Nature has given man not only the ability to clearly see the day and to distinguish colors well. The human eye is also adapted to twilight vision. This is well known to lovers of night hunting, experienced tourists, military, intelligence officers. There are special techniques to improve vision in the dark, even without night-vision devices. However, with some eye diseases in the dark nothing is visible. Features of night vision and their anomalies will be considered by MedAboutMe .

Retinal structure

The retina consists of ten layers of cells. Almost at the outermost of them are special cells capable of perceiving color and light. These are photoreceptors: cones and rods. They differ in structure and functions.

The former are responsible for daylight vision and color perception. Most cones are located in the central part of the fundus. And the place of the best vision – the central fossa of the retina – consists only of them. When you need to consider all the details, the person places the object right in front of the eye. At the same time, the brain receives a clear color image formed in the central fossa of the retina.

The rods are responsible for seeing in the dark and are well aware of the movement. These photoreceptors are grouped at the periphery of the fundus. The rods are less dense than cones. This leads to a lower resolution of the retina in the dark, and, consequently, to lower vision.

The rods can form only a black and white image, but they have a high photosensitivity . These cells are almost 100 times more sensitive to light than cones.

Special cells of the retina

“In the course of evolution, the retina of many living beings has reached the limit of sensitivity. She feels a negligible amount of light – one photon. And it is very important in the dark when the light is measured by only a few photons ”, –   says associate professor of neurobiology and medical engineering Greg Field of Duke University, located in the United States.

The same group of researchers discovered interesting features of cells located in the innermost layer of the retina. Some of them perceive movement in a certain direction. So, there are cells responsible for trapping downward, rightward, and so on.

In the dark, cells that react to an upward movement change their “behavior”. They are activated when moving in any direction. The exact cause of this phenomenon is still unknown. Greg Field suggests that this is especially necessary for those animals that may be victims of predators. The constant activity of such neurons allows you to catch his jump in the dark in time. In humans, there are few such cells, only about 4%, while in rodents their proportion reaches 20-30%.

Light perception mechanism

Once inside the eye, the luminous flux is focused by the cornea and lens at one point on the retina. Then the light passes through all its layers and only in the outer part it meets with photoreceptors.

The main process of perception of light occurs in the outer segment of rods and cones. It is a stack of discs. Each disc consists of a visual pigment surrounded by a membrane. In the rods it is rhodopsin, and in cones it is iodopsin . Rhodopsin consists of retinal (a special form of vitamin A) and opsin protein .

A visual pigment is a special substance that can change its structure when light enters. This triggers a series of chemical transformations and leads to the formation of electrical potential. This impulse is transmitted through the optic nerve to the brain. Here the image is formed and recognized.

With a sharp change in the level of illumination of the eye does not immediately adapt to new conditions. The process of adaptation to bright light is called light adaptation, and to low light it is called dark adaptation. There are special devices for recording light sensitivity during darkadaptation. They are called adaptometers.

It adapts to the light of the eye quickly, it takes 1-3 minutes. But the dark   adaptation   lasts a few hours. Therefore, for a good view in the dark, you should wait at least an hour. At the same time it is necessary to avoid any bright light, including a pocket flashlight. This will allow the eye to adapt to low light as much as possible.

Is it possible to kill with a look?

Interestingly, the eye can not only perceive information, but also affect the world around it. So, in 2003, the Canadian hunter was alone with a hungry grizzly bear. The gun was not available, the beast was very close. In desperation, the hunter, he said, began to look intently into his eyes. The bear, contrary to custom, did not look away. No one knows how long this scene lasted, but in the end the predator fell dead. After the hunter’s story, the beast was found and an autopsy was performed. It turned out he died of cerebral hemorrhage.

This story, of course, is not a recommendation to look at hungry bears, and there are plenty of dark places in it. But it is quite possible that the eyes of a man really do contain a lot of secrets.

Reasons for reducing dark adaptation

Complaints of low vision in the dark can be caused by several reasons.

• Anomalies of refraction.

The vision of most people is not perfect. Particularly common is small myopia, less often.   astigmatism. And some people don’t even realize it. In low light, the pupil expands, much more oblique rays fall into the eye. The existing anomalies of refraction prevent them from focusing on the retina, the image is fuzzy. A person complains of low vision in the dark.

This phenomenon can be compared with the effect of apertured glasses. Their holes are narrower than the pupil in daylight. This gives a higher visual acuity with glasses. Therefore, with a narrow pupil during the day you can see better than with a wide pupil at night.

• Eye diseases.

There is only one ophthalmologic disease, which for a long time is manifested only by this sign. This is retinal pigment dystrophy, it is retinitis pigmentosa.

Bad night vision may accompany other diseases. For example, inflammation of the optic nerve, its atrophy, inflammation of the retina with the adjacent part of the choroid, intraocular foreign body. But in this case there will certainly be other signs indicating the problem. For example, low vision during the day, poor color discrimination, sluggish   eye inflammation.

• Lack of vitamin A in the diet.

From it builds the most important part of the visual pigment rhodopsin. Therefore, its inadequate intake with food leads to impaired night vision. About hypovitaminosis And also they say peeling of the skin, dry and brittle hair, transverse striation of the nails, frequent conjunctivitis or stomatitis.

Retinitis pigmentosa

The cause of the disease is not exactly known. However, most researchers associate it with mutations in the genes encoding rhodopsin. The disease is inherited.

The essence of the disease is the death of photoreceptors. Rods usually suffer first, but there are options for mixed rod and cone dystrophy. As a rule, the process begins with the periphery of the fundus. The outermost layer of the retina, the pigment epithelium, is in close contact with the rods and cones. He is responsible for restoring photoreceptors after absorbing light and photo-isolating them from each other.

The structure of the pigment epithelium is also changing. Individual cells move into the inner layers of the retina. This makes it difficult for the remaining photoreceptors to work.

The disease manifests itself quite early. In about 95% of cases it is diagnosed to 30 years. The first sign is a decrease in vision in the dark. Many patients do not tolerate bright light. Then appear visual field defects. A typical defect is in the form of a wide ring, which leaves only the center and the outer periphery of the visual field free.

The narrowing of the field of view dramatically impairs orientation in space. But visual acuity remains high for a long time. The disease gradually progresses, the outcome is complete blindness.

Modern methods of treatment of the disease

Effective ways to combat the disease does not exist. Actively developed methods of gene therapy. Its essence consists in the introduction of modified viruses into the eye cavity. They carry the “right” genes of photoreceptor cells. Viruses are inserted into the genome of sticks. This restores their normal operation. For the first time such a procedure was performed in America in March 2018. The first patient was a thirteen-year-old boy named Jack. Hogan.

However, gene therapy is still undergoing clinical trials and has not yet entered clinical practice. In addition, the use of stem cells and large doses of vitamin A is being studied.

In the United States in 2011, a special retinal prosthesis Argus was developed for the treatment of patients with retinitis pigmentosa . It is installed on the fundus surgically. The method is applied only in the later stages of the disease.

Of course, the device does not give a full view. However, all patients with prostheses noted an improvement in the recognition of objects and the perception of moving objects. Currently Argus is installed in clinics in Germany, Britain, France and Italy.

Thus, vision in the dark can be reduced for very different reasons. You can start with a trip to the nearest optics to detect refractive errors. If their correction fails, then a thorough examination by a competent ophthalmologist should be carried out.