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ANAT 321
Dr. Brawer
November 26, 2010
Lecture 22
ANAT 321
Lecture 22 – Visual System 2
Dr. James Brawer– November 26, 2010
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Transmission of Visual Information
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Two rules of thumb:
o Wiring of visual system (from retina to cortex), top is top, bottom is bottom, left is left
and right is right
o Retinal image is upside down and reverse of reality
 Upper left visual field will end up on lower right retina
Far left visual field ends up on far right retina of both eyes
o Upper visual field ends up on lower retina
o This is due to the lens, which inverts images
All axons from the ganglion cells in the left eye condense at the optic disk (blind spot) and
contain fibers from both temporal (lateral) and
nasal (medial) retina
o All axons enter the optic nerve (really a
tract)
o Optic nerve leaves retina and nasal fibers
will cross over to other side at optic
chiasm, while temporal fibers keep going
ipsilaterally
o After chiasm both right visual fields are on
the left post-chiasmatic optic tract
Optic chiasm is purely composed of nasal fibers
Lesion in left optic tract  loss of right visual field
in both eyes
Lesion in optic chiasm  loss of temporal visual
field in both eyes (tunnel vision)
o Medial retinas are denervated, so lateral
visual fields are gone
Lesions on the left side can occur near the optic
chiasm where only the temporal fibers are affected
(after left nasal fibers split off, but before right nasal fibers join up)
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ANAT 321
November 26, 2010
Dr. Brawer
Lecture 22
o This will cause loss of loss of right visual field in left eye only
 Optic tracts end up in the lateral geniculate nucleus in the thalamus, which relays to cortex on the
same side  left geniculate nucleus receives entire right visual field
o Optic tract also gives connections to the superior colliculus (some collaterals from axons
going to the Lateral Geniculate, while others are direct axons from the retina)
o Not very clear what the superior colliculus does because you can have damage with no
detectable visual defects
 Optic tract is very compact, whereas the optic radiations (projections from the lateral
geniculate) are extremely spread out  has consequences for lesions and normal vision
Lateral Geniculate Body and Primary Visual Cortex
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A layered structure  has 6 layers
o Layers 6, 4, and 1 receive fibers from contralateral eye
o Layers 5, 3, and 2 receive from ipsilateral eye
o Layers are retina-specific
o Outer 4 layers are parvocellular (small cells)
o Inner 2 layers are magnocellular (large cells)
Parvocellular layers receive information primarily about
colour and form
o We expect these layer to receives axons mainly from
retinal ganglion cells in and around the fovea (where
most cones are)  even though fovea is so small, it
takes up a large area in the lateral geniculate
o This is why we only have focus in a very small area in
the center of the retina  if we had more, we would
require an extensive amount of thalamic volume
 We compensate for a small region of focus by
constantly moving our eyes (we are not conscious of these movements)
o Macular degeneration: a neurodegenerative disease of age when macula/fovea of the
retina degraded  leads to functional blindness
Magnocellular layers receive information primarily about movement and contrast
Therefore, differential processing of visual information begins occurring at the level of the lateral
geniculate nucleus (before even reaching the cortex)
The lateral geniculate nucleus projects to the primary visual cortex, which is in the banks of the
calcarine fissure on the medial surface of the occipital lobe
The primary visual cortex has four quadrants: upper- and lower-right, upper- and lower-left
o Upper-right quadrant receives the upper-right retinas of both eyes and lower-left part of
the visual field of both eyes
o Lesions to the entire right visual cortex would lead to a loss of left visual fields in both
eyes
Primary visual cortex is organized in a retinotopic order
o Retina is sequentially represented in primary visual cortex
o Foveal regions (central regions of retina) end up most posterior in the primary visual
cortex; peripheral regions of retina end up more anterior
o Foveal sparing: lesions to occipital lobe often spare the macular region (unsure why)
Pathway from lateral geniculate to primary visual cortex is very diffuse
o Fibers from upper part of the nucleus fan out towards the upper quadrant in the cortex,
whereas fibers from lower part of nucleus fan out to the lower quadrant
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ANAT 321
November 26, 2010
Dr. Brawer
Lecture 22
o Lesion to temporal lobe may affect these radiations, and can affect the visual field
o Optic radiations that are more ventral project into the temporal lobe and loop around to
the lower bank of the calcarine fissure (this is called Meyer’s loop)
 Visual cortex receives information that has been processed in the retina and lateral geniculate
body, and then dissects and distribute information to various cortices
o Visual association cortex: region of cortex surrounding the primary visual cortex
o When we see a red car driving by, the visual cortex will split up this input into colour
(red), form (car) and movement and send it to various regions of association cortex
 Somehow, our brains have to later consolidate this information
o Bilateral lesions in particular spots of the visual association cortex can lead to loss of the
ability to perceive movement (sees movements as snap shots)  no problem with
identifying objects
 These visual cortices are plastic to some degree
 Prosopagnosia: inability to recognize faces
Superior Colliculus
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It receives projections from the retina, but also from the spinal cord
o Some axons in the anterolateral column (spinomesencephalic tract) end up in superior
colliculus and periaqueductal gray  can be stimulated by pain in foot or hand
Also receives input from primary visual cortex
Superior colliculus gives of a tract (tectospinal tract) that crosses the midline, stay medial and
stems down to the spinal cord
o Involved in visual reflexes (adjustments of head and shoulder)
Blindsight: destruction of visual cortex causes absolute blindness
o But, if put in front of a computer with a yellow dot moving around the screen, they can
use their finger to follow the dot with their trajectory
o This is likely mediated by the superior colliculus (which plays a big role in movement
perception in lower animals)
o Superior colliculus talks to the cortex via the thalamus
 Pulvinar nucleus receives information from the superior colliculus
 Pulvinar widely distributes to visual association cortices around the occipital lobe
Visual Reflexes
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Consensual light reflex: light-to-dark reflex
o Pupils must constrict to restrict the amount of
incoming light
o Light enters the eye and hits the retina
o Retina projects to the pretectum (neurological
tissue anterior to the superior colliculi, which are
in the tectum)
o Pretectum projects bilaterally to nucleus of
Edinger-Wesphal (visceral III)
o Nucleus of Edinger-Wesphal projects to the
ciliary ganglion, which projects to ciliary body
and iris  causes sphincter to contract
o Stimulating only one eye should cause an equal
reflex in both eyes
o Reflex occurs even if cortically blind
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ANAT 321
November 26, 2010
Dr. Brawer
Lecture 22
 Accommodation reflex: far-to-near vision
o Lens must fatten to increase refraction; pupil constricts slightly
o Ciliary body must contract to take tension off of the lens
o This reflex requires a cortex
o We follow the visual pathway through the lateral geniculate to the visual cortex
o Visual cortex projects to the superior colliculus (both colliculi talk to each other)
o Superior colliculus projects to nucleus of Edinger-Wesphal, which projects to ciliary
ganglion
o Ciliary ganglion projects to eye to cause accommodation
o Cortically blind individuals cannot do this reflex
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