Body processes

Scotopic Vision – Function, Task & Diseases

Scotopic vision

It is an everyday phenomenon that when entering a dark room, initially poor eyesight improves as the eyes adapt to the light conditions. This is called dark adaptation and is essential for scotopic vision at night.

What is scotopic vision?

Scotopic vision refers to seeing in the dark. In contrast to photopic vision , it is realized by the rod sensory cells of the retina , because these are particularly well suited for seeing light and dark due to their increased sensitivity to light.

If the rods are damaged due to inherited or acquired changes, there can be a severe reduction in vision in the dark, known as night blindness .

function & task

On the retina of the human eye there are two different types of photoreceptors that are needed for vision: rods and cones . The cones are responsible for color vision in brightness, also known as photopic vision. The rods take over the visual process in low light or at night, i.e. scotopic vision. 

The fact that the rod sensory cells of the eye cannot distinguish between different colors is also the reason for our limited color perception in the dark. However, rods and cones are not evenly distributed across the retina. The highest density of sensory cells and thus the sharpest image resolution is achieved at the so-called yellow spot, the fovea centralis . However, there are only cones there, which are of little use for night vision. Scotopic vision is therefore optimal when the eye is aligned in such a way that the image on the retina does not appear on the yellow spot, but next to it (parafoveal).

In principle, both sensory cell types convert the light into a signal to the brain using the same mechanism . The energy of the incident light causes a structural change in a protein called rhodopsin. This triggers a signal cascade in the cell , as a result of which less glutamate is released. Downstream nerve cells register this and transmit an electrical signal to the brain.

When transitioning to seeing in the dark, for example when entering a dark room, a dark adaptation consisting of four effects takes place. A rapid aspect is the pupillary reflex . When there is little light, the pupil is dilated so that as much light as possible can fall through the opening of the iris onto the retina. In addition, the light sensitivity of the photoreceptors is increased. Your stimulus threshold drops, among other things, due to an increased concentration of rhodopsin, which is only possible in the dark.

On the other hand, in the dark there is a switch from cone to rod vision, since rods per se are already more sensitive to light than cones. This transition takes a certain amount of time and is also known as the Kohlrausch kink.

Finally, with increasing darkness, the lateral inhibition in the retina decreases and thus the size of the receptive fields increases. The result is a stronger convergence of the signals to the downstream ganglion cells, which are responsible for the transmission to the brain and are thus more excited. However, this increased convergence occurs at the expense of the resolving power, i.e. visual acuity .

Diseases & Ailments

A defect or weakening of scotopic vision is called night blindness. In this case, the eye can no longer (sufficiently) adapt to darkness and the ability to see in twilight or darkness is reduced or no longer available at all. This disorder can be congenital (congenital) or acquired. However, night blindness can also occur as an accompanying symptom of other diseases. 

For example, congenital night blindness can be caused by hereditary mutations in proteins that are important for the visual process, such as S-arrestin in Oguchi syndrome. Also genetic is retinitis pigmentosa , a group of hereditary retinal diseases for which causative mutations in over 50 different genes are currently known. The onset of this disease, which usually becomes apparent in childhood, adolescence or young adults, is often indicated by night blindness. In addition to limited scotopic vision, there is often a loss of visual field over the course of retinitis pigmentosa, increased sensitivity to glare and an increasing loss of color vision.

Cataracts also have symptoms that patients describe as night blindness However, the cause here is not a malfunction of the rods in the retina, but a clouding of the lens.

Similarly, in the course of diabetes mellitus , impairment of scotopic vision can occur, which is referred to as diabetic retinopathy . In addition to night blindness, patients with liver amaurosis often have increased sensitivity to glare, nystagmus (involuntary eye trembling) and generally reduced vision.

Distinguishable from these forms of night blindness is the one caused by a lack of vitamin A. Vitamin A is necessary for the body’s own production of the visual pigment rhodopsin. An improvement in this form of night blindness can therefore be achieved by administering vitamin A. However, deficiency-related night blindness is very rare in western industrialized nations, since the need for vitamin A can easily be covered by a balanced diet.

However, with certain risk factors for a vitamin A deficiency, such as various intestinal diseases , inflammation of the pancreas , eating disorders or pregnancy , particular attention should be paid to an adequate supply of vitamin A. In developing countries, vitamin A deficiency due to malnutrition is still a reason for dramatic rates of blindness in children.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.