Plasticity in the nervous system.

Question 1

What is amblyopia?

What are the potential causes?

How is it currently treated?

Answer 1

A decrease in visual acuity in 1 or both eyes, caused by visual deprivation or abnormal binocular interaction.

Strabismus (50%) - abnormal alignment of the eyes
Anisometropia (17%) - differing refractive power of the eyes
Strab and aniso (30%)
Visual deprivation (<3%) - cataracts, ptosis

Occlusion of the good eye: MOTAS 2004 - Increasing dose rate beyond 2 h/d hastened the response but did not improve outcome

Question 2

How does filling in work?

Which parts of the visual cortex are involved in this?

Answer 2

a) Das and Gilbert 1995 - Receptive field (RF) sizes of neurons in adult primary visual cortex are dynamic, expanding and contracting in response to alternate stimulation outside and within the RF over periods ranging from seconds to minutes.

Monocular lesion –> displacement of RFs of V1 cortical binocular cells in the deprived eye.

• These ectopic RFs’ properties match those of the natural field.
• Retinal cells can alter their receptive fields if a scotoma develops. The receptive fields will surround the blank space. (Calford et al 2000)

It’s thought that a retinal lesion uncovers the ectopic field driven by existing, normally sub-threshold lateral connections. The cortex then interprets the information as coming from the original RF.

b) V2 and 3

Question 3

What evidence is there for plasticity in the visual cortex with

a) geniculocortical wiring?
b) intracortical wiring?

Answer 3

a) this fixes at the end of the critical period: in humans this is thought to be 1 year.
Beyond this time, monocular deprivation doesn’t alter the OD columns in layer 4C.
Prior to this, the earlier the monocular deprivation, the greater the shift in OD columns in favour of the sound eye as shown by Horton and Hocking 1997

b) Intracortical plasticity persists into adulthood
- Blakemore et al 1978, showed that OD columns in monkeys can be restored in layers other than 4C, beyond the critical period.
- Further evidence from filling in - axons sprouting and pruning been identified in the region of cortex affected by a retinal lesion.

Question 4

How can we use adult plasticity to restore vision in amblyopia?

Answer 4

Intensive training
- Levi and Polat 1997 - demonstrated fairly task specific, strong improvements in amblyopia adults following intensive training in detection of vernier offsets. - improved vernier acuity, maximal for the trained orientation.

• Li et al 2007 - improved positional acuity and visual snellen acuity (transferred) following intensive training with position discrimination in 2 children with untreated severe amblyopia (9+12) - improvement was beyond what would have been expected with the traditional patch treatment.

Use of video games
-Li et al 2011 - explored the use of video games in adults for 2hours/day for 20 days - found significant improvement in visual and position acuity and also spatial attention. 5x greater than patching alone.

Question 5

What changes occur with…

a) short term monocular deprivation?
b) long term monocular deprivation?

and. ..
c) do anatomical changes in OD columns affect physiology?

Answer 5

a) If reversal within the critical period, you get restoration of OD columns in all layers. If after, OD columns can shift elsewhere.

b) Can restore OD columns even after MD into the post critical period in layers other than 4C, providing there is some visual input into the eye.
- If MD is initiated before eye opening, ODs not restored hence why OD columns of people with uncorrected strabismus or anisometropia who received visual input during the critical period will have normal OD columns, unlike animal suturing models.

c) a loss of binocular cells and orientation selective cells can affect visual acuity.