Central Visual Pathways
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The visual system | Main Anatomy Index | The
Last updated 1 April 2006
Central Visual Pathways
axons --> optic nerve --> optic chiasm (partial decussation) --> optic
tract --> lateral geniculate nucleus
(thalamic relay nucleus for vision) --> internal
capsule --> corona radiata --> primary visual cortex (calcarine sulcus)
- 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.
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
- 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:
- The fibres arising in the temporal retina of the ipsilateral eye;
- 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.
Lateral Geniculate Nucleus
The optic tract curves posteriorly
around the cerebral peduncle, and most of
its fibres terminate here.
This is a 6-layered, dome-shaped
The optic fibres terminate in a precise
- 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
- The contralateral eye: layers 1
(most inferior), 4, and 6
- 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.
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.
Retinotopic Organisation of the
A retinotopic organisation is maintained in the optic radiation.
||Location of Fibres
||Loop farthest into the temporal lobe
||Broad middle area
- The retinotopic organisation in the visual cortex:
||Part of Visual Cortex
||Above calcarine sulcus
||Below calcarine sulcus
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
- The retinal input consists of a substantial
number of fibres in each optic tract that bypass the lateral
- They then pass over the medial
geniculate nucleus in the superior brachium.
- It finally terminates retinotopically in the superior colliculus and in the nearby
- 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:
- The reticular formation;
- The inferior colliculus;
- 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
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.
Processing of Visual Information
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
- 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.
Pupillary Light Reflex
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.
Optic tract axons (afferent
limb) --> superior brachium --> pretectal
area (terminate) --> Edinger-Westphal nucleus
(bilaterally) --> CN III (as
preganglionic parasympathetic fibres) --> ciliary ganglion
--> postganglionic parasympathetic fibres (efferent
limb) --> sphincter pupillae muscle
The Swinging Flashlight Test
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.
Near Reflex (accommodation reflex)
When visual attention is directed to a nearby object 3 things happen in a reflex manner:
- Convergence of eyes, so that the image of
the object falls on both foveae;
- Contraction of the ciliary muscle and a resultant thickening of the lens,
so the image of the object is in focus on the retina;
- 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:
Normal visual pathway --> primary
visual cortex --> visual association cortex
--> superior colliculus and/or pretectal
area --> oculomotor
nucleus (medial rectus) and Edinger-Westphal nucleus
(preganglionic parasympathetic motor neurons)