Visual Pathways (Special Senses) Flashcards

1
Q

Describe the path of information starting at the retina, to the LGN.

A

Information goes down optic nerve to optic chiasm, and then optic tract, and then to LGN (main synapse before entering brain proper).

At the chasm, information on the temporal (outside) half of each 
eyeball remains on the outside half of the chiasm  (does not cross over) but info 
on nasal (inside) side does cross over at the chiasm. As a result, everything on R side of visual world is dealt with by the L half of the brain and vice versa.
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2
Q

Describe the structure of LGN.

A

Six layers:

  • Layers 1 and 2 receive signals that travel by the Magnocellular pathway (through magnocellular cells)
  • Layers 3 through 6 receive signals that travel by the Parvocellular pathway (through parvocellular cells)

Info from contralateral eye goes to LGN layers 1, 4, 6
Info from ipsilateral eye goes to LGN layers 2, 3, 5

From the right retinal hemifield of each eye (the nasal hemifield of the L eye, and the temporal hemifield of the right eye), the visual information is routed to the right LGN.
VICE VERSA

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3
Q

Distinguish between magnocellular and parvocellular ganglion cells.

A

MAGNOCELLULAR Ganglion Cells
• Large axons – lots of myelin – respond quickly
• Receive information from large number of photoreceptors
• Movement, brightness, depth perception
• Project to parietal lobes – the ‘where’ stream

PARVOCELLULAR Ganglion Cells
• Thin axons – less myelin – respond slowly
• Receive information from small number of photoreceptors
• Detail of objects assisting in recognition
• Project to temporal lobes – the ‘what’ stream

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4
Q

Describe the path of visual information after the LGN.

A

Information form the LGN go to the occipital lobe (specifically the striate cortex, first area to receive input from LGN)

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5
Q

Describe the connection between LGN and striate cortex.

A

Line of Gennari – Myelinated fibres running from LGN to synapse in layer 4 of the ‘striate’ cortex

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6
Q

Describe the relationship between spots on the retina and location on the occipital lobe.

A

• Retinotopic map – direct physical relationship between spots on the retina and spots in the occipital lobe (topographic relation)

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7
Q

Define neural tuning, in the context of vision.

A

Specific populations of cells in the occipital lobe will respond to;

  1. Colour
  2. Spatial frequency
  3. Orientation
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8
Q

Describe the retinotopic map in the occipital lobe.

A

Center part of the retina (fovea) corresponds to most posterior aspect of the occipital lobe

Superior part of retina projects to part of occipital lobe inferior to part of occipital lobe to which inferior part of retina projects

Refer to image on slide 10

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9
Q

Identify the different part of the vision pathway (from retina to occipital lobe) which can be damaged, and state the effect on vision. Draw these.

A

1) Lesion in L/R superior temporal retina: corresponding field defect in ipsilateral inferior nasal visual field
2) Complete lesion of L/R optic nerve: total blindness in corresponding eye

3) Chiasmal lesion: bitemporal hemianopia (only damages information that crosses over, i.e. info that comes from nasal retina, which sees the
temporal hemifields, so get bitemporal hemianopia)

4) Lesion of L/R optic tract: contralateral incongruous hemianopia (e.g. if lesion of left optic tract then right incongruous hemianopia)
5) Lesion of inferior optic radiations in temporal lobe: homonymous superior quadrantanopia
6) Involvement of optic radiations: homonymous inferior quadrantanopia (upper L optic radiation would affect bottom R quadrant)
7) Congruous incomplete homonymous heminopia
8) Lesion in R/L hemisphere (e.g. occipital lobe infarct): Homonymous hemianopia in contralateral side

Refer to slide 13 for images

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10
Q

Which is the least common site of hemianopia ?

A

Optic tract

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11
Q

Which kinds of damage along the vision pathway do not affect pupillary light reflex ?

A

Pupillary light reflex not impaired if beyond tract

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12
Q

Identify possible causes of chiasmal lesion, and the associated visual defect. Draw these.

A

1) Pituitary lesion (pituitary is in the midline, and close to the chiasm, so can damage the latter if grows)
2) Craniopharyngioma

EFFECT: BITEMPORAL HEMIANOPIA (Bottom part of retina remains in bottom part of chiasm, and sees top of visual field, and vice versa. Hence, pituitary lesion associated with denser hemifield loss at top of visual field because tend to come from bottom, and vice versa in craniopharygnioma)

Refer to slide 17

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13
Q

Identify a possible cause of retinal lesion. Draw the pattern of visual loss associated with this.

A

Age related macular degeneration

Refer to slide 15

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14
Q

Explain how quadrantanopias occur. Draw these.

A

As information moves from eyeball, to chiasm, to the occipital cortex, it actually splits anatomically. Info which came from the top part of the retina goes to the parietal lobes, and the info from the bottom part
of retina goes through the temporal lobes before gets to occipital lobe.

