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Neuro > Visual System II- Pathways and Target Areas > Flashcards

Visual System II- Pathways and Target Areas Flashcards

1
Q

What do the pathways in the visual system consist of?

A
  1. Central Projections of Retinal Ganglion Cells

2. Subcortical Regions

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2
Q

What do the central projections of retinal ganglion cells consist of?

A
  1. Optic Nerve
  2. Optic Chiasm
  3. Optic Tract
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3
Q

How is the optic nerve formed?

A

Axons of the retinal ganglion cells conveying input from all areas converge on the optic disc–> Penetrate the choroid and sclera to form Optic Nerve.

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4
Q

What is the structure of the optic nerve?

A

Extends from the caudal aspect of the eye to the optic chiasm.

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5
Q

What is the function of the optic nerve?

A

Conveys fibers from the retina of one eye

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6
Q

What happens when there is a lesion in the optic nerve?

A

Interrupts visual input from that eye, which results in an blind spot or blindness of that eye.

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7
Q

What is impacted if a person can only see through 1 eye?

A

depth perception

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8
Q

Where is the optic chiasm located?

A

rostral to the infundibulum

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9
Q

What is the function of the optic chiasm?

A
  • Fibers from the nasal hemi-retinas cross to enter the contralateral optic tract.
  • Fibers from the temporal hemi-retinas enter the ipsilateral optic tract.
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10
Q

How are visual defects named?

A

Always according to the visual field loss, and not according to the area of the retina that is not functioning.

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11
Q

What happens if there is a lesion in the midline of the optic chiasm?

A
  • Nasal Hemi-Retinal fibers are cut
  • Interrupts Temporal Visual Hemifield
  • Loss of peripheral vision, losing monocular crecents on each side of the field
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12
Q

What happens if there is a lesion to the lateral part of the optic chiasm?

A
  • Temporal Hemi-Retinal fiber is cut
  • Loss of Nasal Visual Hemifield on the ipsilateral side
  • Right hemianopia of the left eye OR left hemianopia of the right eye
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13
Q

What is the structure of the optic tract?

A

Extends caudolaterally over the surface of the crus cerebri at the midbrain at its junction with the hemisphere and ends in the lateral geniculate nucleus of the thalamus.

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14
Q

What is the function of the optic tract?

A

Each 1/2 of the brain receives the fibers corresponding to the contralateral half of the visual world.

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15
Q

What would happen if there was a lesion of the optic tract?

A
  • Ipsilateral Temporal Hemi-Retina Fiber and Contralateral Nasal-Hemi Retina Fibers are cut.
  • Loss of Ipsilateral Nasal Visual Hemifield and Contralateral Temporal Visual Hemifield.
  • Results in Contralateral Hemianopia, only see ipsilateral side of the visual field.
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16
Q

What do the subcortical regions consist of?

A
  1. Suprachiasmatic Nucleus
  2. Superior Colliculus
  3. Pretectum
  4. Lateral Geniculate Nucleus
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17
Q

Where do most of the retinal ganglion cells terminate?

A

Lateral geniculate nucleus

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18
Q

What is the structure of the Suprachiasmatic Nucleus?

A

Small collection of neuron cell bodies located above the optic chiasm in the hypothalamus

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19
Q

What is the function of the Suprachiasmatic Nucleus?

A

Controls diurnal rhythms, visceral functions that are influenced by light levels of day/night cycles.

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20
Q

What is the structure of the Superior Colliculus?

A

Alternating gray and white layers located in the tectal portion of the midbrain.

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21
Q

What is the function of the Superior Colliculus?

A
  • Receives extensive cortical input, including auditory and somatosensory inputs
  • coordinates head and eye movements to visual targets
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22
Q

What is the structure of the Pretectum?

A

A collection of neuron cell bodies located rostral to the superior colliculus where the midbrain fuses with thalamus.
-Where retinal ganglion cell projections synapse with pretectal neurons that project bilaterally to the Edinger-Westphal nucei.

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23
Q

How are the Edinger-Westphal nuclei and occulomotor nerve related?

A

The Edinger-Westphal nuclei contain pre-ganglionic parasympathetic neurons that send their axons via the oculomotor nerves to terminate on the ciliary ganglia.

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24
Q

What is the function of the ciliary ganglia?

A

The neurons innervate the sphincter pupillae of the irises of the eyes

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25
Q

What is the function of the Pretectum?

