The lateral geniculate nucleus (LGN) is part of the thalamus that serves as the primary center for processing visual information. It could be thought of as the main relay center from the retina to the part of the cerebrum that integrates and responds to visual stimuli. In humans and other mammals, the visual projection to the lateral geniculate nucleus is one of the two strongest pathways that link the eye to brain. Damage to the LGN may result in some degree of blindness.
Genu is the Latin term for knee. By virtue of its knee-like appearance, the lateral geniculate nucleus was named as such. It is situated on the dorsum of the thalamus, making it part of the central nervous system. The right and left hemispheres of the brain have one lateral geniculate nucleus.
When examined microscopically, the cell bodies in the LGN reveal a layer cake-like arrangement, with the neuronal cell bodies serving as the cake, and the neuropil serving as the icing. The neuropil is a type of gray matter that has nonmyelinated nerve fibers and relatively sparse cell bodies. Typically, the lateral geniculate nucleus has six different layers. The inner two layers are named the magnocellular layers, while the outer four layers are called parvocellular layers.
Magnocellular layers have large cells called magnocellular (M) ganglion cells that receive input from the peripheral retina, and the parvocellular layers have small cells called parvocellular (P) ganglion cells that receive input from the central retina, or fovea. While P ganglion cells are color sensitive and can perceive a high level of detail, M cells are color and detail insensitive. Although M cells are weak in detecting color and detail, they are highly sensitive to motion.
The retina of each eye projects neurons to the lateral geniculate nuclei of both the right and left hemispheres. Each LGN, however, only gets information from one-half of each visual field. This is because of the crossing-over, or decussation, of the axons from the inner or nasal sides of the retinas. The axons from the outer or temporal halves of the retinas remain on the same side.
Visual information from the lateral geniculate nucleus projects to the visual cortices of the two hemispheres as the optic radiation. The left half of the visual field is perceived by the right hemisphere, while the right half is perceived by the left hemisphere, due to the decussation mentioned above. When the signals reach the primary visual cortex located in either hemisphere, the brain is able to reconstitute the image gathered by the cells of the retina. Through the signal transmission from the retina to the LGN to the visual cortex, humans are able to experience conscious visual perception.