Visual Plasticity

Question 1

Lazy eye is a…?

Answer 1

developmental issue

Question 2

LGN input to…?

Answer 2

the primary visual cortex and OD columns

Question 3

OD column development

Answer 3

• before birth, the retinae produce waves of synchronous activity that drive the cortical cells well
• after birth visual input drives co-ordinated retinal activity
• the afferents from each eye fire together independently of the other eye
  
  • LTP!!!
• Receptors: NMDA receptors with Ca2+ and Mg
• synapses are strengthened or weakened depending on the relative amount of input from each eye
• The OD columns will segregate in response to this activity

Question 4

Why is developmental plasticity important?

Answer 4

to treat amblyopia

Question 5

what is amblyopia

Answer 5

• Amblyopia - lazy eye
  
  • ptosis or cataract

Question 6

OD columns and monocular deprivation in monkeys

Answer 6

• if one eye is compromised in early life then the brain makes very few connections with it ⇒ amblyopia
• an eye that may be functional yet has very poor acuity
• acuity even with optional correction

Question 7

OD columns and monocular deprivation

Answer 7

• Cortical cells strongly and consistently driven by this eye (Left)
• cortical cells are not driven by this eye (right)
• Normally - afferents fire together in response to visual stimuli
• eye is compromised at birth - unconditional spontaneous activity only
• good eye - has access to abundant cortical machinery
• bad eye - little access to the cortical machinery hence image cannot be processed in detail

Question 8

Can OD column shift be reversed? provide evidence

Answer 8

• Patching the good eye - forcing to actively work with bad eye
  
  • might shift OD columns around if done EARLY enough but with the danger of compromising vision in the good eye and reducing stereoscopic vision
  • 2 lines snellan chart improvement has been shown with on average ~ 170hrs total @ 4yo and >200hrs @ 6yo

Question 9

OD can be tested physiologically by?

Answer 9

-recording from cells in layer 4C the input layer
-after early deprivation in monkey the great majority of ells are driven by non-deprived eye
-later deprivation doesn’t case a shift in dominance

Question 10

Developmental plasticity is confined to a ‘critical period’

Answer 10

1w to 12w

Question 11

the critical period for MD

Answer 11

first 10w of life
OD pattern becomes fixed at this point

Question 12

What is the best option?

Answer 12

PREVENTION
-effects of MD develop rapidly but not simply reversible

Question 13

all these may develop AY

Answer 13

• ptosis
• cataract
• strabismus
• anisometropia

Question 14

can AY occur without a shift in OD columns?

Answer 14

-strabismic AY = lost bad eye late in life due to lost territory
-anisometropic AY = lost good eye late in life not due to lost territory

Question 15

What is the mechanisms for non-MD AY?

Answer 15

OD cells recorder in PVC area V1

Question 16

Plasticity in the intracortical circuits. Can be filling-in?

Answer 16

• Filling-in a scotoma of retinal origin
• Filling-in involves ectopic receptive fields
• Filling-in scotomas match their surroundings
• Filling-in involves expansion of receptive
• Filling-in invloves activation of existing connections

Question 17

Other mechanisms?

Answer 17

a wave of axon sprouting and pruning is seen in the region of cortex affected by a new retinal lesion
-fMRI as evidence

Question 18

The story so far

Answer 18

• The geniculocortical wiring fixes at the end of the critical period but intracortical wiring doesn’t:
  
  • many silent cortico-cortical connections exist which play a role in normal vision
  • these connections can be revealed in the short system and their synapses are likely capable of plasticity
  • axons may remain capable of growth and retractions changing connectivity
• Hence grey matter circuits can change dramatically even in adult

Question 19

Can adult plasticity be used to restore vision in AY?

Answer 19

• Perceptual learning and AY
  
  • results for intensive training in detecting vernier offsets by amblyopic adults with good eye patched
  • multiple ~1hr session
  • sig improvemets seen
  • fairly task specific e.g. maximal for the trained orientation
  • but it improved snellen acuity too (20/80 → 20/22 vision)
• adult visual performance can be improved simply through practice
• training can improve performance of an amblyopic eye
• likely to be truly effective for strabismic / anisometropic amblyopia

Question 20

Plasticity can be enhanced in adults. HOW?

Answer 20

-AY can be improved by reactivating ‘plasticity genes’