The perception of colors, lights, or swirling patterns when the eyes are closed or in total darkness is a common phenomenon. These visual sensations, called phosphenes (from the Greek words for light and to show), occur without actual light entering the eye, suggesting an internal origin. Phosphenes are not hallucinations but a normal result of how the brain and visual system process signals. Understanding phosphenes reveals how the visual system can be stimulated by forces other than light.
The Mechanism of Phosphenes
Phosphenes arise because the visual cortex does not distinguish between signals generated by light and those generated by internal stimulation. The visual pathway is a one-way street: the retina converts light into electrical signals, which are transmitted along the optic nerve to the visual cortex. The brain interprets any electrical impulse arriving from the optic nerve as “light,” regardless of the signal’s true source.
The retina and neural structures are inherently electrically active. This activity can be triggered by mechanical, chemical, or electrical forces, bypassing the normal process of light hitting the photoreceptors. When this happens, the resulting signal travels the same route to the brain and is perceived as light, often manifesting as shapes, colors, or flashes.
A constant, subtle form of phosphene exists even in the deepest darkness, known as Eigengrau, or “intrinsic gray.” This faint, dark-gray background is caused by the spontaneous, random firing of rod photoreceptors within the retina. The light-sensing protein rhodopsin can spontaneously activate due to thermal energy, sending a minimal signal to the brain. This neural noise provides a baseline activity level perceived as a uniform, dark-gray field rather than true black.
Everyday Causes of Visual Colors
The most recognized cause is mechanical stimulation, often called pressure phosphenes. Applying pressure to the closed eyelid physically deforms the eyeball, mechanically stimulating the retinal cells. This force generates electrical impulses that the brain interprets as bright, swirling colors or geometric patterns.
Phosphenes can also be triggered by sudden changes in blood flow or pressure within the head. Examples include “seeing stars” after a strong sneeze, a heavy cough, or a blow to the head. These events cause a rapid rise in pressure or temporary lack of oxygen, stimulating visual neurons to fire and produce a quick burst of light.
The spontaneous appearance of patterns before falling asleep are known as hypnagogic phosphenes. These are linked to the brain’s transition between wakefulness and sleep. Rapid eye movements (REM) during sleep may also mechanically stimulate the retina, causing brief flashes of light.
When to Consult a Doctor
While most phosphenes are transient and harmless, certain light flashes can signal a medical issue requiring prompt attention. The primary concern is the sudden, new onset of flashes, particularly those resembling lightning streaks or shooting stars in the peripheral vision. These flashes, often described as photopsia, can indicate that the vitreous gel inside the eye is pulling on the retina.
The vitreous gel naturally shrinks and separates from the retina with age, a process called posterior vitreous detachment. If the gel is firmly attached, this pulling action causes mechanical stimulation, resulting in flashes. In serious cases, the tugging can create a retinal tear or a full retinal detachment, which is a medical emergency that can lead to permanent vision loss.
An immediate eye examination is warranted if there is a sudden increase in flashes combined with a shower of new floaters. Floaters are shadows cast by debris in the vitreous. Other warning signs include a shadow or curtain moving across the field of vision or a loss of peripheral sight.
Flashes that appear as shimmering, zigzag lines lasting 10 to 20 minutes are typically characteristic of a migraine aura. This is a neurological event, not an eye condition. Any new, persistent, or dramatic change in visual phenomena should be assessed by an eye care professional.