A

Coordinates the pupillary light reflex

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26
Q

Under normal conditions, how would the pupil of the right eye react if light was flashed into the left eye?

A

Both pupils would constrict

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27
Q

What would it mean if light was directed at the left eye but the right eye did not constrict?

A

Something would be wrong with motor outflow of the right eye

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28
Q

What would it mean if light was directed at the left eye and it did not constrict, but when light was directed at the right eye, both eyes constricted?

A

Something would be wrong with sensory input from the left eye

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29
Q

What would it mean if there was an absence of pupillary light reflexes in an unconscious patient?

A

Damage to the midbrain because the oculomotor nerve originates there.

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30
Q

Describe the structure of the Lateral Geniculate Nucleus

A

A collection of six layers of neuron cell bodies separated by intralaminar layers of axons and dendrites located in the dorsal thalamus.

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31
Q

Where does each layer of the Lateral Geniculate Nucelus receive projections from?

A
  1. Either the contralateral nasal hemiretina or ipsilateral temporal hemiretina.
  2. Either the M or P retinal ganglion cells.
32
Q

How are receptive fields in the lateral geniculate nucleus similiar to the retinal ganglion cells?

A
  • Circular

- Have a concentric center-surround organization

33
Q

How are receptive fields in the lateral geniculate nucleus different from the retinal ganglion cells?

A
  • Larger (due to convergence)

- Show a greater sensitivity to contrasts

34
Q

After the Lateral Geniculate Nucleus, where is the next place the signals project to?

A

Go to the primary visual cortex via the optic radiations that go through the sublenticular and retolenticular parts of the internal capsule.

35
Q

What is the pathway of the fibers representing the inferior retinal quadrants?

A

Inferior retinal quadrants= superior visual quadrants

-Loop into the temporal lobe before turning caudally towards the primary visual cortex

36
Q

What is the pathway of the fibers representing the superior retinal quadrants?

A

Superior retinal quadrants= inferior visual quadrant

-Run caudally straight back toward the primary visual cortex.

37
Q

What is the function of the lateral geniculate nucleus?

A
  • Unclear

- Control the flow of information from the retina to the cortex

38
Q

What would happen if there was a lesion of the lateral geniculate nucleus or the entire optic radiation?

A

Same deficit as seen with a lesion of the optic tract

39
Q

What would happen if there was a lesion of the optic radiation?

A

May involve visual input from the quadrant of the ipsilateral nasal visual hemifield and the contralateral temporal visual hemifield, resulting in a right or left, superior or inferior quadrantanopia.

40
Q

What are the target areas of the visual system?

A
  1. Primary Visual Cortex

2. Extrastriate Visual Areas ( Secondary Visual Cortex and Posterior Parietal Cortex)

41
Q

Where does the Cuneus receive input from?

A

Superior Retinal Quadrant

Inferior visual field

42
Q

Where does the Lingual Gyrus receive input from?

A

Inferior Retinal Quadrant

Superior visual field

43
Q

Describe the structuer of the Primary visual cortex.

A
  • Central part (fovea)= large portion of the primary visual cortex
  • Peripheral Part= located rostrally, smaller part of visual cortex
44
Q

What is the composition of the Primary Visual Cortex?

A
  • Six layers of cell bodies
  • Layer 4 receives input from the lateral geniculate nucleus
  • Layer 2&3 receive input from neurons in the intralaminar layers of the lateral geniculate nucleus, where they innervate patches of cells called blobs
45
Q

How is the primary visual cortex organized?

A
  • 3 major vertically oriented systems:
    1. Orientation Columns
    2. Ocular Dominance Columns
    3. Blobs
46
Q

What are the Orientation columns in the Primary visual cortex?

A

Contain neurons that respond selectively to bars of light with specific axes of orientation.

47
Q

What are the ocular dominance columns in the primary visual cortex?

A
  • superimposed upon the orientation column organization
  • contain neurons that respond selectively to inputs from one or the other eye
  • critical for binocular vision
48
Q

What are blobs?

A
  • Peg shaped region only located within layers 2&3

- Contain neurons that respond to different color stimuli

49
Q

How do the vertically oriented columns in the Primary visual cortex communicate with each other and what do they form collectively?

