Twilight vision: features

Twilight vision (night blindness) is a mechanism of visual perception, which works at a low level of illumination of the surrounding space.In addition to the term "gloomy vision the concept of mesopic vision (derived from the ancient Greek words "average view") can also be used. In this case, the term "twilight" refers to the degree of illumination in the range from the period when the sun has gone already a few degrees beyond the horizon to the amount of light that the moon creates in the clear sky in half phase.By "twilight" when describing the quality of illumination is also the state of the interior, for example, in the case of using a candle to illuminate the space inside the house. Next, we will describe in more detail what is night blindness.

Definition of twilight vision

The human organ of vision has a sufficiently high degree of sensitivity.The visual sensation appears when the light hits the area of ​​the retina of the eye (retina). This organ contains two types of perceiving elements: cones and rods. The cones function, providing the perception of color - the color vision. Sticks do not distinguish colors. But their sensitivity greatly exceeds the sensitivity of the cones.

These sensory elements provide twilight and night vision.The cones are elements of the photosensitive apparatus of day vision. Thus, depending on the degree of intensity of illumination of the surrounding space, either sticks or cones of the retina function. This determines the difference between day, night and twilight vision.

Twilight vision ensures "work" at the same time and rods and cones. In the case of a change in the brightness of the lighting, a greater or lesser "contribution" to the mechanism of perception of the rods and cones changes.

Twilight vision is black and white in quality. It gives only a feeling of darkness and light.

Indicators of the norm

When the degree of illumination of the surrounding space changes, the eyes are "reconfigured" for work under new conditions. Perceiving elements need a certain amount of time to begin to function effectively. The process of "adjustment" is called dark adaptation. The sensitivity of the organs of vision is directly related to the process of adaptation.

Light perception is the ability of the eye analyzer to perceive various degrees of brightness of light.Perceiving elements transform the energy of light radiation into physiological excitation.

The human eye can perceive light of rather low intensity.The minimum value of the light flux, which allows for visual perception, is called the "threshold of irritation". The limit of the minimum difference in the brightness of light between the illuminated objects is the "discrimination threshold".

The "irritation threshold" may vary and depends on the degree of brightness of light that has acted on the person's eyes at the previous time. After some time spent in a well-lit space, the eye can not perceive the environment well. But after a while, objects become discernible.

Maximum visual perception with dark adaptation is achieved within the first 30-45 minutes. With a longer stay in conditions of limited illumination, the photosensitivity of the eyes continues to increase.

The functioning of twilight vision is provided both by rods and cones, and both types of receptors are involved in the formation of the spectral dependence of photosensitivity.When examining the level of twilight vision, the ratio in the perceiving eye system of the quantitative ratio of "rods" and "cones" is taken into account. Normally their ratio should be 1:.

But the change in the brightness of the light flux causes a change in the ratio of the "contribution to work" of rods and cones.The spectral dependence of the photosensitivity also changes: the weakening of the illumination leads to a weakening of the sensitivity to red light and an increase in the perception of blue. The maximum sensitivity in twilight illumination shifts toward the violet edge of the visible spectrum and is located at 510 nm.

Deviations and diseases

Breaking of twilight vision is a disease that in ophthalmology is termed hemeralopia.The disease has no distinction in terms of complexity or intensity. Hemeralopia, which is commonly referred to as "night blindness" is a vision disorder with the following symptoms:

• Weakening of vision in conditions of significant decrease in brightness of illumination.
• Violation of spatial orientation in the dark or twilight.
• Changes occurring in the mechanism of light adaptation.
• Narrowing of the fields of vision.
• Problems associated with the perception of yellow and blue flowers (quite rare).

An additional sign of the presence of "night blindness" is dryness and keratinization of the epidermis around the eye.In addition, the patient may also have brittle hair. Hemerallopia causes the damage caused by various causes of the retina and optic nerve. The problems of twilight vision can be:

1. Symptomatic.This type of pathology is caused by the defeat of photoreceptors, the cause of which are glaucoma, neuritis, pigmentary degeneration and other organic diseases of the retina, optic nerve or vascular shell. In this case, the examination also reveals a change in the fundus and field of vision.
2. Functional.The disease occurs due to the development of hypovitaminosis in the body. The patient also develops xerotic plaques. Plaques appear on the conjunctiva, which is accompanied by exfoliation of the epithelium and hyperkeratosis.
3. Congenital. The features of the development of this type of disease are still unclear. The disease is not accompanied by changes in the fundus.

The reasons for the development of the acquired can be:

• Deficiency in the body of vitamin A, B2, nicotinic acid.
• Diseases of the liver.
• Physical exhaustion.
• Anemia.
• Eye diseases: glaucoma, retinal dystrophy, high degrees of myopia, retinal pigmentary pathologies, optic nerve atrophy, stagnant disk.
• Children's diseases (measles, chickenpox).
• Toxic poisoning.
• Abuse of alcoholic beverages, drug addiction.
• Sunburn of the conjunctiva, an effect on the organs of vision of excessively bright light.
• Disturbances in the visual department of the brain caused by head trauma.

Pregnancy can also cause "night blindness".

Prevention

Congenital "night blindness" can not be cured.To treat other types of hemorrhagia is necessary, taking into account the main cause of the development of the disease. Prevention of hemostalopia:

• Full nutrition, including those products that can ensure the supply of sufficient quantities of essential vitamins.
• Protecting the retina of the eyes. Use of special glasses to protect against excessive exposure to sunlight, as well as in hazardous radiation exposure.
• Treatment of pathological processes that cause hemostalopia.

In the presence of myopia, it is important to carry out a timely correction of vision.With worsening of twilight vision, it is important to conduct a test to determine blood levels of vitamin A, retinol and carotene. It is necessary to get advice from a gastroenterologist and endocrinologist.

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conclusions

In those cases when hemorrhagia is not congenital, it can be caused by a number of causes, related both to the work of the organs of vision, and to other diseases and the general state of the organism.The disease caused by problems with the retina, manifested in connection with the lack of vitamins, as well as exhaustion, alcohol abuse, diseases of the digestive system. Some forms of "night blindness" are explained by eye diseases: atrophy of the optic nerve, glaucoma and even severe myopia.

With the development of hemorrhaphy, day vision remains normal.However, the violation of twilight vision has a dangerous effect on the quality of life of the patient. Such a condition can be detrimental.For example, a person suffering from this disease, in the twilight, practically does not see the dangers on the road. This is extremely important for drivers of vehicles. Also, patients suffering from hememelopia patients may develop phobias associated with darkness. This causes obsessive states and other deformations of the psyche.