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NS Chapter 11: Visual System

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In terms of sensory modalities and portion of the brain, what is the brain mostly dedicated to?
Vision
Pathway of the visual system. Where does it start and where does it end?
Retina-> lateral geniculate nuclei of the thalamus-> primary visual cortex (PVC) of the occipital lobe
What types of disorders do lesions of the visual association cortex cause?
Higher order visual processing disorders.
How is an image formed on the retina?
Light enters the eye through the lens to form image on the retina (that is inverted and reversed)

Superior visual field projects onto upper retina

Right visual field projects to left side of the retina of each eye, and thus the left visual field projects to the right side of the retina of each eye.
Normal visual field: how far does it extend temporally, nasally, superiorly, and inferiorly?
Temporally: 80-90 degrees temporally
Nasally: 50-60
Superiorly: 50-60 degrees
Inferiorly: 60-75 degrees inferiorly
What is the fovea? What is it surrounded by?
Fovea- is the central fixation point for each eye.

Is the area of the retina with the highest visual acuity.

Corresponds to central 1 to 2 degrees of visual space.

It is surrounded by the macula, which is also has high visual acuity and covers the central 5 degrees of visual space.
What is represented on the fovea is projected to where? What will happen if central vision is injured?
It is projected to the occipital pole. If central vision is injured, only will be able to see peripherally.
What is the optic disc formed by? Are there photoreceptors on the optic disc?
Optic disc is formed by axons leaving the retina where they enter the optic nerve.

No photoreceptors over the optic disc. Thus small blind spot is formed here which is located about 15 degrees lateral and slightly inferior to the central fixation point of each eye.

No functional deficit when both eyes are used.

When 1 eye is used, our visual system seems to fill in the blind spot.
The retina has 3 layers? Name the three layers
1. Photoreceptor (outermost layer)
2. Bipolar cell layer
3. Ganglion cell layer (innermost layer)
The photoreceptor layer- what does it contain? What do the two components of this layer function to do?
Outermost layer of the retina
Rods and cones

Rods- can pick up movement in the dark (do not pick up bright)
*for vision in low level lighting conditions
* low resolution
*do not detect color
*outnumber cones by 20:1

Cones (important in detecting color & resolution)
*high resolution
*highly represented in fovea where acuity is highest
*detect color
Second layer of the retina aka middle layer is called? Where does it receive input from?
The second layer is called bipolar cell layer.
Receive input via synapses from photoreceptors.
Third layer of ganglion cell layer- innermost, what is it called?
Where does it receive input from?
What are the two cell types?
Receive input from bipolar cells and sends axons into the optic nerve.

Fire action potentials.

Ganglion cell types:
2 types
-parasol cells: respond to gross stimulus features and movement, large cells bodies, large receptive fields.
- their large diameter fibers project to magnocellular layers of the LGN of the thalamus.

Midget cells
- respond to fine visual detail and colors, small cell bodies, small receptive fields
-their diameter fibers project to the parvocellular layers of the LGN of thalamus
Optic Nerves, Optic Chiasm, and Optic Tracts. What are the differences between on center and off center receptive fields?
*receptive field of a neuron in the visual pathway is defined as the portion of the visual field light causes excitation or inhibition
*many neurons in the visual pathways have center- surround recepetive fields, which are either on center or off center
*off center- receptive fields inhibited by light in the center of their receptive field and excited by light in the surrounding area
* on center- excited by light in the center of their receptive field and inhibited by light in the surrounding area.
*there are neurons with more sophisticated receptive field properties to further respind to selective orientations of stimulation from light.
Where does the optic nerve receive input from and where does it exit?
receives input from retinal ganglion cell axons (part of the innermost layer) and exits in optic canal
The optic chiasm, what is it? The nasal retinal fibers are responsible for which fields of vision?
Optic Chiasm is the partial crossing of fibers where the optic nerves meet
-nasal (medial) retinal fibers crossover in chiasm
-thus, nasal (medial) fibers are responsible for temporal (lateral) hemifields of vision.
The left hemi- retinas of both eyes end up in the left optic tract, and R hemiretinas of both eyes end up in R optic tract. Thus, ipsilateral hemiretinas represent contralateral or ipsilateral visual fields?
Contralateral visual fields.
For example R hemiretinas represent the left visual field.
If one has a lesion of the eye, retina, or optic nerves, what is the visual field defect?
It will produce monocular vision field defects
Lesions of optic chiasm therefore often produce what types of visual field defects?
Bitemporal visual field defects, i.e bitemporal hemianopsia
Because of the cross over in the chiasm, lesions posterior to the chiasm (optic tracts, LGN, optic radiations, or visual cortex) will produce what type of visual field deficits?
Will produce homonymous visual field defects.