Damage exclusively to right temporal lobe (e.g. due to CVA), will result in L visual field problem, specifically affecting the superior hemifield (because that info has come from lower part of retina), so
L superior quandratnanopia (R inferior temporal lobe
problem)

Parietal lesions (e.g. CVA) lead to inferior quadrantanopias affecting the side of the visual field contralateral to the lobe involved.

Refer to slide 20

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15
Q

Identify techniques to test visual field.

A

SIT OPPOSITE PATIENT, AT SAME HEIGHT

-Ask patient, ‘while looking at me, can you see all of my face, and the surrounding room ? ‘
-Present your open palsms in the temporal quadrants ‘point to the hand that is waving’
-Present targets to the patient:
With the patient’s R eye covered/your L eye closed (and vice versa)
In the plane midway between two
Moving from periphery to the center along a line of 45o
Without crossing the vertical or horizontal meridian
-Start with a white cotton bud, moving all the way from periphery to fovea
-Map out macular function with a red cotton bud ‘tell me when you see this as red’
-Map out the blind spot, moving from non-seeing towards seeing areas

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16
Q

Describe the path of visual information in the brain following its arrival in the occipital lobe.

A

There are two separate functional streams of visual information that complement each other creating the overall sense of vision:

1) VENTRAL PATHWAY- Vision perception
-Concerned with the “what” of the visual inputs.
-Anatomically, the ventral stream projects to the temporal lobe (including infero-temporal cortex)
-In this stream:
• Distinguishes spatial patterns, objects and faces (also processes colors and shapes)
• Stores visual memory
• Recognises significance of objects and faces

2) DORSAL PATHWAY- Vision for action
-Concerned with the “where” and “how” of visual inputs.
-Anatomically, the dorsal stream projects mainly to the parietal lobe.
-In this stream:
• Integrates motion vs object locations (to facilitate navigation through that space)
• Coordinates visual guided action for skilled
movements (i.e. manipulation of objects and facilitation of eye-hand coordination)
• Guides visual attention

17
Q

Distinguish between the roles of L and R temporal lobes.

A

R side of temporal lobe more concerned with face recognition, orientation, and route finding whilst L side more concerned with abstract shapes and forms, letters, and numbers

18
Q

As part of the process of vision, what occurs following the dorsal and ventral streams ?

A

Frontal lobe effects motor change, allowing you to decide on an appropriate plan of (motor) action

19
Q

Which stream is involved in the optical illusion on slide 30 ? How so ?

A

Dorsal stream (parietal lobes), struggling to appraise that overall visual scene, due to too much information

20
Q

What sorts of visual impairments are found in pre-mature babies ?

A

In premature babies, dorsal stream is affected in premature baby brain injury. This is associated with Simultanagnosia, form of visual dysfunction in which they are very bad at appraising overall visual scene, dealing with complexity and clutter, and seeingone thing amongst many, need to simplify visual world for them to allow them to see clearly.

21
Q

What is the Ebbinghaus illusion ?

A

Optical illusion of relative size perception

22
Q

How can you acquired a simultanagnosia ?

A

From stroke affecting both parietal lobes.

23
Q

Identify possible effects of loss of R temporal lobe on vision.

A

R temporal lobe responsible for face recognition, orientation, and route finding. Hence loss of this results in:

  • Loss of face recognition; Prosopagnosia
  • Loss of route finding; Topographic Agnosia
24
Q

Identify possible effects of loss of L temporal lobe on vision.

A

L temporal lobe responsible for abstract shapes and forms, letters, and numbers.

• Inability to read words; Alexia

25
Q

Define cerebral achromoatopsia, and explain its causes.

A

Colour vision loss

Possibly acquired, due to stroke affecting both lobes (also results in dorsal stream damage, so inability to survey whole vision scene)

26
Q

Describe development of vision from a neonate to adult vision.

A

NEONATE: very poor vision, no color, very poor contrast, no conscious awareness of objects (knowing what they are), very poor acuity
-Only aspect of vision that is hardwired in neonates is the idea of orientation of things that look like faces (from week 1, will fix on mom’s face at close ranges, and will prefer shapes on slide 49, resembling a face, to shapes in slide 50)

Over 4, 6, 9 months, vision improves.

18 MONTHS: Vision quite adult like, except visual acuity which is still a little reduced (about 6/18)

27
Q

Graph binocular acuity as a function of age.

A

Refer to slide 58.

-First year or two of life, massive acceleration in quality of vision

28
Q

What may happen to long term development of vision if a neonate receives a knock affecting vision ? if insufficient oxygen or blood supply fo brain as a neonate ?

A

May cause vision not to develop normally in the first few months, then develops late (delayed visual maturation) but may end up with still normal vision
HOWEVER, if insufficient oxygen or blood going to brain as a neonate, may get irreversible damage to brain, and to vision

29
Q

Which cells are damaged in glaucoma ?

A

Ganglion cells

30
Q

What proportion of the brain is devoted to vision ?

A

40%

31
Q

What is the main cause of irreversible blindness in the world ?

A

Glaucoma