Phosphene
Based on Wikipedia: Phosphene
Seeing Light That Isn't There
Press gently on the side of your closed eye. A ring of colored light appears on the opposite side—glowing, pulsing, entirely real to your perception. But no light has entered your eye. No photons have struck your retina. You've just created a phosphene.
The word comes from Greek: phos meaning light, and phainein meaning to show. It's the phenomenon of seeing light without any light actually being there. And if you've ever rubbed your eyes, sneezed hard, stood up too quickly, or taken a blow to the head, you've experienced one.
Isaac Newton documented this simple eye-pressing experiment centuries ago. Hermann von Helmholtz, the nineteenth-century physician and physicist who helped establish the principle of conservation of energy, published detailed drawings of the patterns he saw when pressing his own eyes. These weren't idle curiosities to them. They were windows into how vision actually works.
The Ancient Recognition
Phosphenes have been known since antiquity. Greek texts describe them. This makes sense—any human with eyes and fingers could discover pressure phosphenes accidentally. But understanding what causes them required waiting for modern neuroscience.
When you press on your closed eye, you're mechanically stimulating the cells of your retina. The retina doesn't know the difference between being stimulated by light or by pressure. A signal is a signal. It dutifully sends information to your brain, which interprets it as it always does: as vision.
What you see varies. Some people describe a darkening of the visual field that moves opposite to the direction of rubbing. Others see diffuse colored patches drifting in the darkness. Some report well-defined bright circles near or opposite the pressure point. Many describe an ever-changing grid of scintillating light, deforming and shifting like a crumpling screen, occasionally dotted with dark spots. A sparse field of intensely blue points is another common experience.
The most striking aspect: if you stop rubbing and quickly open your eyes, the phosphenes persist briefly. You can see them superimposed on the actual visual scene, a ghost image that gradually fades as your retina settles back to normal.
Seeing Stars
The phrase "seeing stars" isn't poetic license. It's a literal description of another common phosphene experience.
A hard sneeze can do it. So can vigorous laughter, a deep heavy cough, blowing your nose forcefully, or—most famously—getting hit on the head. The sudden change in pressure, the mechanical shock traveling through your skull, the brief disruption of blood flow to your visual system: all of these can trigger those characteristic flashing points of light.
Standing up too quickly produces a related effect. Your blood pressure drops momentarily as your cardiovascular system struggles to pump blood up to your brain against the sudden demand. Your visual cortex, briefly starved of oxygen and glucose, starts firing erratically. You see stars, your vision tunnels, and if it's severe enough, you might faint.
This is what distinguishes "seeing stars" from simple pressure phosphenes. The retina might be involved, but so might the neurons in your visual cortex itself, or other parts of the complex chain that converts light into conscious perception. Metabolic stress—too little oxygen, not enough blood sugar—can make neurons fire spontaneously, producing light that exists only in your mind.
The Prisoner's Cinema
There's a more unsettling way to experience phosphenes: remove all visual input for long enough, and your visual system starts making things up.
Prisoners in solitary confinement, people trapped in total darkness, explorers lost in caves—they all report it. After extended periods without any visual stimulation, the visual system becomes hypersensitive. Random neural firing, normally drowned out by the constant flood of actual visual information, becomes perceptible. Patterns emerge from noise. Light appears where none exists.
The phenomenon is called the prisoner's cinema, and it's been documented across cultures and centuries. It's not madness. It's your brain's visual processing system, designed to find meaning in input, desperately searching for something to process and finding only its own internal noise.
Meditators have long reported similar experiences. In certain Buddhist traditions, the lights and patterns seen during deep meditation are called nimitta, and they're considered signs of deepening concentration. Whether you interpret them as spiritual phenomena or neurological artifacts depends on your framework, but the underlying mechanism is likely similar: a quiet mind, closed eyes, reduced sensory input, and a visual system that never quite stops looking for something to see.
Electricity and the Brain
In 1929, the German neurologist Otfrid Foerster discovered something remarkable. If you applied small electrical currents directly to the brain's visual cortex, patients reported seeing light. Not real light—phosphenes.
This was more than a curiosity. It was a proof of concept.
In 1968, Giles Brindley and Walpole Lewin took this further. They implanted a matrix of stimulating electrodes directly into the visual cortex of a fifty-two-year-old woman who was completely blind. Using small pulses of electricity, they could create phosphenes at will—points, spots, and bars of light, some colorless, some colored, appearing in specific locations in her visual field depending on which electrodes they activated.
