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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