The anatomical structures involved in vision include the eyes, optic nerves and tracts, optic thalamus, primary visual cortex, and higher visual areas of the brain. The eyes are motor organs as well as sensory; that is, each eye can turn directly toward an object to inspect it. The two eyes are coordinated in their inspection of objects, and they are able to converge for near objects and diverge for far ones. Each eye can also regulate the shape of its crystalline lens to focus the rays from the object and to form a sharp image on the retina. Furthermore, the eyes can regulate the amount of light reaching the sensitive cells on the retina by contracting and expanding the pupil of the iris. These motor responses of the eyes are examples of involuntary action that is controlled by various reflex pathways within the brain. See also Eye (vertebrate).

The process of seeing begins when light passes through the eye and is absorbed by the photoreceptors of the retina. These cells are activated by the light in such a way that electrical potentials are generated. These potentials serve to generate nerve responses in various successive neural cells in the vicinity of excitation. Impulses emerge from the eye in the form of repetitive discharges in the fibers of the optic nerve, which do not mirror exactly the excitation of the photoreceptors by light. Complex interactions within the retina serve to enhance certain responses and to suppress others. Furthermore, each eye contains more than a hundred times as many photoreceptors as optic nerve fibers. Thus it would appear that much of the integrative action of the visual system has already occurred within the retina before the brain has had a chance to act.

The optic nerves from the two eyes traverse the optic chiasma. Figure 1 shows that the fibers from the inner (nasal) half of each retina cross over to the opposite side, while those from the outer (temporal) half do not cross over but remain on the same side. The effect of this arrangement is that the right visual field, which stimulates the left half of each retina, activates the left half of the thalamus and visual cortex. Conversely the left visual field affects the right half of the brain. This situation is therefore similar to that of other sensory and motor projection systems in which the left side of the body is represented by the right side of the brain and vice versa.

The visual cortex includes a projection area in the occipital lobe of each hemisphere. Here there appears to be a point-for-point correspondence between the retina of each eye and the cortex. Thus the cortex contains a “map” or projection area, each point of which represents a point in visual space as seen by each eye. Other important features of an object such as its color, motion, orientation, and shape are simultaneously perceived. The two retinal maps are merged to form the cortical projection area. This allows the separate images from the two eyes to interact with each other in stereoscopic vision, binocular color mixture, and other phenomena. In addition to the projection areas on the right and left halves of the cortex, there are visual association areas and other brain regions that are involved in vision. Complex visual acts, such as form recognition, movement perception, and reading, are believed to depend on widespread cortical activity beyond that of the projection areas. See also Brain.

Night animals have eyes that are specialized for seeing with a minimum of light. This type of vision is called scotopic. Day animals have predominantly photopic vision. They require much more light for seeing, but their daytime vision is specialized for quick and accurate perception of fine details of color, form, and texture, and location of objects. Color vision, when it is present, is also a property of the photopic system. Human vision is duplex; humans are in the fortunate position of having both photopic and scotopic vision. Some of the chief characteristics of human scotopic and photopic vision are enumerated in the table.

Characteristic

Scotopic vision

Photopic vision

Photochemical substance

Rhodopsin