By 1974, Brindley and Rushton had turned this into something functional: a visual prosthesis that used phosphenes to depict Braille dots. The woman couldn't see the world, but she could read, after a fashion, through patterns of electrically induced light.
This opened a tantalizing possibility. If blindness resulted from damage to the eyes or optic nerves, but the visual cortex remained intact, could you bypass the broken parts? Could you feed visual information directly into the brain?
Restoring Sight
Researchers have been working on this for decades, with varying success.
William H. Dobelle, an American biomedical researcher, developed one of the first practical visual cortex prostheses. He implanted electrode arrays in blind patients and connected them to cameras. The system couldn't restore anything close to normal vision—it produced only a field of phosphene dots, like a very low-resolution display. But it was enough for some patients to navigate, to locate doorways, to move through the world with more independence than they'd had before.
Mark Humayun took a different approach, focusing on retinal implants rather than cortical ones. His work led to the development of the Argus II, a device that stimulates the retina directly. It too produces phosphenes rather than true vision, but patients learn to interpret these patterns of light, translating them into useful information about their environment.
Dick Normann's work at the University of Utah focused on high-density electrode arrays—the "Utah array"—designed to interface with neural tissue. Animal research using these arrays has helped researchers understand how many electrodes you need, how close together they can be, and how to stimulate the brain without damaging it.
The challenges are substantial. Electrodes cause scarring over time. Infections are a constant risk with any implanted device. Seizures can occur when you stimulate the brain electrically. The resolution remains poor—even the best systems produce something closer to a flickering pointillist sketch than a photograph. But for people living in total darkness, even this can be transformative.
Magnets and the Mind
You don't have to implant electrodes to create phosphenes electrically. There's a non-invasive approach: transcranial magnetic stimulation, or TMS.
The principle is electromagnetic induction. A rapidly changing magnetic field generates an electrical field. If you position a powerful magnetic coil against someone's skull and pulse it, you induce tiny electrical currents in the brain tissue beneath. Aim it at the visual cortex, and you get phosphenes.
Researchers use this technique to study how different parts of the visual system work. Stimulate above the calcarine fissure—a groove in the brain where much of the primary visual cortex is folded—and phosphenes appear in the lower part of the visual field. Stimulate below, and they appear above. The visual cortex contains a map of the visual world, inverted and reversed, and TMS lets you probe that map non-invasively.
There's ongoing debate about exactly where these magnetically induced phosphenes originate. Some researchers argue the magnetic field stimulates the retina directly through current spread from the scalp electrodes. Others maintain the effect is truly cortical. The phosphenes produced by alternating currents that oscillate at specific frequencies—transcranial alternating current stimulation—tend to appear in the peripheral visual field, which adds another layer of complexity to the question.
Light in Space
Astronauts see phosphenes too, but not from pressure or magnetic fields. They see them from radiation.
In space, outside Earth's protective atmosphere and magnetic field, high-energy cosmic rays constantly bombard everything. When these particles—mostly protons moving at near-light speeds—pass through the eye or brain, they can directly stimulate visual neurons. Astronauts report seeing flashes of light, especially when they close their eyes in the dark.
This isn't just a curiosity. It's a concern for long-duration missions. Those cosmic rays are depositing energy in brain tissue. Each flash represents particles passing through cells. Over months or years, the cumulative radiation dose becomes significant. The phosphenes are a visible reminder that space is fundamentally hostile to human biology.
Similar flashes have been reported by patients undergoing radiotherapy for cancer. When radiation beams pass through or near the eyes, some patients see blue flashes. Analysis has suggested these may actually be Cherenkov radiation—the optical equivalent of a sonic boom, produced when charged particles travel through a medium faster than light does in that medium. Your eye contains fluid. High-energy particles passing through can exceed the local speed of light and produce actual photons. In this case, you're seeing real light, but light that exists only inside your eyeball, generated by your cancer treatment.
Medications and Side Effects
Phosphenes can be a side effect of certain medications. Ivabradine, a drug used to treat heart conditions, is known to cause them. The British National Formulary lists phosphenes as an occasional side effect of at least one anti-anginal medication.
This makes sense pharmacologically. Any drug that affects ion channels or neural excitability might potentially trigger spontaneous firing in the visual system. The heart and the nervous system share some of the same molecular machinery. Alter that machinery with medication, and unexpected perceptual effects can result.
