The experience of seeing flashes, spots, or “stars” is a phenomenon known as phosphenes. These visual sensations are produced not by external light sources, but by the internal stimulation of the visual system. Understanding why the brain interprets these non-light stimuli as light requires examining the fundamental mechanisms of vision. This article explores the physiological triggers responsible for these effects, including sudden changes in blood pressure and physical impacts on the eye and head.
How the Brain Creates Light Sensations
The fundamental mechanism behind phosphenes rests on the principle of neural non-specificity within the visual pathway. The cells responsible for sight, including photoreceptors in the retina and neurons in the visual cortex, process light signals. When these cells are activated, the brain automatically interprets the resulting electrical impulse as vision.
The visual system cannot differentiate between an electrical signal generated by light and one generated by other means, such as mechanical pressure or lack of oxygen. Consequently, any sufficient stimulation of the visual pathway is uniformly perceived as light. This is why phosphenes can manifest in various forms, including diffuse colored patches, simple flashes, or scintillating spots, even when the eyes are closed.
Triggers Related to Sudden Blood Pressure Changes
One common cause of phosphenes is a temporary disruption of blood flow to the visual system, often linked to sudden changes in body position or internal pressure. When a person stands up too quickly, the circulatory system may momentarily fail to compensate for gravity, leading to a sudden drop in blood pressure, a condition known as orthostatic hypotension.
This drop results in a brief period of reduced blood flow and oxygen supply (ischemia) to the brain and retina. The visual neurons in the occipital lobe become metabolically deprived and can fire spontaneously and erratically. The brain misinterprets these random firings as a burst of light, producing the characteristic “stars.”
Similar phosphenes can be triggered by actions that dramatically increase internal thoracic or abdominal pressure, like a strenuous cough, a forceful sneeze, or intense straining. This is often an example of the Valsalva maneuver, which temporarily impedes blood return to the heart and elevates pressure within the eye and head. This brief mechanical and circulatory stress can stimulate the retina or visual cortex, causing a momentary flash of light.
Triggers Related to Physical Impact and Eye Stress
Physical trauma to the head is a well-known cause of seeing stars, often due to the mechanical stimulation of the visual processing centers. A concussive force causes the brain, which is suspended in cerebrospinal fluid, to rapidly accelerate and then decelerate against the inside of the skull. This impact mechanically stimulates the occipital lobe, forcing neurons in the visual cortex to fire in an uncontrolled manner, which the brain perceives as bright flashes or stars.
A less severe but more common mechanical trigger is applying direct pressure to the eyeball, such as rubbing the eyes hard. This pressure directly stimulates the photoreceptor cells in the retina, generating pressure phosphenes.
Stress on the retina caused by the vitreous humor, the gel-like substance that fills the eyeball, is also a trigger. As a person ages, the vitreous naturally liquefies and shrinks, sometimes pulling away from the retina in a process called posterior vitreous detachment. If the vitreous tugs on the light-sensitive retina, it causes the retinal cells to fire, resulting in flashes of light.
Visual disturbances can also be caused by electrical waves that spread across the visual cortex, a common precursor to a migraine headache. This phenomenon, known as a migraine aura, usually lasts between five and sixty minutes. The aura can involve bright, shimmering spots, zigzag lines, or even a temporary blind spot, and is thought to be caused by a wave of intense electrical activity.
When to Seek Medical Attention
While many instances of seeing stars are fleeting and harmless, certain symptoms demand immediate medical attention. A sudden onset of new phosphenes, especially when accompanied by a shower of new floaters, can be a warning sign of a serious ocular issue. These symptoms suggest the vitreous gel is pulling hard enough on the retina to cause a tear.
If the flashes are accompanied by a dark curtain or shadow moving across the field of vision, this may indicate a retinal detachment, which requires urgent treatment to prevent permanent vision loss. Any phosphenes that occur after a head or eye injury, or those that persist for an extended period, should also be evaluated by a healthcare professional. Other concerning symptoms that warrant consultation include phosphenes paired with severe headache, nausea, double vision, or a temporary loss of consciousness.