Lateral Geniculate Nucleus - LGN Output

LGN Output

Information leaving the LGN travels out on the optic radiations, which form part of the retrolenticular limb of the internal capsule.

The axons that leave the LGN go to V1 visual cortex. Both the magnocellular layers 1-2 and the parvocellular layers 3-6 send their axons to layer 4 in V1. Within layer 4 of V1, layer 4cβ receives parvocellular input, and layer 4cα receives magnocellular input. However, the koniocellular layers (in between layers 1-6) send their axons to layers 4a in V1. Axons from layer 6 of visual cortex send information back to the LGN.

Studies involving blindsight have suggested that projections from the LGN not only travel to the primary visual cortex but also to higher cortical areas V2 and V3. Patients with blindsight are phenomenally blind in certain areas of the visual field corresponding to a contralateral lesion in primary visual cortex; however, these patients are able to perform certain motor tasks accurately in their blind field, such as grasping. This suggests that neurons travel from the LGN to both the visual cortex and higher cortex regions.

Steamboat Jim (talk) 23:01, 26 November 2012 (UTC)==Function in visual perception== The functions of the LGN are multiple. Its unigue folding contributes to its utility by performing a range of anatomical calculations without requiring mathematical computations. These include both temporal correlations/decorrelations as well as spatial correlations. The resulting outputs include time correlated and spatially correlated signals resulting from summing the signals received from the left and right semifields of view captured by each of the two eyes. These signals are correlated in order to achieve a three-dimensional representation of object space as well as obtain information for controlling the precision (previously auxiliary) optical system (POS) of the visual modality.

The outputs serve several functions.

• A signal is provided to control the vergence of the two eyes so they converge at the principle plane of interest in object space.

• A signal is provided to control the focus of the eyes based on the calculated distance to the principle plane of interest.

• Computations are achieved to determine the position of every major element in object space relative to the principle plane. Through subsequent motion of the eyes, a larger stereoscopic mappping of the visual field is achieved.
  • A tag is provided for each major element in the central 1.2 degree field of view of object space. The accumulated tags are attached to the features in the merged visual fields forwarded to area 17 of the cerebral cortex (often described as the "primary" visual cortex or V1)
  • A tag is also provided for each major element in the visual field describing the velocity of the major elements based on its change in coordinates with time.
  • The velocity tags (particularly those associated with the peripheral field of view) are also used to determine the direction the organism is moving relative to object space.

These position and velocity tags are extracted prior to the information reaching area 17. They costitute the major source of information reported in blindsight experiments where an individual reports motion in a portion of the visual field associated with one hemisphere of area 17 that has been damaged by laceration, stroke, etc.

The output signals from the LGN determine the spatial dimensions of the stereoscopic and monoscopic portions of the horopter of the visual system.

It has been shown that while the retina accomplishes spatial decorrelation through center surround inhibition, the LGN accomplishes temporal decorrelation. This spatial-temporal decorrelation makes for much more efficient coding. However, there is almost certainly much more going on.

Like other areas of the thalamus, particularly other relay nuclei, the LGN likely helps the visual system focus its attention on the most important information. That is, if you hear a sound slightly to your left, the auditory system likely "tells" the visual system, through the LGN via its surrounding peri-reticular nucleus, to direct visual attention to that part of space. The LGN is also a station that refines certain receptive fields. Experiments using fMRI in humans reported in 2010 that both spatial attention and saccadic eye movements can modulate activity in the LGN.