Chapter 7: Visual physiology

The visual pathway (pp. 192—196)

Ask Yourself

What you need to know

  1. Destinations of optic nerve fibres (pp. 192—193)
    • Lateral geniculate nuclei
    • Superior colliculi
    • Suprachiasmatic nuclei
    • Pretectum
    • Pregeniculate
    • Accessory optic system
  2. The geniculo-striate pathway (pp. 193—196)
    • Representation of the visual field
    • Structure of the LGN
    • Signal processing in the LGN

Destinations of optic nerve fibres

The fibres of retinal ganglion cells form the optic nerve, which terminates at six locations in the brain in vertebrates. In primates the major projection (90% of ganglion cell fibres) is to the lateral geniculate nuclei (LGN, also called dorsal LGN or LGNd), with the remaining 10% projecting to other subcortical nuclei (Perry et al., 1984). The destinations of optic nerve fibres are summarised below:

The geniculo-striate pathway

In primates the two eyes face forwards (see Fig. 7.1, FP p. 180), so an object will project images onto the retina of both the right eye and the left eye. Binocular neurons in the visual cortex receive signals from both eyes and compare the images from the left and right eyes. Binocular convergence is achieved through partial decussation. Fibres originating in the left visual field of each eye converge on the right hemisphere, and fibres originating in the right visual field converge on the left hemisphere.

Research Activity: Damage to the visual pathway

The LGN has six major layers, numbered from 1 to 6. Layers 1 and 2, the magno layers, contain large cell bodies and receive projections from parasol ganglion cells. Layers 3 to 6, the parvo layers, contain small cell bodies and receive projections from midget ganglion cells. Konio layers are sub-layers of the LGN magno and parvo layers that contain the smallest cell bodies and receive projections from bistratified ganglion cells. Cells in each layer receive inputs from the same eye, and are arranged topographically (see Fig. 7.10, FP p. 195). Three major layers receive inputs from the ipsilateral eye, and the other three receive inputs from the contralateral eye. Retinal input accounts for only 10% of the input to the LGN.

The LGN appears to sharpen centre–surround responses, to promote synchronous activity of ascending signals, and to emphasise changes in stimulation (Sillito & Jones, 2002).

So What Does This Mean?

The fibres of retinal ganglion cells form the optic nerve, which carries responses from the retina towards the brain. In mammals, 90% of optic nerve fibres terminate in the lateral geniculate nuclei (LGN), forming the geniculo-striate pathway. The remaining 10% of fibres terminate at a number of sub-cortical nuclei. Each LGN contains four magnocellular layers (direct projection from parasol ganglion cells), and two parvocellular layers (projections from midget cells), each separated by a koniocellular layer (projections from bistratified cells). LGN cells project to V1. Overall, the LGN appears to modulate the flow of information from the retina to visual cortex.

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