Photopic vision refers to normal color vision via the so-called M, L, and S cones, which are photosensorically optimized for the green, red, and blue regions, respectively. Photopic vision requires a minimum brightness of about 3 to 30 cd/sqm and occurs mainly in the fovea centralis, a small area in the retina. The fovea centralis contains the greatest density of cones for sharp color vision, whereas the regions outside the fovea centralis are mainly occupied by so-called rods on the retina, which are much brighter but only provide scotopic vision.
What is photopic vision?
Photopic vision means sharp color vision. It occurs sensory with the help of the L, M, and S cones, which are optimized for the red, green, and blue spectral regions, respectively, and reach their greatest density on the retina in the area of the fovea centralis, which is about 1.5 millimeters in diameter. The particularly sharp vision in the fovea centralis comes about through the nervous interconnection of the color cones of almost 1:1. Almost every single cone is connected to a separate nerve fiber, so that every incident photon can be located relatively precisely in the brain. The counterpart to photopic brightness vision is scotopic vision in relative darkness, which occurs by means of the light-sensitive rods located mainly outside the fovea centralis on the retina. Although the rods are extremely sensitive to light, they are not able to distinguish colors. This means that scotopic vision is equivalent to monochromatic vision. In addition, scotopic night vision is associated with some blurring because many rods must share one nerve fiber at a time, so the brain cannot locate incident photons as precisely as with cones.
Function and task
We humans are among the diurnal creatures for whom vision is one of the most important sources of information. The ability to see sharply in color even allows nonverbal communication to a certain extent. Strong emotions such as excitement, fear or anger are expressed in facial expressions, on the skin by blushing in certain areas and in visible body language. The detection of nuances in nonverbal communication requires the sharpest possible color vision, i.e. photopic vision. Photopic, binocular vision also enables spatial vision and thus facilitates orientation in three-dimensional space, including distance estimates. Photopic vision has been optimized by evolution to provide humans with the best possible protection from enemies and other dangers and to make it easier for them to find food. Provided an appropriate minimum illumination of 3 to 30 cd/sqm, photopic vision is a nearly all-encompassing aid in (almost) all life situations and serves as an orientation aid for the brain in case of incompatible multisensory information. In such cases, photopic vision serves as a master stimulus to which all other sensory impressions are aligned in case of doubt, which in many cases can lead to problems such as spatial disorientation.
Diseases and disorders
Photopic vision depends, on the one hand, on the surrounding light conditions, as well as on the function of the individual organic components related to the functionality of the L, M, and S cones. Even if all the components involved are in perfect condition, optical illusions can occur that make it difficult for us to orient ourselves and can even trigger discomfort or even vomiting. While our brain can compensate for short-term discrepancies between vision and vestibular messages, problems with discomfort set in with longer-lasting incompatibilities that can result from optical illusions. The discomfort or a subsequent vomiting can be interpreted as a protective mechanism. It is intended to prevent any ingested psychogenic or hallucinogenic substances, on which the discrepancies between sensory feedbacks may be based, from causing more harm. Diseases and disorders that directly affect photopic vision cover a broad spectrum of causes. Visual disturbances may develop because of a deficiency in the supply of oxygen and nutrients to the cones due to simple circulatory disturbances.The visual disturbances can even be included in a diagnosis as an indicator of circulatory disorders. In many cases, infectious diseases or genetic degeneration of the retina or cones are the cause of reduced vision, as in age-related macular degeneration (AMD). It involves a gradually progressive degeneration of the color sensors in the macula, the yellow spot or fovea centralis. Another eye disease, called glaucoma, also known as glaucoma, causes visual impairment and even vision loss due to progressive damage and degeneration of the optic nerve.