Visual field defect occuring in the same portions of the visual field for each eye.
What is contralateral superior quadrantantanopia? What is contralateral inferior quadrantanopia?
Visual field of the same side of both eyes is affected (aka pie in the sky). Contralateral inferior quadrantanopia is pie on the floor, same side of both eyes is also affected.
Optic tracts-> mid brain (lateral side)-> LGN of the thalamus

axons of retinal ganglion cells traveling in optic tracts synapse on neurons in LGN which then project where?
Project to the primary visual cortex.
What are the names of the different layers of the lateral geniculate nucleus?
Layers 1-2 called the Magnocellular Layers (ventrally located) and Layers 3-6 which are called the Parvocellular layer (dorsally located).
The magnocellular layer of the lateral geniculate nucleus is responsible for what?
Relays information from parasol cells of the retina (motion and spatial analysis)
The parvocellular layers of the lateral geniculate nucleus is responsible for what?
Relays information from midget cells of the retina (detailed form and color)
Does information from the left and right eyes remain segregated even after passing through LGN?
Yes because axons from left and right retina synapse onto different layers of the LGN. Eg. Right LGN= Right hemiretinas of both eyes=left visual field
What are the extrageniculate pathways?
They are a minority of fibers in optic tract that bypass the LGN to enter the supperior colliculus and the pretectal areas to form the extrageniculate visual pathways.
The pretectal areas is involved in which reflex?
Pupillary light reflex (direct response of vasoconstriction and consensual vasodilation of the opposite eye)
Pretectal area and the superior colliculus are involved in which functions?
Involved in directing visual attention and eye movements toward visual stimuli (via projections to brainstem and lateral parietal cortex and frontal eye fields).
What are optic radiations?
Axons leaving LGN sweep over and around the lateral ventricle to project back to PVC (primary visual cortex)

These axons fan out over a large area as they project back to PVC

Axons from ipsilateral and contralateral retinal layers of LGN are intermingled in optic radiations cause homonymous defect affecting the contralateral visual field.
Slide 23 Ipsilateral optic radiations carry info from contralateral or ipsilateral visual fields? Lesions of the ipsilateral radiations cause what type of visual field defect.
They carry information from contralateral visual fields. Lesions of the ipsilateral radiations would cause a contralateral homonymous hemianopia visual field defect.
What forms Meyer's Loop? Inferior optic radiations carry information from where?
Temporal lobe lesions cause what type of defect?
Fibers of the inferior optic radiations arc forward into the temporal lobe forming Meyer's loop.

Inferior optic radiations carry information from inferior retina (superior visual field)

Temporal lobe lesions cause contralateral homonymous superior quadrantopia "pie in the sky" visual field defect.
Superior optic radiations carry information from where? What type of defect does a lower parietal lobe lesion cause?
Superior optic radiations carry information from superior retina (or inferior visual field)

Lower parietal lobe lesion causes contralateral homonymous inferior quadrantanopia (pie on the floor, visual field defect).
Primary visual cortex is which brodmann's area. Secondary and tertiary visual cortices are which brodmann's areas? Where does primary visual cortex lie in relation to upper and lower banks?
Brodmann's area 17. Secondary brodmann's area 18 and tertiary 19. Primary visual cortex lies on the upper and lower banks (both sides) of the calcarine fissure in the occipital lobe (medial view).
What is the latin name of the upper bank called?
What is the latin name of the lower bank called?
Cuneus ("wedge")-upper bank/gyrus
Lingula ("little tongue")-lower bank/gyrus
Superior optic radiations project to the upper or lower bank of the calcarine fissure? What kind of defect does an upper bank lesion cause?
upper bank of the calcarine fissure. Upper bank lesion causes contralateral homonymous inferior quadrant defects.
If one has a lesion to the right upper bank of the primary visual field defect?
Left inferior quadrantanopia (aka pie on the floor).
Inferior optic radiations project to where? Lower bank lesions cause which types of defects? Lesion to the right lower bank of the primary visual would cause what visual field defect?
Inferior optic radiations project to the lower bank of the calcarine fissure. Lower bank lesions cause contralateral superior quadrant defects. Left superior quadrantopia (aka pie in the sky)
How are the fovea and macula represented at the occipital pole?
Fovea and macula are represented at the occipital pole. The fovea occupies about 50% of primary visual cortex, as the fovea contains the highest density of photoreceptors and has corresponding high visual acuity.
Thus the fovea and macula have a disproportinate cortical representation despite the small retinal area.
The three parallel channels that process visual input process which aspects of vision?
The three parallel channels process
1)analysis of motion
2) form
3) color
What is the pathway of travel for motion/spatial analysis?
parasol cells-> magnocellular layers-> primary visual cortex (layer 4 C alpha to layer 4B)-> Visual association cortex (area 18)-> Higher order visual association cortex (dorsal parieto-occipital cortex).
Dorsal pathways of analyzing motion, form and color answer what types of questions and dorsal pathways projects to where?
Dorsal pathways project to the parieto-occipital association cortex. Pathway answers the question of where by analyzing motion and spatial relationships between objects as well as between body and visual stimuli.
Ventral pathways project to where?
Ventral occipotemporal association cortex. Pathway answers the question of "What?" by analyzing form, with specific regions identifying colors, faces, letters, and other visual stimuli.
Assessment of Visual Disturbance
Two major steps needed:
1) Nature of the visual disturbance
-time course and positive phenomena (e.g. bright colored lights) or negative phenomena (regions of decreased vision) present.

