The Lateral Geniculate Nucleus




(1)
University of Sydney, Sydney, Australia

 




Overview


The lateral geniculate nucleus (LGN) is located in the dorsal posterolateral thalamus.

1.

Function



  • All visual information for conscious perception travels through the LGN [1].


  • The LGN regulates the flow and strength of visual information sent to the visual cortex.


  • This regulation is influenced by extraretinal inputs to the LGN [2].


  • The LGN codes visual attention for the visual cortex [3].

 

2.

Connections (see Fig. 12.​3. The Optic Nerve)



  • Retinal ganglion cell (RGC) axons reach LGN neurons via the optic nerve, chiasm, then tract [4].


  • LGN neurons send axonal projections to the visual cortex via the optic radiations.

 


Structure (Fig. 13.1)




A347009_1_En_13_Fig1_HTML.gif


Fig. 13.1
Laminated structure of the lateral geniculate nucleus

The LGN consists of six layers that each receive monocular input [5].

1.

Characteristics of layers



  • Layers 2, 3, and 5 receive ipsilateral inputs.


  • Layers 1, 4, and 6 receive contralateral inputs [6, 7].


  • Each layer has a distinct population of neurons characterized by size: [8]

    (a)

    Layers 1 and 2 have large (magnocellular, M) neurons [5].

     

    (b)

    Layers 3–6 have small-medium (parvocellular, P) neurons

     

    (c)

    In between each layer exists koniocellular (K) neurons that are smaller than P cells [9].

     

 

2.

Visuotopic maps



  • Each layer represents a map of the contralateral visual hemifield [10].


  • The maps are aligned to one another; congruent points in visual space can be joined by a straight line passing through each layer (Fig. 13.1) [5].


  • Within each layer, the superior field is represented laterally and the inferior field medially [11].


  • The peripheral field is represented anteriorly and the central field posteriorly.


  • The central field is magnified compared to the periphery.


  • There are only two P layers in the peripheral field representation.

 


Projections to the LGN




1.

Retinal projections to the LGN (10–20% of LGN inputs) [12] (see Chap. 8. The Retina)



  • The M layers receive afferent fibers from M (parasol) retinal ganglion cells [13].


  • The P layers receive afferent fibers from P (midget) retinal ganglion cells.


  • The ratio of M:P retinal ganglion cell afferent fibers to the LGN is approximately 1:10 [13].


  • M, P, and K LGN layers receive inputs from multiple ganglion cell classes. The diversity is greatest for K layer cells [14].

 

2.

Extraretinal projections to the LGN (80–90% of LGN inputs)



  • The LGN receives extraretinal input from the:

    (a)

    Primary visual cortex [2]

     

    (b)

    Extrastriate visual cortex

     

    (c)

    Superior colliculus

     

    (d)

    Pretectal area

     

    (e)

    Thalamic reticular nucleus (TRN) [5, 15]

     


  • These modulate the flow of visual information to the visual cortex.


  • Inputs from the colliculus and pretectum mostly target the koniocellular layers.

 


Projections from the LGN






  • Most axonal output from the LGN terminates in the primary visual cortex (V1) [4].


  • A minority of axons terminate in the extrastriate cortex. These may be responsible for residual vision (or “blindsight”) in patients who have damaged V1 [16].


  • A significant proportion of LGN outputs terminate in the nearby TRN, which are involved in inhibitory feedback loops that influence visual signal modulation [17].


LGN Signal Processing




1.

LGN neural receptive fields

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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on The Lateral Geniculate Nucleus

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