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The role of visual experience on development of visual cortex
- The first synapses in visual cortex are found at 23 weeks of gestation
- numbers peak at 8 months postnatal and then decline, reaching adult levels at 10 years of age.
- The fovea is not fully developed until at least 4 years of age
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If cataract is not removed in the first 1-2 months, there will be irreversible loss of visual abilities in the affected eye
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Each lateral geniculate layer gets input from only one eye, and the geniculate cells relay this output to the cortex in alternating left-eye right-eye zones.
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Most of he cells in layer 4 of the cortex are monocular
they respond only to input through the left or to the right eye
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Connections from cells in layer 4 converge in other layers
- Almost all the cells in the other layers of the cortex are binocular
- binocular cells repined best when both of the eyes are looking at the object
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Ocular dominance columns
- Most cells respond more stingily to one eye than to the other
- About half of the cells are left-eye dominated and half right-eye dominated
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Each ocular dominance column is about 1-2mm wide
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The development of ocular dominance columns
- The connections from the geniculate sort themselves out by making and breaking connections
- most before birth
- continues the first few years of life
- the basic segregation of geniculocortical proections is complete at 4-6 months postnatal
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Result of a cataract that is not corrected in time
fast majority of cells in layer 4 will respond only to the unaffected eye
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Axons that fire together, wire together!
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Model of activity-mediated segregation
- Initially, inputs from both eyes via the LGN connect to each cell in layer 4
- By the time of birth, about half the cells have more left-eye input, while others have more right-eye input
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meridional amblyopia
permanent deficiencies in acuity for lines of particular orientations
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uncorrected astigmatism can lead to
- permanent deficiencies in acuity for lines of particular orientations
- this defect need not be corrected until early grade school age
- but don't really know
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Strabismus (cross-eyedness or wall-eyedness)
- can lead to loss of stereopsis and suppression of the vision in one eye (amblyopia)
- Strabismus is often treated by surgery followed by patching the eye with the stronger vision
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