If we close our eyes, can we still see things? If so, what might we see? To find the answer, why not try closing your eyes for half a minute right now and feel the visual experience in the dark—what do you see?
Some people might see something like a faint red halo, which is likely due to the bright light in your environment. Even with eyes tightly closed, some light can still penetrate the thin eyelids. As this light passes through the blood-filled capillaries, it might take on a red hue. Moreover, some people might still vaguely sense the outline of their phone or computer screen after suddenly closing their eyes, resulting from a kind of “afterimage” formed by staring at the screen for a long time.
Even if you are in a darker environment and let your eyes adapt to a state of no bright light before closing them, you may still perceive faintly glowing lines or specks twisting, drifting, or flicking in your field of vision. So, what exactly are these?
The bright phenomena visible even without light exposure are called “Phosphenes.” This concept comes from two Greek roots— “phos” meaning light and “phainain” meaning to show.
In ancient Greece, philosophers began discussing this phenomenon and offered their understanding. Plato described it as an innate, non-burning flame within the eye, a gentle glow naturally occurring in the body determined by the creator’s design. However, modern scientific research has given us a deeper understanding of vision.
Today, the scientific community understands that we are able to see the world because light from the environment passes through the eyeball, is refracted by the lens, and forms an image on the retina. The retina contains millions of photoreceptor cells that convert the received light signals into electrical signals and transmit them to the brain’s visual center, which processes this information, allowing us to see the outside world.
In theory, when we close our eyes in the dark, the eyes no longer receive external light, but there may still be some weak light inside the eye. Most of the body’s cells may emit a very small amount of photons during regular metabolic activities and biochemical reactions. For instance, the respiratory process of mitochondria, lipid peroxidation reactions, and chemical reactions related to peroxisomes can produce excited state free radicals that, when returning to the ground state, might emit faint light. If these photons’ wavelengths fell within the visible range of the human eye and were in sufficient quantity, they might activate the photoreceptor cells, causing the brain to receive signals and thus see these lights.
The observed ripples or light spots are actually a direct manifestation of the photons produced by the cellular metabolic processes.
Aside from normal cell metabolism, external stimuli can also lead to a rapid increase in cellular free radicals, producing phosphenes. For example, rubbing your eyes hard and seeing spots or bright points is because the rubbing action generates more photons in eye cells. Other instances like sneezing, blowing your nose, or the “seeing stars” during a migraine are caused by the same principle. Notably, astronauts influenced by cosmic rays in space also experience phosphenes, with investigations showing that over 80% of astronauts have had this perception during certain missions.
Phosphenes are usually caused by stimulation of the visual system, but sometimes they are a result of direct stimulation of the cerebral cortex, such as the visual hallucinations during functional magnetic resonance or electroencephalogram exams. A study found that certain blind individuals, despite being unable to see, under specific conditions, such as wearing special devices that process external camera imagery through a computer, are able to perceive phosphenes and experience bright stripes that reveal the contours of objects.
Now, let’s discuss the “afterimage,” mentioned at the beginning. This phenomenon occurs when after staring at something for a long time, we close our eyes or move our gaze and the pattern of the object still stays in our vision. This happens because the retinal cells responsible for imaging created more excited state molecules due to prolonged exposure to the object. If these molecules fail to release the photons promptly through quenching, these photons get released when we close our eyes or turn away, causing the afterimage phenomenon.
While phosphenes are most often a normal visual occurrence, sometimes they can also be a precursor to diseases, such as an early symptom of demyelination in the optic nerve, potentially leading to multiple sclerosis.
Scientists advise: abnormal vision requires medical attention
According to research by Katrina Schmid, an expert on eyes and vision from the Queensland University of Technology, she pointed out in a popular science article that if the visual stripes observed when people close their eyes significantly differ from the usual, this may be a warning signal for eye health. In this case, it is recommended that the individual seek medical attention promptly for a professional eye health examination.
Changes in the colors or patterns that are typically seen when closing one’s eyes may be related to various factors, including light stimulation, changes in eye pressure, and the nervous system’s response. If abnormal visual phenomena occur, it could be an early sign of an eye disease or other health problem. Therefore, staying alert to such phenomena and consulting with a medical expert in a timely manner is crucial for maintaining ocular and overall health.
