Vision is one of the special senses that allows the body to detect light and and relay messages to the brain to perceive images.
When light enters the eye, it passes through the cornea, the aqueous humor, the lens, and the vitreous chamber before it reaches the retina, where the light stimulus is received by sensory receptors. As the light travels through the eye, the rays are bent, or refracted, by the convex-shaped cornea and the lens. The purpose of the lens is to fine-tune the light refraction. When focusing on near objects, it can change shape so the light reaches a particular location on the retina. This process is called accommodation. Accommodation of the lens is not necessary when objects are viewed at a long distance because the eye does not need to focus light over long distances.
Rods and cones are the two types of photoreceptors of the retina. They are aptly named for the shape of their outer segment. The outer segment contains the light-sensitive pigments, and the inner segment contains most of the cellular organelles. This is where the G-protein activates the light response when the photoreceptor turns off. Interior to the inner segment are the nucleus and the synaptic terminals. Both rods and cones are partially embedded in a pigmented layer at the back of the retina that prevents light from scattering in the eye. Rods are more light-sensitive and can detect dim light. They are primarily distributed on the sides of the retina. When rods process light, they form images in shades of gray and receive light from the periphery, thus allowing for peripheral vision. Cones are less sensitive to light and are activated only under conditions of bright light. They are highly concentrated on the posterior region of the retina and are responsible for clear color vision. There is a patch on the most posterior region of the retina made up primarily of cones. This region is called the macula lutea, or macula, and in the center is a depression called the fovea centralis, or fovea. The fovea is made up exclusively of densely packed cones and allows for sharp, clear vision. It is free of blood vessels and other obstructions, which allows for greater visual acuity of the photoreceptors.
The Process of Vision
Both rods and cones contain photopigment proteins that change shape when stimulated by light. A vitamin A derivative absorbs light and modulates protein molecules called opsins, which are activated at different wavelengths of light. Rods have rhodopsin which is very sensitive to low light levels, and cones have three types of opsins: blue (S-cones), green (M-cones), and red (L-cones). The letters in the names refer to the wavelength they detect—short, medium, and long. As cones and rods are stimulated, the internal processing in the retina allows the eye to perceive various colors, intensities, and edges.
Relative Sensitivity of Photoreceptors
The photoreceptors are nonneural cells that are sensory receptors for light. They receive the light stimulus at the back of the eye and, when activated, release a chemical to signal the bipolar neurons. A single bipolar neuron can receive information from multiple photoreceptors and in turn can activate multiple larger ganglion cells, neurons near the surface of the retina. Bipolar cells are found between the rod and cone cells, and the ganglion cells. Once the ganglion cell receives information that has been gathered from the photoreceptors and processed by the bipolar, horizontal, and amacrine cells found in the inner layer of the retina, it sends the signal via action potential down its axon through the optic nerve. Portions of the two optic nerves cross at the optic chiasm, located at the bottom of the brain below the hypothalamus, so that signals coming from the left side of both eyes are processed on the left side of the brain, and signals coming from the right side of both eyes are processed on the right. Most visual processing occurs in the occipital lobe of the brain. This processing forms an image, and then the information is sent to the temporal lobes for identification, the parietal lobes to determine location, and the frontal lobe to determine any needed responses to the image.