Visual Pathway and Pupillary Light Reflex

Question 1

Describe the visual system?

Answer 1

1. Eyes register optical information
2. Pathways to occipital cortex
3. Two pathways from V1 (primary visual cortex)
   - “What” pathway to temporal cortex (what do I see?)
   - “Where” pathway to parietal cortex (where do I see it?)
4. Convergence on frontal cortex

Question 2

What is visual agnosia?

Answer 2

• Inability to identify objects and/or people
• Caused by damage to inferior (lower) temporal lobe
• Disruption of the “what” pathway

Question 3

What is visual neglect?

Answer 3

• Inability to see objects in the left visual field
• Caused by damage to right parietal lobe
• Disruption of the “where” pathway

Question 4

Parts of the visual system?

Answer 4

1. Eye (optics of image formation)
2. Retina (light transduction)
3. Lateral Geniculate Nucleus (waystation)
4. Area V1
5. Higher cortical areas (features)
6. Cortical pathways (what/where)

Question 5

Function of retina?

Answer 5

light transduction
- Light is detected by the sensory retina by Photoreceptors (rods and cones) which transfer information to Ganglion cells
- Ganglion cell axons enter the brain as millions of bundles forming the optic nerve

Question 5

Function of the optic tract?

Answer 5

Optic nerve fibers from the optic chiasm continue as the optic tract & terminate in the lateral geniculate nucleus of thalamus.
- Each tract contains axons that carry input from the contralateral visual field.
- Left Optic Tract receives from R. visual field
- Right Optic Tract receives from the L. visual field

Question 5

Function of the lateral geniculate nucleus (waystation)?

Answer 5

• Primary termination of Optic Tract fibers
• Each LGN receives input from the contralateral visual field.
• Optic Tract Projections to pretectum for reflexes

Question 6

Function of lateral geniculate nucleus?

Answer 6

• Primary termination of Optic Tract fibers
• Each LGN receives input from the contralateral visual field.
• Optic Tract Projections to pretectum for reflexes

Question 8

Diploplia?

Answer 8

Double vision due to failure of the image to be aligned on corresponding points of the left & right retinae

Question 9

Amblyopia?

Answer 9

• Decreased visual acuity in the absence of anatomical defects in the visual pathway
• Secondary to strabismus; to avoid diplopia, the vision in one eye is suppressed at the level of the cerebral cortex
• Occurs in children with the critical period variously reported as ending by age 3/4 yrs and up to 8 yrs.

Question 10

Scotoma?

Answer 10

Area or “island” of visual loss within the visual field

Question 11

Quadrantanopia?

Answer 11

Defective vision or blindness in approx. one-fourth of the visual field

Question 12

Hemianopsia?

Answer 12

Defective vision or blindness in approx,. one-half of the visual field

Question 14

Heteronymous defects?

Answer 14

Visual defects involving parts of both the left & right visual fields

Question 14

Congrous defects?

Answer 14

: Visual defects are equivalent in each monocular visual field

Question 15

Incongruous defects?

Answer 15

Visual defects are NOT equivalent in each monocular field

Question 16

Altitudinal defects?

Answer 16

Visual defects are in the upper or lower aspect of the visual fields

Question 17

What is RAPD?

Answer 17

relative afferent pupil defect
- Marcus Gunn Pupil = afferent defect due to damage to the Optic Nerve
- Tested with the Swinging Flashlight Test
- Intact consensual light reflex in ‘good’ eye but paradoxical pupillary dilation to direct light that ‘swings’ from the good eye to the eye with the damaged optic nerve

Question 18

Causes of RAPD?

Answer 18

anything causing damage to the afferent portion of the pathway
1. optic neuritis
2. anterior ischemic opric neuropathy
3. glaucoma
4. optic nerve tumours
5. orbital disease
6. ischemic retinal diseases
7. Ocular ischemic syndrome
8. chiasmal compression
9. optic tract lesion