Central Visual Pathways

The visual system | Main Anatomy Index | The cerebellum

Last updated 1 April 2006

Central Visual Pathways

• In addition, a considerable number of optic tract fibres project to the midbrain.
• A few also go to the hypothalamus.

• Throughout this pathway, the numbers of fibres and areas of representation for the macula are disproportionately large for the macula's actual size.
• This reflects the relatively small amount of convergence, which reflects its specialisation for high acuity.

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• The unmyelinated axons of ganglion cells collect at the optic disk.
• They pierce the sclera in a region called the lamina cribrosa.
• Here they acquire myelin sheaths, forming the optic nerve.

• The optic nerve is actually a tract of the CNS and has meningeal coverings.
• The sclera continues as its dural sheath, lined in turn by arachnoid and pia.

• The subarachnoid space around the optic nerve communicates with subarachnoid space generally.
• Increases in intracranial pressure are transmitted to the optic nerve.
• Such an increase in pressure can cause detectable swelling of the optic disk.
• This swelling is called papilledema, and can be a valuable diagnostic sign.

• This is where the 2 optic nerves partially decussate.
• It is located just anterior to the infundibular stalk.

• All fibres from the nasal half of each retina cross to the contralateral optic tract.
• All fibres from the temporal half of each retina pass through the lateral portions of the chiasm without crossing and enter the ipsilateral optic tract.

• The result of the partial decussation of the optic chiasm is that each optic tract contains:

1. The fibres arising in the temporal retina of the ipsilateral eye;
2. And the fibres of the nasal retina of the contralateral eye.

• This delivers all the information from the contralateral visual field to each optic tract.
• Also, much of the basis for depth perception involves a comparison of the slightly different views seen by our two eyes.
• The optic chiasm also brings together information from comparable areas of the 2 retinas.

• Fibres arising in corresponding areas of the 2 retinas are located near each other.
• This relationship continues throughout the remainder of the visual pathway.
• Damage to the optic tract or more central parts of the pathway tend to cause comparable visual deficits in both eyes.

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• The optic tract curves posteriorly around the cerebral peduncle, and most of its fibres terminate here.
• This is a 6-layered, dome-shaped nucleus.
• The optic fibres terminate in a precise retinotopic pattern.

• The pattern is about the same in each layer so that a given point in the visual field is represented in a column of cells extending through all 6 layers.
• Each layer receives input from only one eye:

• The contralateral eye: layers 1 (most inferior), 4, and 6 (most superior).

• The ipsilateral eye: layers 2, 3, and 5.

• Layers 3-6 contain small neurons that receive their inputs from the numerically dominant class of small ganglion cells sensitive to colour and form.
• These layers are referred to as the parvocellular layers.
• This entire subdivision of the visual system is the parvocellular system.

• Layers 1 and 2 contain larger neurons that receive their inputs from a separate class of larger ganglion cells that are more sensitive to movement and contrast.
• This subdivision, including the magnocellular layers (1 and 2) of the lateral geniculate nucleus, is referred to as the magnocellular system.

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• Fibres arising in the lateral geniculate nucleus project through the retrolenticular and sublenticular parts of the internal capsule.
• They curve around the lateral wall of the lateral ventricle and terminate in the cortex adjacent to the calcarine sulcus.

• The optic radiation is often called the geniculocalcarine tract.

• Not all of these fibres pass directly backward to the occipital lobe.
• Rather, they form a broad sheet covering much of the posterior and inferior horns of the lateral ventricle.
• Fibres representing superior visual quadrants loop out into the temporal lobe (Meyer's loop) before turning posteriorly.
• As a result, temporal lobe damage can produce a visual deficit.

• A retinotopic organisation is maintained in the optic radiation.

• The retinotopic organisation in the visual cortex:

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• Not only does the retina project to the lateral geniculate nucleus but also the superior colliculus.
• In lower vertebrates the collicular (or tectal) pathway is the more important, but in primates it is much less so.
• The main inputs to the superior colliculus are visual, one from the retina and the other from the primary visual cortex (striate cortex).

• The retinal input consists of a substantial number of fibres in each optic tract that bypass the lateral geniculate nucleus.
• They then pass over the medial geniculate nucleus in the superior brachium.
• It finally terminates retinotopically in the superior colliculus and in the nearby pretectal area.

• The cortical input consists of cells in area 17 that project to the superior colliculus (again via its brachium).
• It ends in a pattern that coincides with the retinotopic map in the colliculus.

• Efferent connections of the superior colliculus include projections to:

1. The reticular formation;
2. The inferior colliculus;
3. The cervical spinal cord (the tectospinal tract).

• Of interest the superior colliculus also projects to the posterior thalamus, notably to the lateral geniculate nucleus and the pulvinar.
• The pulvinar, in turn, projects to the visual association cortex.

• The function of the human superior colliculus is poorly understood.
• It is presumed to play a role in certain reflexes, such as orienting the head to visual (or other) stimuli, and in certain kinds of eye movements.

• There are, however, no known clinical conditions in humans attributed specifically to damage to the superior colliculus.

• These fibres provide photic input that is involved in many neuroendocrine functions of the hypothalamus.
• These fibres end in a small hypothalamic nucleus above the optic chiasm called the suprachiasmatic nucleus.
• There is considerable evidence that the suprachiasmatic nucleus of the hypothalamus is a "master clock" for the timing of many (but not all) circadian rhythms.

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• The processing of visual information begins at the level of the cells of the retina.
• Already at this level, the image is analysed for features that are related to detection of edges, movement and contrast.

• In general the parvocellular system (colour, detailed form) projects to more ventral portions of areas 18 and 19.
• The magnocellular system (location, movement) is to the more dorsal portions.
• These various qualities begin to be sorted out in the divisions of the lateral geniculate nucleus.
• However, the details are incompletely understood and the two systems are far from independent.

• Light directed into one eye cause both pupils to constrict.
• The response of the pupil of the illuminated eye is called the direct pupillary light reflex.
• That of the other eye is called the consensual pupillary light reflex.

• The patient seated in a dimly lit room.
• A light source is quickly moved back and forth from one eye to the other.

• For example, if the right optic nerve is damaged, when the left eye is illuminated, both pupils will constrict.
• When the right eye is illuminated, the light reflex arc will be less effectively activated, and both pupils will dilate.

• When visual attention is directed to a nearby object 3 things happen in a reflex manner:

1. Convergence of eyes, so that the image of the object falls on both foveae;
2. Contraction of the ciliary muscle and a resultant thickening of the lens, so the image of the object is in focus on the retina;
3. And pupillary constriction, which improves the optical performance of the eye by reducing certain types of aberration and by increasing its depth of focus.

• Unlike the pupillary light reflex, the near reflex requires the participation of the cerebral cortex.
• The pathway involved is poorly understood but is generally considered to follow:

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