Visual Plasticity Flashcards
How are development, plasticity, learning, reorganization and recovery from injury related?
They share overlapping mechanisms that are differentiated with age, environment and external challenge
Why is the visual system a model platform to study plasticity, learning, reorganization and recovery from injury?
rich stimuli conditions possible, high degree of functional organization, and importance of visually guided behavior
What is synaptic plasticity?
ability of the synapse between two neurons to change in strength or effectiveness
Where is the first site of binocular interaction in the visual pathway?
primary visual cortex
What did intravitreal injection of 3H proline (a transynaptic tracer) reveal in an adult monkey?
pattern of ocular dominance columns (layer 4 of V1 separated by eye preference)
What did intravitreal injection of 3H proline (a transynaptic tracer) reveal in a fetal monkey?
no ocular dominance columns, a continuous band of labeling in layer 4 of V1
At what week does the normal ocular dominance segregation begin in development?
week 3
By what week is the ocular dominance pattern established in development?
week 13
What is ambylopia?
a weak or lazy eye
Why do children who have congenital cataracts who undergo surgery at 10 years old remain functionally blind in the affected eye when adults undergoing an analogous procedure have restored good vision?
deficit occurred within the critical period, relates to the improper neural development of higher level function whereas in adults that is already established when the cataract occurs
In monocular deprivation, which side of the visual cortex is underdeveloped?
the side contralateral to the deprived eye
What happens to the ocular dominance columns of the undeprived eye and the deprived eye due to monocular deprivation?
the undeprived eye takes over territory normally occupied by the deprived eye (whose bands have shrunk)
What do the axonal arbors in the visual cortex look like after short term monocular deprivation?
they are shrunken and poorly developed
What is the critical period?
time window in development when connections are highly susceptible to input stimulus
How does monocular deprivation at different ages affect response?
The younger monocular deprivation occurs at, the more prominent the skewing is. The older the monocular deprivation occurs at, the more even distribution of cells in visual cortex
What does binocular deprivation lead to?
extension of the critical period and few binocular cells
What does binocular deprivation reveal about the mechanism behind the response of the visual cortex?
the issue is not disuse, but rather competition
Is critical period unique to visual development? What other systems have critical periods?
No, also involved in language acquisition and speech development, sensory-motor coordination, and social development
What is strabismus?
deviated eye, cross eye
Is strabismus a peripheral or central disorder? Ambylopia?
peripheral disorder, central disorder
How does strabismus develop?
involves extraocular muscles causing misalignment of the 2 eyes, preventing binocular vision and convergence
What is ambylopia?
lazy eye
What does ambylopia result from?
imbalance or lack of visual coordination between 2 eyes during development, leading to poor vision or acuity in 1 eye compared with the other strong eye
What does ambylopia lead to?
improper development of ocular dominance columns and improper binocular connections in visual cortex
What else can cause ambylopia besides strabismus?
refractive errors, cataracts
What two distinct issues must be corrected for ambylopia?
- must correct source of binocular imbalance to address ambylopia
- must cover good eye to weaken it/strength bad eye for match in strength
What does normal development of the eyes require?
match in strength of input from each eye and coordination of inputs
What is synchronous input?
seeing the same thing at the same time
What is stereopsis?
depth vision
What causes poor stereopsis?
improper binocular vision
How does the brain compensate for diplopia?
suppression or alternation
What is suppression?
to compensate for ambylopia, suppress one eye’s input at a time to avoid diplopia, equivalent to monocular deprivation
What is alternation?
both eyes have input to the visual cortex but take turns being suppressed, equivalent to binocular deprivation
What is the pattern of distribution of cells in binocular deprivation?
large number of cells for columns 1 and 7 (ipsilateral and contralateral eyes), no binocular cells, cells are only fed by one eye or the other
What is Hebb’s hypothesis?
cells that fire together wire together
How does Hebb’s hypothesis relate to plasticity?
it is closely linked to synaptic competition and adjusting synaptic strength
How do multiple synapses driven by a common source that are synchronously activated affect the synaptic strength? What is an example?
- activities reinforce each other and strengthens these synapses to be maintained and encourage more to form driven by this same source
- for example the nondeprived eye in monocular deprivation
In artificial electrical stimulation to optic nerves, asynchronous pulsing of the optic nerves causes…?
the visual cortex to be organized toward the extreme of ocular dominance, no binocularity in between
In artificial electrical stimulation to optic nerves, synchronous pulsing of the optic nerves causes…?
no ocular dominance columns in V1, all binocular vision because there is no way to differentiate which of the 2 optic nerves are being driven
What receptor defines the critical period?
NMDA
What do antagonists of NMDA cause?
reduced ocular dominance plasticity and refinement of receptive field properties
What experiment showed activity-based competition leads to segregation even in a system that normally lacks segregation?
3 eyed frog experiment
What was the ocular dominance-like pattern in the 3 eyed frog experiment sensitive to? What does this indicate?
NMDA blockade, prevented formation of the columns indicating NMDA’s role in ocular dominance patterning/plasticity
How is the critical period dependent on inhibitory interneurons?
If inhibitory interneurons of a different age are introduced, the critical period mirrors the age of the injected interneurons
(time window of plasticity correlates to age of inhibitory interneurons)
Why do we not normally see our blind spot in the retina?
the cortex craves input and will cover up issues seeking input by pulling info from other areas, cortical fill-in corrects blind spot
What happens after an artificial blind spot is created by burning the retina with a laser?
within minutes, the cortical territory represented by the lesion starts having activity again even though sensory input is gone, “cortical fill-in”
What occurs after retinal detachment?
retina is welded back in place with many little blind spots that cause no noticeable deficits because the V1 cortex fills them in by reorganizing and rewiring
What did the somatosensory experiments with digit amputation and digit stitching together show?
lesions lead to quiet cortex where lesion was early on, but cortex became alive again very soon after and got input from neighboring digits
How is there sensation in phantom limbs after amputation of limbs?
empty cortex in the homunculus won’t tolerate being quiet so it pulls sensory input from the area closest to it (for example, hand will pull from face so may feel sensation on missing hand if face is touched)
Does blindness lead to enhanced auditory processing?
unused visual cortex may grab information from neighboring areas as input, see this effect modestly
Will an adult blind person ever see with retinal prosthesis?
Even if it is perfect, the visual cortex is already wired incorrectly due to abnormal development and is beyond the critical period so visual processing functions would not be able to be repaired
