The size of a peanut, situated either side of the rear end of the thalamus, it receives most of its input from the contralateral half of the visual field of the retina via the optic nerve that crosses at the optic chasm, and relays it to primary visual cortex from which it also receives inputs (see figure below for further details). It has six layers of cells: three receive inputs from the left (L) eye that alternate with three from the right (R) eye. In the left geniculate, they ordered LRLRRL and RLRLLR in the right geniculate. The layers contain two types of cells: those in layers 1 and 2 are large and termed magnocellular (M), while those in layers 3, 4, 5, and 6 are smaller and termed parvocellular (P). In between the M and P layers is a zone of smaller cells, which is made up of interlaminar, or koniocellular (K), layers. Providing a third input to the primary visual cortex, K cells are functionally and neurochemically distinct from M and P cells. The lateral geniculate is recognized as a key ‘way station’ along the route that visual signals follow in that it contains inhibitory circuits that selectively turn individual signals off and regulate exactly which visual information is transmitted to the primary visual cortex for processing. Surprisingly, the function the lateral geniculate nucleus remains unclear. It is possibly involved in color perception, and may assist the visual system to focus attention on salient information (e.g., hearing a sound on the right of body, the auditory system possibly informs the visual system via the lateral geniculate to direct attention to that part of space). In this respect, it may be involved in intersensory coordination.
