Visual system
The visual system is what allows us to see. It consists of:
- The eye, especially the retina
- The optic nerve
- The optic chiasm
- The visual cortex
The task of the visual system is to interpret what would otherwise be a two-dimensional image of the world as a moving, colored three-dimensional world.
This article mostly describes the visual system of mammals, although other higher animals have similar visual systems.
The eye
The eye is a complex biological mechanism, considered by some to be miraculous. The eye functions remarkably like a CCD camera, taking visible light and converting it into a stream of information that can be transmitted via nerves.
Light enters the eye, passing through the cornea and the pupil (controlled by the iris) and is refracted by the lens. The lens inverts the light and projects an image onto the retina.
The retina consists of a large number of "receptor" cells and contain a particular protein molecule called rhodopsin. When rhodopsin is struck by a photon (a particle of light) it transmits a signal to the cell; the more photons strike the cell, the stronger the signal will be. In some animals, like humans, cone cells contain rhodopsin molecules attuned to specific wavelengths of light; i.e., a blue cone cell contains rhodopsin most attuned to blue-wavelength light and will most strongly be stimulated by blue-wavelength light, while a yellow-red cone cell will only be weakly stimulated by blue-wavelength light. This gives the ability to distinguish color.
Optic nerve
Following some rudimentary processing (mostly involving color boundaries), the information about the image received by the eye is transmitted to the brain via the optic nerve. In humans, the optic nerve is the only sensory system that is connected directly to the brain and does not connect through the medula, due to the necessity of processing the complex visual information quickly.
Optic chiasm
The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the brain. At this point the information from both eyes is combined and split according to the field of view. The corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively (the brain is cross-wired), to be processed. That is, though we might expect the right brain to be responsible for the image from the left eye, and the left brain for the image from the right eye, in fact, the right brain deals with the left half of the field of view, and similarly for the left brain. (Note that the right eye actually perceives part of the left field of view, and vice versa).
Visual cortex
The visual cortex is the most massive system in the human brain and is responsible for higher-level processing of the visual image.