Visual

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. 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

Optical layout of the eye 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.

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.

S. Ramón y Cajal, Structure of the Mammalian Retina, 1900

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.

The retina consists of a large number of photoreceptor cells which contain a particular protein molecule: the photopigment 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 cone opsin molecules attuned to specific wavelengths of light; i.e., a blue cone cell contains opsin 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

Information flow from the eyes (top), crossing at the optic chiasma, joining left and right eye information in the optic tract, and layering left and right visual stimuli in the lateral geniculate nucleus. V1 in red at bottom of image. (1543 image from Andreas Vesalius' Fabrica)

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 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

Six layers in the LGN

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. Layers four and six (4 & 6) of the LGN also connect to the opposite eye, but to the P cells 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. 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).

Optic radiations

Visual cortex: V1, V2, V3, V4, V5 (also called MT)

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 β. The M layer neurons relay to V1 layer 4C α. 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.

Hippocampus' location in the brain (red).

"Lesions 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#Role in spatial memory and navigation, and the Fusiform gyrus in the temporal lobe of the cortex.

See also

• Edinger-Westphal nucleus
• Memory-prediction framework
• Visual perception

References

• David H. Hubel (1989), Eye, Brain and Vision. New York: Scientific American Library.
• David Marr (1982), Vision: A Computational Investigation into the Human Representation and Processing of Visual Information. San Francisco: W. H. Freeman.
• R.W. Rodiek (1988). "The Primate Retina". Comparative Primate Biology Vol. 4 of Neurosciences. (H.D. Steklis and J. Erwin, editors.) pp. 203-278. New York: A.R. Liss.
• Matthew Schmolesky, The Primary Visual Cortex
• Martin J. Tovée (1996), An introduction to the visual system. Cambridge University Press, ISBN 0521483395 (References, pp.180-198. Index, pp.199-202. 202 pages.)
• Andreas Vesalius (1543) De Humanis Corporis Fabrica (On the Workings of the Human Body)
• Torsten Wiesel and David H. Hubel (1963), "The effects of visual deprivation on the morphology and physiology of cell's lateral geniculate body". Journal of Neurophysiology 26, 978-993.