The Lateral Geniculate Nucleus

Optic nerve fibres from the eyes terminate at two bodies in the thalamus (a structure in the middle of the brain) known as the Lateral Geniculate Nuclei (or LGN for short). One LGN lies in the left hemisphere and the other lies in the right hemisphere.

[Projections to the LGN | LGN layers | LGN cell types ]

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Projections to the LGN

This picture shows the visual field at the top, divided into a left half and a right half, the two eyes in the middle, and the two LGNs as stacks at the bottom.

Follow the route taken by visual information in the left visual field (red area at the top - everything to the left of your point of fixation). You can see this part of the visual field with both the left eye (via its nasal or 'nose-side' retina) and with and the right eye (via its temporal or 'temple-side' retina).

The optic nerve fibres from the right eye's temporal retina arrive at the LGN on the same side of the head (ipsilateral), and the optic nerve fibres from the left eye's nasal retina cross over to the opposite side of the head (contralateral) to join them in the same LGN (coloured red). A similar partial crossing over (known as "partial decussation" in the jargon) happens to the optic nerve fibres carrying information about the right visual field (blue).

This apparently complicated arrangement is engineered so that the right LGN receives information about the left visual field, and the left LGN receives information about the right visual field.
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LGN Layers

An anantomic slice through the six layers of each LGN looks like this. This layered structure is exquisitely precise in two ways. First, cells in different layers that align (like the numbers in the picture) have receptive fields in the same area of retina. Second, optic nerve fibres from the two eyes are segregated in different layers. If you look carefully at the projections to the LGN, you will see that ipsilateral fibres arrive in layers 2, 3, and 5, while contralateral fibres arrive in layers 1, 4, and 6 (no-one knows why).
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LGN cell types

All cells in the LGN have concentric receptive fields, just like the ganglion cells whose fibres terminate in the LGN. Layers 1 and 2 are made up of cells with large bodies ("magnocellular") that have monochromatic responses (ie. mediate responses to light and dark), while layers 3 to 6 are made up of cells with small bodies ("parvocellular") that mediate colour vision.
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Created by George Mather, University of Sussex (.georgem@biols.susx.ac.uk)Some of the images and text used in these pages were originally developed at the Department of Psychology, York University, as part of the GRASP project.