Dark adaptation (also: dark adaptation) refers to the adaptation of the eye to darkness. As a result of various adaptation processes, light sensitivity increases in this process. Dark adaptation may be impaired due to congenital or acquired disease.
What is dark adaptation?
Dark adaptation refers to the adaptation of the eye to darkness. The human eye is good at adapting to different lighting conditions. It is functional during the day and night. If the light conditions of the environment deteriorate, the eye adapts to the increasing darkness. This process is called dark adaptation. Several processes take place: The eye switches from cone to rod vision, the pupil dilates, the rhodopsin concentration in the rods increases, and the receptive fields of the ganglion cells expand. These adaptations increase the eye’s sensitivity to light, enabling vision in the dark (scotopic vision). At the same time, visual acuity is reduced compared to daytime vision. In addition, differences in brightness can be perceived in the dark, but colors can hardly be distinguished. Complete adaptation takes about 10 to 50 minutes. However, it depends on the preceding light conditions and can also take significantly more time.
Function and task
When entering a dark room, initially nothing or almost nothing is visible to the human eye. After a few minutes, however, the eye has adapted to the new lighting conditions to the extent that outlines become recognizable. It can take 50 minutes or more to reach maximum vision in the dark. Meanwhile, several processes occur in the eye to adapt. Three of the four processes involved in dark adaptation take place in the retina of the eye. The retina contains sensory cells that function as receptors. They register the light that enters the eye through the pupil. They convert this stimulus into electrical signals, which they transmit to the nerve cells behind them (ganglion cells). Each of these ganglion cells covers a specific area of the retina whose stimuli it receives. That is, each ganglion cell receives information from a specific group of receptors. Such an area is called a receptive field. The smaller the receptive field, the higher the visual acuity. The electrical signals received by the ganglion cells are transmitted via the optic nerve to the brain, where they are processed. There are two types of receptors in the retina for registering light: cones and rods. They are specialized for different tasks. The cones are responsible for daytime vision (photopic vision), the rods for twilight and night vision. The rods contain the pigment rhodopsin (visual purple). This changes chemically when light falls on it, thus initiating the process by which the stimulus is transformed into an electrical signal. In brightness, this transformation requires a lot of rhodopsin, which causes its concentration to decrease. In darkness, on the other hand, rhodopsin regenerates. It is responsible for the light sensitivity of the rods. The higher the concentration of rhodopsin, the more light-sensitive the rods and thus the eye. Four different processes take place during dark adaptation:
- 1. the eye switches from cone vision to rod vision. Since rods are more sensitive to light, they are better able to perceive dim light sources. Whereas in cone vision colors can be distinguished and contrasts detected and visual acuity is high, in rod vision only differences in brightness can be perceived.
- 2. in darkness, the pupil dilates. Thus, more light falls into the eye, which the rods can convert into signals.
- 3. rhodopsin concentration gradually regenerates. As a result, light sensitivity increases. Until the greatest possible light sensitivity is established in the dark, about 40 minutes pass.
- 4. receptive fields expand. As a result, the single ganglion cell receives information from a larger area of the retina. This also results in increased light sensitivity, but also results in decreased visual acuity.
Diseases and complaints
Several congenital or acquired diseases can negatively affect dark adaptation and night vision.If vision in the dark is very limited or no longer possible, this is called night blindness (nyctalopia). In some cases, there is also an increased sensitivity to glare. However, daytime vision is not impaired. As a rule, both eyes are affected in night blindness. Congenital night blindness can be due to various causes. It can be a sign of pathological retinal changes, such as those that occur in retinopathia pigmentosa. In this disease, the sensory cells in the retina are gradually destroyed. The rods are the first to perish, causing an increase in night blindness. Congenital stationary night blindness, on the other hand, results from mutations in the genetic material that cause the rods not to function properly. Congenital night blindness cannot be treated. In acquired night blindness caused by vitamin A deficiency, the function of the rods is also disturbed. Vitamin A is a component of rhodopsin, which is critical for rod function. A deficiency interferes with the regeneration of the pigment. It occurs when either too little vitamin A is supplied or the body cannot absorb the vitamin from food. Night vision can also be disturbed by various other diseases. These include cataracts, which make twilight vision difficult due to clouding of the lens. Retinal damage can occur as a result of diabetes mellitus. Because various muscles and nerves are involved in the process of dark adaptation, muscular and neurological diseases (such as muscle paralysis and optic neuritis) can also interfere with dark adaptation.
