Visual Field Disorders
15 important questions on Visual Field Disorders
In what 4 structures (roughly) does visual information go through? (starting from the eye and ending with V1)
2. optic chiasm
3. LGN
4. V1
What are 4 components of the visual system?
- visual pathway
- primary visual cortex (V1)
- visual association cortex
What are the differences and similarities between the eye and a camera?
- image is inverted and focused onto a light-sensitive surface
- amount of life is controlled
Differences:
- lens change shape (thinner/ fatter) rather than distance to focus
- retina is curved and is not uniformly sensitive
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What is the topographical organisation of the visual system?
How many rods and cones are in the retina?
6 million cones
What are the characteristics of rods?
- sensitivity of rods to low light results from the fact that they pool their responses to influence the next layer of cells
= means you can see in low light but you sacrifice detail
(rods also respond to bright light, but the pigment quickly depletes and the rods cease to function until it is replenished - p. 185 neuropsychology textbook)
What are the characteristics of cones?
- high acuity vision because few receptors funnel their input to a postsynaptic cell
- 3 types of cones:
1. one that responds to long wavelengths (red part of spectrum)
2. one that responds to medium wavelengths (green part of spectrum)
3. one that responds to short wavelengths (blue part of spectrum)
= the activity of these 3 types of cones gives us the ability to see in colour
What is the consequence of unequal distribution of rods and cones on the peripheral visual field and the fovea?
Fovea: lots of cones and few rods = colour vision and high acuity but poor light sensitivity
What are the 2 visual pathways?
- visual perception
2. Retino-tectal: 10% of optic neurons (contains the pulvinar nucleas of the thalamus and the superior colliculus of the midbrain)
- visual attention and orienting
How many layers is the LGN made up of?
- the M cell sends output to the 2 bottom layers
- the P cell sends output to the 4 top layers
What are the 2 types of optic radiations?
2. Superior - fibres representing superior visual field i.e. info below eye level
What is retinal detachment?
Result = peripheral vision loss & visual distortion - not detecting light from the right part of the space
- can be reattached if detected early
What is meant by perimetry and what are 2 methods to measure this?
1. Confirmation - the patient confirms what he can't/ can see
2. Perimeter (more accurate than method 1)
What is hemianopic dyslexia?
Severe reading difficulties despite intact language functions
Types of errors:
- omission of part of sentence or words e.g. "sunshine" - would omit "sun" or "shine"
- substitution of part of word e.g. would read "room" as "roof" etc
What does the extent of reading difficulties depend on?
i.e. left or right?
i.e. foveal sparing or splitting?
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