A

By means of horizontal connections that link cells within a layer
-Form hypercolumns which represent visual properties of one region of the visual field

50
Q

What are the 4 different types of neurons seen in the Primary Visual Cortex?

A
  1. Concentric Center-Surround
  2. Simple
  3. Complex
  4. Hypercomplex
51
Q

Describe the Concentric-Center surround neurons.

A
  • Same as retinal ganglion and lateral geniculate neurons

- Associated with layer 4

52
Q

Describe the simple neurons.

A
  • Circles that are overlapping
  • Represent the receptive fields of many concentric center-surround lateral geniculate neurons
  • Respond best to a bar of light with a specific orientation in a specific position within the receptive field (excitatory and inhibitory zones)
  • Associated with neurons in layer 4
53
Q

Describe the complex neurons.

A
  • Respond best to a bar of light with specific orientation, but the specific position within the receptive field is less crucial
  • No excitatory or inhibitory zones
  • Represent the receptive fields of several neurons with orientated simple fields
  • Associated with neurons in layers 2, 3, 5, and 6.
54
Q

Describe a hypercomplex neuron

A
  • Respond to stimuli similar to neurons with complex fields, however lengthening the bar of light past a limit results in a weaker response
  • Associated with neurons in layers 2, 3, 5, and 6.
55
Q

What is macular sparing?

A

Occurs because the area of cortex representing the macula is large and has a collateral blood supply

56
Q

What are the two major parallel processing pathways, and how are they organized?

A
  1. Dorsal Pathway
  2. Ventral Pathway
    - Hierarchically organized
57
Q

Where does the dorsal pathway receive input from?

A

The Magnocellular (M) pathway in the primary visual cortex

58
Q

What kind of information does the dorsal pathway process?

A

Information related to motion (location, speed, and direction) and depth.
-“where” pathway

59
Q

What region does the dorsal pathway go to?

A

Middle Temporal Area–>Posterior Parietal Cortex

60
Q

How do we understand that something is moving?

A

The middle temporal area contains neurons with large receptive fields that are sensitive to spots or bars of light by detecting differences in luminance, texture, and color

61
Q

What mechanism does depth perception require, and what are the two categories of depth perception?

A
  • One that converts a 2D retinal image into 3D
    1. Far field (>100’)
    2. Near Field ()
62
Q

How do we perceive depth in a far field?

A

With one eye by relying on monocular depth cues

63
Q

What are some examples of monocular depth cues?

A
  • size of an object
  • parallel lines converging
  • size perspective
  • motion parallax
64
Q

How do we perceive depth in a near field?

A

with both eyes, by utilizing the disparity in the retinal images to estimate distance.
-V1, V2, V3, V5 contain neurons that are sensitive to binocular disparity, they respond when the inputs from the two eyes are spatially disparate on the two retinas

65
Q

A lesion in the middle temporal area could result in what two injuries?

A
  1. Cerebral Akinetopsia

2. Decreased depth perception

66
Q

What is Cerebral Akinetopsia?

A

a deficit in detecting the speed of movement. An individual with such a deficit would have difficulty pouring a drink, following a dialogue, and crossing the street.

67
Q

A lesion of the posterior parietal cortex would result in what?

A

Visual neglect, a deficiency in attentiveness to the left visual field.

68
Q

Where does the ventral pathway receive input from?

A

Parvocellular and Magnocellular pathways in the Primary Visual Cortex.

69
Q

What type of information does the ventral pathway process?

A

Info related to high acuity and color

-“what” pathway

70
Q

What region does the ventral pathway go to?

A

Inferior Temporal Cortex

71
Q

How do we process high acuity and color information?

A

The inferior temporal cortex has very large receptive fields that are sensitive to a variety of shapes and colors OR are selective only for shape or color.
-some neurons only respond to specific complex stimuli such as the hand or face

72
Q

A lesion of the inferior temporal cortex would result in what type of injuries?

A
  1. Associative Agnosias

2. Cerebral Achromatopsia

73
Q

What is associative agnosias?

A

Deficits where objects are perceived but devoid of meaning.

74
Q

What is prosopagnosia?

A

The inability to recognize faces

75
Q

What is cerebral achromatopsia?

A

Deficits in seeing color

76
Q

Explain how visual attention works.

A
  • Only a fraction of the information available is processed by the two retinas
  • limits the info that reaches the highest centers of processing
  • filters out features and sharpens the perception of others

Neuro (31 decks)

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