2) Visual field
Description of the regions for each eye involved
How is visual acuity reported?
Visual acuity is often reported with Snellen notation of 20/x. "x" is distance which a normal individual can see the smallest line of the eye chart seen at 20 feet.

Visual field defects usually do not affect visual acuity.
In assessment of visual disturbances what is one of the most important things to do?
Is to distinguish whether it is a monocular or binocular visual disturbance.

pts often describe a disturbance as being in one eye in reality the left or right visual field is affected for both eyes.

Blurred vision is hard to interpret without further description and examination; it could be attributed to a variety of things, from a corneal problem/dz to a visual cortex lesion.
What is the difference between a positive and negative phenomena?
Positive phenomena includes two types. Simple phenomena and formed phenomena.

Simple phenomena includes lights, colors, or geometric shapes caused by disturbances located anywhere from eye to cortex.

Formed phenomena include people, animals, or complex scenes, arising from inferior temporal-occipital visual association cortex

Negative phenoemna- region of vision that a person cannot see, including scotoma or a homonymous defect,
Formed visual hallucinations arise from what part of the cortex? What are some causes of visual hallucinations?
Formed visual hallucinations arise from the inferior temporo-occipital visual association cortex. Can be from many causes: toxic or metabolic disturbances (drugs), withdrawal from alcohol or sedatives, focal seizures, complex migraine, neurodegenerative conditions, psychiatric disorders.

Formed hallucinations can also appear as release phenomena:
patients with visual deprivation in all or part of their visual fields caused by ocular or CNS lesions may report seeing objects, people, or animals in the region of vision loss, especially in the early stages of deficit.
What does visual field testing test for?
Tests for crude deficits in the visual fields.

Confrontation testing at bedside.

Test each quadrant while making sure patient's eyes stay centrally fixated.

Test each eye using wiggling fingers and having patient count number of fingers being held up.

Fields recorded as if viewing own visual field.

Blink to threat can be used on lethargic/uncooperative patients.
What causes monocular scotoma?
Can be from lesion of the retina.
Area of retina damage would reflect damage to the associated visual field.
Location, size, and shape will vary depending on location and the extent of the lesion.

Causes: retinal infarcts, hemorrhage, degeneration, and infection.

Could also be from incomplete damage to optic nerve (e.g. trauma, or small accidental cut of nerve during surgery in area of optic nerve).
What can cause monocular vision loss?
Can be from complete lesions of the optic nerve.
-all fibers of the optic nerve of the respective eye are carrying information from the entire retina of the eye.
-causes: glaucoma, optic neuritis, neuropathy, elevated ICP, tumors, and trauma
- a lesion that is severe enough to involve the entire retina would also produce a monocular vision loss.
*entire retina= all photoreceptors for that eye.
For bitemporal hemianopia, what area tends to be damaged? What is the chiasm made up of?
There tends to be damage to the optic chiasm.

Chiasm is made up of nasal (medial) retinal fibers which carry info for temporal (lateral) visual fields.

Causes: pituitary adenoma, hypothalamus glioma, other misc. tumors

Optic chiasm lies just in front of pituitary gland thus making it susceptible pituitary tumors or other lesions in this area.
Contralateral homonoymous hemianopia, where is the lesion?
Can be from lesions of the optic tracts (D).

Optic tract carries information for the contralateral visual field.

For example, right optic tract carries information for the left visual field.

Optic tracts: behind the chiasm where nasal (medial) retina fibers for each eye (responsible for temporal (lateral) hemifields) have crossed over to the other side.

