a structural and functional unit of the photosensitive membrane of the photoreceptors of the retina, the rods and cones. The first stage of visual perception—the absorption of quanta of visible light—takes place in the visual pigment. A molecule of visual pigment (molecular mass, approximately 40,000) consists of a chromophore that absorbs light and opsin, a complex of protein and phospholipids. The chromophore of all visual pigments is an aldehyde of vitamin A, or A,—retinal or 3-dehydroretinal. The two forms of opsin (rod and cone) and the two forms of retinal unite in pairs and form four types of visual pigment that differ from one another in their absorption spectra: rhodopsin, or visual purple (the most common rod visual pigment; maximum absorption 500 nanometers [nm]), iodopsin (562 nm), porphyropsin (522 nm), and cyanopsin (620 nm). The primary photochemical link in the mechanism of vision consists in the photoisomerization of retinal, which, under the action of light, changes its curved configuration to a flat one. A chain of dark processes follows this reaction; they lead to the emergence of a visual receptor signal, which is then synaptically transmitted to the sequent nerve elements of the retina—the bipolar and horizontal cells.