When you close your eyes, you often see a swirling, shifting display of colors, dots, and patterns instead of total blackness. This common experience is entirely normal and offers a glimpse into the internal workings of your nervous system. The lights and shapes you perceive are not stray light leaking through your eyelids, but the result of internal stimuli acting upon your visual pathway. This phenomenon is rooted in the complex neuroscience of how the eye and brain constantly process information, even in the absence of external visual input.
Phosphenes: The Science of Seeing Internal Light
The general term for the perception of light without actual light entering the eye is “phosphenes,” derived from the Greek words for “light” and “to show.” A phosphene is a visual sensation generated entirely within the nervous system, meaning the stimulus originates somewhere in the eye or brain rather than from the outside world. This highlights a fundamental principle of vision: seeing is the brain’s interpretation of electrical signals, not merely the passive reception of light. Phosphenes can be induced by various means, including mechanical force, electrical currents, magnetic fields, or the random firing of neurons. This internal stimulation proves that the sensation of light is simply an electrical message sent to the brain.
The Visual System’s Resting Noise
The faint, shifting colors and graininess seen when the eyes are closed are primarily caused by spontaneous neural activity, sometimes called “dark noise.” The cells responsible for light detection—the rods and cones in the retina—are never truly silent. Even in total darkness, these photoreceptors and the retinal ganglion cells fire randomly at a low, baseline rate. This constant, chaotic signaling is the brain’s equivalent of static on a radio, generating a low level of electrical noise. The brain interprets this random firing as a faint, shimmering field of color, typically appearing as subtle blues, greens, or grays that drift and change shape.
The spontaneous firing of cells creates a background of visual information that the brain must constantly filter out to perceive a clear image when the eyes are open. This low-level activity is thought to be partly thermal, where light-sensitive molecules in the photoreceptors spontaneously activate as if they had absorbed a photon. This constant activity ensures the visual system remains sensitive and ready to respond instantly to the arrival of actual light.
Phosphenes from Mechanical Pressure
A stronger, more distinct type of phosphene occurs when you apply physical force to the eye, such as gently rubbing your closed eyelids. This physical stimulation generates a bright, short-lived visual event known as a pressure phosphene. The retina is highly responsive to mechanical deformation; applying pressure physically stretches and deforms the photoreceptor cells, mimicking the effect of light hitting them. This mechanical stress causes the cells to generate a strong electrical signal, which is then sent to the brain as a burst of perceived light.
Because the pressure is localized, the resulting phosphene often appears as a bright, temporary spot, ring, or structured grid opposite the point of pressure. Pressure phosphenes can also be triggered by rapid eye movements, sneezing, or a heavy cough, which cause a sudden, temporary increase in pressure within the eye. These actions briefly distort the retina, leading to the familiar sensation of “seeing stars” or bright flashes of light.
When Internal Colors Signal Other Events
Phosphenes can also arise from non-mechanical internal stimuli that affect the visual pathway. One common example is the complex visual disturbance that precedes a migraine, known as a scintillating scotoma. This phenomenon is a type of phosphene caused by a wave of abnormal electrical activity in the brain’s visual cortex, not the eye. A scintillating scotoma typically begins as a small blind spot that expands over several minutes, featuring a shimmering, zigzag pattern of flickering lights.
This visual event is caused by cortical spreading depression, a slow wave of neuronal depolarization that sweeps across the occipital lobe. The brain interprets this spreading electrical disturbance as a structured visual pattern, which may occur with or without a subsequent headache.
Sudden, dramatic, or persistent new phosphenes, especially those described as flashes of light, should be reviewed by an eye care professional. While most phosphenes are benign, persistent flashes can sometimes indicate stress on the retina or the vitreous gel pulling on the retinal tissue. In rare cases, this could be an early warning sign of a serious issue, such as a retinal tear or detachment.