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Improved night vision means more sensitivity to light, but it's not necessarily a good thing...
Written by | chestnut
Review | Twenty-seven
In nature, many animals have better night vision than humans.
For example, some fish can see infrared rays that are invisible to our naked eyes; for example, cats, although they can't see infrared rays, have a large number of light-sensitive cells in their eyes that adapt to the dim environment, and the light it needs to see things is roughly equivalent to 1/6 of what humans need.
In contrast, humans usually have limited vision in the dark. If a person's night vision ability suddenly increases, and he sees a scene that he could not see in the dark before, he may be alarmed. Some cancer patients have a similar experience after undergoing a certain therapy.
They don't know why they have extraordinary night vision. But scientists then discovered why.
Why is human night vision poor?
As you may have heard, the photoreceptor cells in the human retina are divided into cone cells and rod cells.
There are three kinds of cone cells, each with different visual pigments, we can see a variety of colors, it is by the cooperation of these three kinds of cone cells. In contrast, rod cells have only one visual pigment that receives light, which does not help us feel the colored world, but is more sensitive to light, allowing humans to see something when the light is weak.
Rod cells on the top and cone cells on the bottom (Image: Bioninja)
Our nighttime vision is dominated by rod cells. In rod cells, the pigment sensitive to light is called rhodopsin; the group that plays a major role in rhodopsin is called retinal. When visible light is illuminated on rod cells, the structure of retinal will change, and a series of biochemical reactions will follow, which can help the cells convert the received light into electrical signals. When this signal reaches the brain and is "understood" by the brain, we see something.
By the way, retinal is also called vitamin A aldehyde. The symptoms of night blindness that people experience in the absence of vitamin A are also associated with retinal.
Of course, under normal circumstances, even ordinary people who are not blind at night, "night vision ability" is very limited. After all, retinal, no matter how sensitive to light, is limited to the scope of visible light, and it is not sensitive to infrared. At night, the visible light is weak, and people can see less.
Night vision shoot (Credit: Credit: Dr Aaron Greenville via ABC News)
So, if retinal also reacts to infrared, can't humans see more clearly in the dark?
How do we "modify" what we see as retinal? It reminds scientists that there is a cancer treatment called photodynamic therapy (PDT) that may be able to achieve such a function. Some cancer patients have reported strange side effects after receiving this therapy — their night vision has increased. Patients are suddenly able to see some eerie silhouettes in the dark, which makes them feel uneasy.
Why does cancer therapy enhance night vision?
The team associated PDT therapy because when it is implemented, doctors often inject or apply a photosensitizer called dihydroporphyrine e6 (Chlorin e6, Ce6) to patients. Then, by focusing an infrared laser on the tumor, the photosensitizer Ce6 is activated, turning the oxygen in the tissue into reactive oxygen species that can destroy cancer cells.
Scientists speculate that the reactive oxygen species produced by Ce6 after being irradiated by infrared light can not only kill cancer cells, but also react with the retinal in rod cells, change the structure of retinal, and trigger a series of subsequent reactions: as in the case of visible light, the light is turned into an electrical signal to the brain. In this way, humans may use infrared rays to fill the gap of visible light and see clearer pictures in the dark night.
To confirm this idea, the team did molecular simulations of how Ce6 reacts after it enters the eye tissue: in the model, they simulated the motion of each atom, as well as the formation and breaking of chemical bonds.
Molecular simulation. A is the rhodopsin embedded in the cell membrane, and the Ce6 molecule; B-D is the three persistent binding sites of rhodopsin and Ce6 (Source: original paper)
When the team inserted a virtual pigment molecule, rhodopsin (which contains retinal), into the cell membrane, the system could predict how the pigment molecules, Ce6 molecules, and water molecules would interact.
The model ran for months and performed millions of calculations, and the scientists finally concluded that the reactive oxygen species produced by the photosensitizer Ce6 under infrared irradiation can indeed make retinal isomerized, just like when exposed to visible light. If it is in reality, such a biochemical reaction will probably be completed in nanosecond time.
The isomerization of retinal, from 11-cis (top right) to all-trans (bottom right), occurs when visible light is irradiated; molecular simulations suggest that when Ce6 is irradiated by infrared light, the retinal aldehyde reacts in the same way (Source: original paper)
The research team believes that they have found the principle of unexpectedly enhanced night vision after receiving PDT therapy in humans.
Can it really be used?
For the drug Ce6 used in PDT therapy, enhancing the night vision ability of cancer patients may not be crooked. After all, Ce6 is a chlorophyll derivative. Chlorophyll is no stranger to everyone, the reason why it can make plants appear green is because the red and blue-purple light bands are absorbed, and the absorption of green light is very small.
In the animal kingdom, there are also deep-sea creatures such as the Malacosteus niger, which use chlorophyll derivatives to help rod cells absorb far-red or infrared light in order to see something that other animals can't see in the dark world.
In addition, the Pacific barrel eye fish (Macropinna microstoma) also relies on some green pigments to see the world in very low light. Because the fish's head is transparent, we can easily observe the large green in its eyes.
Pacific barrel-eye fish, the green part is the pigment in the eye
(Image source: Wikimedia Commons)
So, can ordinary humans use Ce6 to unlock stronger night vision?
In recent years, studies have shown that Ce6 can effectively treat night blindness and improve the vision of patients with eye diseases in low light. Although scientists at that time did not fully understand the mechanism of Ce6's ability to improve night vision, this did not prevent them from understanding the effect of Ce6.
In 2015, another group of bio-hackers developed "night vision eye drops," the main ingredient of which is also Ce6. When one member of the team was given the eye drop, his entire eyeball was stained black, but as the drug was absorbed, the eyeball returned to its original color within seconds.
Eyes that darken after eye drops (Image: Gabriel Licina)
Within an hour of the drop, the researcher could see the shape of a hand 10 meters away in the dark, which the non-drip control could not do. In addition, when the drug-taking researcher and the four non-drip companions were scattered in the woods, 50 meters apart, he could recognize others with 100% accuracy, while the four controllers were only about 1/3 accurate in identifying others.
The next day, his vision returned to its original state, and no significant side effects were found 20 days later.
Even so, the experiment is still dangerous. If Ce6 makes the eye more sensitive to light, more light entering the eyeball can also cause cellular damage. So after the researcher was dripped, the buddies immediately covered his eyes with black contact lenses and didn't remove them until the test started.
So, don't try it yourself.
If you had super night vision, what would you like to see?
Original paper: https://pubs.acs.org/doi/abs/10.1021/acs.jpclett.9b02911
bibliography
[1]https://news.cnrs.fr/articles/seeing-in-the-dark
[2]https://www.nature.com/articles/s41598-017-03247-1
[3]https://scienceforthemasses.org/2015/03/25/a-review-on-night-[4]enhancement-eyedrops-using-chlorin-e6/
[5]https://www.businessinsider.com/biohackers-created-eye-drops-that-enhance-night-vision-2015-3
[6]https://www.sciencealert.com/scientists-have-figured-out-how-to-inject-human-eyes-with-night-vision
This article is reprinted from Universal Science (ID: huanqiukexue) with permission, please contact the original author for secondary reprinting.