Vision is the special sense concerned with reception and interpretation of light stimuli reaching the eye; it is the principal sense in humans. Light reaches the cornea and then passes through this, and also the aqueous humor, the lens, and the vitreous humor, before impinging on the retina. Here there are two basic types of receptor: rod cells, concerned with light and dark distinction, and cone cells, with three subtypes corresponding to three primary visual colors: red, green, and blue. Much of vision and most of the cones are located in the central region, the macula, of which the fovea is the central portion; gaze directed at objects brings their images into this area. When receptor cells are stimulated. nerve impulses pass through two nerve cell relays in the retina before passing back toward the brain in the optic nerve.
Behind the eyes, information derived from left and right visual fields of either eye is collected together and passes back to the opposite cerebral hemisphere, which it reaches after one further relay. In the cortex are several areas concerned with visual perception and related phenomena. The basic receptor information is coded by nerve interconnections at the various relays in such a way that information about spatial interrelationships is derived with increasing specificity as higher levels are reached. Interference with any of the levels of the visual pathway may lead to visual symptoms and potentially to blindness.