When you close your eyes, you can still see strange ripples, is there something wrong with your eyes? It may be sick, but it is most likely normal.
If we close our eyes, can we still see? If we can, what do we see? To answer the above question, you might as well close your eyes for half a minute now, and then come back and tell me what you see.
Some people will see a bright layer of reddish light, which may be due to the brighter environment you are in right now. Even if you close your eyes, there is still light through the thin eyelids and perceived by your eyes. And because the light passes through the blood-filled capillaries in your eyelids, it has a reddish color.
There are also some people, although their eyes are closed, they can still vaguely see the shape of the mobile phone or computer screen for a short time. This is because you have been staring at the screen for a long time, and suddenly you close your eyes and see the "afterimage" of the screen of your phone or computer, a phenomenon that we will explain later.
But even if you get to a darker place, get your eyes used to the absence of light, and then close your eyes, you may still see a gleaming ripple or spot that is twisting, swimming, or flickering. So, what are these things?
Renderings of light illusion art (Image source: Wikimedia)
See the light inside the eye
This phenomenon of seeing light even when no light enters our eyes is called "phosphene", which comes from the Greek word "phos" for "light" and "phainain" for "appearance". As early as thousands of years ago, ancient Greek philosophers had their own understanding and interpretation of this phenomenon. Plato believed that it was a flame that existed in our eyes, but its property was not to burn, but to be designed by the Creator to be a soft glow that would normally appear in people's bodies.
But today, we have a much deeper understanding of human vision. Modern science tells us that we can see the world because light from the outside enters our eyeballs, is refracted by the lens and imaged on the retina. The retina is made up of millions of light-sensitive cells that, after receiving light signals from the outside world, convert them into electrical signals that are transmitted by nerves to the visual center of the brain. After the brain processes this information, it allows us to see images of the outside world.
人类眼球结构（图片来源：NIH National Eye Institute Public Domain）
Although after we close our eyes in the dark, theoretically no more light from the outside world enters our eyes, in fact, there will still be some light inside our eyes at this time.
Most of the cells in our body may release extremely small amounts of photons when they carry out some normal metabolic processes and biochemical reactions. For example, the respiration process in mitochondria, the lipid peroxidation process, peroxisome-related reactions, etc. These reactions produce molecules in an excited state, such as free radicals, and the quenching of these molecules may produce trace photons.
These photons may be at different wavelengths, some of which are at visible wavelengths. And if these photons at visible wavelengths exceed a certain size in our eyes, they can cause photoreceptors to perceive them, which can send signals to the brain to see the image of these photons.
The effect is the ripple or light spot you see.
An example of cell metabolism producing photons (Image source: Császár et al, 2016)
Blind people can see
In addition to the normal metabolic processes of the cell, external stimuli (mechanical force, electrical and magnetic stimuli, etc.) may also cause a rapid increase in free radicals in the cell in a short period of time and trigger photovisual hallucinations. As a common example, when you rub your eyes vigorously, you may also see spots of light or bright spots. This is because the action of rubbing your eyes causes the eye cells to be mechanically compressed, producing more photons, which triggers light hallucinations.
In addition to this, sneezing, blowing nose, and migraines can sometimes make you "star-popping", which is the same principle. That is, the cells are stimulated to produce more photons, resulting in optical hallucinations.
Astronauts in space may also experience optical hallucinations due to cosmic rays (high-energy particle radiation). A survey of astronauts shows that more than 80 percent of astronauts serving in NASA or European Space Agency (ESA) programs perceive light hallucinations on a regular basis, at least on some missions.
Although the vast majority of optical hallucinations are produced by stimulation of eye cells, there are also cases when the cerebral cortex is directly stimulated, resulting in related visual hallucinations, for example, some people may also see bright writhing ripples during a functional MRI or EEG.
An interesting phenomenon is that some blind people can also perceive light hallucinations, but most of them are patients who have been able to see things but have acquired blindness. As a result, some researchers are trying to use this to restore a little vision to blind people, that is, by having them wear special equipment, use a camera to capture the surrounding visual images, convert them into electrical signals after computer processing, and then transmit them to the brain, so that these blind people can see bright streaks similar to light hallucinations, and these bright streaks can show the general outline of the object captured by the camera.
"Aftershadow"
Then we go back to the "afterimage" mentioned at the beginning, which is scientifically known as "afterimage". In life, if we stare at an object or a pattern for a while, and then look away or close our eyes, sometimes we will find that we can still see the shape of the pattern, but it is more blurry and the color may not be the same.
闭眼能看到视后像(图片来源:Salari et al, 2017)
This is because when we stare at an object for a long time, the cells in the retina that are responsible for imaging the object receive a lot of light from it and become more active, and these cells may also produce more excited molecules. If we suddenly close our eyes or look away before these molecules have time to quench the photons, and then these photons are released and received by the retina, the image of the original object will stay in front of your eyes for a while, which is the principle of aftervision, which can also be seen as a kind of optical illusion.
Although in most cases, it is normal to see shiny ripples with your eyes closed, sometimes the occurrence of photovisual hallucinations can be a precursor to a related disorder. For example, optical hallucinations are one of the early symptoms of demyelination of the optic nerve, and these patients may later develop multiple sclerosis. Katrina Schmid, a scientist who studies eyes and vision at Australia's Queensland University of Technology, cautions in an article that if you close your eyes and see streaks that are very different from usual, you may want to consider seeing a doctor to see if there is something wrong with your eyes.
bibliography
[1]https://pubmed.ncbi.nlm.nih.gov/26544101/
[2]https://www.sciencedirect.com/science/article/abs/pii/S1350946217300435?via%3Dihub
[3]https://theconversation.com/curious-kids-why-do-we-see-different-colours-when-we-close-our-eyes-154378
[4]https://dukeeyecenter.duke.edu/news/whats-really-happening-when-we-see-stars-after-rubbing-our-eyes[5]https://www.sciencedirect.com/science/article/pii/S0042698905006735?via%3Dihub
[6]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2189731/
[7]https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2021.780344/full
[8]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4921351/
[9]https://pubmed.ncbi.nlm.nih.gov/15947465/
Planning and production
Source丨Global Science