Visual system

The visual system is the part of the nervous system which allows organisms to see. It interprets the information from visible light to build a representation of the world surrounding the body. The visual system has the unenviable task of reconstructing a three dimensional world from a two dimensional projection of that world. Note that different species are be able to see different part of the light spectrum; for example, some can see into the ultraviolet, while others can see into the infrared.

This article mostly describes the visual system of mammals, although other "higher" animals have similar visual systems. In this case, the visual system consists of:


 * The eye, especially the retina
 * The optic nerve
 * The optic chiasm
 * The optic tract
 * The lateral geniculate nucleus
 * The optic radiations
 * The visual cortex

Eye
The eye is a complex biological device. The functioning of a CCD camera makes an apt metaphor for the workings of the eye, which takes visible light and converts it into a stream of information that can be transmitted via nerves.

Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil (controlled by the iris) and is further refracted by the lens. The lens inverts the light and projects an image onto the retina.

Retina
The retina consists of a large number of photoreceptor cells which contain a particular protein molecule called an opsin. In humans, there are two types of opsins, rods and cones. Either opsin absorbs a photon (a particle of light) and transmits a signal to the cell through a signal transduction pathway, resulting in hyperpolarization of the photoreceptor. (For more information, see photoreceptor).

Rods and cones differ in function. Rods are found primarily in the periphery of the retina and are used to see at low levels of light. Cones are found primarily in the center (or fovea) of the retina. There are three types of cones that differ in the wavelengths of light they absorb; they are usually called short or blue, middle or green, and long or red. Cones are used primarily to distinguish color and other features of the visual world at normal levels of light.

In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, who will then conduct action potentials to the brain. A significant amount of visual processing arises from the patterns of communication between neurons in the retina. About 130 million photoreceptors absorb light, yet roughly 1.2 million axons of ganglion cells transmit information from the retina to the brain. The processing in the retina includes the formation of center-surround receptive fields of bipolar and ganglion cells in the retina, as well as convergence and divergence from photoreceptor to bipolar cell. In addition, other neurons in the retina, particularly horizontal and amacrine cells, transmit information laterally (from a neuron in one layer to an adjacent neuron in the same layer), resulting in more complex receptive fields that can be either indifferent to color and sensitive to motion or sensitive to color and indifferent to motion.

The final result of all this processing is five different populations of ganglion cells that send information to the brain: M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus; P cells, with smaller center-surround receptive fields that are sensitive to color and shape; K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth; another population that is intrinsically photosensitive; and a final population that is used for eye movements.

Optic nerve
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.

Different populations of ganglion cells in the retina send information to the brain through the optic nerve. About 90% of the axons in the optic nerve go to the lateral geniculate nucleus in the thalamus. These axons originate from the M, P, and K ganglion cells in the retina. This parallel processing is important for reconstructing the visual world; each type of information will go through a different route to perception. Another population of photosensitive ganglion cells sends information to the pretectum for regulating circadian rhythms, and a final population sends information to both the pretectum and the superior colliculus for controlling eye movements (saccades).

Optic chiasm
The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the frontal lobe 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).

Optic tract
Information from the right visual field (now on the left side of the brain) travels in the left optic tract. Information from the left visual field travels in the right optic tract. Each optic tract terminates in the lateral geniculate nucleus (LGN) in the thalamus.

Lateral geniculate nucleus
The lateral geniculate nucleus (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN consists of six layers in humans and some other primates such as macaques. Layers 1, 4, and 6 correspond to information from one eye; layers 2, 3, and 5 correspond to information from the other eye. Layer one (1) contains M cells, which correspond to the M (magnocellular) cells of the optic nerve of the opposite eye, and are concerned with depth or motion. Layers four and six (4 & 6) of the LGN also connect to the opposite eye, but to the P cells (color and edges) of the optic nerve. By contrast, layers two, three and five (2, 3, & 5) of the LGN connect to the M cells and P (parvocellular) cells of the optic nerve for the same side of the brain as its respective LGN. The six layers of the LGN are the area of a credit card, but about three times the thickness of a credit card, rolled up into two ellipsoids about the size and shape of two small birds eggs. In between the six layers are smaller cells that recieve information from the K cells (color) in the retina. The neurons of the LGN then relay the visual image to the primary visual cortex (V1) which is located at the back of the brain (caudal end) in the occipital lobe.



Optic radiations
The optic radiations carry information from the midbrain lateral geniculate nucleus to layer 4 of the visual cortex. The P layer neurons of the LGN relay to V1 layer 4C &beta;. The M layer neurons relay to V1 layer 4C &alpha;. The K layer neurons in the LGN relay to large neurons called blobs in layers 2 and 3 of V1.

There is a direct correspondence from an angular position in the field of view of the eye, all the way through the optic tract to a nerve position in V1. At this juncture in V1, the image path ceases to be straightforward; there is more cross-connection within the visual cortex.

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. It lies at the rear of the brain (highlighted in the image), above the cerebellum. The interconnections between layers of the cortex, the thalamus, the cerebellum, the hippocampus and the remainder of the areas of the brain are under active investigation. Currently, much of what is known stems from patients with damage to known areas of the brain, with a corresponding study of the cognitive functions which have been spared.

Affecting the Parahippocampal Cortex Yield Spatial Memory Deficits in Humans", Cerebral Cortex, Vol. 10, No. 12, 1211-1216, December 2000

Zeineh et al., "Dynamics of the Hippocampus During Encoding and Retrieval of Face-Name Pairs", Science 2003 299: 577-580.

See also:Hippocampus, and the Fusiform gyrus in the temporal lobe of the cortex.