Some diseases also cause phosphenes. Multiple sclerosis can damage the optic nerve or visual pathways, creating spurious signals that the brain interprets as light. Phosphenes that occur with movement or sound can indicate optic neuritis—inflammation of the optic nerve. In these cases, phosphenes aren't just curious phenomena; they're diagnostic clues.
Ancient Art and Altered States
In 1988, anthropologist David Lewis-Williams and researcher Thomas Dowson published an influential and controversial paper. They argued that the non-figurative art of the Upper Paleolithic—the geometric patterns found in caves alongside the more famous animal paintings—depicts phosphenes and related visual phenomena.
Their theory: prehistoric artists were recording what they saw during altered states of consciousness, probably induced by hallucinogenic substances, but possibly also through rhythmic drumming, dancing, sensory deprivation, or other techniques that can produce similar effects. The spirals, zigzags, grids, and dots that appear in cave art worldwide aren't random decoration. They're representations of entoptic phenomena—the geometric patterns generated by the visual system itself, independent of external input.
The evidence is circumstantial but intriguing. The same basic geometric forms appear across cultures that had no contact with each other, suggesting they arise from shared human neurology rather than shared cultural transmission. Psychedelic drug users often report remarkably similar geometric hallucinations in the early stages of their experiences, before the more complex visions take over. And some of these forms closely resemble what researchers have documented from laboratory-induced phosphenes.
Not everyone accepts this interpretation. Some archaeologists argue Lewis-Williams overreached, reading too much into simple decorative patterns. But the theory has been influential, suggesting that some of humanity's earliest artistic impulses may have been attempts to represent the strange lights we see when we look inward rather than outward.
Brain to Brain
In a remarkable recent experiment, researchers used phosphenes as part of a system for direct brain-to-brain communication.
The system, called BrainNet, works like this: Two people—the "senders"—wear electroencephalography (EEG) caps that detect their brain activity. A third person—the "receiver"—wears a transcranial magnetic stimulation helmet. All three watch screens showing a Tetris-like game. The senders can see whether falling blocks need to be rotated, but they can't control the game. Only the receiver can actually rotate the blocks.
To send a signal, the senders focus on one of two flashing icons, one flickering at fifteen hertz and the other at seventeen hertz. Focusing on a specific frequency produces a detectable pattern in the EEG. This signal is transmitted to the TMS helmet, which stimulates the receiver's visual cortex, producing a phosphene. The receiver perceives different phosphenes for the two frequencies and rotates the block accordingly.
It worked. In experiments with five different groups of three people each, the system achieved eighty-one percent accuracy. Information passed from brain to brain, with phosphenes serving as the language.
This is still a long way from telepathy. The bandwidth is incredibly low—essentially one bit of information at a time, with significant effort from all participants. But it's a proof of concept for something that would have seemed like science fiction a few decades ago: using artificially induced visual experiences as a channel for communication between minds.
What Phosphenes Tell Us
Most vision researchers understand phosphenes as the normal activity of the visual system responding to abnormal stimulation. Press on your eye, and you mechanically activate the same retinal cells that normally respond to light. Stimulate the visual cortex electrically or magnetically, and you activate the same neural circuits that normally create visual perception. The system doesn't know the difference. A signal is a signal.
This understanding has practical implications for the development of visual prostheses. It also has philosophical ones. Your perception of light, of color, of the visual world, is a construction. It's what your brain makes from the signals it receives. Normally those signals come from light striking your retina. But they don't have to. The experience of seeing is more fundamental than the physical process we usually associate with it.
An ancient theory held that light is generated within the eye itself—that seeing involves something flowing outward as well as inward. Modern neuroscience discarded this idea, but one researcher has half-seriously revived a version of it, proposing that phosphenes might involve the perception of biophotons, the extremely weak light emissions that living cells produce. The theory isn't widely accepted, but it points to how strange phosphenes can seem even to those who study them professionally.
In the end, phosphenes are reminders that vision is an active process. Your brain isn't a passive camera. It's constantly generating expectations, filling in gaps, creating a coherent visual world from incomplete and ambiguous information. Sometimes, when the input is absent or corrupted, you can see that generative process directly: light that comes from nowhere, patterns that exist only in the mind, a private cinema playing behind closed eyes.
The next time you rub your eyes and see sparks, consider what you're witnessing. Not just a curiosity, but a glimpse of how vision works—and how it can be harnessed, studied, and perhaps one day restored to those who have lost it.