Chapter 7: Visual physiology
The visual pathway (pp. 192—196)
- How does the construction of the visual pathway relate to our experience of depth perception?
- Why does brain injury affecting left striate cortex result in a loss of part of the visual field in both eyes?
- Although the majority of fibres in the visual pathway project to LGN, what is the functional significance of the remaining 10% of fibres?
What you need to know
- Destinations of optic nerve fibres (pp. 192—193)
- Lateral geniculate nuclei
- Superior colliculi
- Suprachiasmatic nuclei
- Accessory optic system
- 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:
- Lateral geniculate nuclei receive the major projection from the retina in primates (see Fig. 7.1, FP p. 180), and then project to V1 (striate cortex). These signals are thought to be important for conscious visual awareness.
- The superior colliculi also receive major projections from the retina. They are thought to be the most important projection site in many non-primate species, with roles in integrating visual and auditory signals, and in directing visual attention.
- The suprachiasmatic nuclei form part of the hypothalamus, and project to the pineal gland, regulating the brain's "biological clock".
- The pretectum is thought to have a role in the regulation of pupil diameter.
- The pregeniculate is adjacent to the LGNd and probably corresponds to the ventral LGN (LGNv) of other vertebrates. It does not project to cortex, and may have a role in regulation of the biological clock.
- Some nerve fibres branch away from the main tract and terminate in midbrain nuclei that form the accessory optic system, which is thought to have a role in stabilising the retinal image (Rodieck, 1998).
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
Related Site: More information
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.
Related Site: More information about 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.