Sight or visual perception is the function of the optic pathway, which describes the journey that nerve impulses take from the retina to the occipital cortex.
The entire pathway, made up of the retina, the optic nerve, chiasm and tracts, the lateral geniculate bodies, the optic radiation and the visual (striate cortex), can be regarded as part of the central nervous system.
Impulses are transmitted from the photoreceptors in the retina (rods and cones) to bipolar cells which then synapse to the ganglion cells. Axons of the ganglion cells converge at the optic disc to form the optic nerve. The nerve passes posteriorly from the eyeball surrounded by the three meningeal layers (pia, arachnoid and dura matter). It leaves the orbit to reach the chiasm, through the optic canal which lies within the lesser wing of the sphenoid bone.
The optic chiasm lies in the floor of the third ventricle, directly above the pituitary gland. Nerve fibres from the nasal half of each retina cross the midline and enter the tract at opposite sides. Nerve fibres from the temporal half of the retina pass on the same side.
The two tracts exit the chiasm extending backwards towards the lateral geniculate body (LGB), a sensory relay nucleus in the thalamus. 90% of the optic nerve fibres which terminate here are involved in conscious visual sensation. The other 10% do not enter the LGB but pass medially into the midbrain to enter the superior colliculus (fibres involved in the visual body reflex) and the pretectal nucleus (fibres involved in the pupillary light reflex). In the LGB axons of the retinal ganglion cells synapse with cell bodies of neurones forming the optic radiation.
Axons of the optic radiation transmit the impulses from the LGB to the visual cortex. Information from the superior portion of the visual fields is carried in fibres which sweep around the inferior horns of the lateral ventricles in the temporal lobe (Meyer's loops). Those of the inferior portions of the visual field travel in the parietal lobe and loop very little.
The visual cortex is the last part of the optic pathway and processes the information received from the retina. A large portion of it is occupied by the area which corresponds to the macula lutea of the retina. The cortex is supplied by the posterior cerebral artery, but the areas associated with the macula are also supplied by the middle cerebral artery.
The visual fields of each eye overlap considerably and it is in this overlapping region where the binocular vision occurs. An object in the binocular visual field can be seen with both eyes.
Light rays from an object in the temporal half of the visual field falls on the nasal half of the retina while light rays from an object in the nasal half of the visual field falls on the temporal half of the retina. Thus, after the cross over in the chiasm, visual information from the right half of the visual field is conveyed to the left side of the brain and vice versa.
Unilateral field loss (A): caused by damage to one optic nerve due to trauma, ischaemia or inflammation.
Bitemporal hemianopia (B): can occur with damage to the optic chiasm. This is commonly due to compression from pituitary adenomas, craniopharyngiomas or suprasellar meningiomas. An aneurysm of the anterior communicating artery can also compress the chiasm from above.
Homonymous hemianopia (C and D): usually seen in stroke patients but can also be due to trauma or tumours affecting the optic tract or radiation. If there is macula sparing (G) then the lesion is more likely to be in the occipital cortex due to its greater representation there as mentioned above.
Quadrantanopia: upper quadrantanopias suggest a lesion in the temporal lobe affecting Meyer's loop while lower quadrantanopias suggest a lesion in the parietal lobe. The most common cause of both are cerebrovascular disorders.
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Purves D., Augustine J.G., and Fitzpatrick D. (Eds) (2004), Neuroscience, 3rd Ed. Sinuaer Assc, Inc, MA pp 259-269
Figure 1&4: Purves D., Augustine J.G., and Fitzpatrick D. (Eds) (2004), Neuroscience, 3rd Ed. Sinuaer Assc, Inc, MA pp 259-269
Figure 2: J Simon (2010), From eye to brain: Researchers map functional connections between retinal neurons at single-cell resolution, Salk Institure http://www.physorg.com/news205589653.html)
Figure 3:John Wiley & Sons inc http://eyeoptic.wikispaces.com/file/view/optic-pathway.gif/225278604/optic-pathway.gif
Figure 5: J L Fitzaerleysity (2007) Lectures: Visual Pathways, University of Minnesota Duluth http://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Figures/VisualPathways.jpg
Figure 6 (adapted): Factsheet: Visual Field Loss (2007), Vision Australia http://www.visionaustralia.org.au/info.aspx?page=1516
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