Unlike most animals, humans rely most heavily on their sense of vision to perceive the world.

In the process leading to vision, light enters the eye through the cornea, then passes through the pupil (in the center of the iris) and the lens, which focuses it onto the retina. The lens changes its shape to allow light to be focused sharply on the retina. Directly behind the lens and on the retina is a depressed spot called the fovea, which lies at the center of the visual field.

The retina of each eye contains the two kinds of receptor cells responsible for vision: rods and cones. Rods, chiefly responsible for night vision, respond to varying degrees of light and dark but not to color. Cones respond to light and dark as well as to color or different wavelengths of light, and operate mainly in daytime. Only cones are present in the fovea.

Rods and cones connect to nerve cells, called bipolar cells, leading to the brain. In the fovea, a single cone generally connects with one bipolar cell. Rods, on the other hand, share bipolar cells. The one-to-one connection between cones and bipolar cells in the fovea allows for maximum visual acuity, the ability to distinguish fine details. Vision is thus sharpest whenever the image of an object falls directly on the fovea; outside the fovea, acuity drops dramatically.

The sensitivity of rods and cones changes according to the amount of available light. Light adaptation helps our eyes adjust to bright light; dark adaptation allows us to see, at least partially, in conditions of darkness. An afterimage can appear until the retina adapts after a visual stimulus has been removed.

Neural messages originating in the retina must eventually reach the brain for a visual sensation to occur. The bipolar cells connect to ganglion cells, whose axons converge to form the optic nerve that carries messages to the brain. The place on the retina where the axons of the ganglion cells join to leave the eye is the blind spot.

At the base of the brain is the optic chiasm, where some of the optic nerve fibers cross to the other side of the brain.

The human vision system allows us to see an extensive range of colors. Hue, saturation, and brightness are three separate aspects of our experience of color. Hue refers to colors (red, green, blue, etc.), saturation indicates the vividness or richness of the hues, and brightness signals the intensity of the hues. Humans can distinguish only about 150 hues but, through gradations of saturation and brightness, we can perceive about 300,000 colors.

Theories of color vision attempt to explain how the cones, which number only about 150,000 in the fovea, are able to distinguish some 300,000 different colors. One clue lies in color mixing: Additive color mixing is the process of mixing only a few lights of different wavelengths to create many new colors; subtractive color mixing refers to mixing a few pigments to come up with a whole palette of new colors.

Based on the principles of additive color mixing, the trichromatic theory of color vision holds that the eye contains three kinds of color receptors that are most responsive to either red, green, or blue light. By combining signals from these three basic receptors, the brain can detect any color and even subtle differences among nearly identical colors. This theory accounts for some kinds of colorblindness. People referred to as dichromats have a deficiency in either red-green or blue-yellow vision; monochromats see no color at all. People with normal color vision are referred to as trichromats. By contrast, the opponent-process theory maintains that receptors are specialized to respond to either member of the three basic color pairs: red-green, yellow-blue, and black-white (dark and light).

Drawing on elements of the two theories, current knowledge holds that while there are three kinds of receptors for colors in the retina (for violet-blue, green, and yellow light), the messages they transmit are coded by other neurons in the visual system into opponent-process form.

In addition to humans, many other primates, including tree shrews, monkeys, and apes, also distinguish colors quite well. In contrast to humans, however, most are dichromats–experiencing the whole world only in terms of reds and greens or blues and yellows. Rodents, such as hamsters, rats, and squirrels, appear to be completely colorblind. Some reptiles, fish, insects, birds, and shellfish can distinguish color, but differ in which colors they can see; for example, bees can see ultraviolet light but not red. Knowing an animal is sensitive to light of a certain wavelength does not mean we know how that animal actually experiences color.