Causes: tumors, infarct, demylelination

Lesions to LGN also can cause contralateral homonymous hemianopia.
What are the three possible causes of contralateral homonymous hemianopia?
1) lesions of the optic tracts caused by tumors, infarct or demyelination
2) lesions of the entire optic radiations caused by infarcts, tumors, demyleination, trauma, and hemorrhage.
3) lesions from the entire primary visual cortex due to termination of optic radiations on lower and upper banks of calcarine fissure caused by PCA infarcts, tumors, and infection. (slide 50)
What causes contralateral superior quadrantopia ("pie in the sky")
1) Can be caused by lesions in the temporal lobe
-inferior optic radiation (blue temporal/ meyer's loop) carry info for superior contralateral visual field

E.g. right inferior optic radiations carry info for the left superior visual field.

Cause: MCA inferior division infarcts

2) Can be from lesions to the upper bank of the calcarine fissure.

Causes: PCA infarcts, hemorrhage, tumors, infections, trauma to occipital pole
With homonymous hemianopsia, is the lesion anterior or posterior to the optic chiasm?
posterior to the optic chiasm.
Is it possible to have visual defects affecting the fovea and macula?
Yes, however because the fovea has a large retinographical representation (beginning in the optic nerve and continuing to the primary visual cortex)

This, partial lesions of the visual pathways sometimes result in cental visual field being spared- thats because partial lesions would not usually affect entire fovea and macula representation in the primary visual cortex.

Macular sparing- i.e. the visual field represented by the macula is intact ("macula is spared")
What is a central scotoma?
Central scotoma is when there is a partial lesion to the PVC (primary visual cortex) specifically to the occipital pole, would primarily involve the area represented by fovea and macula. This results in a visual field loss in the center of the visual field.

E.g., a head injury involving hitting the back of the head at the bottom of the occiput could involve the area representing the central visual field in the primary visual cortex on both sides of the posterior occipital lobe.
Where does the retina receive its blood supply from?
Mainly from the branches of opthalmic artery

Impaired blood flow can be caused by emboli, stenosis, and vasculitis.

Central retinal artery (branch of the opthalmic artery) supplies inner retinal layers.
Retinal Artery has two main branches: what are the names of those branches?
The superior branch covering the superior half and the inferior branch covering the inferior half.

Occlusion of one of the arteries can cause altitudinal scotoma in one eye.

An altitudinal scotoma in one eye can result from occulsion to one of these branches.
-occulsion of the superior or inferior branch of the right central retinal (branch of opthalmicO???? slide 57
What is amaurosis fagax or "browning out"? What is the common cause of this?
It is a transient occlusion of the superior or inferior branch of the retinal artery caused by emboli.

Is sometimes described as "like a window shade" moving down or up over the eye.

This symptom should be treated like any TIA and should be considered a warning sign for impending retinal or cerebral infarct.

Common cause is ipsilateral ICA stenosis causing artery to artery emboli.
Optic Tracts, optic chiasm, and optic nerves receive blood supply from where?
Optic tracts, optic chiasm, and optic nerves receive blood supply from numerous small branches of anterior cerebral artery (ACA) and middle cerebral artery (MCA).

Clinically significant infarcts of these are therefore rarely seen.
Where does the lateral geniculate nucleus receive blood supply from? If blood supply is affected to the lateral geniculate nucleus, what impairments might this cause?
Has variable blood supply from several vessels including anterior choroidal (branch of ICA), thalamogeniculate, and posterior choroidal arteries (branches of PCA).

Impairments include:

1. Contralateral hemiparesis (due to involvement of the nearby posterior limb of internal capsule) and/or
2. associated hemisensory loss (due to involvement of nearby thalamic somatosensory radiations).
What type of deficit occurs if there is an infarct of LGN?
Contralateral homonymous hemianopia
Where do optic radiations which pass through parietal lobe receive blood supply from?
Optic radiations passing through parietal lobe receive blood supply from superior divisions of MCA
Damage/infarct to parietal (superior) optic radiations cause what type of visual field deficit?
Damage to left parietal optic radiations causes what?
Contralateral inferior quadrantopia-pie on the floor

Right inferior quadrantopia
Optic radiations passing through temporal lobe receive blood supply from?
Receives blood supply from inferior divisions of MCA.
Damage/infarct to the temporal (inferior/Meyer's loop) optic radiations cause what type of visual field deficit?
Contralateral superior quadrantopia "pie in the sky"
Damage to left temporal optic radiations causes what?
Right superior quadrantopia.
What artery supplies to the primary visual cortex ?
PCA
Large infarcts involving the entire ipsilateral primary visual cortex will cause what visual field defect?
Contralateral homonymous hemianopia
Right PCA infarct of entire primary visual cortex would cause what?
Left homonymous hemianopia.
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