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What gave evidence of brain plasticity?
Evidence from strabismus
What is plasticity?
Brain can change in response to experience
What is strabismus?
What does strabismus lead to?
- During early life, it leads to pronounced visual defects
- Particularly in stereopsis
What is stereopsis?
What kind of visual defects in stereopsis occurs due to stabismus?
Can't effectively focus on one thing
What happens to the misaligned eye in children with strabismus?
- Children appear to suppress the image from that eye
- Don't use that eye b/c circuitry isn't normal
How can strabismus be treated?
- Covering the healthy eye for several hours a day
Why would you cover the healthy eye to treat strabismus?
- Forces them to see out of misaligned eye until surgery
- Strengthens misaligned eye
What does the strabismus treatment with covering the healthy eye imply?
There's active competition btwn the eyes for representation in the brain
What is binocularity?
- Ability of neurons to see out of both eyes
- Object image hits both retinas
What is the advantage of front facing eyes?
- If you consider the visual field, they overlap a lot
- Regions of overlap = binocular/3D vision
Which neurons are monocular?
Lateral Geniculate Nucleus neurons
Where are LGN neurons located?
In the thalamus
How many layers does the LGN have?
How do the LGN layers respond to eyes?
Alternating layers respond to left or right eye
What kind of receptive field do LGN neurons have?
Where do binocular neurons emerge?
In the visual cortex
How does binocularity give you depth perception?
- Relative distance to an object determines binocular disparity
- So by computing binocular disparity, the relative distance can be estimated
What is binocular disparity?
- Distance btwn images formed on retina
- Relative offset btwn images
How are cortical neurons (neurons in the visual cortex) driven?
By both eyes
What most likely forms the fundamental basis of stereopsis?
The fact that neurons in the visual cortex can be driven by both eyes
What's the point of this image?
Even though one eye is closed/not being used, neurons in the cortex can still be driven by either the left or the right eye
Which neurons are selective for binocular disparity?
Some neurons in V1
How is depth perception simulated?
Relative offset of objects mimics visual disparity
What does it mean to be selective for binocular disparity?
Firing depends on the distance between the images formed on retina
Why is binocular disparity selectivity useful?
Can code how far away an object is
Define ocular dominance
Most cortical neurons prefer input to one eye over the other
What does the frequency histogram of ocular dominance look like?
What happens to ocular dominance (OD) when you close one eye (say, the contralateral eye) early in life?
Those neurons that previously preferred contralateral eye shift to prefer ipsilateral eye
What plays a significant role in ocular dominance after deprivation?
Activity AND competition play a significant role
What does the preferential shift in ocular dominance after early deprivation suggest?
Input from each eye competes during development
Which region does monocular deprivation cause changes in?
- Eye closing doesn't alter receptive fields in retina or LGN
- Changes must be taking place in the cortex
What happens when you close both eyes?
- No shift in ocular dominance occurs
- Total visual deprivation decreases the number of cortical neurons that respond to visual stimulation
What hypothesis does the lack of change after binocular dominance support?
How are neurons that prefer the same eye arranged?
What do neurons situated along the vertical (perpendicular) axis have in common?
Lots of similar ocular preferences
What is this image showing us?
- Visualization of ocular dominance columns- trans-neuronal tracers
- Columns revealed in tangential sections of V1 after injection of tracer in one eye
- White - axons coming from LGN corresponding to labeled eye
- Black - other eye
Which neurons show more binocular disparity?
The neurons on the border of the columns in the tangential sections of V1
What happens to the ocular dominance columns following monocular deprivation (MD)?
- Columns restructure
- Expansion of territory of columns labeled for open eye
What does the restructuring of ocular dominance columns demonstrate?
Anatomical basis of plasticity
Is competition necessary AND sufficient? How was this tested?
- All their neurons are monocular (always cross at optic chiasm)
- Artificially drive ocular dominance by transplanting third eye
- New eye fights with other eye for territory
- Shows competition is necessary and sufficient for ocular dominance
What happens to ocular dominance columns after surgically induced strabismus?
- Sharpens the columns
- Circuits stay completely separate
- Bimodal distribution to either right or left eye
What does the sharpening of OD columns imply?
Visual defects in children with strabismus is due to ocular dominance columns that are too sharp
When does the sharpening of OD columns generally occur?
- During critical period when alterations make a big difference
- Plasticity is high
Can OD columns change after the critical period?
What is the synaptic mechanism(s), or hypotheses, of ocular dominance plasticity?
- LTD: non-active synapses are actively weakened
- LTP: active synapses are strengthened
Why are the mechanisms of ocular dominance plasticity important?
Might be fundamental to the activity-dependent wiring of neural circuits throughout the brain
In the Hebbian models for OD plasticity, what is the activity pattern of the open eye during monocular deprivation?
- Respond to visual stimuli
- Strong or correlated firing
In the Hebbian models for OD plasticity, what is the activity pattern of the closed eye during monocular deprivation?
- Weak or no responses
- Weak or uncorrelated firing
In the Hebbian models for OD plasticity, what is the result of strong or correlated firing from the open eye?
- Stronger synapses
- New synapses added
In the Hebbian models for OD plasticity, what is the result of weak or uncorrelated firing?
- Synapses weakened
- Synapses removed
What is the non-Hebbian, homeostatic model for OD plasticity?
The idea that neurons scale up or down their synaptic inputs to maintain a stable level activity
Describe homeostatic OD plasticity in terms of firing
- If there's less firing from weakened pathway, neuron scales up active synapses to make them stronger and more numerous
- This brings the firing back to normal
What causes the homeostatic OD plasticity?
Since closing one eye reduces net synaptic drive to binocular neurons, active synapses scale up their strength to maintain total drive of cell
Why is visual experience early in life critical?
For normal development of the visual system
Where does binocular info first start to converge?
In the visual cortex
Describe the preferences of neurons in V1
- Prefer one eye or the other
- Neurons with the same preference are arranged in columns
How does altering visual experience by closing one eye affect the representation of the opened eye?
Expands the representation
There's a competition between the two eyes for representation in the cortex
What kind of plasticity mechanisms are likely to be involved in visual system development?
- Homeostatic plasticity