The visual pathway is the network of nerves that convey the light striking the eyes into the brain in the form of chemical and electrical information. The optic nerve carries signals indicating color, brightness, and motion from the retina to a relay center in the mid-brain called the thalamus. From here, neurons reach the visual cortex of the brain's occipital lobe, which assembles a neural map or chart of the visual fields of both eyes. The visual pathway's primary task of converting light information into a picture of the outside world is moderated by neurons of the visual cortex.
In the eye, the visual pathway begins when light passes through the cornea, pupil, and lens, where it is inverted and projected onto the retina. Specialized cells, called photoreceptors, comprise the retina. There are two kinds of photoreceptor cells in the mammalian retina: rod cells, which detect the relative intensity of light and operate best in darkness; and cones, which are color sensitive. When light strikes either of these cell varieties, they undergo a chemical reaction resulting in signals to the bipolar cells directly behind them.
From the retina, visual information passes onto the bipolar cells and then to the optic nerve's ganglion cells. The optic nerve, which begins at the retina, is the sole visual pathway to the brain. Light information is conveyed as an electrical action potential through the neurons. These nerves represent the light's wavelength as its color and its intensity as its brightness, using a special kind of code to convey this information to the brain.
The two tracts of the optic nerve — one from each eye — cross one another before they enter the brain. The right and left visual tracts coming from the eyes pass to the left and right hemispheres of the brain, respectively. A small bundle of neurons follows a separate visual pathway to convey information about light and dark to the neural regions that regulate the body's circadian rhythm, including sleeping and waking patterns. The majority of nerves in the visual pathway proceed to the thalamus in the mid-brain, where all visual information is sorted and then relayed to the cerebral cortex.
The visual cortex is a very large region of the brain, occupying much of the occipital lobe. Here, many neurons are highly specialized to signal only when an object is seen with a specific color, angle, or location in the visual field of the eyes. The entire field of both eyes is represented in the cortex as a large map composed of these specialized cells arranged together, where the information conveyed by the visual pathway is sorted out and organized. Object recognition and the many complex aspects of conscious visual perception are distributed widely across the brain.
