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From the retina to the brain, there will be projections both _____ and _____.
Ipsilateral (to the same side) and contralateral (opposite side).
What is found in the lateral geniculate nucleus (LGN)?
- Located in the thalamus, there are specific regions devoted to projections from ipsilateral side and contralateral side.
- These extend even farther into the visual cortex, in layer 4 of the V1 region.
How to regions in the visual cortex respond bilaterally?
This involves neurons innervated by projections from both eyes.
How can you measure the monocular and binocular-ness of a given cell? What makes something monocular? What makes something binocular?
- You can measure the level of monocular or binocular-ness of a given cell by recording the cell and seeing the degree that a stimulus to the right eye and left eye drive the response in a neuron.
- If a neuron only responds one eye, its completely monocular.
- If it responds equally to both eyes, it is completely binocular.
According to the ocular dominance columns (cat pictures), what is considered normal?
Most cells in visual cortex can be simulated by light in either eye because, for most of the visual field, light from an object reaches corresponding spots on both retinas. So most cortical cells become binocular as the two eyes are stimulated by experience.
According to the ocular dominance columns (cat pictures), what is considered monocular deprivation? Does the deprivation apply to adulthood?
- When one eye is closed in development, it quickly loses its connection to visual cortex. If deprived long enough, the animal will become blind in that eye.
- EVIDENCE OF CRITICAL PERIODS- Similar deprivation in adulthood has virtually no effect on the connections from the eye or the ability of the cat to see.
According to the ocular dominance columns (cat pictures), what is considered one eye deviated?
If the two eyes are not aligned properly, then light from the visual field may still reach both eyes, but it does not reach the corresponding parts of the two retinas. So each cortical cell comes to listen to only one eye or the other. This cat will have very poor depth perception.
According to the ocular dominance columns (cat pictures), what is binocular deprivation?
Ironically, briefly depriving both eyes during development may have less of an effect on the connections from the eye to the cortex than depriving one eye. The reason is that the two eyes are evenly matched in their competition for connections to the cortex. However prolonged binocular deprivation in development will lead to total blindness.
What would happen if you were to perform some of the experiments done to the cats? What does this formation depend on?
- You would see effects in the size of the ipsilateral and contralateral layers in the LGN.
- This formation is activity dependent.
- It depends on a balance of inputs from the left and right side to setup a normal balance of activity in monocular and binocular neurons.
What would happen if you stitch one eye?
You would disrupt this appropriate signaling, and signals from one eye will take over the majority of the LGN and the V1, causing most cells to be monocular, and dominated by the non-stitched eyes.
The ocular dominant regions in the cortex or the LGN are dependent on what?
- The ocular dominant regions in the cortex or the LGN are time dependent.
- There is a critical period when these columns can grow.
- If you stitch the eye of a kitten, the ocular dominance columns could look very monocular. However, if you do this in an adult Cat, the normal ocular dominance have already been solidified, so they wont disappear - the circuit is resistant to change.
Eye specific segregation of visual inputs: Inputs from right and left eyes are segregated where?
In LGN and V1 (layer 4).
Eye specific segregation of visual inputs: How do binocular neurons in V1 receive input from both eyes?
Via layer 4 neurons.
What does a tracer molecule injected in one eye that is transported across multiple synapses provide?
This provides a way to trace eye-specific projection into the cortex.
What blocks segregation of OD columns?
Binocular injection of TTX blocks segregation of OD columns.
What happens if both eyes are sutured closed (binocular deprivation)?
OD columns are normal.
What does the segregation of inputs depend on? What does it not depend on?
- Segregation of inputs depends not on the absolute level of activity,
- but on the balance of activity from the eye-specific inputs.
- Activity-dependent competition
What are the mechanism for normal development?
- Initially: axon terminals from the two eyes overlap.
- Locally, inputs from one eye becomes stronger and dominate.
What are the mechanism for molecular deprivation?
- Inputs from open eye are more active, take over more territory.
- Inputs from deprived eye are less active, lose territory.
During which time is the effect of monocular deprivation permanent?
During critical period.
During which time does the effect of monocular deprivation have no effect?
After closure of critical period.
What/when is the critical period?
A period in early life during which reorganization of a neural circuit (or behavior) is susceptible to external sensory inputs (eg monocular deprivation).
What does the critical period window depend on?
The critical period window depends on the species and circuit.
Changes that occur during this period are irreversible.
- Changes that occur during the critical period are irreversible.
- Important implications for treating e.g. congenital cataracts, strabismus in young children.
The circuit is resistant to change during this time.
Once the critical period closes, the circuit is resistant to change.