause this sudden visual disturbance, registering a false light signal.
Circulatory Changes and Blood Pressure
One of the most frequent everyday causes of phosphenes involves sudden changes in the body’s circulatory system. When a person stands up too quickly, they may experience a sudden, temporary drop in blood pressure called orthostatic hypotension. Gravity causes blood to pool momentarily in the lower extremities, reducing the volume of blood flowing back toward the brain and eyes.
This brief interruption of blood flow leads to a temporary state of oxygen deprivation, or hypoxia, in the visual cortex and the retina. The sensitive neural tissue malfunctions when starved of oxygen and glucose, causing the cells to misfire. This cellular misfiring is perceived as shimmering spots, blurred vision, or a temporary “gray-out” until the body restores normal blood pressure and circulation.
A similar mechanism can be triggered by intense physical straining, such as during a heavy cough or lifting a very heavy object. This action, known as the Valsalva maneuver, temporarily increases pressure within the chest and abdomen, which can restrict blood return to the head. The resulting drop in blood flow to the eyes can induce phosphenes, manifesting as a shower of spots or a momentary dimming of vision until the pressure normalizes.
When Flashes Indicate a Serious Eye Condition
While many causes of phosphenes are benign, flashes of light can signal a serious condition originating within the eye. The central portion of the eye is filled with a gel-like substance called the vitreous humor, which is connected to the retina. As a person ages, the vitreous naturally shrinks and becomes more liquid, leading to posterior vitreous detachment (PVD).
PVD involves the vitreous gel pulling away from the retina, and this mechanical tugging stimulates the light-sensitive retinal cells. The resulting sensation is perceived as flashes of light, often described as lightning streaks or camera flashes, typically seen in the peripheral vision. While PVD is common and usually harmless, the traction on the retina can sometimes cause a retinal tear.
A retinal tear, if left untreated, can progress to retinal detachment, where the retina pulls away from its underlying supportive tissue. This condition is an ocular emergency because the detached retina is cut off from its blood supply, risking permanent vision loss. Any sudden onset of flashes or stars, especially when accompanied by a shower of new floaters or a dark shadow spreading across the field of vision, requires immediate medical evaluation to preserve sight.