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The visual system is what allows us to see.
It consists of:
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.
Light is inverted by the lens and projected onto the retina; blue-attuned cone cells will be most strongly stimulated
by blue light, while yellow/red-attuned cone cells will not be. |
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 medulla, 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).
Lateral geniculate nucleus in the thalamus
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.
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